1. Technical Field

The present invention introduces a process and apparatuses for washing a mass of crystal-mother liquor mixture to thereby produce a mass of highly purified crystals. A highly efficient fractional solidification process is obtained by combining the present crystal washing process with a crystallization operation and a crystal melting operation; an efficient purification system is obtained by combining a crystal washer of the present invention with a freezer and a melter. The crystallization operation may either be a direct contact operation or an indirect contact operation.

2. Background art

^■ — In chemical industries, crystals obtained by fractional solidif cation process are purified broadly by two types of separation processes: (a) by counter 5 washing of crystals, and (b) by centrifugal separation of mother liquor. These two types of crystal purification processes are reviewed in this section.

2 - A. COUNTER-WASHING OF CRYSTALS

In recent years, a considerable number of fractional 0 crystallization techniques have been developed. These techniques are described in details in the book entitled "Fractional Solidification", edited by M. Zief and W.R. Wilcox and published by Marcel Dekker, Inc., New York in - 1967. There is a step of separating crystals from mother -5 liquor and washing the crystals in a fractional solidifi¬ cation process, and the product purity depends on how effectively this step has been conducted.

Column crystalliztion was conceived by P.M. Arnold and described in U.S. Patent 2,540,977 (1951) . The prσ- 0 cess is conducted in a system that comprises a freezing section, a purification section and a melting section. The crystals and the adhering liquid are conveyed from the freezing section through the purification section to the melting section, where melt liquid is formed. A 5 fraction of this melt liquid is removed as the high melting product. The remaining fraction is returned to the purification section as a free liquid for differen¬ tial countercurrent contacting with the crystals.. The impurities in the crystals, in the adhering liquid and in 0 the retained liquid are transferred to the free liquid and are removed from the column as constituents of the low melting product. Thus, it is seen that column cry¬ stallization is analogous to distillation in a packed tower. One of the best examples of the successful com- 5 ercial application of countercurrent column fractional

crystallization is the Phillips process that has been described by D.L. McKay in Chapter 16 of the "Fractional Solidification" book described. The process is based on inventions made by P.M. Arnold (U.S. Pat. 2,540,999 ₅ (1951), 2, 540,083 (1951), and U.S. Pat. Re 24,038

(1955)), J. Schmidt (U.S. Pat 2,617,274 (1952) and U.S. Pat. Re 23,810 (1954)), J.A. Weedman (U.S. Pat. 2,727,001 (1956)), and R.W. Thomas ( U.S..Pat. 2,854,494 (1958)). In this process, chilled slurry feed, from a scraped-sur- 10 face chiller, enters at the top of a purification column. The crystals are forced down by means of a piston and impure liquid is removed through a wall filter. Wash liquor, produced by melting purified crystals at the bottom of the column, is transported upwards counter-cur¬ ie rently to the crystals. The wash liquor may be pulsed upwards. Schildknecht column crystallizer, described in chapter 11 of the "Fractional Solidification" book des¬ cribed, uses a spiral placed in a column defined by two concentric tubes and is rotated to convey the crystals in o the desired direction.

The counter washer used for sea water desalination by Colt Industries comprises a vertical column and screens provided at the vertical wall. An ice-brine slurry enters at the bottom of the vertical column. The 25 ice crystals are carried upward by the stream of brine. Further up, the ice crystals consolidate into a porous plug which moves continuouly upward by means of a pressure difference maintained across the plug. Excess brine drains through screens that are located about 0 midway between the top and the bottom of the column.

Wash water is introduced at the top of the column. Its velocity down the column with respect to the upper velocity of the ice plug is oαJ.y that much above z-ero to compensate for dispersion of the fresh water-brine 5 interface. The purified ice is harvested at the top .of the column, A detailed description is given in United States Office of Saline Water R and D Report No. 491, issued in October 1969. f

J.w. Mullin has described the TNO process on page 250_, Vol. 7 of "Encyclopedia of Chemical Technology", edited by Kirk and Othmer and published by Wiley Co. In this process; separation is effected by countercurrent washing coupled with repeated recrystallization facilitated by impacting the crystals during their transport through a vertical column. Impacting is achieved by balls bouncing on sieve plates in the vertical column.

The Brodie Purifier has been used in commercial operations since 1974 and is available through Nofsinger Corp., in Kansas City, Missouri, U.S.A. The Brodie Purifier uses several rotating helical ribbon tubular crystallizers and has a recovery section, a refining section, a purifying section and a crystal melting section. Feedstock enters the plant at the feed inlet point, located between the recovery and refining sections. The internal stream is continuously cooled under controlled conditions as it flows through the tubes of the recovery section. The portion of the feed that has been depleted of product component (and contains most of the impurities) leaves the plant as residue. Crystals of product material are produced and settle in their own mother liquor. These crystals are mechanically conveyed towards the refining section, countercurrent to the internal liquid stream, by a low speed helical ribbon. As the crystals are transported through the richer mother liquor, they continue to grow in size and also increase in purity. After passing through the refining section, the crystals settle by gravity in the purifying section. Here, a heater melts the purified crystals; a portion of the melt is extracted as product, and a portion is refluxed and rises coun±ercurrent to the bed of crystals in the purifying section. Cooling is achieved by means of a closed coolant system, flowing countercirerent to the process liquid. Small ^" heat imputs are made to all unjacketed and unscraped surface to prevent uncontrolled crystallization.

2 - B. Centrifugal SepaLation of Mother Liquor from

Crystals *- '

Commercial centrifuges may be divided into two broad types,, viz. centrifugal filters and sedimentation 5 centrifuges, each of which can be further subdivided _* according to the means provided for advancing and discharging the separated solids and liquid phases. The cost of a centrifuge depends on the centrifugal force it delivers. A centrifuge with a low centrifugal force is Q quite inexpensive. Examples are washers used in washing clothes. A centrifuge with a high centrifugal force can be very expensive. Centrifuges used in chemical industries for crystal purifications deliver centrifugal forces that are 2,000 to 100,000 times that of a 5 gravitational force and are rather expensive. Detail descriptions of centrifuges used in separating solid-liquid mixtures have been presented by Charles M. Ambler in Section 4.5 of "Handbook of separation Techniques for Chemical Engineers", edited by P.A. 0 Schweitzer and published by McGraw Hill Co. in 1979, and have also been presented by A.C. Lavanchy et al in Vol. 5 of the "Encyclopedia of Chemical Technology", edited by Kirk and Othmer and published by John Wiley and Sons, Co.

2 - B - a. Centrifugal Filters

5 The centrifugal filter supports the particulate solids phase on a porous septum through which the liquid phase is free to pass under the action of centrifugal force. An important parameter is the permeability of the filter cake under the applied centrifugal force. Except 0 for very special applications, they are generally applied only to the dewatering of ^" relatively free-draining solids. Feed slurry concentration and particle s ze distribution are important factors in centrifugl filter . performance. The performance of a centrifugal filtering 5 operation is„..measured by the amount of mother liquor retained on the drained cake, and is expressed by S-value

defined as volumn of mother liquor retained per volumn of solid. The_S-value at a given period of centrifuging decreases as the centrifugal force increases. Therefore, in a conventional centrifugal filter, a high centrifugal force is applied to obtain a low S-value and thereby reduce the amount of retained mother liquor in the drained cake. Such a centrifuge is expensive. Crystals on a centrifugal filter may be washed with a wash liquid.

Efficiency of washing on a centrifugal filter is usually rather poor, because the residence time of wash liquid is short and the cake on the filter is not properly agitated.

2 - B - b Sedimentation Centrifuges

The sedimentation type of centrifuge simulates and amplifies the force of gravity, by a factor of from 2 to 5 orders of magnitude in the commercial sizes. As in the gravitational field, there must be a difference between the density of the dispersed particle and that of the liquid phase it displaces to cause this particle to migrate—away from the axis of rotation if the difference is positive, and toward it if the difference is negative. In the sedimentation type centrifuge, the amount of mother liquor contained in the drained cake is an inverse function of retention time and applied centrifugal force and aproaches an asymptote. Solid-liquid mixtures with particles in the range of 10 μm to 5000 lϊlmay be processed.

Continuous solid bowl centrifuges manufactured by . Pennsalt Chemical Corporation (Sharpies) and Bird Corporation belong to this category. The two principal elements of the Solid Bowl Cen'l^rifugal are the rotating bowl which is the settling vessel and the conveyor which discharges the settled solids. The bowl has adjustable overflow weirs at its larger end for discharge of clarified effluent and solids discharge ports on the opposite end for discharging dewatered solids.

-7- 3. Disclosure of Invention

Separation by the conventional fractional solidification process rarely achieve product purities indicated by phase equilibria for a variety of reasons. High impurity levels result because mother liquor is often occluded in crystal imperfections and is entrapped, in crystal agglomerates. The crystal is further contaminated by the large amount of mother liquor held in the crystal mass by surface tension and capillary forces. Impurities are also adsorbed on the crystal surface. A further source of impurity in the crystals is the major or minor amount of solid solubility. In the present disclosure, the term "free liquid" is used to refer to the liquid that will readily drain from or pass through the solid bed, the terra "retained liquid" is used to refer to the liquid that will not readily drain or pass through the solid bed, the term "intercrystalline space" is used to refer to the spaces not occupied by the crystals, and the term "intercrystalline free space" is used to refer to that part of the intercrystalline space which is not occupied by the retained liquid. Thus, an intercrystalline free space may be occupied by free liquid entirely, by a gas phase entirely, or by free liquid and a gas phase. In an abridged form, the term "solid phase" is used to refer to a solid-liquid mixture that is transported together and is denoted as a K-stream. A solid phase may be a slush, a wet cake, or a drained cake depending on the amount of liquid contained in the solid phase.

This application is related to a group of inventions which are so linked as to form a single general inventive concept. Therefore, there are several modes of carrying out the inventions. In making the present invention, several fundamental requirements to be met in an efficient crystal washing operation have been recognized, methods of effectively meeting these requirements have been found and several of these methods have been

incorporated in each mode of carrying out the present invention.

- Ft is noted that ήer ^' e are two types of crystal washing beds, depending on whether the intercrystalline free space is filled entirely with liquid or not. A bed of the former type is referred to as a submerged bed; a bed of the latter type is referred to as a drained bed. Some modes of carrying out the present invention deal entirely with submerged beds, some deal entirely with drained beds and some deal with both submerged and drained beds.

In a process of the present invention, purification is accomplished in a purification zone that has a first end and a second end; the solid phase is transported from the first end to the second end; the free liquid is transferred from the second end to the first end and in the direction that is substantially countercurrent to the transfer direction of the solid phase. The purification zone may have several discrete sub-zones, or it may be a continuous zone. There is an impurity concentration profile established in the purification zone, impurity concentration decreasing in the direction from the first end to the second end. At a given location, the impurity concentration in the retained liquid is higher than that in the free liquid.. Impurities in the retained liquid region has to be first transferred to the free liquid region and then be removed from the region either by a counter washing or a draining operation. In most cases, a part of the melt of the purified solid is used as the initial wash liquid. However, other liquid can also be used.

In making the present. invention, it has been recognized that there are several fundamental requirements that have to be met in order to be able to accomplish an efficient washing operation. It i difficult for a process that meets only a part of these

requirements to produce a mass, of highly purified crystals economically. These fundamental requirements are as follows:

(a) Requirement No. 1

S Retained liquid is the liquid that can not be readily displaced or drained from the bed. At a given position, the impurity concentration in the retained liquid is higher than the free liquid that is in the proximity of the retained liquid. The impurities in the

10 retained liquid do diffuse into the free liquid. But this process is slow. There is a need to enhance the transfer of impurities from the retained liquid to the free liquid. Futhermore, this has to be accomplished in a proper way so that it does not interfere meeting other

L5 requirements. Methods that can be used in meeting this requirement are referred to as "means for transferring impurities from the retained liquid to the free liquid," or simply as "means for transferring impurities" and are also referred to as Type A operations. Type A operations

20 will be described in detail in the following paragraph.

A region in which a Type A operation is conducted is referred to as an A-n sub-zone. There are operations, each of which can be used to meet both requirements No.l and No.2 simultaneously. These operations will be 5 referred to as Type AB operations. A Type AB operation may be used in the place of a Type A operation. Therefore, the term "means for transferring impurities" will be used to include both Type A operations and Type AB operations.

0 (b) Requirement No. 2

This is the requirement that the amount of impurities in the intercrystalline free space be reduced. Ways of meeting this requirement are referred to as "means for reducing the amount of impurities from the

intercrystalline free space" or simply as "means for reducing impurities" and are also referred to as Type B operations. A 'Type B operation may be a displacement operation, a draining operation, or a compaction operation. A region in which Type B operation is conducted is referred to as a B-n sub-zone. A Type AB operation described may also be used in the place of a Type B operation. Therefore, the terra "means for reducing the amount of impurities from the intercrystalline free space" will be used to include Type B operations and Type AB operations.

(c) Requirement No. 3

When the impurities in the free liquid of a bed are to be reduced by a displacement operation, it is important that the bed.is properly consolidated so that the level of channelling be held down to a very low level. In a purification column with alternating A-sub zones and B-sub zones, the level of channelling in B-s b zones has to be kept low, but the levels of channellings in A-sub zones are not critical. In a purification column with AB sub-zones in which both Type A and Type B operations are conducted simultaneously, the level of channelling has to be kept low.

(d) Requirement No. 4

In a purification zone, solid phases are transported in the general direction from the first end to the second end. Any transport of the solid phase in the reverse direction will reduce the washing efficiency. Therefore, a Type A operation has to be^localized in a narrow region transverse to the general direction of transporting the solid phase.

fe) Requirement No. 5

In a purification zone, the impurity level diminishes in the general directon from the first end to the second end and the free liquid is displaced in the direction from the second end to the first end. Any movement of liquid in the reverse direction, i.e. from the first end to the second end, will reduce the effectiveness of crystal washing. Since the solid phase with its retained liquid is transported in the reverse direction, the damage caused by this transport has to be kept low. This damage can be reduced by (1) maintaining a low mother liquor to solid ratio in the transported solid phase, or (2) counter washing the solid phase during its transport, or (3) both of the two approaches described.

(f) Requirement No. 6

There are profiles of impurity concentration and temperature established in a counterwasher—the impurity concentration and temperature respectively decreases and increases from the first end toward the second end. This is so because the freezing temperature of a solution increases as its impurity concentration decreases. Therefore, the solid phase meets successively with ^' armer and purer free liquids. The crystals in the solid phase are heated to thereby remove heat from the free liquid. Due to this heat removal, a mass of crystal is formed from the liquid surrounding the crystals. This phenomenon is referred to as a wash-front solidification. Depending on the initial permeability of the bed and the concentration gradient, this additional solidification may reduce the permeability so. much as to make an— effective counterwashing difficult. Under such a situation, there is a need to break up the bed, or use a multistage washing operation using successively purer wash liquids.

As has been described, a Type A operation and a Type B operation are used respectively to meet Requirements No. 1 and No. 2 and a Type AB operation is used to meet Requirements No. 1 and No.2 simultaneously, without significantly infringing other requirements. These three types of operations are described as follows:

(a) Type A Transfer Operations

A Type A operation is used to enhance the transfer of impurities from the retained liquid to the free liquid, while preventing a longitudinal bulk mixing of the two phase mixtures. A Type A operation is to be used in combination with a Type B operation. When a localized and ^" transverse agitation (Type A operation) is to be used in combination with a consolidated bed counterwashing operation or a compaction operation (Type B operation), it is desirable to keep the mother liquid to solid ratio of the agitated bed, denoted as M/S ratio, at a low value. It is preferrable to keep the M/S ratio lower than 2.0:1 or even lower than 1.5:1. It is not necessary to have an M/S ratio in excess of 2:1. It is recommended to gently agitate a bed with a low M/S ratio that is even less than 1:1. This approach is incorporated in the first mode of carrying out the invention to be described. A localized agitation is accomplished by using one or more agitation elements that are connected to and retained by a solid object such as a moving arm, a moving shaft and a supportive wall. When a soaking and agitation operation (Type A operation) is to be used in combination with a draining operation to form a drained cake (Type B operation) , the M/S ratio used is not as critical and a higher M/S ratio may be used. This approach is incorporated in-.the second mode of carrying out the invention to be described.

(b) Type B Transfer Operation ».

A Type B transfer operation is an operation by which the amount of impurities in the intercrystalline free space is reduced. A Type B transfer operation may be accomplished by a counter washing operation, a draining operation , a compaction operation, or any combination of the above operations.

A Type A operation and a Type B operation form a conjugate set of two step operation and several two step operations are repeated. The process may be conducted in a purification zone that has several stages, each stage having an A-subzone and a B-subzone. Type A operations and Type B operations take place respectively in the A-subzones and B-subzones. By having discrete A and B sub-zones and by forming compacted beds in the

B-subzones, other requirements described are also met.

(c) Type AB Operation

A Type AB operation is an operation that meets both Requirements No. 1 and No. 2 without significantly interfering other requirements. In the present disclosure, the term "Type A operation" is used to include a "Type AB operation", and the term "Type B operation" is also used to include a "Type AB operation." A Type AB operation is a localized agitation operation combined with a counterwashing operation which is so conducted as not to cause an excessive channelling. Some examples of Type AB operations are as follows:

(i) Ultrasonic vibrations coupled with counter¬ washing; (ii) Thin wire or blade agitation coupled with counterwashing; (iii) static mixing coupled with counterwashing.

ln a first mode of carrying out the invention, a feed crystal-mother liquor mixture is purified in a purification zone by a wash liquid by conducting localized agitated crystal washing operations and stationary crystal washing operations alternately and transferring the solid-phase and free liquid in substantially countercurrent directions. An agitated crystal washing operation is a Type A transfer operation; a stationary crystal washing operation is a Type B transfer operation. The purification zone contains a set of stationary washing sub-zones and a set of agitated washing sub-zones and has a first end and a second end, which are respectively defined as the upstream end and downstream end relative to the normal direction of the movement of the solid phase. The two sets of sub-zones are laid alternately along the longitudinal direction of the purification zone from the first end to the second end so that an agitated sub-zone is interposed between two stationary sub-zones. Therefore, the purification 0 zone comprises a plurality of processing stages. Each processing stage say n-th stage, has an agitated washing sub-zone, denoted as A-n sub-zone, and a stationary washing sub-zone, denoted as B-n sub-zone, the two sub-zones forming a conjugated set of sub-zones. In most

- _> g cases, the wash liquid used is a mass of the melt of purified crystals.

The mass of crystals and the mass of liquid in each agitated sub-zone are gently agitated repeatedly to cause agglomerates to spread, turn and fold, so that the

^•' -* ^" * impurities in the retained liquid is released to the free liquid. Agitation is accomplished by moving blades that are connected to a moving shaft. The M/S ratio, defined as the ratio of the mass of lquid to the mass of " crystals, in the two phase mixture in an agitated washing 5 sub-zone is maintained at a low value that is in the range that is less than 2:1 or even less than 1.5:1 and is higher than that of a consolidated packed bed so that the consistency of the two phase mixture is of a high

value. Therefore, the two phase mixture is a thick ixturev Because of the th ckness of the two phase mixture and the gentle and transverse nature of the agitation, an intimate local mixing of the two phases is accomplished, suppressing a long range mixing of the two phases in the main transport direction of the solid phase. It is desirable that the amount of mother liquor in the solid phase transferred from the sub-zone is low. By keeping a low M/S ratio, the amount of mother liquor transferred is kept at a low value.

The mass of crystals in a stationary sub-zone is compacted by a compacting means to form a bed with an enhanced degree of compaction that is substantially higher than the degree of compaction attainable by a natural formation of the bed. A properly enhanced degree of compaction of the bed in a stationary sub-zone is important from the standpoints of an efficient counter washing operation of reducing the amount of impurities from the free liquid and of reducing the amount of mother liquor in the solid-phase transferred from the sub-zone.

Due to the substantially countercurrent transfers of the solid phase and the free liquid, concentration profiles of the impurities are established both in the free liquid and in the liquid retained by the crystals, the impurity concentrations decreasing from the first end toward the second end. Therefore, a transfer of an impure liquid in this direction reduces the efficiency of the crystal washing operation. Since the solid phase is transferred in this direction, it is important either to reduce the amount of liquid that is transferred with the crystals or counterwash he crystals during an inter sub-zone transfer of the solid phase. Means for accomplishing these functions are incorporated in the apparatuses of this invention. Because of the properly conducted alternating stationary and agitated washing operations and the properly conducted inter sub-zone transfers of the solid phase and the free liquids the

impurity concentrations of the liquids retained in the solid phases transferred between sub-zones decrease sequentially, nearly following a geometric sequence rather than approaching a limiting value. Thus, a mass of highly purified crystals can be obtained from the second end of the purification zone. It is noted that there is also a temperature profile established in the purification zone, the temperature increasing from the first end toward the second end.

It is of interest to compare- the performance of a conventional column crystallizer with that of the first mode of carrying the present invention just described. It is noted that a conventional column crystallizer has a deep stationary washing bed. hen the concentrations of the impurities in the retained liquid and in the free liquid in the purification zone of a conventional column crystallizer are plotted as functions of the bed depth, thay tend to approach asymptote values. Therefore, the purity attainable in a coventional column crystallizer has a practical limit. It seems that adding depth to a column crystallizer has diminishing return to its improvement in performance. The bed formed in the column is not agitated and simply descends through the column. Therefore, impurities in the retained liquid are not quickly released to the free liquids, and a channelling pass formed tends to stay as a channelling pass. In contrast, in a process of the present invention, the bed is alternately agitated and reformed to enhance release of impurities from the retained liquid and accomplishes an effective counter washing in each stage. The height of each stationary ^' ashing sub-zone is short, yet an effective washing is accomplished therein. ι -

In a second mode of carrying out the present invention, the purification is accomplished by several properly and alternately conducted agitated crystal washing and crystal draining operations and substantially countercurrent transfers of the solid phases and free

liquids. An agitated crystal washng operation is a Type

A transfer operation; a crystal draining operation is a

Type B transfer operation. The system used comprises an initial crystal draining zone, a main purification zone that comprises a plurality of processing stages, and a crystal melting zone. Each processing stage, say n-th stage, has a washing sub-zone, denoted as A-n sub-zone and a draining sub-zone, denoted as B-n sub-zone, the two sub-zones forming a conjugated set of sub-zones.

Therefore, the purification zone contains a set of crystal washing sub-zones and a set of crystal draining sub-zones. These two sets of sub-zones are laid sequentially and alternately along a line that may either be staight or tortuous; the solid phase and the free liquid are transferred alternately through the two sets of sub-zones in countercurrent directions.

A processing stage, say n-th stage, comprises an agitated crystal washing sub-zone, denoted as A-n sub-zone and a crystal draining sub-zone, denoted B-n sub-zone. The two sub-zones in a stage are said to be conjugated sub-zones. The wet solid phase (K^)JJ leaving a washing sub-zone, A-n sub-zone, is drained in its conjugate sub-zone ( B-n sub-zone) and is separated into a mother liquor (Ig)fl ^{and a} drained solid phase (Kg) _π . The drained solid phase is transferred to the next following washing sub-zone, A-(n+l) sub-zone, in the direction toward the second end. The drained solid phase (Kg), contains a mass of crystals (Sg)η and a mass of retained liquid (MB ) _π . The mother liquor (Ig) _n is divided into a first portion (Jg and a second portion (Lg) _π , which respectively become an intra-stage ^" recycle liquid and inter-stage transfer liquid. The former is recycled to the original washing sub-zone and the latter is transferred to the next washing sub-zone, A-(n-l) sub-zone, in the direction toward the first end. The ^• ratio of the mass of inter-stage transfer liquid and the mass of the retained liquid in the drained solid phase, viz. the ratio of the mass of (Lg) _π to the mass of ( g) _n

is an important factor in determining the effectiveness of a cry-stal purification operation: the e fectiveness increases as this ratio, (Lβ)π/(Mg) _π , increases. This ratio may be denoted as a wash ratio, R _π . The wash ratio can be increased by increasing (Lg) _π -, by decreasing

(Mg)- , or by both. A high (Lg) _π value requires recycling a large amount of crystal melt and thereby requires an increased energy consumption. Therefore, it is desirable to attain a desired wash ratio by decreasing (Mg) _π , viz. increasing the degree of draining crystals. In practice, there is an optimal degree of drainage: a high degree drainage requires an expensive centrifuging operation and a low degree of drainage results in a small wash ratio and a low degree of crystal purification.

It is noted that in a crystal draining operation, a major amount of free liquid is drained from an intercrystalline free space and a gas phase occupies a major fraction of the free space. Therefore, a major amount of the impurities is removed from the intercrystalline free space. There is some retained liquid in the drained solid phase. When a wash liquid is added to the drained solid phase, the wash liquid becomes the free liquid and impurities in the retained liquid is transferred to the free liquid. This operation may be accomplished by a simple soaking operation or by a soaking operation coupled with a gentle agitation. Therefore, when a draining operation is used for a Type B transfer operation, a simple soaking operation with or without a gentle agitation may be used for a Type A transfer operation. Therefore, the second mode of operation described may be modified by replacing the soaking operation for the agitated crystal washing operation.

There are multistage alternate washing and draining operations in the second mode of operation. It is important to note that some moderate degrees of washing and some moderate degrees of draining in these operations

are sufficient to give a high overall degree of purification and that such* a multistage process is superior to purifications by conventional column crystallizers, in which purifications are accomplished mainly by a stationary crystal washing, and is also superior to purifications by centrifuges, in which purifications are accomplished mainly by mother liquor drainage. In other words, a superior process is obtained by repeating properly balanced crystal washing and crystal draining operations. Similar statements can.be made for a second mode of operation with the modifications described in the preceeding paragraph.

A process of the present invention may either be a continuous process or a batch process. In a batch process, purification may be accomplished by a batchwise countercurrent operation conducted in one or more crystal washing vessels. A mass of feed mixture is introduced into a vessel to form an initial bed (Kg)o , which may either be a submerged bed or a drained. The bed is then subjected alternately to Type A and Type B operations. In other words, the bed is subjected to a multistage operation, each stage, say n-th stage, having a Type A operation, denoted as A-n operation, and a Type B operation, denoted as B-n operation. The Type A, Type B and Type AB operations described earlier can also be used in a batch process. The Type A operations are sequen¬ tially denoted as A-l, A-2, , A-n, , and A-M operations; the Type B operations are sequentially denoted as B-l, B-2, , B-n, , and B-N operations.

The bed after an A-n operation is denoted as (K^)R , and the bed after a B-n operation is denoted as (Kg)η . Fol¬ lowing the terminology used in a continuous process, these beds are referred to._as "inter sub-zone transfer

^" i solid phase" or simply as a "solid phase". In a system with a multitude of crystal washing vessels, the first end and the second end of the purification zone refer respectively to the position at which the feed mixture is introduced and the position at which the initial wash

liquid is introduced. Therefore, the positions of the first e_nd and the second end -are shifted with time. In such a system, a crystal washer successively becomes A-l,

B-l, A-2, B-2, , A-n, B-n, , A-N and B-N sub-zones, the bed in a crystal washer successively becomes (K _A )| ,

(K )j, (K _A ) ₂ , (K _B )2 , , (K _A ) _π , (Kg) _π , , (K _A )j| and

(K )^ . Following the terminology used in a continuous process, the transformation of the bed in a crystal washer after these operations will also be referred to as

"inter sub-zone transfer of the solid phase".

In a third mode of carrying out the invention, the purification is accomplished by a batchwise counter¬ current operation conducted in a multitude of crystal washing vessels. A mass of feed crystal-mother liquor mixture is introduced into a vessel and drained to form an initial drained bed (Kg)g and a mother liquor (Lg)Q . The mother liquor (Lg)Q is discharged from the system as a residue. The initial drained bed (Kg)Q is then subjected to a series of soaking and draining operations using washing liquids (Lβ)l , ⁽ Lg)2 , , ( ^L B^M ^and

( )ty+ι and successively becomes drained beds (Kg)j ,

⁽ K ⁾ 2 _f 1 ^{and (K} B ^J N ^b ? releasing mother liquors (Lg)j , (Lβ)2 t , and (Lg)fl re¬ spectively. The mass of drained solid phase after the N-th stage soaking and draining operations becomes the purified solid phase. The purified solid phase is melted to become a melt mass. A part of the melt mass becomes a purified product and the remainder becomes the initial washing liquid (Lβ)|ι|+1. The first mother liquor (Lg)j is discharged from the system as a residue. The remaining mother liquors (Lg)2 / (Lg)3 , , (L )N-l and (Lg)N become the wash liquids described. It is noted that, being drained beds, the int ^' ercyrstalline spaces of the beds ⁽ Kg ⁾ ₀ , ⁽ Kg ⁾ j , (Kβ)N-l ^and ( ^K B>N ^are substantially filled by gas .phases. Therefore, the ratio of the mass of mother liquor to the mass of the solid phase of each of these drained beds is low. The third mode of operation may be modified by replacing another

Type A operation, such as a localized agitated washing operation in the place oil the soaking operation, and replacing another Type B operation, such as a counterwashing operation, in the place of the draining operation. Operational procedures are similar to what have been described, except that the solid beds, ( g) _j ,

(Kg)2 , 1 (Kg) _| ι are submerged beds.

In a fourth mode of carrying out the present invention, the purificaton is accomplished in a 0 purification zone that comprises a set of agitated crystal washing sub-zones and a set of solid-phase transfer sub-zones that are laid in an alternating sequence from a first position to a second position. An agitated crystal washing operation is a Type A transfer

15 operation and a solid transfer operation is a Type B operation. It has the following features:

(a) The solid phase is transported sequentially through both of the two sets of sub-zones from the first position to the second position;m

2Q (b) A part of the free liquid is transported through the two sets of sub-zones in the direction substantially countercurrent to the transport direction of the solid phase; and (c) The remaining portion of the free liquid phase

25 is transported through the purification zone in the direction substantially countercurrent to the main transport direction of the solid phase, bypassing the solid-phase transfer sub-zones. This mode of operation has an

30. improved washing «efficiency and provides a low pressure drop operation. It is also able to handle a mixture with small crystals.

In a fifth mode of carrying out the invention, the purification is conducted in a purification zone that has

placed alternately. The solid phase passes through both the non-agitated washing sub-zones and the solid-phase transfer sub-zones, while a major part of the free liquid passes only through the nonagitated washing sub-zones, bypassing the solid-phase transfer sub-zones. In this mode of operation, a Type A operation and a Type B operation are simultaneously conducted in a solid-phase transfer sub-zone and a Type B operation is conducted in a non-agitated washing sub-zone.

In some modes of operations, a Type A transfer operation and a Type B transfer operation are conducted simultaneously. Examples of systems used are a counter washing column agitated by transverse ultrasonic vibrations and a counter washing column equipped with rotating vertical thin wires or thin blades and a counterwashing column provided with static mixers. These modes of agitation are explained as follows:

(a) Counter Washer Agitated by Ultrasonic Vibrations.

When a crystal-liquid bed is agitated by ultrasonic vibrations, transfer of impurities from the retained liquid region to the free liquid region is greatly enhances without causing channelling passes to form substantially. Therefore, a counter washing operation can be applied to such a bed simultaneously. Thus, requirements No. 1 and No. 2 can be met in the same processing zone.

(b) Counter Washer Agitated by Thin Blades or Thin Wires. ^•

When a blade of substantial cross section is pushed through a bed, the solid and liquid in front of the blade are pushed away and solid and liquid move into - he rear region behind the blade. While solid and liquid are moving into the rear region, liquid tend to move in faster forming channelling passes. The size of a channelling passage formed decreases as the cross section

of the blade and the rpm deer-eases. Therefore, it is possible to agitate a coβn-ter washer with moving thin wires or thin blades to meet requirements No. 1 and No. without forming channelling passages excessively.

(c) Counter Washer with a Static Mixer.

Static mixers or in-line motionless mixers have recently been introduced by Kenics Corporation and Koch Engineering Corporation. The Kenics Mixer is a series o fixed, helical elements enclosed within a tubular housing. The fixed geometric design of the unit produce the following unique patterns of flow division and radia mixing simultaneously:

(i) Flow Division In laminar flow, a processed material divides at th leading edge of each element and follows the channels created by the element shape. At each succeeding elemen the two channels are further divided, resulting in an exponential increase in stratification. The number of ssttrriiaattiioomns produced in 2 ⁿ where n is the number of elements.

(ii) Radial Mixing In either laminar or turbulent flow, rotational circulation of a processed material around its own hydraulic center in each channel of the mixer causes radial mixing of the material. All processed material is continuously and completely intermixed, resulting in virtual elimination or radial gardients in temperature, velocity and material composition. Due to the multiple striations caused by the multiple flow division and due to the radial mixing caused by rotational circulation, a static mixer is effective in enhancing transfer of impurities from the retained liquid zone to the free liquid zone. Formation-o£ channelling passages can be suppressed in a static mixer. Therefore, counterwashing can be applied to a moving bed within a static mixer to meet requirements No. 1 and No. 2 simultaneously. Since there is a substantial frictional loss in transporting a bed through a static mixer, there is a need for pushing

the bed through the mixer. Therefore, there is a need for balancing the frictional loss and the degree of enhancing impurity transfer obtained. It is noted that a large motionless mixer with multiple elements can be used in the crystal purification.

It is noted that there are two types of solidifica¬ tion processes which may be respectively referred to as solvent crystallization (or crystallization from melt) and solute crystallization. An example for the former case is fractional solidification of a feed mixture containing 90% p-xylene and 10% m-xylene. In this case p-xylene is the solvent and it is p-xylene that crystallizes out. The major component of the crystals is referred to as the first component. Therefore, p-xylene is the first component. In purification of a feed solid-liquid mixture in this case, a substantially pure p-xylene (the first component) is used as the initial wash liquid. An example for the latter case is crystal¬ lization of first solute, say potassium chloride, from an aqueous solution containing potassium chloride (first solute) and soldium chloride (second solute) . The major component of the crystals, i.e. potassium chloride is the first component. In purification of a feed solid mix¬ ture, water or an aqueous solution of potassium chloride is used as the initial wash liquid. The initial wash . liquid used is not a melt of the first component.

4. Brief Descripton of Drawings

Higure 1 illustrates'-a unconsolidated bed of crystals. It shows agglomerates of crystals separated by inter agglomerate free liquid, it also shows crystals, 5 retained liquid and intra -agglomerate free liquid within each agglomerate. Figure 2 illustrates a consolidated bed of crystals.

Figure 3-A illustrates the fundamental requirement No. 1 of transferring impurities from the retained liquid 0 to the free liquid; Figure 3-B illustrates the fundamental requirement No. 2 of reducing the amount of impurities from the intercrystalline free space. Figures 4-A through 4-C. illustrate an agitated crystal washing and a counterwashing operation. Figures 5-A through 5-C illustrate a crystal soaking operation and a mother liquor draining operation.

Figure 6 illustrates the purification zone of a first mode of carrying out the present invention. The purification zone contains a set of stationary washing _Q sub-zones (five are shown) and a set of agitated washing sub-zones (five are shown), which are laid alternately so that an agitated sub-zone is connected to two stationary sub-zones. It is noted that rotating blades are the agitation elements and they are connected to a rotating 5 shaft. Figure 7 compares the performances of crystal washing operations conducted in a conventional column crystallizer and in a crystal purification column of Figure 6. The first line and the second line respectively illustrate the concentration profiles of the 0 impurities in the retained liquid and in the free liquid in a conventional column crystallizer as functions of the depth of the purification section. They show that the impurity concentrations of the liquids tend to approach limiting values. Therefore, the purity of the harvested 5 crystals also tend to approach a limiting value.- The third and and fourth line respectively illustrate similar

concentration profiles in a purification column of Figure

6 as_ functions of the zo*ιe number. They show that the impurity concentrations in the liquids at corresponding points in the sub-zones sequentially decrease, nearly following a geometric sequence. Therefore, a mass of highly purified crystals can be harvested from this purification column.

Figure 8 illustrates a crystal purification system that has a main processing zone that comprises a plurality of processing stages, stage 1 through stage 5 being shown, and a crystal melting zone. A processing stage, say n-th stage, in the main processing zone has a washing sub-zone, denoted as A-n sub-zone, and a crystal draining sub-zone, denoted as B-n sub-zone, the two sub-zones forming a conjugated set of sub-zones. A drained feed mixture (Kg)g is introduced into the main processing zone. The solid phases are transferred successively through the main processing stages, passing through the washing sub-zone and then the draining sub-zone in each stage as (K _A )j , (Kg)ι , (K _A )2 , ( ) ,

( _A )3 , (K )3 , (K _A ).^ , (K ) , (K _A )5 and (K )5 streams, and the purified and drained solid phase from the last stage (Kg)g is melted in the crystal melter and becomes a crystal melt. A fraction of the crystal melt becomes a purified product D, and the remainder is introduced into the main processing zone as a wash liquid (Lg)g . Simple filtration devices are shown in the crystal draining sub-zones. In this system, an agitated washing is a Type A transfer operation and a draining operation is a Type B transfer operation. It is noted that the solid phase leaving a B-n sub-zone is a drained cake. Figure 9 illustrates the performance of the system of Figure 8. It shows how the impurity concentration (Zg)jιj leaving the last stage is related to the impurity concentration of the retained liquid in the feed (Zg)g , the wash ratios and the number of stages. The figure shows that a high degree purification can be accomplished by an operation

in which the washing ratio is relatively low and the number of stages is practicable.

Figure-,.10 illustrates an. elong ted processing zone in which the transverse agitation of the two phases is

„ 5 accomplished by using multitude of small agitating blades rotated by multiple shafts. This system may be regarded as a system of Figure 6 in which the agitated sub-zones and non-agitated sub-zones are greatly reduced in size. This system may also be regarded as a system in which an

20 agitated crystal washing and counterwashing take place simultaneously. Figure 11 illustrates a system in which an agitated washing and a counterwashing take place simultaneously in each sub-zone. The system illustrated may have a single continuous purification zone or have

15 several purification sub-zones. It shows that the solid an free liquid phases are transported in countercurrent directions and the two phases are agitated in the transverse direction to cause local mixing of the two phases. The ratio of the mass of liquid to the mass of

20 crystals in a transverse agitation region is maintained at a low value which is less than 2:1 or even less than 1.5:1 and is higher than that of a packed bed, so that the consistency of the two phase mixture is high. Figure 12 illustrates an elongated purification zone in which 5 the transverse agitation of the two phases is accomplished by ultrasonic vibrations.- Figure 13 illustrates a system in which a localized agitation of the two phases and a counterwashing operation are conducted simultaneously. The localized agitation is

30 accomplished by a multitude of rotating thin wires or thin blades to suppress formations of channelling paths.

Figure 14 illustrates « crystal purification unit having a crystal feeding zone, a crystal purification zone and a crystal melting zone. The purification zone " contains a set of stationary .washing sub-zones (five are shown) and a set of agitated washing sub-zones (four are shown) , which are laid alternatley so that an agitated

sub-zone is connected to two stationary sub-zones. Figure 14a illustrates a cross-section taken at a stationary sub-zone. It is seen that baffles are provided in the sub-zone to help maintaining a compacted bed and preventing the bed from being agitated by the agitators in the neighboring agitated sub-zones. Figure 14b illustrates an agitator used to promote local mixing of solid and liquid in an agitated sub-zone. Figure 14c illustrates a rotary unit with tilted blades used for compacting the bed in a stationary sub-zone. Figure 14d illustrates a rotating perforated disk with shaving knives that may be used to support the bed in a stationary sub-zone and transfer the solid phase from the sub-zone.

Figure 15 illustrates a crystal purification unit, in which a rotary perforated disk with shaving knives is used to separate a stationary sub-zone from its next following agitated sub-zone. The bed in the stationary sub-zone is supported by the rotating perforated disk and is compacted by a rotating blade; the inter sub-zone transfer of the solid phase is accomplished by the shaving action of the knives provided in the rotary disk. Figure 16 illustrates a very simple crystal purification unit: there is no perforated disk to separate the sub-zones, there is no rotary unit with blades to compact the bed in a stationary sub-zone and there is no rotary shaving knives for inter sub-zone transfer of the solid phase. There is only one compacting means at the top of the column. In this system, the crystals in all sub-zones including the agitated sub-zones form a continuous bridged structure through which force can be transmitted. Therefore, the downward force applied by the compacting means at the top of the purification zone and the entire weight of -the solid bed above Λ given level are applied to the bed at the level through the bridged structure and thereby compact the bed thereat. ^"

Figure 17 illustrates a drained bed washing column.

There are a set of agitated gub-zones, denoted as A-l through A-5 sub-zones, and a set of non-agitated sub-zones, denoted as B-l through B-5 sub-zones. These two sets of sub-zones are laid alternately upward. A feed crystal-mother liquor mixture is fed at the bottom and the solid phase is pushed upward by the blades in the agitated sub-zones. Wash liquid is added at the top and drain through the bed downward. Figure 17a illustrates a cross-section of an agitated sub-zone; Figure 17b illustrates a cross-section of a non-agitated sub-zone; Figure 17c shows an extended view of the column. The rotating blades in an agitated sub-zone agitates the two phase mixture in the sub-zone and push the drained bed upward. For a large drained bed crystal washer, it is desirable to use several washing rings in an agitated washing sub-zone. A column illustrated can also be used as a submrged bed washer. In this case, a feed crystal-mother liquor mixture may either be added at the top or bottom of the column.

Figure 18 illustrates a centrifugal crystal puri¬ fication unit. There are a first rotating body and a second rotating body. The first rotating body includes a rotating cylindrical vessel and rotating screens (or rotating bowls) attached to it. The second rotating body includes a rotating shaft, rotating arms and blades. The rotating vessel define a purification zone that contains an initial draining zone (B-zero zone) and a main processing zone that contains a plurality of processing stages 1, 2, , N-l, N, N being 4 in the figure. The n-th stage comprises an A-n crystal washing sub-zone and a B-n crystal draining sub-zone. There is a rotating screen or a rotating bowl for draining mother liquor from crystals in each draining sub-zone and there is a rotating arm unit with blades in each crystal washing sub-zone to agitate the crystals and transfer crystals to its conjugate draining sub-zone. The first rotating body is rotated at a first rpm such that mother liquors are

drained from masses of crystals to moderate degrees in the draining sub-zones. The second rotating body is rotated at a second rpm that is either somewhat higher or lower than the first rpm such that the rotating arms agitate the crystal-liquid mixtures in the washing sub-zones and transfer crystals to their conjugate draining sub-zones. The solid phase is transferred from the left to the right; interstage transfer liquid is transferred from right to left.

Figure 19 illustrates a plate-type crystal washing column that looks like a plate-type distillation column. There are a first set of plates, a second set of plates and a set of transfer sections each containing a solid phase transfer conduit and liquid transfer conduit. The solid phase is transported radially outward on the first set of plates, radially inward on the second set of plates and downward through the solid phase transfer conduits; the free liquid is transported radially inward on the first set of plates, radially outward on the second set of plates and upward through the liquid transfer conduits.

5. Modes of Carrying out the Invention

5-1 Introduction

Figure 1 illustrates an unconsolidated bed of crystals; figure 2, illustrates a consolidated bed of crystals. In an unconsolidated bed, there are agglomerates 1 of crystals separated by inter agglomerate free liquid 2. Within a crystal agglomerate, there are crystals 3 and a mass of liquid. The part of the liquid that will easily drain from or pass through the agglomerate is called the intra-agglomerate free liquid 4, the remaining liquid is retained by the crystals and is called the retained liquid 5. A counterwashing of a unconsolidated bed is inefficient, because the displacing liquid tends to preferentially pass through the passages filled by the inter agglomerate free liquid. This is referred to as "Channelling". By compacting an unconsolidated bed, a consolidated bed illustrated by Figure 2 is obtained. In the process of compacting, agglomerates are pushed together so that sizes and number of channelling passes are greatly reduced. In the consolidated bed, there are crystals 3, free liquid 4 and retained liquid 5.

There is an optimal range of the degree of compaction for an efficient crystal washing operation — a certain degree of compaction is needed to suppress the degree of channelling, and a proper permeability is needed for a practical rate of displacement.

Figure 3-A illustrates the fundamental requirement No. 1 of transferring impurities from the retained liquid to the free liquid. In the figure, crystals in a crystal agglomerate are collectively illustrated by a circled region 5, the retained liquid is illustrated lay a ring 6, and the inter-agglomerate and intra-agglomera _. te free liquid are illustrated by the region 7 outside of the rings of the retained .liquid. The requirement No. 1 is

to transfer impurities from the retained liquid region 6 to the free liquid region ¹ .7. Figure 3-B illustrates the fundamental requirement No. 2 of reducing the amount of impurities from the intercrystalline free space. In the figure, there are crystal region 5, retained liquid region 6, and free liquid region 7.

Figures 4A, 4B, and 4C illustrate the process of successively subjecting a submerged bed to a,gentle localized agitation (Type A operation) and a counterwashing operation (Type B operation) . This two step operation is to be repeated. Figure 4A illutrates the state of the bed before the Type A operation; Figure 4B illustrates the state of the bed after the Type A operation and also illustrates the state of the bed before the Type 3 operation; Figure 4C illustrates the state of the bed after the Type 3 operation. Figures 4-A and 4-B illustrate that the impurity concentration in the retained liquid is reduced from (Cp)ι to (C )2 and the impurity concentration in the free liquid is increased . from (Cg)ι to (Cg)2 by the Type A operation. Figures

4-B and 4-C illustrate that the impurity concentration in the free liquid is reduced from (Cg)ι to (Cgh by the Type B operation. During the Type B operation, the impurity concentration in the retained liquid may change somewhat from (CR)2 to (CR)3.

Figures 5A, 5B and 5C illustrate the process of successively subjecting a drained bed to a soaking operation (Type A operation) and a draining operation (Type B operation) . This two step operation is to be repeated. Figures 5A and 5B respectively illustrate the states of the bed before and after the soaking operations; Figures 5B and 5C respectively illustrate the states of the bed before and after the draining operation. Figure 5A illustrates that in the drained bed, the intercrystalline free space 7 is filled with a gas phase. Figure 5B shows that, after the soaking operation, the intercrystalline free space is filled with

a liquid phase and the impurity concentration in the

• - ^• retained ^" liquid has been reduced from (C^) _* to (CR)2«

Figure 5C shows that after the draining operation, the intercrystalline free space is filled with a gas phase again. The amount of impurity in the free space has been reduced, because the mass of free liquid in the free space has been reduced.

In order to properly describe the condition of the bed formed in a stationary sub-zone in a purification column of the present invention, it is important to review the mechanisms by which a bed is formed from a suspension and it is convenient to define the following terms: "naturally formed bed", "bed with enhanced compaction" and "bed without enhanced compaction".

When a suspension is allowed to settle in a tank batch-wise, particles of different sizes do not settle the same distance during a given settling period. A particle may settle for a period of time before it is supported by bridging with other particle. A coarse particle comes to rest earlier than a small particle does. A small particle may settle part of the time on top of the bed of coarse material and part of the time through the interstices between large particles. This action, called consolidaton trickling, represents the settling of fine particles, whereas coarse particles are self-supported and do not settle. The settling of fine particles is much slower during consolidation than during suspension, but the effect may be important in determining the degree of compaction of the resulting bed. Finally, all particles, coarse and fine, come to rest and a self-supported bridged structure of particles is formed. The degree of compaction of the bed obtained may be measured by the porosity of the bed: the lower is the porosity of the bed, the higher is the degree of compaction. Degree of compaction of a bed may also be measured by the permeability of the bed: the lower is

- 34 - the permeability of the bed, the higher is the degree of compaction.

Increasing the static pressure to a naturally formed bed does not substantially affect the degree of compaction, because forces applied to various parts of the outer surface of a particle balance out. However, forces transmitted through the self-supported structure of the bed can cause the structure to collapse and cause the bed to assume a more consolidated structure and ^■ -- thereby raise the degree of compaction. . A bed formed i from a suspension by sedimentation without application of an external force other than the gravity forces on the particles, is referred to as a "naturally formed bed" or "bed without enhanced compaction". Within a deep ' ³ naturally formed bed, the degree of compaction at a low level may be substantially higher than that at a high level because the weight of the bed transmitted through the structure of the bed does compact the bed at the low level. Therefore, the average degree of compaction of a naturally formed bed is a function of the depth of the bed.

In a purification column of the present invention, there are a set of stationary washing sub-zones and a set of agitated washing sub-zones laid alternately. The

25 degree of compaction in each stationary sub-zones is an important factor affecting both the effectiveness of crystal washing and the amount of liquid carry over in the inter sub-zone transfer of the solid phase. In order to properly characterize the bed in a stationary washing

30 sub-zone, it is convenient to refer to a reference bed. The degree of compaction of an actual bed can then be compared with the degree of compaction in the reference bed. The reference bed used in this specification is a "steady state naturally formed bed", which may be ^" simply

~ς referred to as a "naturally formed bed". This reference bed is defined in the following paragraph.

When a stationary washing sub-zone is connected at the first end to an up-stream agitated sub-zone by a perforated plate and is also connected at the second end to a down-stream agitated sub-zone by another perforated plate and then a solid phase is introduced into the sub-zone at the first end and a solid phase is removed from the sub-zone at the second end, and a free liquid is introduced to the sub-zone at the second end and another free liquid is removed from the sub-zone at the first end, so that a steady state is attained in the sub-zone, the bed so formed in the sub-zone is referred to as "steady state naturally formed bed" or simply as "naturally formed bed". There is no force applied to the self-supported structure of the crystals of the bed formed in the sub-zone other than the frictional drag of the moving fluid and gravity forces applied to the bed.

There are two ways by which the degree of compaction of the bed formed in a stationary washing sub-zone can be increased or enhanced substantially beyond the natural degree of compaction. As has been described, a bed can be compacted by transmitting force through the self-supported structure of the solid particles in the bed. One way to compact the bed is to have a mechanical means of applying force to the structure of the bed. Mechanical means such as a piston, a screw conveyor, and a rotating unit with tilted blades may be used to compact the bed. The other way is to transmit the weight of the beds in those sub-zones, including both stationary and agitated, above a given sub-zone through the structure of the bed in the sub-zone to thereby compact the bed at the given sub-zone. In order to transmit the weight of the beds in the higher sub-zones to the structure of the bed in the given sub-zone, no perforated plate should be used in separating the sub-zone -from its up-stream agitated sub-zone and the structures of the beds in the higher sub-zones should be such that the weight of the beds in the sub-zones can be transmitted through the beds. A bed whose degree of compaction is substantially higher than

that of the corresponding naturally formed bed is referred, _. to as a "bed with enhanced compaction".

5-II Performances of the First Mode and Second Mode of Carrying out the Present Invention

The Performances of the first mode and second mode of carrying out the present invention are described in this section.

5-IIa Performance of the First Mode Process

Figure 6 illustrates a general system in which the first mode process can be conducted and Figure 7 compares its performance with that of a conventional column crystallizer.

Figure 6 illustrates a first mode purification column 8 bound between the first end 9 and a second end 10. The purification zone contains a set of agitated washing sub-zones, 11a through lie, denoted respectively as sub-zones A-l through A-5, and a set of stationary washing sub-zones, 12a through 12e, denoted respectively as sub-zones B-l through B-5. In each stationary sub-zone, there are radial and vertical baffles 13 dividing the sub-zones into compartments. There are agitating blades 14 (agitating elements) connected to and rotated by a shaft 15 in each agitated sub-zone. The agitating blades may be tilted to compact the beds in the stationary sub-zones. Then, the blades perform the functions of agitating the beds in the agitated sub-zones and compacting the beds in the stationary sub-zones. It is noted that static mixers described may be installed in the stationary mixing zone to enhance impurity transfer.

A crystal-mother liquor feed 16 is introduced at the top of the column; a wash liquid 17 is introduced at the bottom; a mass of purified solid phase 18 is discharged at the bottom; an impure liquid 19 is removed at the

top. The bed descends successively through the stationary and agitated sub-zones and free liquids flow upwards through the sub-zones. The solid phases entering A-l through A-5 agitated sub-zones are respectively denoted as (Kg)g through (K ) streams; the solid phases leaving A-l through A-5 agitated sub-zones are respectively denoted as (K _A ) _j through (K _A )5 streams; the free liquids entering A-5 through A-l agitated sub-zones are respectively denoted as (Lg)j through (Lg) _j streams; the free liquids leaving A-5 through A-l agitated sub-zone are respectively denoted as ( _A )5 through (L _A )1 streams. The purified solid phase 18 is melted; a part of the melt becomes the purified product and the rest becomes the wash liquid (L _A )β .

For an effective crystal washing operation in a stationary sub-zone, the bed in the sub-zone needs to be compacted to a degree that is substantially higher than the natural degree of compaction. An enhancement of bed compaction in this system is accomplished by the two ways described earlier.

Since the impurity concentration decreases in the downward direction, transfer of an impure liquid in this direction impairs the performance of the column. Unavoidably, the liquid retained in the solid phase transferred between sub-zones does move in this unfavorable direction. The unfavorable effect caused by the movement of the retained liquid can be reduced by either one or both of the following ways:

( ^* i) reduce the amount-of retained liquid in the inter sub-zone transfer solid phase;

(ii) counter wash the solid phase while it is transferred between two sub-zones. Both ways are used in the-.column illustrated.--

One of the best examples of the successful commercial application of countercurrent column fractional crystallization is the Phillips process

described. The column used contains a crystal forming zone, a deep non-agitated crystal purification zone and crystal melting zone. The bed in purification zone is not agitated. Because of the countercurrent transfers of

5 the solid phase and the free liquid, impurity concentration profiles are established in the purification zone. Line I, 20-21-22, in Figure 7 illustrates how the impurity concentration in the retained liquid varies with the depth of the column; line

0 II, 23-24-25, illustrates how the impurity concentration in the free liquid varies with the depth of the column. It is shown that both of these lines approach some limiting values. Since the solid phase discharged from the bottom of the column is melted to become the purified

5 product and since the solid phase contains the retained liquid, the impurities in the retained liquid become the impurities in the purified product. Therefore, the purity attainable in a conventional column crysallizer has a practicable limit. Therefore, it seems that adding !0 depth to a column crystalizer has diminishing return in it performance. The major reasons are that the bed formed in the column is not agitated and simply descends through the column, impurities retained within crystal agglomerates are not quickly released to the free liquid, .5 and a channelling pass formed tends to stay as a channelling pass.

Line III, 20-26-27-28, and line IV, 23-29-30-31, respectively illustrate the impurity concentration profiles established in the retained liquids and free

30 liquids in a purificaton column of the present invention. The column contains eight agitated washing sub-zones, denoted as A-l, A-2, , A-7 and A-8 sub-zones, and - eight stationary washing sub-zones, denoted as B-l, B-2, and B-8 sub-zones. (Zg)g and (X _A )g respectively

35 represent the impurtiy concentrations in the retained liquid of the solid-liquid feed and in the wash liquid; (Z _A )j and (Zg) _j respectively represent the impurity concentrations in the retained liquids of the solid

phases transferred at the lower ends of A-i and B-i sub-zones ^" ;- (X _A ) _j and (Xg) _j respectively represent the impurity concentrations in the free liquids transferred at the upper ends of A-i and B-i sub-zones. It is illustrated in the figure that the impurity concetrations in liquids at corresponding positions in these sub-zones form sequences that are nearly geometric sequences. Thus, [(Z )Q , (Zg) _j , , (Zg)g] , [(Z _A ) _j , (Z _A )2 , ,

< ^z A ⁾ 8]'[ ^(χ B>l' ^(X B ^} 2 '—' ^(X B>8 ^] ' [< ^X A>1 ' ^(X A>2 ,— > ^(χ A>8 , (X»)g _] form four sets of sequences. It is important to note that the impurity concentrations in the liquids in each set successively diminish without approaching a limiting value. Therefore, by having a proper number of sub-zones, the impurity level can be reduced to a very low level, say in the part per million or even lower level.

It will be shown that the depth of each sub-zone is rather short: the depth of a stationary sub-zone may be from a few inches to 1 or 2 feet, and the depth of an agitated sub-zone may be shorter. Therefore, a high performance column that produces super-pure chemicals may be a relatively short column. Futhermore, it will be shown that a relatively mild agitation enhancing local mixing may be used in an agitated sub-zone. To summarize, in a purification column of the present invention, alternative stationary washing and agitated washing operatons and inter sub-zone transfers of the solid phases and the free liquids are properly conducted so that the bed moving through the column refreshes itself, keeping up the performance of the column.

5-IIb Performance of the Second Mode. Process

Figure 8 illustrates a general system in which the second mode process can be conducted and Figure 9 illustrates the performance of the system.

The second mode of operation is based on the discovery-that, by using a multistage processing with alternate washing and draining operations in each stage and substantially counter-current transfers of the solid 5 phases and free liquids, a high degree purification of a mass of crystals can be accomplished by substantially reduced degrees of washing and draining in each stage compared respectively to crystal purification mainly by washing alone as in a column crystallizer and to crystal

10: purification mainly by draining alone as in a centrifugal sedimentation and in a centrifugal filteration. Therefore, a crystal purification process of the present invention gives an improved overall economy over conventional ways of purifying crystals. In this

L£ disclosure, "substantially reduced degree of washing", and "moderate degree of washing" are used to describe the degree of washing to be attained in each processing stage. These words are used equivalently to describe that the degree of washing to be attained in the washing

20; sub-zone of each processing stage of the present process is substantially lower than the degree of washing needed to attain the same overall degree of purification when washing alone is the main mechanism of purification, such as in the washing column of a column crystallizer. Also,

2 ^* 5- "substantially reduced degree of draining" and "moderate degree of draining" are equivalently used to mean that the degree of draining to be attained in the draining sub-zone of each processing stage is substantially lower than the degree of draining needed to attain the same

30 ^' overall degree of purificaton when draining alone is the main mechanism of purification, such as in a centrifugal separation, centrifugal sedimentation or centrifugal filtration.

Performances of sedimentation centrifuges and 3 ^' 5 ^" centrifugal filters are described in Section 4.5 of "Handbook of Separation Techniques for Chemical Engineers" quoted earlier. These performances may be used as reference performances.

Figure 8 illustrates a crystal purification system of the second mode process. ^t . he system comprises a main processing zone 31 and a crystal melting zone 32. The main processing zone comprises a plurality of processing stages, stages 1 through 5 being shown. A processing stage, say n-th stage, in the main processing zone, has a washing sub-zone, denoted as A-n sub-zone, and a crystal draining sub-zone, denoted as B-n sub-zone, the two sub-zones forming a conjugated set of sub-zones. Therefore, the stages 1 through 5, 33a, 33b, 33c, 33d and 33e, have crystal washing sub-zones 34a, 34b, 34c, 34d and 34e, denoted respectively as A-l, A-2, A-3, A-4 and A-5 sub-zones and have draining sub-zones 35a, 35b, 35c, 35d and 35e provided with filters 35f, denoted respectively as B-l, B-2, B-3, B-4 and B-5 sub-zones.

The main processing zone is enclosed within the boundary illustrated by the dashed lines and has a first end that is on the top end and a second end that is the bottom end. Crystal washing in each washing sub-zone may be agitated washing or packed bed washing. Crystal draining in each draining sub-zone may be a simple gravity filtering operation or a centrifugal draining operation. A centrifugal draining operation may either be a centrifugal sedimentation operation or a centrifugal filtering operation. A simple gravity filtration with a filter 35f is illustrated in each draining sub-zone of the figure. A feed solid-liquid mixture ( _A ) is drained in the initial draining zone B-0 not shown in the figure to form a filtrate (Lg)(j which is discharged from the system and a drained solid phase (Kg)Q which is introduced into the main processing zone at the first end. A part of the melt formed in the crystal melter ( tø) _(j is introduced into the main processing zone at the second end. The drained solid phases leaving B-0 zone and B-l through B-5 sub-zones are respectively denoted as < ^K B ^} 1 ' ^(K B>2' < B>3 ' (Kg)4 ^' , ^* ' and (Kg)g streams ; ^" the solid-liquid mixtures or simply solid phases leaving A-l through A-5 sub-zones are respectively denoted as (K _A )ι , (K _A )2 , ( _A )3 , ( _A )4 and ( _A )g streams; the liquid streams

leaving B-l through B-5 sub-zones are respectively denoted -Jg)j , dg^ ' ^(I B^3 *. ^(I βU ' ^{and (I} B ^} 5 ^strearπ s. A part of the first drained liquid stream (ig) becomes intra-stage recycle stream ^a °d is recycled to A-l subzone and the remainder (Lg)j is discharged from the system. Part of the (Iβ) ' ^B^3 ' (Ig) ^and ( ^x >5 ^streams become intra-stage recycle liquids ( g)2 t ( ^J g)3 ' t ^J BH and Jg)5 and are respectively recycled to A-2, A-3, A-4 and A-5 sub-zones respectively; the 'remainders become inter-stage transfer liquids ( g)2 , ( g)3 , ( g)4 and

(Lg)g that are respectively transferred to A-l, A-2, A-3 and A-4 sub-zones. The drained solid phase from the last stage (Kg)g is melted in the crystal melter 32. A part of the melt becomes the purified product D and the remainder becomes a wash liquid (Ltø)g which is introduced into A-5 sub-zone. In general, the operations conducted in n-th processing stage are as follows:

(a) The drained solid phase (Kg)π—1 from (n-l)th stage, the inter-stage transfer liquid (Lg)π+1 from the (n+l)th stage and the intra-stage re¬ cycle liquid (Jg) _π are brought in the A-n s.ub zone and the resulting mixture is discharged as (K _A ) _π solid phase.

(b) The (K _A ) _π solid phase is drained in the (B-n) sub-zone and becomes a drained solid phase (Kg)n and a mother liquor (Iβ)π •

(c) A fraction of the mother liquor becomes an intra stage recycle liquid (Jg) _π and is recycled to the A-n sub-zone, and the remainder becomes an inter-stage transfer liquid (Lg) _π and is introduced into A-(n-l) sub-zones. The liquid streams (Lg)Q and (Lg)ι that are obtained from the B-0 and B-l sub-zones become residue streams that contain substantially all o.f the impurities in—the feed mixture.

The ratio of the mass of inter-stage transfer liquid and the mass of the retained liquid in the drained solid

phase, viz. the ratio of thetmass of (Lg) _π to the mass of (Mg) _π is an important factor in determining the effectiveness of a crystal purification operation: the effectiveness increases as this ratio, (Lg) _|j /(Mg) _fl , increases. This ratio may be denoted as the wash ratio, R{]. The wash ratio can be increased by increasing (Lg) _π , by decreasing (Mg)r _j , or by both. A high (Lg)π - value requires recycling a large amount of crystal melt and thereby requires an increased energy consumption. Therefore, it is desirable to attain a desired wash ratio by decreasing (Mg) _π , viz. increasing the degree of draining in the B-n sub-zone. In practice, there is an optimal degree of drainings: a high degree of drainage requires an expensive centrifuging operation and a low degree of drainage results in a small wash ratio and a low degree of crystal purification.

The impurity concentration in the product Xp is related to the impurity concentration in the retained mother liquor (Z ) from the last stage. The impurity concentration (Zg)»ι leaving the last stage is related to the impurity concentration of the retained liquid in the feed (Zg)g , the wash ratios, Rj , R2, , Rj^j, and the number of stages N. Figure 9 shows the relation relating (Zg)jι to Rfi and N, when it is assumed that (Zg)Q = 0.1 and that the wash ratios are all equal.

Lines 36a through 36i respectively show Z^ as functions of number of stages N, when (Zg)g = 0.1 and R _Λ ' _S are 1, 1.5, 2.0. 3.0, 4.0, 5.0, 6.0, 8.0 and 10.0 respectively. It is seen that a high degree purification can be obtained by using a reasonably low wash ratio and a reasonable number of stages. For example, with (Zg)w = 0.1, R _n =2.0, the number of .stages required to reduce (Z )^ to 1 ppm is 18; with R _π = 3.0, N is around 11; with R _π = 4, N is around 8. When ( gJ^/tKg)^ = 0.1, R _π - 2.0 can be obtained by recycling about 20% of the crystal melt as wash liquid 3.0 can be obtained by recycling about 30% of the crystal melt, etc.

In a system of the present invention both the equipment cost per stage and the cost of operating a stage are reduced to such low values that the overall cost of running the multistage system is low. Yet the system is able to achieve a high degree purification of crystals. In order to achieve a low cost operation of each stage, the degree of drainage to be attained in each draining sub-zone is kept at a moderate degree. The degree of draining attained in each draining sub-zone is substantially lower than the degree of draining needed to attain the same overall degree of purification when draining alone is the main mechanism of purification.

It has been described that, in the third mode of carrying out the invention, the purification is accomplished by a batchwise countercurrent operation conducted in a multitude of crystal washing vesssels. The initial drained bed (Kg)g is subjected to a series of soaking and draining liquids. The performance of a third mode operation can be analyzed in a way similar to that of second mode operation. The results are also similar to those of the second mode of operation.

In this section, the operation of the first mode process and the second mode process have been described and their performances have been presented. Operations of these modes and other modes are described in the following section without presenting their performances.

5-III Other Modes of Carrying out the Invention

Figure 10 illustrates an elongated processing zone

37 in which the transverse agitation of the two phases is accompl-lshed by using multitude of small agitating blades

38 rotated by multiple shafts 39. Long range mixing of the two phases in the transport direction is suppressed by maintaining a low value of M/S ratio and thereby maintaining a high consistency value for the two phase mixture and using gentle agitation by small blades.

Figure 11 illustrates a general system in which Type AB operations described earlier are conducted. In the system, solid phase is transported from the first end 40 toward the second end 41 and the free liquid is trans¬ ported from the second end toward the first end. Transverse agitation and counterwashing are conducted simultaneously in sub-zones AB-1 through AB-10. These sub-zones may be discrete sub-zones or they, may form a continuous zone. Figure 12 illustrates a system of Figure 11, in which the desired transverse agitation is accomplished by ultrasonic vibrations. Several transducers used 40 are the agitating elements and are connected to the vessel wall 41. Figure 13 illustrated another system of Figure 11, in which the desired localized agitation is accomplished by rotating thin wires 42 or thin blades that are connected and rotated by rotating arms 43.

Figure 14 illustrates a crystal purification system having a crystal feeding zone 44a denoted as A-l sub-zone, a main crystal purification zone 45 similar to the system illustrated by Figure 6 , and a crystal melting zone 46, denoted as M-6 sub-zone. The purification zone comprises a set of agitated sub-zones 44a through 44e, denoted respectively as A-l 'through A-5 sub-zones, and a set of stationary sub-zones 47a through 47e, denoted respectively as B-l through-, B-5 sub-zones, in each stationary sub-zone, there are radial and vertical

baffles ^" ~48 dividing the sub-zone into compartments. There is a central shaft 49, a crystal feeding means 50, a first set of solid phase transfer means 51, a set of agitating means 52, a second set of solid phase transfer 5 means 53, a solid phase discharging means 54 and another agitating means 55 for agitating crystals in the melter. It is seen that all the feeding, agitating and transfer means are attached to the central shaft and are rotated by it. A heating coil 56 provided with a heating medium 10. inlet 57 and a medium outlet 58 is installed in the crystal melting zone.

For an effective crystal washing operation in a stationary sub-zone the bed in the sub-zone needs to be compacted to a degree that is substantially higher than 15 the natural degree of compaction. This is accomplished by activating the solid feeding means 50, and solid-phase transfer means 53. Therefore, these solid-phase transfer means are also used as bed compacting means.

Figure 14a illustrates a cross section taken at a 20 stationary sub-zone. It is seen that there are vertical and radial walls 48 separating the sub-zone into compartments 59a through 59f, in which crystal beds are formed and descend through. The walls are provided to help maintain a compacted bed and preventing the bed from 25 being agitated by the agitators in the neighboring sub-zones.

Figure 14a and 14c illustrate one type of agitator that can be used in an agitated sub-zone to promote local mixing of the crystal-liquid mass to thereby break up

30 crystal agglomerates, mix crystals and the free liquid to thereby release the impurities from the crystal mass into the free liquid and help recrystallize the crystals. It is noted that local mixing 'of the crystal mass ^" and free liquid is important in this operation. The agitator 44

35 shown has radial arms 60 and blades 61 that are tilted with respect to the direction of its movement. The

agitator is rotated so that ttte'motion is substantially coplanar. The blades on the first and third arms are tilted in one direction and the blades on the second and fourth arms tilted in the opposite direction. As the crystal bed descends through the agitated sub-zone, the crystals are moved alternatively inward and outward and are mixed intimately with the upward moving free liquid. It is noted that the motion of a crystal mass in the sub-zone is substantially coplanar, even though there is some downward component to follow the descent of the bed.

Figure 14c illustrates the structures of the solid phase transfer means 51, 53 shown in Figure 14. A transfer means has blades 62 tilted in the downward direction. The first transfer means 51 at the bottom of a stationary sub-zone shaves a mass of solid bed that sticks out of the sub-zone and thereby transfers it to the next agitated sub-zone. The second transfer means 53 at the bottom of an agitated sub-zone takes in a mass of crystal-liquid mass from the sub-zone, compacts it to release a major fraction of the liquid and transfer the compacted mass of crystals to the next stationary sub-zone. The second transfer means also serves to compact the bed in the next stationary sub-zone.

Figure 14d illustrates a rotating disk 63 with perforations 64 provided with shaving knives 65. Such a rotating perforated disk may be used simultaneously to support the bed in a stationary sub-zone and to transfer the solid phase from the sub-zone. It can therefore be used in the place of a first solid transfer means 51 in Figure 14. In Figure 14, the rotating perforated disk 54 is shown as a solid transfer means transferring the solid phase from the last stationary sub-zone into the crystal melting zone.

Figure 15 illustrates a modified crystal purification unit. This unit is similar to that of Figure 14 and has the following modification: a rotary

perforated plate 66 provided with shaving knives 63 similar to that illustrated by Figure 14d is used to separate a stationary sub-zone from its next following agitated sub-zone. The bed in the stationary sub-zone is supported by the rotating perforated disk 66 and is compacted by a rotating blade 67 that is illustrated by Figure 14c. The crystal-liquid mass in an agitated sub-zone is agitated by an agitator 68.

Figure 16 illustrates yet another modified crystal purification unit. This unit is also similar to that of Figure 14 and has the following features: (1) ^■ there is no perforated plate, rotary or non-rotary, to separate a stationary sub-zone from its neighboring agitated sub-zone, (2) no rotary blade is used to compact the bed in a stationary sub-zone, and (3) there is no rotary shaving knives for inter sub-zone transfer of the solid phase.

An agitator 68 is used to agitate an agitated sub-zone. There is only one bed compacting means 69 at the top of the column. In this unit, the crystals in all sub-zones including the agitated sub-zones form a continuous bridged structure through which forces for compacting the bed can be transmitted. Therefore, the downward force applied by the compacting means at the top of the purification zone and the entire weight of the solid bed above a given level are applied to the bed at the level through the bridged structure and thereby compact the bed thereat. The agitator may also be provided with tilted blades to compact the bed further.

Figure 17b illustrates a drained bed washing column.

There are a set of agitated sub-zones, denoted as A-l through A-5 sub-zone, and a set of non-agitated sub-zones, denoted as 3-1 through B-5 sub-zones. ^" These two sets of sub-zones are laid alternately upward. A feed crystal-mother liquor mixture is fed at the bottom and the solid phase is pushed upward by the blades in

the agitated sub-zones, wash liquid is added at the top and drain- through the bed downward. Figure 17a illustrates a cross-section of an agitated sub-zone;

Figure 17b illustrates a cross-section of a non-agitated sub-zone; Figure 17c shows an extended view of the column. The rotating blades 70 in an agitated sub-zone agitates the two phase mixture in the sub-zone and push the drained bed upward. There are baffles 71 in the non-agitated sub-zones to keep the beds therein from rotating with the beds in the agitated sub-zones.

In-line motionless mixer or static mixer described may be placed in the non-agitated sub-zones. For a large drained bed crystal washer, it is desirable to use several washing rings in an agitated washing sub-zone. The columns illustrated can also be used as submerged bed washers by introducing the feed crystal-mother liquor at the top and introducing the wash liquid at the bottom.

Figure 18 illustrates a centrifugal crystal purification unit of the present invention. There are a first rotating body 72 that is rotated at (rρm) _j , and a second rotating body 73 that is roatated at (rpm)2. There are an initial draining zone, denoted as B-0 zone, and a main processing zone having a first end 74a and a second end 74b within the first rotating body. The zone inside the first rotating, body is divided into sub-zones by baffles 75a, 75b. Four processing stages are shown in the figure: the washing sub-zones 76a, 76b, 76c, 76d are respectively denoted as A-l through A-4 sub-zones; the draining sub-zones 77a, 77b, 77c, 77d are respectively denoted as B-l through B-4 sub-zones. There are centrifugal filters 79 in the initial draining zone and all draining sub-zones. These filters are part of the first rotating body and are rotated at the first rpm, (rpm) _j . hen size of crystals is small, one may use sedimentary centrifuges in' €he place of centrifugal filters. There are rotating arms 80 with agitating means 81 and crystal transfer means 82 in the washing sub-zone. These are part of the second rotating body and are

rotated at the second rpm, (rpm)2. The first rpm is of such a moderate value that solid.phases introduced into the draining sub-zones are drained to moderate degrees. The centrifugal force applied to the centrifuges is also moderate and is substantially lower than that of a conventional centrifuge used to accomplish equivalent overall degree of crystal purification. The second rpm is either somewhat greater or less than that of the first rpm, so that the content in the washing sub-zones are properly agitated by the agitating means 81 and solid phases (K^) _π are transferred by the transfer means 82 to the corresponding draining sub-zones.

In operation, a feed solid-liquid mixture (Kfl)g is introduced into B-0 zone and is drained and becomes (Kg)n and (Lβ)Q. (Lg)ø is an impure liquid and is discharged from the system; the solid phase (Kg)ft. is introduced into the main processing zone. The drained solid phase from the last stage (Kg)* is taken to the crystal melter 78 and is melted therein. A portion of the melt becomes the purified product D and the remainder (L )5 is transferred to A-4 sub-zone. The operation conducted in the n-th stage are similar to those described earlier for the system of Figure 8 and are described briefly by referring to the second stage as follows:

(1) ( ^K B^1 ^{is mix} ® ^d with (L )3 and g)2 in the A-2 sub-zone and is agitated by the agitating means 81 and become a solid-liquid mixture (K^)2.

(2) The solid -liquid mixture. β)2 is scraped up of A-2 sub-zone and transferred to B-2 sub-zone by the transfer means 8 ^* 2.

(3) (K 2 ^S drained in the B-2 sub-zone by the centrifugal filter 79 and form a drained solid phase (K )2 and a mother liquor (Iβ)2 •

(4) A portion of (Ig)j ^' -becomes an intra-stage recycle liquid ( g) and is returned to the

A-2 sub-zone through the opening 8ia on the baffle 75b.

(5J The remainder of ^ e mother liquor becomes an inter-stage transfer liquid (Lg and is transferred to the A-l sub-zone.

Operations in other stages are similar. It is noted that the agitation in the washing sub-zones and intra- stage transfer of (K») _n are made possible by rotating the second rotating body at a rpm that is somewhat greater or less than that of the first rotating body. It is also noted that the first rotating body is rotated at a moderate rpm so that the solid-liquid mixtures (K*) _π 's are drained to moderate degrees and give moderately drained solid phases (Kg)η's. The rpm of the first rotating body is chosen by a simple optimization study based on computations that are used in constructing -Figure 7. A higher rpm gives a higher degree of drainage, resulting in a lower (Mg) _π /(Sg) _n ratio and a higher wash ratio. However, a higher rpm also require a higher equipment cost and operating cost. Due to the multistage operation, a high degree purification of a mass of crystals can be obtained when the degrees of draining are rather moderate. The centrifugal force used, is substantially lower than that used in a conventional centrifuge that gives as equivalent overall degree of purification. The centrifugal force used in the former may be less than 30% or even less than 10% of that of the latter. The drained solid phase from the last stage (Kgty is collected into a stationary chamber 83 and then introduced into the crystal melter 78. A portion of the melt becomes the purified product and the remainder (Lg)* is transferred to A-4 sub-zone by a pump 84. (kg)ø and (Lg)ι are collected into a stationary ring 85 and are discharged from the stationary ring.

Figure 19 illustrates a plate column crystal washing system. There are a set of'agitated crystal washing sub-zones 86, denoted as A-l through A-5 sub-zones, and a set of transfer sub-zones 87, denoted as B-l through B-4 sub-zones, and a crystal melter 88, denoted as M-6

sub-zone. The A-2 sub-zone has two component sub-zones, denoted as A-2a and A-2b sub-zones, and the A-3 and A-4 subzones also have two component sub-zones, denoted as A-3a and A-3b sub-zones and A-4a and A-4b sub-zones respectively. The transfer sub-zones, denoted as B-l,

B-2, B-3 and B-4 have solid transfer sub-zones 89, B-la, B-2a, B-3a and B-4a and liquid transfer sub-zones 90, denoted as B-lb, B-2b, B-3b and B-4b. The remainder 91 of the transfer sub-zones, denoted as B-lc through B-4c, are blanked off and not used. The solid phase is transported sequentially through A-lb, B-la, A-2a, A-2b, B-2a, A-3a, A-3b, B-3a, A-4a, A-4b, B-4a and A-5a sub-zones and enter the melter M-6; the free liquid is transported sequentially through A-5a, B-4b, A-4b, A-4a, B-3b, A-3b, A-3a, B-2b, A-2b, A-2a, B-lb and A-lb sub-zones bypassing B-4a through B-la sub-zones. The solid phase is moved inward on the plates in A-lb, A-2b, A-3b and A-4b sub-zones and is moved outward on the plates in A-2a, A-3a and A-4a sub-zones. Some free liquid streams ⁽ LA ^*> 2a ' ^ ^L A^3a ' ^ ^L AUa ^and ^ ^L A^5a ^aEe separated'from the two phase mixtures by the filters provided in the A-2a, A-3a, A-4a and A-5a sub-zones and are introduced into A-lb, A-2b, A-3b and a-4b sub-zones and therefore bypass B-la, b-2a, b-3a, and B-4a subzones respectively. The two phase mixture entering B-la sub-zone is compacted so that a part of the liquid (Lg) _ja leaves the sub-zone and passes through B-lb sub-zone to A-la sub-zone. Similar operations take place in B-2a and B-3a sub-zones. The pressure drop in this system is small, because a major fraction of the liquid phase bypasses the compacted beds in B-l, B-2, B-3 and B-4 sub-zones. For the same reasons, this system can purify a solid-mother liquor mixture containing small crystals.

6. Industrial Applications

The present process is an energy conserving process that can accomplish a super purification of a crystallizing component. The process may therefore be used to produce superpure monomers for the polymer industries and high purity chemicals for the electronic, pharmaceutical, pesticide and other industries. It may be used to separate azeotropic mixtures.