^' APPARATUS FOR THE RECOVERY OF VAPORIZED PHTHALIC ANHYDRIDE

FROM GAS STREAMS

This invention relates to purification by conden- sation and recovery of phthalic anhydride (PA) from gas streams, in which it exists as a condensable vapor, either as formed by oxidation of naphthalene or o-xylene admixed with other oxidation products or as released by PA storage tanks in the breather vent gas, or as generated by any manu- facturing process, in a heat transfer system of improved

^• efficiency, and more particularly, recovery of the. phthalic anhydride in a heat-pipe switch condenser system, comprising a group of heat pipes which in alternate operation the phtha lic anhydride is condensed to solids in one mode, while the condensed solid phthalic anhydride is melted out in a simul¬ taneously operated companion unit, the units being cyclicall alternated, with great heat economy both with this more ef¬ ficient exchanger system and by using air as the cooling phase for the condensation of PA solids and by using hot combustion gas or heated air for melting the condensed PA solids, especially using waste heat such as incinerator ex¬ haust gas for supplying the heat needed for melting out to recover the condensed PA solids in the companion mode and apparatus. The process hereof becomes more efficient for the production and recovery of phthalic anhydride and is greatly

improved in economy over the prior art practices. The heat- pipe exchanger, although known per se as a type of exchanger has not been used either for PA recovery or comparable sys¬ tems. Moreover, the heating and cooling systems used in the present PA. recovery are alternately ambient air for condens¬ ing the PA vapor, and hot gases, usually from combustion of waste gases, particularly even exhaust gases available from incineration of PA reactor tail gas, for supplying heat needed to melt the condensed PA solids. Particularly, the improved economy in the construction and operation of the system is available from the fact that no circulating water or. other low temperature heat transfer medium is needed to cool the gas and condense the PA to the solid state, nor is steam or other high temperature heat transfer medium needed to melt out the recovered solid, so the heat exchanger sys¬ tem can be located and operated, not only without these auxiliary heat transfer media systems, an important economy in itself, but also it can be located in places where coolin water is unavailable or costly.

Prior Art

In prior art practices the condensation of PA to a solid state is typically performed in shell and tube heat exchangers, or in arrangements of tube bundles in large en¬ closures with alternate cooling and heating of the tube sur- faces by passage of a heat transfer medium through the tubes, one unit of which cools the gas stream to condense the P

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vapor on the tube surfaces, and after selected periods the unit is alternated to the other mode of passing a second heat transfer fluid through the tubes at a temperature- high enough to melt the PA. There are three types of phthalic anhydride gas streams conventionally to be treated, the so-called (a) re¬ action gas stream formed in catalytic oxidation of naphtha¬ lene or o-xylene; (b) vent gas stream which emanates from P storage tank breather vents; and (c) any manufacturing pro- cess gas stream that generates PA-laden vapors, each posing the problem of condensing the gaseous phthalic anhydride to solid form, and the subsequent warming of that solid conden¬ sate to- recover the solid phthalic anhydride in molten form.

TEMPERATURES In the case of reaction gas, the initial tempera¬ ture of the gas PA stream is in the approximate range of 135-250°C and the exit gas temperature to which the gas stream is cooled is usually in the range of 45-75°C, prefer¬ able to about 55-65°C. Vent gases reach a lower entering temperature, such as 135-165°C with the desired exit gas temperature in the range of 40-60 C. The process gas stream typically have an entering temperature in the range of 135- 200 C, depending upon the source of the process gas. Conse¬ quently, for the stated low exit temperature the exchanger surfaces at the outlet end should be maintained in the range of about 35-60°C with higher values applicable to reactic

gas and lower values to vent and process gas streams.

The surface temperature of the exchanger required for melting the deposited PA solids is in the range of 135- 190°C from any of these sources. Merely by considering the extreme temperatures to be needed in each exchanger operatio from low to high as stated, separate systems are required fo the two levels of heat transfer medium to meet these two ranges, i.e. as low as 35-60°C and as high as 135-190°C. These separate low temperature and high temperature fluid circulating systems that - re needed for conventional exchang er practice typically depend on cooled water and high pres¬ sure steam, or a cold and a hot circulating fluid, and that was the practice of the prior art in making such temperature practically available for handling these PA recovery streams.

AUXILIARY UNITS

Moreover, the low temperature circulating system must further be provided with means for dissipating the heat removed from the hot PA gas stream, and conventionally heat exchange with cooling water was employed for this, but the cooling water also required a cooling tower for dissipating the heat. The high temperature heat transfer system, in con trast, required supply of high level heat to the circulating fluid, and this could be accomplished only by an indirect ga fired or oil heater, or by use of a boiler in the case of heating with steam.

It is clear that the facilities required in the

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prior art to alternately cool and heat the heat exchangers involved great initial investment, as well as high operating and maintenance costs. Moreover, conventional indirect ex¬ changers of that type have low thermal efficiencies with ac- companying low economy. In the case of reaction gas it is most uneconomical to utilize available waste heat in the tail gas incinerator exhaust usually associated with such systems. Vent gas streams in most instances require processing at re¬ mote tankage areas where cooling water is not available and a cooling tower for this purpose is needed. The isolated locations of such equipment items further present maintenance problems complicated by the isolation.

Heat-pipe exchangers for PA recovery, according to the present invention, simplify the heat exchanger system by eliminating the need for an intermediary low temperature heat transfer system, such as cooling water or other medium, and impart greater thermal efficiency to the system, so that particularly in the case of reaction gas, the available waste heat from the reaction process is useful in the melt- ing step to virtually eliminate the need for other fuel. In consequence, the present invention provides lower initial investment, lower operating costs and substantial fuel sav¬ ings. Each of these objects is achieved in the novel heat exchanger system using the heat-pipe exchanger both in re- covery and melting of PA applied in alternate operating modes. It uses air directly as the cooling medium and either heated air or hot exhaust gas for the heating medium in con-

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trasting great efficiency and economy for this purpose.

The heat-pipe exchanger hereof seems uniquely adapted for recovery of PA from the several gas streams listed to achieve these benefits. The term "heat pipe" as commonly understood, refers to a conduit pipe, duct or tube, usually but not necessarily circular in cross-section, and sealed at both ends to provide a closed circuit. It in¬ cludes a wick structure within the duct or tube and usually upon the inside surface, and the tube is evacuated and charged with a small quantity of a working fluid. Typical working fluids are the common fluorocarbon fluids such as R-ll and R-113, and hydrocarbons such as toluene. Heat ab¬ sorbed at one end of the heat-pipe causes the working fluid to vaporize, and the vapor flows under the driving force of the slightly increased pressure to the cold end of the tube where it condenses and thereby releases its heat to the pipe wall. The circuit is completed by the condensed working fluid returning to the hot end of the heat pipe by capillary movement through the internal wick structure. A heat-pipe exchanger includes a substantial multiplicity of such heat- pipes, arranged so that all of the cold ends are exposed ex¬ ternally to the gas stream being heated, and the opposite hot ends are exposed to the gas stream from which heat is being extracted, in gas-to-gas heat exchange of the two streams, each conducted simultaneously through adjacent enclosures about opposite tube ends in countercurrent flow with the heat pipes in a bundle extending into separate enclosure ducts,

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one for each stream, and having a common dividing wall. The total heat-pipe exchanger consists of such tubes sized to accommodate the exchanger needs of the streams to be treated. The advantages of the heat-pipe exchangers applicable to PA recovery are:

1. They operate with small temperature difference between cold and hot ends of the heat-pipes under conditions of high heat transfer rates;

2. They have high thermal efficiency in gas-to- gas exchange at low pressure drops;

3. They have large surface areas that are avail¬ able in heat-pipe bundles of reasonable dimensions through the use of extended surface or finned tubes for the accumula¬ tion of the condensed PA solids; 4. Bidirectional control of the aspect of heat pipe operation is an inherent feature such that the heat transfer can be reversed by reversing the hot and cold ends, in this case by switching the hot and cold gas or air streams. 5. The working fluid contained in the sealed pipes can be readily selected as to be operable over the full temperature range of both condensing and melting steps from about 35-225°C.

Because of these characteristics, ambient air is useful directly as a cooling gas stream for the PA condensing step. To avoid cooling reaction gas below a temperature approaching the water dew point of about 35-40°c, a portion of heated exit air is recycled to the blower and mixed with

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incoming ambient air to maintain a combined inlet temperature in the range of 30-55°C, preferably 35-40°C.

Again, heated air or gas is useful as the high temperature heating medium applied during the melting step of the PA. For this several options are available. One such is to recirculate through the air side of the heat-pipe exchanger heated air at a temperature in the range of about 150-300°C, preferably 175-225°C. The air is heated by sev¬ eral alternate means, using a gas or oil fired in-line burner, or electrically using an electric resistance heater, or using a steam coil. A further option is available with reaction gas systems that employ a tail gas incinerator by using the incinerator exhaust gases adjusted to a temperature range of 200-300°C as the heating medium for melting the PA. Typi- cally, the incinerator exhaust gases will have a heat con¬ tent sufficient to supply all of the necessary melt-step heat in a reasonable adjusted cycle time. Moreover, an aux¬ iliary burner may be provided to pass combustion gases into the hot gas duct to supplement the heat as needed for the usual cycle time, or even to accelerate to shorter periods the melting time required to melt the PA using a higher temperature as provided by the extra heat, and periodically to remove other high melting impurities that may accumulate on the heat transfer surfaces for short excess-temperature heating periods. Still another option available for treat¬ ment of reactor gases, useful in the absence of a tail gas incinerator, is to use any available hot flue gas stream

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from an oil, gas or coal fired heater or steam boiler.

In vent gas applications, heating of the air for the melting step is most readily provided by use of an elec¬ tric resistance heater, although a steam heater can also be -used, with total air recirculation to minimize the energy requirement. In these several alternatives, the pre'sent method allows complete elimination of the need for separate cooling water systems, usefully serving remote tankage area locations, and it also makes possible elimination of steam supply and steam condensate systems.

Wide variations in methods for designing gas-to¬ gas heat-pipe exchangers for other services are known. In designing for condensation of the accumulated solid PA while maintaining the desired thermal efficiency and acceptable pressure drop, it is found that the number of tube rows may be increased in depth by a factor of 1.5 to 5, preferably 2 to 3 with respect to the usual gas-to-gas exchanger re¬ quirements. Alternatively, it is preferred to increase the face area of the PA condensing side of the exchanger by a similar factor, or to increase the face area in combination with an increase in the number of tube rows, to minimize pressure build up on the condensing side. In this manner vent gas, process gas or reaction gas streams containing va¬ porized PA in the typical concentrations, ranging from 30-75 g/Nm at a temperature in the range of 135-250 C, can be cooled with ambient air in a heat-pipe exchanger to an exit temperature of 45-75°C, preferably 55-65 C with a PA recovery

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efficiency of greater than 99% and the condensed solid is easily and efficiently melted off of the exchanger surfaces either by heated air or an exhaust gas stream or other heat alternatives listed above. The invention is further described in relation to the drawings wherein:

Fig 1 schematically shows, in plan view, an ar¬ rangement of a heat-pipe switch condenser system with one exchanger A in the condensing mode and the second heat-pipe exchanger B of the same system in the melting mode. Both heat-pipe exchangers 10 are each disposed in a divided hous¬ ing 12 and 18. A number of heat-pipes are disposed in a bundle 14, extending horizontally, or at a slight angle, from one housing 12 to the other housing 18, separated by a partioning wall 20 which prevents any interchange of exchang¬ er gases from the chamber 12 to the chamber 18. The incoming hot gas stream, either reaction gas, vent gas or process gas carrying recoverable PA enters through duct 22, the left valve 24 being opened and the right closed. The PA laden stream passes into the housing 12 through that portion of ex¬ changer bundle 14 contained therein, referred to as the "condensing side", at a temperature in the ranges stated above, leaving through duct 28, the valve 30 being open.

The left blower 32 draws ambient air through ducts 34 and 36 as well as recycle flow in line 42, the valves 38 and 40 being adjustably opened. The proportioned flows are adjusted by the settings of valves 38 and 40, to provide the

inlet air at a temperature adjusted by recycle to housing 18 as cooling air to pass through the opposite portion of bundle 14, referred to as the "air side". The entering cooling air from line 44 at the left side of the exchanger thus passes in countercurrent flow to the PA-laden gas, and by the mecha¬ nism of heat transfer characteristic of heat pipes described above, cools the gas for condensing PA, the residual gas leaving at 28. The heated air from the air side 18 leaves by way of line 46, the valve 48 being opened. In an alter- nate melting stage a duct 50 for removal of melted PA after first depositing in mode A operation is connected to a duct 52 through a valve 54 for disposal of the molten PA with¬ drawn to storage. The duct 54 is closed to exchanger A during the condensing cycle, while the PA is accumulated as solid sublimate therein.

As the hot entering PA-laden gas passes through the condensing side of the heat-pipe bundle 14, it transfers its heat to the heat pipes and condenses the PA vapor to the solid state. The heat extracted in gas cooling and in the condensation of the PA is conducted by the heat-pipe vapori¬ zation of internal working fluid to the opposite end of the tube bundle 14 where it is cooled and condensed by ambient air passing through housing 18. The lower temperature cool¬ ing air stream passing through the air side from line 44 cools and condenses the internal working fluid which then flows by capillary action through the wick structure therein, returning as liquid working fluid to the PA condensing side

IΓGREA

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of heat-pipe bundle 14 contained in housing 12. A portion o the air heated in passage through the airside leaves by"«a of 46 and another portion controlled by valve 40 is recycled through line 34 for admixture with fresh ambient air to pro- vide a temperature control for the entering ambient cooling air in line 44. During the cooling of the PA-laden reaction gas, the PA is condensed as a solid on the heat-pipe tube surfaces in the condensing side. Somewhat prior to the point at which the accumulated solids adversely affect the heat transfer or the pressure drop, the exchangers are each switched to the alternate mode, that is, mode A formerly condensing PA is switched to melting mode B and mode B for- - merly melting is switched to condensing PA. In this illus¬ trative drawing, both left and right hand sides of Fig 1 hav similar exchangers simultaneously operating, one to condense PA and the other to melt and remove the condensed PA solids, shown in the figure as mode A on the condensing cycle and mode B on the. melting cycle, which are alternatively switche to the opposite cycle periodically. During the melting .cycle exchanger B is isolated by valve 24 and the 3-way valve 54 is positioned to allow flow between ducts 56 and 52 for discharge of the melted PA. In the case of heat being provided by hot combustion gas or incinerator exhaust gas (combustion equipment and incinerato not shown) , this hot gas stream flows through duct 58, valve 60 being open, enters the air side housing 18 of the exchanger B

through line 44, and' exits through valve 48, the recycle valve 40 being closed. The direction of heat transfer in the heat pipes is now reversed from that of the condensing mode. Heat is extracted from the hot gas stream passing through the air side in housing 18, causing the internal working fluid in the heat pipes to vaporize and to flow to the other side of the tube bundle-contained in housing 12, on the external surfaces of which is deposited the solid PA condensed during the previous cycle. The condensing working fluid raises the temperature of the tube sufficiently to melt the accumulated PA as defined above. The condensed working fluid in the heat pipes then returns to the hot ends of the tube bundle in the ^' air side to complete the internal flow cycle as described above. In the alternate case of heat being provided by heater 62 (right side) , which can be a gas or oil fired in¬ line burner, a steam coil, or an electric resistance heater, blower 32 is used to recycle the heated air stream through duct 44, exchanger housing 18, returning to the blower through ducts 42 and 34, and valve 40, valve 38 being closed. It will thus be observed that the system illustrat¬ ed comprises equivalent exchangers, one of which is operating in condensing mode A simultaneously with the other exchanger operating in melting mode B. The system described herein, however, is not limited to using heat-pipe exchangers in pairs. In special cases of intermittent operation, a single exchanger may be employed. On the other hand, for large re-

action gas installations, the system may comprise three or more exchangers, either in a balanced configuration with an equal number of exchangers alternately in mode A and mode B, or an unbalanced configuration with several exchangers in mode A and with a smaller number operating alternately in mode B on a shorter cycle time.

E X A M. P L E S

EXAMPLE 1

Reaction gas formed by catalytic oxidation of naph- thalene flowing at a rate in the range of 10,000 - 20,000

N /hr. , typically - 1,750 Nm /hr. , at a temperature of 145 C and containing 48.4 g/Nm of phthalic anhydride (PA) vapor is passed into a mode A operating heat-pipe exchanger as shown in Fig 1. It is cooled in the exchanger to an outlet temperature of 60 C to condense 99+% of the PA, depositing PA solids upon the tube surfaces of the condensing side of the heat-pipe exchanger. The ambient air circulated through the air side of the exchanger is flowing at a rate of 20,000

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- 40,000 Nm /hr., typically 36,000 Nm /hr., adjusted to an exchanger inlet temperature of 38°C by partial recycle of outlet air. The exchanger face dimensions on the condensing side of the exchanger are 3.66 m by 3.05 m and on the air side, 2.44 m by 3.05 m. The tubes with an extended surface

2 area of 0.59 m /m are arranged in two banks of eight rows deep, each containing 208 tubes. The air blower has a power requirement of 10 - 20 KW. In mode B the exchanger for the

melting step passes tail gas incinerator exhaust with a tem erature of 204°C at a rate of 10,000 - 20,000 Nm /hr, ^' typi-

3 cally 17,000 Nm /hr. , and which is circulated through the a side of the heat-pipe exchanger to warm the tube surfaces t a temperature of 166 C in about twenty minutes, at which temperature the deposited PA solids of mode A are melted an discharged from the exchanger. The total cycle time is abo ninety minutes for deposition and melting out at these rates forty-five minutes for each mode.

EXAMPLE 2

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Vent gas from PA storage tanks containing 41 g/Nm of vaporized PA, at a rate of 20 - 50 Nm /hr. , typically 34 Nm /hr. , and a temperature of 150 C, is cooled in mode A operation to 49 C condensing 99+% of the contained PA on the dual heat-pipe, exchanger system as shown in Fig 1. It is operated with the following characteristics: Ambient air is circulated through the air side of the heat-pipe exchanger at a flow rate of 200 - 500 Nm /hr., typically 345 Nm ³ /hr. ^' - ^• adjusted to an inlet temperature of 38 C ^' by partial air re- cycle. The face dimension of the condensing side and the air side of the exchanger are 0.30 m by 0.22 m each. The

2 heat pipes with an extended surface area of 0.59 m /m are arranged in a single bank with a depth of fourteen rows for a total of. forty-nine tubes. The maximum air power require- ment is 0.3 KW. For the melting step mode B, the air is passed at 200 - 500 Nm ³ /hr. , typically 360 Nm ³ /hr. , at a temperature of 177 C and circulated through the air side using an electric resistance heater as the source of energy for heating the air. The tube surfaces reach a temperature

of 166°C in about fifteen minutes. The PA condensing step of mode A is applied over 'a period of 4 to 8 hours, typicall six hours, and the melting step is completed in forty-five minutes. As thus described, phthalic anhydride is recovered in a two mode system, condensing the PA from commercially available streams, such as reaction, vent or process gases, by cooling in a heat-pipe exchanger or exchangers for depo¬ sition of the PA in yields of better than 99%, using ambient air or ambient air mixed for temperature adjustment with re¬ cycle air, and preferably simultaneously the deposited PA sublimate is separately melted in the same type of exchanger or exchangers, preferably in alternate cycles using ambient air for cooling and heated ambient air or combustion gases to melt the PA for recovery in-a heat-pipe exchanger system with high operating efficiency.

The system may be modified, as will occur to one skilled in the art, for application of known variations of heat pipe structures and operations in the general exchanger art in the recovery of phthalic anhydride, and it is intende that the description and examples be regarded as illustrativ and not limiting, except as defined in the claims-

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