“INDUSTRIAL PROCESS FOR RECOVERY OF THE SOLID FRACTION, RICH IN DIMETHYLOLPROPIONIC ACID, FROM THE MOTHER LIQUORS OF THE SAME

Technical field of the invention

This invention relates to recovery of the solid fraction, rich in dimethylolpropionic acid, present in the mother liquors of the said acid.

Specifically, the invention is intended for a process for recovery, substantially quantitative and with a high degree of purity, of the dimethylolpropionic acid remaining in the final residual mother liquors obtained from its synthesis and its purification by crystallization and filtration.

Background of the invention

Dimethylolpropionic acid (DMPA, or 2,2-bis (hydroxymethyl) propionic acid [CAS 4767-03-7]) is a white crystalline solid which can be used, for example, as a starting product or as an intermediate for a wide variety of synthesis procedures. A particularly preferred use thereof is, for example, in the field of coatings and adhesives to convert binders or resins which are themselves soluble in solvents into aqueous binders.

One of the important problems connected with the industrial synthesis of dimethylolpropionic acid is constituted by the fact that the particular characteristics of density, viscosity, thixotropy of the process water and its components compromise the possibility of efficiently using filtration techniques for the effective insulation of the product. For this reason, a significant portion of the same is always left in the final residual mother liquors derived from the synthesis and recovery (purification) of the product after crystallization and decantation/filtration.

The average % composition by weight (w:w) of the aforementioned mother liquors is, in fact, substantially corresponding to the one reported in the following Table 1.

Table 1 - Average % composition (w:w) of mother liquors obtained from the synthesis and purification of DMPA

From what is shown in Table 1, it therefore appears that the quantity of residual dimethylolpropionic acid (DMPA) in the suspension of the aforementioned mother liquors is about 20% by weight, or even more, indeed a non-negligible quantity.

Attempts made up to date to also recover this significant portion of the product have involved, in particular, the use of classical concentration-filtration techniques both under pressure and under vacuum. For example, filtration systems have been used under pressure through candle filters and rotary filters under vacuum, but without success (low permeabilities). Even filtration attempts using a filter press did not lead to appreciable results due to the thixotropy of the mixture which caused clogging in the cloth and collapse of the permeability. Technical problem

Consequently, it would be very useful (and, in fact, a strongly felt need in the sector) to have a method available for (a system that is able to) recover in an efficient, simple and substantially quantitative way and with a high degree of purity the residual dimethylolpropionic acid present in the reaction and purification mother liquors, thus determining an almost complete reduction/elimination of the waste produced and a consistent increase in the reaction yield, with a consequent reduction in the supply of hazardous raw materials (for example, propionic aldehyde), with a like level of productivity, and with consequent reduction of the overall production costs.

The object of the present invention is to give a satisfactory response to the technical problem described above.

Summary of the Invention

The present inventor has now found that the exploitation of the density difference between the product of interest (dimethylolpropionic acid, DMPA) and the other compounds present in the mother liquors through the use of a suitable, opportunely set, centrifugal extractor/decanter, has quite unexpectedly revealed itself capable of providing an advantageously adequate response to the technical problem described above.

Consequently, one object of the present invention is a method for recovering the solid fraction, rich in dimethylolpropionic acid from the mother liquors obtained from its synthesis and purification, through the use of a suitable centrifugal extractor/decanter, as described in the relative attached independent claim. Another object of the present invention is the use of the aforementioned centrifugal extractor/decanter to recover the solid fraction rich in dimethylolpropionic acid from the mother liquors obtained from its synthesis and purification, as described in the relative attached independent claim.

Further advantages of the present invention may be deduced from the attached dependent claims, as well as from the continuation of the description and the relative attached claims.

Brief Description of the Drawings

Further characteristics and advantages of this method, and relative plant, according to the invention, will be clarified by the description that follows of some of its embodiments, provided by way of non-limiting examples, with reference to the attached drawings, where:

- Figure 1 illustrates the appearance of the suspension of the process mother liquors after the removal (recovery) of most of the dimethylolpropionic acid, for example, by decantation or filtration using a belt press filter, at the end of the reaction;

- Figure 2 illustrates the appearance of the solid dimethylolpropionic acid recovered, in good quantity and with a good degree of purity, from the aforementioned mother liquors through the centrifugal extractor/decanter at substantially ambient temperature (19.6 °C);

- Figure 3 illustrates the appearance of the residual opalescent liquid, after decantation from the centrifugal extractor/decanter, strongly enriched in oligomers and reaction by-products;

- Figures 4 and 5 illustrate two embodiments, in sectional view, of the centrifugal extractor/decanter of the present invention;

- Figure 6 illustrates a detail of the above apparatus (Figures 4 and 5), which illustrates operation of the horizontal cylindrical drum, equipped with a helicalcochlea, of the centrifugal extractor/decanter of the present invention.

- Figure 7 illustrates a detail of the above apparatus (Figures 4 and 5), which illustrates an embodiment of the discharge for the solid phase, substantially dehydrated and with a good degree of purity, conveyed by the movement of the cochlea.

Detailed Description of the Invention

As previously anticipated, this invention is intended for a method for recovering in significant quantities (preferably, almost quantitatively) the solid fraction, highly rich in dimethylolpropionic acid (DMPA), from its mother liquors, in particular, obtained from its synthesis and purification, through the use of a suitable extractor/centrifugal decanter, set opportunely.

The centrifugal extractor/decanter is, as known in the art, a device for carrying out, for example, the separation of different phases. Structurally, it is not a specific object of this invention as it is already known per se. In any case, a brief description is given below, and in the attached figures, for the purpose of greater clarity.

As illustrated in Figs. 4 and 5 attached, in a preferred embodiment of this invention, the centrifugal extractor is substantially composed of a rotating unit (10). Said rotating unit is generally driven by an electric motor (20). The rotating unit is supported, for example, by two blocks of bearings on a frame so that it can rotate.

As always highlighted in Fig. 4, the rotating unit comprises inside a drum formed by a cylindrical portion (11) which continues with a conical portion (12). A variable pitch helical cochlea (13) is positioned inside the drum itself (11, 12). The helical cochlea (13) is, in turn, rotatable thanks to a further drive unit (21), with relative transmission, which creates precisely the desired/necessary differential centrifugal speed between the cochlea and the drum (the rotational speed of the cochlea is lower than that of the drum).

There is therefore a difference in rotational speed between the helical cochlea and the drum within which the cochlea is rotatably placed (the direction of rotation of the cochlea and drum is the same, but their rotational speeds differ, see above).

Separation takes place inside the horizontal cylindrical drum equipped with acochlea.

The feeding of the suspension of mother liquors, mentioned above, takes place through a fixed inlet pipe and is gently accelerated by an inlet distributor (see in Fig. 5 the section showing “Feet/”). The centrifugal forces applied to the rotating elements (cochlea and drum) cause the solids to settle on the drum wall. As previously mentioned, the helical cochlea rotates in the same direction as the drum, but at a suitable, different speed (lower, as described below), thus moving and compressing the solids towards the conical end of the drum and causing the liquids to flow to the opposite direction. The solid fraction, consequently relatively dry, of the product (substantially comprising dimethylolpropionic acid with a good purity content), conveyed by the movement of thecochlea, leaves the drum through appropriate drains for solids as shown in Fig. 5 and, in greater detail, in Fig. 6 and in Fig. 7.

The key to good centrifugation performance lies in an adequate flow of solid sediments moving (i.e., being pushed and compressed by thechoclea) to the opposite direction with respect to the liquid portion of the mother liquors. The design of the cochlea is therefore important. The sliding pitch is the distance between the sliding blades of a helical revolution, i.e., the advancing movement made by the sliding during a rotation. During the separation process, an accumulation of relatively dry solid material forms along the drum in the narrow (conical) section of the drum to form what is called the 'beach' . The cochlea, during its rotating movement, collects and pushes the material of the ‘beach’ towards the exit. The differential speed is given by the lower number of revolutions per minute made by the cochlea compared to the drum. The slower the cochlea (i.e., low differential speed), the longer the residence time along the ‘beach’ and the greater the degree of humidity that the solid will bring with itself.

Appropriate setting of the machine parameters such as, for example, 'differential speed' between drum and cochlea, 'drum rotation speed' , 'flow rate of the incoming fluid material', 'distance between drum and cochlea', 'opening of the outlet holes' (or 'weir') of the liquid (the larger the holes, i.e. the greater the radius between the center of the drum and the weir end, the greater the residual, dirty liquid coming out and the larger the slightly wet beach at the other end of the drum; that is, the drier the solid coming out), 'temperature entering and exiting the drum ' (the temperature has an influence on solubility of the dimethylolpropionic acid material), made it possible to obtain the recovery of a solid, dusty dimethylolpropionic acid (see Fig. 2), with a very low moisture content and a good degree of purity.

The average % composition by weight (w:w) of the aforementioned dimethylolpropionic acid recovered from the aforementioned process and purification mother liquors through the centrifugal extractor above was substantially approximately equal (± 10%; preferably, ± 5%; more preferably, ± 3%; more preferably, ± 2%; or even ± 1%) to the one described in the following Table 2.

Table 2 - Average % composition (w:w) of the solid fraction of dimethylolpropionic acid recovered from the mother liquors of synthesis and purification

It is therefore clear from Table 2 that the solid dimethylolpropionic acid extracted/recovered from the centrifugal extractor has a low water content (on average about 4%) and good purity (on average, about 83%). Furthermore, the solid dimethylolpropionic acid thus recovered does not stick as it is characterized by a floury, non-sticky consistency. Furthermore, the solid dimethylolpropionic acid thus recovered was found to be perfectly soluble in hot water according to the typical solubility curve of dimethylolpropionic acid.

The possibility of obtaining dimethylolpropionic acid separated from the above process mother liquors, with the qualitative-quantitative characteristics described above, has advantageously allowed its reuse, for example, by direct reintegration in the reaction phases (for example, dilution in the aldol condensation reaction) and/or finishing the main preparation process (for example, complete dissolution in water, passage over resins, precipitation, drying and flaking). All this has led to a significant reduction (almost a substantial zeroing) of the waste produced and an increase in the reaction yield with a consequent reduction in the supply of dangerous raw materials (for instance, propionic aldehyde), with the same productivity. All this has also advantageously reflected on the environmental impact of the industrial synthesis and on the overall costs of the industrial preparation process of dimethylolpropionic acid.

As regards the operational setting of the centrifugal extractor, this depends on the model of extractor used and its operational capacity for the quantitative treatment of mother liquors. Consequently, the setting must be adjusted from time to time and case by case, possibly also obtaining different results depending on the type and size of the plant (decanter) used.

In an illustrative, but not strictly limiting, embodiment, the average operating parameters, advantageously adoptable for general setting of the possible centrifugal extractors that can be used for the purposes of this invention, substantially correspond to those listed in the following Table 3.

Table 3 - Average characteristics of the setting parameters of a centrifugal extractor of this invention

In detail, from the columns of the previous Table 3, it is clear that in:

- Column 1 - The “mother liquor inlet temperature (t)” (as well as the one of extraction/exit of the solid and liquid phases after separation) is in any case, for reasons of solubility of the DMPA in water, generally close to or equal to ambient temperature (in which the range 20-25 °C is preferably considered as “ambient (t)”) or, preferably, lower; for example, it does not exceed 20 °C; or it is 20 °C; or it is 19.6 °C; or it is 19 °C; or it is 18 °C; possibly, even lower, for example, down to 15 °C;

- Column 2 - The flow rate of the mother liquors fed to the extractor (“Feed flow rate”) is generally between 0.5 and 10 mc/h (or m ³ /h); or from 1 to 8 mc/h; or from 1 to 5 mc/h; or from 1 to 4 mc/h; or from 1 to 3 mc/h; or from 1 to 2 mc/h; or 1 mc/h;

- Column 3 - “drpm (differential)” is the differential centrifugal speed between the different drum and cochlea speeds and is expressed as a variable number on a numerical scale from 1 to 20 where each numerical value corresponds to a speed such that each unit means one rotation per minute of the cochlea less than the drum (for example, if drpm = 4, the cochlea would rotate at four revolutions per minute of the cochlea less than the drum. On the other hand, if, absurdly, drpm were = 0, no solid separated from the mother liquors would exit);

- Column 4. “rpm” is the centrifugal speed of the rotating unit of the centrifugal extractor/decanter of the invention and is expressed in rpm.

- Column 5. “r” is the radius of the cochlea, expressed in cm, which varies from 120 cm, preferably from 200 cm upwards depending on the type of apparatus used (that is to say, its size/capacity and the relative setting chosen, for example, thanks to the insertion of suitable plates, as known in the art. For example, r = 120 refers to tests carried out by the inventor using a pilot centrifugal decanter with a radius of 120 cm).

As anticipated, the values of the above parameters may vary for each type of machine used. In a particularly preferred embodiment of the invention, the ranges indicated above have proved to be particularly advantageous when adopted for the appropriate setting of centrifugal decanters of the series of decanters S200, S350, S700 from the company Polat Makina San. Tic. A.S. (Turkey). However, the same ranges, as well as their possible variations, can easily also be applied to other models of centrifugal extractors known commercially, based on the experience of a person skilled in the art.

Setting of the centrifugal extractor of this invention can be carried out by the operator manually, or also from a suitable control panel, for example, with computerized management, possibly located in a suitable remote-control unit or station.

The decanted liquid coming out from the appropriate outlet port for the liquid phase (as already indicated above, located opposite to the one of the solid phase outlet port) of the centrifugal extractor drum, is opalescent (see Fig. 3) and stable, that is, without further forming separable solid deposits at about 20 °C.

The average % composition by weight (w:w) of the aforementioned opalescent liquid thus decanted is substantially equal (± 5%; or ± 2%; or ± 1%) to the one described in the following Table 4. Table 4 - Average % composition (w:w) of the liquid decanted from the mother liquors of synthesis and purification

It is therefore evident from Table 4 that the centrifugal extractor of the present invention practically allowed the complete separation (i.e., the recovery) of the solid fraction (substantially, dimethylolpropionic acid with a good degree of purity and low moisture content, as previously illustrated) free from the colloidal oligomeric part, which instead followed the final decanted liquid, together with the reaction by-product.

Advantageously, and quite unexpectedly, the liquid fraction thus obtained has chemical characteristics suitable for its direct sale also in other, different commodity sectors.

In fact, elimination of the insoluble solid fraction, mainly composed of dimethylolpropionic acid, made it possible to obtain a substantially homogeneous mixture of liquid oligomers dissolved in water. By way solely of a non-limiting example, this mixture can advantageously be used in different commodity sectors such as, for example:

- the environmental sector, as a product for the production of biogas;

- the animal feed sector;

- the industrial ecology sector, leading to the elimination of waste production for this type of chemical process, as well as for other similar processes. Furthermore, the method of the present invention could equally be applied also to suspensions of other process mother liquors formed, for example, by the byproducts of reactions such as aldol condensation, oxidation of complex aldehydes, the Cannizzaro reaction. More generally, this separation principle of this invention can be applied advantageously to those complex aqueous and organic matrices where the characteristics of density, viscosity, thixotropy affect the possibility of using purification techniques based on filtration (even under heavy conditions).

Consequently, another object of the present invention is the use of the aforementioned liquid fraction in other, different industrial sectors, in particular in the industrial sectors described above.

In light of all the above, another object of the present invention is the use of a centrifugal extractor/decanter such as, for example, those described above (and with the types of settings described above) to recover almost quantitatively and with an acceptable degree of purity the solid fraction rich in dimethylolpropionic acid from the mother liquors of synthesis and purification thereof.

Similarly, another object of the present invention is the use of said centrifugal extractor/decanter to recover precious (solid) fractions from complex aqueous and organic matrices whose characteristics of density, viscosity, thixotropy affect the possibility of using purification techniques based on filtration.

In view of all the above, this invention is intended in particular, but not exclusively, for the following characterizing points:

[1] A method for recovering the solid fraction, rich in dimethylolpropionic acid, from the mother liquors of its synthesis and purification, said method comprising: - feeding said mother liquors into a centrifugal extractor/decanter comprising a rotating unit (10) which comprises internally a drum (11, 12) consisting of a cylindrical portion (11) and a conical portion (12), joined together, and equipped with a helical cochlea (13), in which both drum and cochlea rotate in the same direction but at different centrifugal speeds from each other;

- carrying out, inside the rotating drum, the separation of said solid fraction from the liquid fraction of said mother liquors through the rotary movement of the cochlea which moves and compresses the solids towards the conical end of the drum, causing the liquids to flow in the opposite direction;

- collecting the solid phase, thus separated from the liquid phase, through appropriate drains for solids; and, at the same time,

- collecting the liquid phase, decanted from the solid phase, through special drains for liquids.

[2] The method in accordance with the previous point [1], in which the aforementioned mother liquors have the following initial average % composition

(w:w):

Component % (by weight)

Water 20-30 (preferably, ~ 30);

Dimethylolpropionic acid 10-30 (preferably, 19-20);

Trimethylolethane 10-25 (preferably, ~ 15);

Oligomers > 30 (preferably, ~ 30).

[3] The method in accordance with the previous point [1] or [2], in which the aforementioned mother liquors are fed into said centrifugal extractor/decanter at a feed rate of 0.5 - 10 mc/h.

[4] The method in accordance with one of the points above, in which the feeding temperature (t) of said mother liquors is lower than or equal to the ambient temperature.

[5] The method in accordance with one of the previous points, in which the centrifugal speed of the cochlea (13) is lower than the one of the drum (11, 12) and the differential centrifugal speed (drpm) between the different speeds of the drum and the cochlea, expressed as a variable number on a numerical scale from 1 to 20, ranges from 2 to 8.

[6] The method in accordance with one of the previous points, in which the centrifugal speed (rpm) of the rotating unit (10) is 3,600 - 4,600 rpm.

[7] The method in accordance with one of the previous points, in which the radius of the cochlea (13) is variable from 120 cm up.

[8] The method in accordance with one of the previous points, in which the setting of said centrifugal extractor/decanter can be carried out manually or from a control panel, with or without computerized management, possibly placed in a suitable remot- control unit or station.

[9] The method in accordance with one of the previous points, in which the said solid phase, thus separated from the liquid phase, has the following average % composition (w:w) (± 10%):

Component % (by weight)

Residual water 4;

Dimethylolpropionic acid (DMPA) 83;

Trimethylolethane 9;

Oligomers 4.

[10] The method in accordance with one of the previous points, in which the said liquid phase, decanted from the solid phase, has the following average % composition (w:w) (± 5%):

Component % (by weight)

Water 35.8;

Residual dimethylolpropionic acid 4.4;

Trimethylolethane 16.4;

Oligomers 43.3;

[11] Use of the liquid phase in accordance with the previous point [10] in different industrial commodity sectors.

[12] Use of the liquid phase in accordance with the previous point [11] in the following industrial commodity sectors:

- the environmental sector, as a product for the production of biogas;

- the animal feed sector;

- the industrial ecology sector.

[13] Use of a centrifugal extractor/decanter, in accordance with what has been described in one or more of the previous points and description, to carry out the separation/recovery of the solid fraction, rich in dimethylolpropionic acid, from the mother liquors of the synthesis and purification of the same.

[14] A method, in accordance with what has been described in one or more of the previous points and description, to carry out the separation of solid fractions from aqueous and/or organic matrices in which the characteristics of density, viscosity, thixotropy affect the possibility of using filtration-based purification techniques.

[15] Use of a centrifugal extractor/decanter, in accordance with what has been described in one or more of the previous points and description, to carry out the separation of solid fractions from aqueous and/or organic matrices in which the characteristics of density, viscosity, thixotropy affect the possibility of using filtration-based purification techniques.

Industrial Applicability

The method of the present invention has made it possible to recover in a substantially quantitative way and with a high degree of purity the solid fraction, rich in dimethylolpropionic acid, from the final residual mother liquors derived from its synthesis and purification by crystallization and filtration.