Process for the crystallization of dicarboxylic acids

Details Process for the crystallization of dicarboxylic acids Process for the crystallization of dicarboxylic acids

1999-07-28

2002-04-23

Geist, Gary (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

C562S593000, C562S530000

Reexamination Certificate

active

06376709

ABSTRACT:

The present invention relates to a process for the crystallization of dicarboxylic acids.
More specifically, the present invention relates to a novel process for the crystallization of dicarboxylic acids employing the use of additives which help increase the yield and purity of crystals.
The object of the invention is to achieve the desired increased size, high purity and greater yield of crystals.
The other object of the invention is to achieve a crystallization process in which crystallization is at a faster rate as compared to the conventionally known processes.
Still another object of the invention is to achieve the desired results by consuming less energy.
The embodiment of the invention resides in the crystallization of di-carboxylic acids in a novel crystallizer comprising a cylindrical impeller and impeller shaft.
Surfactants and/or salts are used as additives to achieve the desired increased size of crystals and high yield.
Addition of surfactants as additives for the crystallization of dicarboxylic acids results in obtaining bigger crystals with high purity, whereas addition of salts as additives for the crystallization of dicarboxylic acids results in the greater yield of crystals.
The addition of surfactants and salts result in obtaining high yield crystals of having bigger crystal size.
BACKGROUND OF THE INVENTION
The modification of the crystal morphology and structure is achieved by introduction of additives which stick on one of the preselected crystal faces thereby inhibiting the growth of the crystal predictably perpendicular to the surface on which they adhere. The size of the crystal structure is drastically varied by carrying out crystallization in electrolyte solutions.
U.S. Pat. No. 5,296,639 claims a process for purification of adipic acid during crystallization by modifying crystal morphology through the introduction of additives which are among caproic acid and surfactants, thereby decreasing the incorporation of impurity on the crystal system.
U.S. Pat. No. 5,827,700 teaches a process of recovery and crystallization of citric acid from impure process stream by introducing very strong salts.
Juetten, in his thesis entitled “The Enhanced Crystallization of Dicarboxylic Acids in Electrolyte Solutions”, Michigan State University, 1992, performed crystallization experiments on aqueous solutions using dicarboxylic acids in order to determine by trial-and-error which electrolytes caused “salting out”.
One of the major drawbacks associated with the use of salts, either buffer salts or acid salts is the instant precipitation, which results in the smaller size of the crystals. In the subject invention to overcome this drawback, the selection of salts with the surfactants has resulted in achieving the desired crystal size along with better yield of crystals.
The present invention relates to a process for crystallization of dicarboxylic acids in a system, which is under continuous shear force that is created in the annular space between the impeller and the crystallizer wall. The system consists of selection and addition of additives in a manner to avoid any effect on the crystal purity.
To achieve the objectives of increased size, high purity and greater yield of crystals, in the present invention crystallization of diacids is carried out in a solution consisting of a solvent or a mixture of solvents, crude diacid and the additives in a crystallizer.
The crude diacids consists of one solute in major amount and impurities in minor amounts. When re-crystallized, the subject process provides a highly purified crystals of the diacids, as the amounts of impurities present, if any, are totally dissolved in the solvent/solution.
The solvents used in the present invention are selected preferably from water, any aliphatic monobasic acid having 2-4 carbons comprising primary and secondary hydrogen, any primary alcohol having 2-4 carbon or any combination thereof. Preferably, the solvent used in the subject process is water or water with small amount of acetic acid, wherein the ratio of water to acetic acid is more than 1:1. More preferably, the solvent with less than 10% of acetic acid is used. The preferred range of the solvent to the crude diacid is between 10 to 1 to 0.1 to 1. Most preferred range of the solvent to the crude diacid is 5 to 0.2
The additives used in the subject invention are selected from surfactants, and/or buffer salts and acid salts.
The surfactants addition results in obtaining bigger crystals with high purity. The surfactants are selected from non-ionic surfactants, anionic surfactants, cationic surfactants and zwifterionic surfactants. Preferably, the anionic surfactants are used. The preferred anionic surfactants can be selected from Sodium dodecyl sulfate (SDS), Sodium dodecyl benzene sulfonate (SDBS) and Sodium bis (2-ethyl hexyl) sulfosuccinate (AOT).
The nonionic surfactant used in the present invention are selected from hexoxy ethylene glycol mono-n-dodecyl ether (C
12
E
6
) and Tween 20.
The cationic surfactants can be selected from cetyl trimethyl ammonium bromide (CTAB) and dodecyl dimethyl ammonium bromide (DDAB).
The zwitterionic surfactants are selected from Phosphatidyl choline (PC) and Phosphatidyl ethanolamine (PE).
The amount of surfactants that is required for achieving good crystal size and shape is between 0.005% to 2.0% of the weight of the total solution. It has been observed that the concentration of surfactant below the critical micellar concentration (CMC) in the solution has achieved the desired results successfully. Most preferably, the amount of surfactant used is between 0.01% to 1.0% i.e. below the CMC.
To increase the yield of crystals the buffer acids and/or buffer salts are used as additives. The addition of salts results in increased rate of crystallization thus resulting in greater yield of crystals. The buffer salts exhibit resistance to the change in pH and acid salts increases the pH. The buffer salts used in the present process are selected from Na
2
HPO
4
, NaH
2
PO
4
, KH
2
PO
4
and K
2
HPO
4
. Preferably these salts are selected from Na
2
HPO
4
or K
2
HPO
4
or a combination thereof. The amount of buffer salts taken is between 0.0025% to 1.0% of the weight of the total solution.
The acid salts used in the present invention are selected from NaHSO
4
and KHSO
4
, more preferably KHSO
4
. The amount of acid salts taken is between 0.0025% to 1.0% of the weight of the total solution.
Hence, in the subject crystallization process, both the acid and buffer salts along with the surfactants are used to achieve the desired average crystal size as surfactants makes the crystal grow and the salts precipitate out of the di-carboxylic acid.
The presence of surfactants results in growth of crystals, thereby results in obtaining crystals of higher purity. Salts present in the system results in the increased yield i.e. the weight of the crystals formed is more than the weight of crystals formed in a conventional process. The same is shown in the comparative table:
TABLE 1
with
with
surfactant
surfactant +
Con-
only
salts
with salts
ven-
(0.5%
0.25 + 0.2
only (0.5
tion-
wt of
5% wt of
% wt of
al
solution)
solution)
solution)
Yield (%)
60.95
62.75
76.67
81.23
Purity (%)
99.8
100
99.92
99.8
Time for crystallization
1.00
1.00
1.00
1.00
(hrs)
Crystal size (microns)
655
1814
1041
310
The amount of surfactants and buffer or acid salts is between 0.005% to 4.0% of the weight of the total solution.
As can be clearly seen from the Table-1, the presence of surfactants helps only in the growth of the crystals and the size of the crystal is increased by three times approximately. Increased size results in increasing the purity of crystals. Here, the yield increases but the increase is not considerable.
In case of the use of salts only as additives, the size of the crystals decreases, but the yield increases.
However, in the presence of both the salts and surfactants, there is increase in the yield, purity and size of the crystals.
In accordance with the present invention the diacid is crystallized in medium consisting of

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