Dewatering process

Details Dewatering process Dewatering process

2001-01-22

2002-11-12

Manoharan, Virginia (Department: 1764)

Distillation: processes, separatory

Distilling to separate or remove only water

From organic compound

C203S057000, C203S060000, C203S061000, C203S099000, C203SDIG001, C203SDIG002, C568S492000, C568S493000

Reexamination Certificate

active

06478929

ABSTRACT:

The present invention relates to a process for the removal of water from solutions containing formaldehyde for the production of formaldehyde.
Formaldehyde is a commodity chemical that is conveniently produced and transported in the form of formalin solutions. Formalin solutions contain typically between 30% and 60% formaldehyde, the balance of the solution being mostly water, usually with some methanol present. The formaldehyde is predominantly present as complexes with water or methanol in the form of glycols of hemiformals. There are a number of methods described in the patent literature for the dehydration of formaldehyde solutions with the object of producing a dry monomeric formaldehyde stream. For example U.S. Pat. No. 4,962,235 describes the purification of a formaldehyde/water/methanol mixture by distilling in the presence of a polyalkylene oxide such that formaldehyde vapour is produced at the top of the column and polyalkylene oxide, water and methanol are removed from the bottom of the column.
NL-A-6814946 describes a process for recovering formaldehyde from aqueous formaldehyde streams by contacting the stream with C
6
-C
10
aliphatic alcohol which reacts with the formaldehyde to form a hemiformal which can be separated from the water and subsequently dissociated back to the alcohol and formaldehyde components.
U.S. Pat. No. 3,174,912 describes a process for the removal of water and coloured organic impurities from a dilute aqueous formaldehyde mixture by distillation in the presence of acetone. A mixed stream containing acetone and formaldehyde and a small amount of water is removed from the top of the column and separated by partial condensation into a stream containing a relatively small amount of formaldehyde in acetone and a stream containing a larger proportion of formaldehyde in acetone.
It is, however, desirable to avoid the introduction of additional chemical compounds into a process which requires purified formaldehyde because the additional compound may need to be removed from the process at a subsequent stage.
One process in which formaldehyde is used is that for the production of methyl methacrylate by the reaction of methyl propionate with formaldehyde in the presence of methanol. The use of formaldehyde as a readily available formalin solution introduces water into the reaction which may have a deleterious effect on the catalyst used and is likely to promote the hydrolysis reactions of the methyl propionate reactant and of the methyl methacrylate product. Water is produced as a by-product of the methacrylate synthesis reaction and it is therefore desirable to reduce to a minimum the amount of water that is introduced into the reaction zone with the feeds so that the level of water in the reactor is maintained as low as possible. Processes for the production of methyl methacrylate, in which propionic acid or its methyl ester is reacted with formaldehyde or methylal in the presence of methanol, are disclosed in U.S. Pat. No. 3,535,371, U.S. Pat. No. 4,336,403, GB-A-1107234, JP-A-63002951. However there is no disclosure in these references of how to prepare the feed materials, particularly the formaldehyde, for the required reaction.
U.S. Pat. No. 4,040,913 discloses the use of water entraining compounds such as benzene, toluene or methylisobutyl ketone, and FR-A-2409 975 discloses the use of saturated C
4
-C
6
hydrocarbons. GB-A-1301533 discloses the use of alkanols being C
3
-C
6
compounds but the alkanols are not used as water entraining compounds.
It is therefore an object of the invention to provide a process for the preparation of a formaldehyde feed product from a formaldehyde-containing solution.
It is a further object of the invention to provide a process for treatment of a formaldehyde-containing solution to yield a formaldehyde feed suitable for use in a methyl-methacrylate production process which contains a reduced amount of water compared to the original formaldehyde-containing solution.
According to the invention, a process for separating a formaldehyde-containing product from a formaldehyde solution comprising formaldehyde, water and methanol, wherein said formaldehyde-containing product contains substantially less water than said formaldehyde solution, comprising distilling said formaldehyde solution in the presence of a water entraining compound.
The water entraining compound is selected such that it is capable of dissolving water, formaldehyde, methanol and hemiformal compounds formed by the reaction of formaldehyde with methanol. The water entraining compound is preferably a saturated or unsaturated carboxylic acid or ester or a carbonyl compound which is substantially unreactive towards formaldehyde under the conditions of the distillation and also able to entrain the water, preferably by forming a minimum boiling azeotropic mixture with water Preferably the water entraining compound forms a heterogeneous minimum boiling azeotrope with water. Suitable compounds include C
4
-C
8
alkanoic acids and their lower alkyl, e.g. C
1
-C
8
, esters, and ketones having at least 4 carbon atoms such as diethyl ketone. Particularly preferred compounds are esters and methyl propionate and methyl methacrylate have been found to be especially useful in certain processes. It is greatly preferred to use a compound which is intended to be introduced into a process in which the dewatered formaldehyde product is intended to be used.
One particularly preferred compound for use in dewatering a formalin solution to produce formaldehyde suitable for use in a process to react formaldehyde with methyl propionate in the presence of methanol is methyl propionate. In one preferred form of the invention we therefore provide a process for separating a formaldehyde-containing product from a formaldehyde solution comprising formaldehyde, water and optionally methanol, wherein said formaldehyde-containing product contains substantially less water than said formalin solution, comprises distilling said formaldehyde solution in the presence of methyl propionate.
According to a second aspect of the invention we provide a process for the production of methyl methacrylate by the reaction of formaldehyde with methyl propionate in the presence of methanol and of a suitable catalyst, wherein said formaldehyde is produced from a formaldehyde solution by means of distilling said formaldehyde solution in the presence of methyl propionate. Suitable catalysts are known in the art and include silica catalysts having alkali metal sites.
The process of this aspect of the invention is beneficial, in that formaldehyde feed is recovered as a complex with methanol in a methyl propionate rich stream and water is removed from the process substantially free of organics. Integration of the process for producing formaldehyde of the present invention with a methyl methacrylate producing process as described has the additional advantage that the overall energy requirement for the combined process may be reduced compared with other formaldehyde dewatering methods.
Although we have found that the process of the present invention is particularly suitable to provide a formaldehyde feedstock for subsequent reaction for methyl methacrylate production, the invention is not limited to the production of formaldehyde for such a process and may be suitable to produce dewatered formaldehyde for other applications.
The formaldehyde solution is preferably standard formalin which usually contains water and formaldehyde in approximately equal proportions, usually with a small quantity of methanol. At least some of the formaldehyde is normally present as various water-formaldehyde or methanol-formaldehyde adducts. Normally the term “formaldehyde” will be used here to refer to the total formaldehyde, whether present as free formaldehyde or in the form of such adducts. The composition of formalin solutions may vary and the process of the invention may be operated for a variety of different formalin compositions.
It is preferred that the formaldehyde solution is pre-mixed with methanol before the distillation

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