Process for producing butyl acrylate

Details Process for producing butyl acrylate Process for producing butyl acrylate

1999-08-06

2001-01-30

Killos, Paul J. (Department: 1623)

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

Organic compounds

Carboxylic acid esters

Reexamination Certificate

active

06180819

ABSTRACT:

The present invention relates to an improved process for producing butyl acrylate. More specifically, the invention relates to a new method of distilling, and of recovering and recycling normal butanol (“BuOH”), acrylic acid (“AA”), and normal butyl acrylate (“BA”) from one or more process streams in an acid-catalyzed esterification process for BA. The invention encompasses two new process components, one related to the hydrolytic recovery of valuable reactants from their higher boiling adducts, and a second component related to improved distillation of a crude product yielding BA substantially free of AA. The hydrolytic recovery component of the invention also is useful in processes for producing selected acrylic esters, in addition to BA. Most specifically, the invention relates to a highly efficient, continuous process for producing BA in high purity and high yield.
Direct esterification of AA with an alcohol is an equilibrium process. The equilibrium constant determines the net rate and extent of conversion of AA and alcohol; for continued high rates of conversion the mixture must not approach equilibrium. Conventionally, an excess of alcohol over AA is employed and water of esterification is removed distillatively as its azeotrope with alcohol and ester to maintain a high rate of conversion of AA. The azeotrope is removed via a distillation column mounted directly on an esterification reactor. In the case of methyl or ethyl esters, the water of esterification, excess alcohol, and product ester are removed from the head of the distillation column and are substantially free of AA. Water extraction removes the alcohol which is concentrated distillatively for recycle to the reactor. The washed ester is azeotropically dehydrated and finally distilled to provide the pure ester product. In butyl acrylate production, however, the separation of acrylic acid from water of reaction, excess alcohol, and product ester is more difficult, and the distillate from the esterification reactor in a continuous process typically contains 1-3% AA. This AA typically is extracted into aqueous caustic. Although it is possible to recover some of this AA from the resulting aqueous salt solution by acidification with a strong acid followed by extraction into an organic solvent, e.g. butyl acrylate or butyl acrylate/butanol mixture, significant loss to a large aqueous waste stream is unavoidable. The butyl acrylate and excess butanol are next azeotropically dehydrated wherein excess butanol is separated from the product ester as a butanol/butyl acrylate azeotrope for recycle to the esterification reaction. A final distillation provides pure butyl acrylate. In all cases a small bleed stream is removed from the esterification reactor and a small bottoms stream is taken from the final product distillation to remove high boiling byproducts and inhibitor residues from the process. These streams are stripped to recover free AA, alcohol, and alkyl acrylate values, but little or none of the values present within the high boiling byproducts are recovered. Thus, the conventional processes for producing C
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-C
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esters suffer from yield losses to high boiling byproducts, and the C
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process further suffers from direct losses of AA because of the difficulty in separating AA from butanol, water, and ester.
In the art of recovering and recycling reactants from their higher boiling adducts formed during processing (so called “heavy ends;” in BA production these include, for example, butyl &bgr;-butoxypropionate and esters of sulfuric acid), there has been only limited success. For example, in ethyl acrylate (“EA”) production from ethylene and AA, U.S. Pat. No. 4,968,834 ('834) describes a process for recovering EA from a “spent black acid” stream containing sulfuric acid residues and other adducts bled from the bottom of a distillation column. The '834 process uses an alcoholic solvent to facilitate an overhead distillative recovery of ethyl acrylate, and treats the black acid residues with an aqueous alkanol mixture. No materials are directly returned to the EA-producing reactor nor to the distillation column which generates the black acid stream. The '834 process thus provides partial recovery of ethanol, EA and AA, but only by an aqueous treatment which is isolated from the reactor of the ethylene-AA process. Other processes employ distillation units (often designated “bleed strippers”) to partially recover free AA, BA, and BuOH from reaction bleeds, but to the extent that heavy ends are recovered in that operation, they remain chemically in the higher boiling (heavy end) form and are not transformed to the desired valuable AA, BA, and BuOH forms.
Distillation is commonly used in BA production. For example, U.S. Pat. No. 4,012,439 ('439) describes a continuous process for BA in which a reactor esterification mixture is distilled through an AA separation column to give an overhead mixture of BA, butanol, and water, and, from the column bottom, a concentrated AA stream which is returned to the reactor. While separating the overhead mixture from AA, the '439 process recycles a very high proportion (>97%) of aqueous phase distillate to the head of the AA-separating column. This high proportion of aqueous recycle (i.e. having an aqueous reflux ratio of about 32:1) disadvantageously requires a large column and a large expenditure of energy in returning large volumes of water to the process.
Thus, in the acid-catalyzed production of acrylic acid alkyl esters (“alkyl acrylates”), particularly of BA, there remain significant energy use and reactant recovery problems. There are needs for a process which would recover reactants from their higher boiling, heavy end, adducts formed during the production of acrylic esters, e.g. BA, which would recycle recovered reactants and the ester to the esterification reactor or elsewhere in the process for reuse. Further needs include methods making more efficient use of the water of reaction, both in facilitating distillative separation of acrylic ester from AA and in more efficiently recovering and recycling unreacted AA, particularly if these steps were accomplished with reduced energy use. Meeting one or more of these needs would provide increases in process and/or material use efficiencies. Additionally, if such improved processes led to reduced dibutyl ether (DBE) byproduct in comparison to known processes, even greater process efficiency would result.
We have discovered a high yield process for producing alkyl acrylates, using BA as a preferred example, which achieves these desirable ends. Our new process provides for the recovery of “values,” that is, reactants and alkyl acrylate product, from the heavy ends produced in the process. Our new process includes the use of at least one of the following process components: 1. recovering values from a hydrolysis reactor unit (“HRU”) fed with a source of heavy ends, as from an esterification reactor; 2. recovering additional values from a cracking reactor preferably used in conjunction with the hydrolysis reactor; and 3. specific to a continuous BA process, distilling by use of an acrylic acid separation column in an efficient new way and providing recovery of BA which is substantially free of AA. Our new process advantageously provides very low levels of DBE in product BA because the esterification reactor is operated under mild temperature and pressure conditions, and at relatively low acid catalyst levels.
Thus, in the broadest use of the hydrolytic recovery component of the invention, there is provided a method of recovering AA, a C
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alkyl acrylate, and a C
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-C
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alkanol from heavy ends produced during production of the C
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-C
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alkyl acrylate, comprising the steps of:
a) feeding a total aqueous and heavy end feed stream comprising the heavy ends, water, residual acid catalyst, and optionally a strong acid selected from a mineral acid or sulfonic acid, to a hydrolysis reactor maintained at 90° to 140° C., 50 to 1000 mm Hg pressure, and a residence time of 0.5 to 20 hours based on the total aqu

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