Abstract:
A process for the production of n-butyl acrylate (BuAcA) including the steps of esterifying acrylic acid (HAcA) with n-butanol (n-BuOH) in a reaction zone, obtaining a condensate from the overhead vapors from the reaction zone which separates into organic and aqueous phases, feeding the organic phase comprising a major portion of BuAcA and minor portions of BuOH, water and light ends impurities to a finishing fractionation zone from which is obtained a side stream of a finished high purity BuAcA and an overhead condensate comprising BuAcA, BuOH, water and light ends, recycling most of the overhead condensate from the finishing zone to the reaction zone, feeding the remainder of such condensate to a purge recovery zone, obtaining an overhead condensate from the purge recovery zone which separates into organic and aqueous phases each containing a portion of light ends produced in the reaction, recycling a portion of the organic phase as reflux to the purge recovery zone and discarding the remainder. The entire aqueous phase of the overhead condensate of the purge recovery zone is discarded. The discarding of a portion of the organic phase and the entire aqueous phase of the overhead condensate of the purge recovery zone has the effect of preventing or retarding the buildup of light ends in the system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an improved process for the production and purification of n-butyl acrylate. 
     2. Background Information and Description of Related Art 
     n-Butyl acrylate (BuAcA) is a commodity chemical useful for the production of acrylic resins and polymers and copolymers for paint formulations, solvent coatings, textile and leather finishes and adhesives and binders. One method of synthesizing BuAcA is to react n-butanol (BuOH) with acrylic acid (HAcA) in the presence of an acid catalyst, e.g., a sulfonic acid, in at least one reaction zone to produce a mixture of BuAcA, water and several by-product impurities, and subjecting the product mixture to purification in at least one distillation zone, together with any of various schemes of reflux and recycle streams among the reaction zone(s) and distillation zone(s) to obtain a BuAcA of relatively high purity. When any of these processes are operated, low boiling impurities produced in the reaction zone, i.e., those having boiling points near or below BuAcA, such as butyl acetate and dibutyl ether and characterized as “light ends”, or “lights” must be systematically disposed of to prevent them from interfering with the described reaction and compromising the purity of the BuAcA. However, this is often difficult to accomplish, e.g. by purging an appropriate stream, without also losing an unduly large amount of BuAcA product and unreacted BuOH, and/or incurring an unfavorably large cost in energy consumption or purification equipment to prevent such loss. Thus, any improved process, which accomplishes an adequate disposal of light ends while keeping the loss of BuAcA and unreacted BuOH and/or the incremental cost of energy and equipment to a minimum, is very desirable. 
     U.S. Pat. No. 4,012,439, issued Mar. 15, 1977 to Erpenbach et al., discloses a process for producing n-butyl acrylate by reacting acrylic acid with n-butanol in liquid phase and in contact with an acid cation exchanger as a catalyst. The process utilizes a single reaction zone and three distillation zones. 
     U.S. Pat. No. 4,280,010, issued Jul. 21, 1981 to Erpenbach et al., discloses a continuous process for making alkyl acrylates free from ether by reacting a C 1 -C 4 -alkanol in a molar ratio of 1:1 to 1:2 in liquid phase at 80° to 130° C. under 100-760 mmHg in the presence of a catalyst. The process employs a single reaction zone and two distillation zones for separating the alkyl acrylate from unreacted alkanol and ether by-product. 
     U.S. Pat. No. 4,814,493, issued Mar. 21, 1989 to Dougherty et al., teaches a process for the production of an n-butyl acrylate by reaction of n-butanol with acrylic acid in the presence of an esterification catalyst and soluble manganese or cerium. The system for carrying out the process includes a reactor from which a butyl acrylate reaction product is sent to a finishing tower wherein the reaction product is separated into a pure butyl acrylate stream, a volatiles stream a portion of which is sent to a butanol recovery tower, and a residue stream which is recycled to the reactor. A butanol stream from the butanol recovery tower is recycled to the reactor, and a residue stream from such tower is removed from the system. 
     South African Patent No. 9704628 issued Mar. 25, 1998, discloses a process wherein acrylic acid (HAcA) and n-butanol (BuOH) are reacted in two reactors in series, and the product separated in two distillation columns to produce a stream comprising butyl acrylate (BuAcA), dibutyl ether (DBE), butyl acetate (BuAc), and BuOH. Such stream is further separated in a splitter distillation column to provide an overhead fraction containing DBE, BuAc, and BuOH, and a bottoms fraction containing BuAcA and heavy ends components, e.g., low molecular weight polymers of HAcA, and/or BuAcA, and Michael addition products (“heavies”). The overhead fraction from the splitter column is subjected to further distillation in the presence of water to separate BuOH and BuAcA from lights containing DBE and BuAc, with the BuOH and BuAcA being recycled to either or both reactors and the lights being sent to waste treatment, while the bottoms fraction from the splitter distillation is rectified in a butyl acrylate distillation column to separate BuAcA product from heavies which are recycled to either or both reactors. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with this invention, n-butyl acrylate (BuAcA) is produced by a process comprising reacting, generally amounts of n-butanol (BuOH) with acrylic acid (HAcA) in a reaction zone from which an overhead vapor mixture is withdrawn comprising BuAcA, BuOH, water and light ends which are organic by-product impurities having boiling points near or somewhat below that of BuAcA, and composed mainly of n-butyl acetate (BuAc) and di-n-butyl ether (DBE). BuAcA, BuOH, and water are removed by a combination of binary and ternary azeotropes. The vapor mixture is condensed to form a substantially organic liquid phase comprising a major portion of BuAcA and minor portions of BuOH, water and light ends, and a substantially aqueous liquid phase comprising mainly water and a minor amount of BuOH. The organic phase is fed to a finishing fractionation zone, from which a side stream of BuAcA product is withdrawn. Most of the overhead stream of the finishing fractionation zone composed of a major portion of the total of BuAcA and BuOH present and minor portions of water and light ends, is recycled to the reaction zone. However, to prevent an undesirable buildup of light ends in the system, a minor portion of such overhead stream is purged and sent to a purge recovery fractionation zone to effect separation of light ends from the valuable BuAcA and BuOH in the purge stream. 
     The overhead vapors from the purge recovery fractionation zone are condensed to form separate organic and aqueous liquid phases, with each phase containing a portion of the light ends produced in the system. The majority of the light end impurities remain in the organic phase compared to the aqueous phase. A portion of the organic phase and the entire aqueous phase, are discarded such that the total amount of light ends in such discarded streams is comparable to the amount of light ends produced in the reaction. The remainder of the organic phase is recycled as reflux to the purge recovery zone. Alternatively the overhead vapors are condensed and forwarded to a receiving vessel, a portion of the resultant stream is discarded and the remainder recycled as reflux to the purge recovery unit. By “discard” what is meant is to remove the stream, or portion thereof, from the reaction process. The stream may be further processed for proper disposal or further treated for recovery and reuse. 
     The residue from the purge recovery zone, containing the bulk of the BuAcA and BuOH in the purged portion of the condensed overhead from the finishing zone, and having a reduced light ends content is recycled to the reaction zone. 
     The process of the invention accomplishes the prevention of a buildup of light ends in the system while minimizing the loss of BuAcA and unreacted BuOH, and keeping the cost of energy and new equipment to accomplish this purpose, at a relatively low level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a schematic representation of a preferred embodiment of the overall process of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the synthesis of n-butyl acrylate (BuAcA) in accordance with this invention, close to equimolar amounts of acrylic acid (HAcA) and n-butanol (BuOH) are fed to the reaction zone together with an acid esterification catalyst, e.g., about 1 to about 5 wt % based on the weight of total reaction mixture. Examples of the esterification catalyst include alkyl- or arylsulfonic acid such as methanesulfonic, benzenesulfonic or toluenesulfonic acid. HAcA and BuOH are fed to the reaction zone e.g. about 0.85 to about 1.3 moles of BuOH per mole of HAcA. Stoichiometric excess of BuOH over HAcA is preferable to minimize the polymerizable free HAcA in the system, e.g. about 0.85 to about 1.3 moles of BuOH per mole of HAcA. To minimize polymerization in the process, a polymerization inhibitor is preferably also fed to such zone and may be mixed with the HAcA. Those of skill in the art will appreciate the type and amount of inhibitor necessary to achieve non-polymerization, said type and amount being based on a variety of factors. The inhibitor may be, for example, phenothiazine or a phenolic compound such as a dihydroxybenzene or a mono-lower alkyl dihydroxybenzene, e.g. hydroquinone or its monomethyl or monoethyl ether. 
     In an embodiment of the invention the esterification of HAcA with BuOH to produce BuAcA takes place primarily in the base reaction portion of the reaction vessel with the top portion of the vessel containing fractionation stages, e.g. in the form of trays and bubble caps or packing, to separate the bulk of the BuAcA and unreacted BuOH from residue at the base of the column comprising primarily low molecular weight polymers of HAcA and BuAcA, and catalyst and polymerization inhibitor. Also, remaining in the reactor are some quantities of free HAcA and BuAcA. The reaction vessel operates at a temperature, e.g., in the range of about 20 to about 150° C. and a pressure in the range of about 50 to about 400 mmHgA. 
     The overhead vapors from the reaction vessel consist mainly of BuAcA, lower amounts of BuOH and water and a minor amount of light ends, i.e. by-product organic compounds having somewhat lower boiling points than BuAcA, such as n-butyl acetate (BuAc), n-butyl propionate (BuPr), DBE, secondary butyl acrylate (SBuAcA) and isobutyl acrylate (IBuAcA). The overhead vapors are condensed and separated into two phases, a substantially organic phase having a major portion of BuAcA, reduced amounts of BuOH and water, and the light ends, and a substantially aqueous phase comprising mainly water and BuOH. The organic phase is sent to the finishing fractionization zone (the finishing tower) and the aqueous phase to a butanol recovery zone (the butanol recovery tower) wherein the BuOH is separated from the water for recycle to the reaction vessel. 
     The residue from the reaction vessel comprising mainly of low molecular weight polymers of HAcA and BuAcA, some free HAcA, BuOH, and catalyst is fed to a heat treater operating at a temperature, e.g., of about 150 to about 250° C. and a pressure, e.g., of about 80 to about 150 psig, where some of the polymers are converted to monomers. See Horlenko, U.S. Pat. No. 3,868,410 for further discussion regarding heavy end components. After a residence time of, e.g., about 1 to about 50 min, most of the heat treated residue is recycled back to the reaction vessel, with a slip stream, e.g., of about 5 to about 20 wt % of the heat treated residue being removed to a heavy ends removal distillation unit, operating at a temperature of about 125 to about 200° C. and a pressure of about 100 to about 700 mmHgA. The volatiles from the heavy ends removal unit are recycled to the reaction vessel while the residue from such unit is discarded. 
     The organic phase of the overhead condensate from the reaction vessel comprises a major amount, e.g., about 80 to about 95 wt % of BuAcA, and minor amounts, e.g., about 5 to about 15 wt % of BuOH, about 1 to about 3 wt % water, about 1 to about 10 wt % of light ends, and about 0.001 to about 0.05 wt % of HAcA, is fed to the finishing tower, such tower operating at a temperature, e.g., of about 20 to about 150° C. and a pressure, e.g., of about 50 to about 400 mmHgA. Finished BuAcA having a purity of, e.g., about 99.0 to about 99.99 wt % is withdrawn as a vapor side stream from the finishing tower, and overhead vapors comprising, e.g., about 10 to about 80, preferably about 35 to about 70 wt % of BuAcA, e.g., about 10 to about 80, preferably about 25 to about 70 wt % of BuOH, and e.g., about 1 to about 10 wt % of light ends, are withdrawn from the top of the finishing tower. The overhead vapors from the finishing tower are condensed and a portion of the condensate, e.g., about 0 to about 75, preferably about 5 to about 50 wt % is purged and sent to a purge recovery fractionation zone (the purge recovery tower) with the remainder being recycled to the reaction vessel. Here, “greater than about 0” means at least a small quantity, which is large enough or sufficient to operate the purge recovery tower. The residue from the finishing tower contains a major amount of BuAcA and is recycled to the reaction vessel. 
     The aqueous phase of the reaction vessel overhead condensate containing, e.g., about 1 to about 10 wt % of BuOH, and about 90 to about 99 wt % of water, is fed to the butanol recovery tower operating at a temperature, e.g., of about 70 to about 120° C. and a pressure, e.g., of greater than about 0 to about 5 psig. Alternatively, the aqueous phase of the reaction vessel overhead condensate may be discarded. The vapors from the top of the tower are primarily a binary azeotrope of BuOH and water which is condensed into two immiscible phases, one being predominantly BuOH which is recycled to the reaction vessel and the other a solution of a minor amount of BuOH in water which is recycled to the butanol recovery tower as reflux or discarded or recycled to the reaction vessel. The residue from the tower is composed primarily of water containing very little BuOH and is discarded. 
     The purged portion of the finishing tower overhead condensate contains an amount of light ends which, when discarded, is sufficient to prevent an undesirable buildup of light ends in the system. However, to minimize wastage of BuOH and HAcA utilized in the process, the purge stream is distilled in the purge recovery tower to separate a major portion of BuOH and BuAcA from the light ends. More specifically, the purge recovery tower operates at a temperature, e.g., of about 30 to about 150° C., preferably about 65 to about 110° C., a subatmospheric pressure of, e.g., about 50 to about 500 mmHgA, preferably about 200 to about 350 mmHgA. The primary feed to the purge recovery tower is the purged portion of the finishing tower overhead condensate, described previously. Also entering the purge recovery tower is a portion of the substantially organic phase of the overhead condensate of the purge recovery tower as reflux, to be described in greater detail hereinafter. 
     The overhead vapors from the purge recovery tower are condensed and the condensate separated into organic and aqueous phases, with the organic phase being, e.g., about 90 to about 99 wt % of the total overhead stream and containing, e.g., about 10 to about 60 wt % of BuAcA, about 30 to about 70 wt % of BuOH, about 3 to about 20 wt % of light ends, and about 5 to about 15 wt % of water, and the aqueous phase being the remainder of the total overhead containing, e.g., about 85 to about 98 wt % of water, about 2 to about 15 wt % of BuOH, and generally less than about 1.0 wt % of light ends. As previously mentioned, a portion of the condensate organic phase, e.g., about 10 to about 80 wt %, is recycled to the purge recovery tower as reflux, the remainder being discarded, with the entire condensate aqueous phase also being discarded. 
     Alternatively the overheard vapors are condensed and forwarded to a receiving vessel, a portion of the resultant stream is discarded and the remainder is recycled as reflux to the purge recovery unit. The composition of the stream coming out of the receiving vessel is about 5 to about 60 wt % BuAcA, about 10 to about 70 wt % BuOH, about 3 to about 20 wt % light ends, and about 10 to about 50 wt % water. 
     The purge recovery tower is operated such that the total amount of the light ends in the discarded portion of the overhead condensate organic phase and the entire discarded aqueous phase is comparable to the amount of light ends produced in the system, so that there is no undesirable buildup of light ends in the system. 
     The residue composition from the purge recovery tower contains from about 10 to about 80 wt % of BuAcA and e.g., about 10 to about 80 wt % of BuOH, as well as relatively smaller amounts of water and light ends, and is recycled to the reaction vessel. 
     The process of the invention may be further described with reference to the drawing, which is a schematic diagram of a preferred embodiment of the inventive process. 
     Into reaction vessel  1  are fed n-butanol (BuOH), acrylic acid (HAcA) and an acid esterification catalyst such as methanesulfonic acid (MSA) through lines  2 ,  4 , and  5  respectively. A polymerization inhibitor is also typically employed throughout the process, and added at appropriate locations and concentrations. Those of skill in the art will recognize appropriate locations and concentrations to utilize. Conditions of temperature and pressure are maintained in reaction vessel  1  to cause a reaction of HAcA with BuOH present in some excess to form n-butyl acrylate (BuAcA) and water, and small amounts of various by-products such as light ends including n-butyl acetate (BuAc) and di-n-butyl ether (DBE) and heavy ends such as low molecular weight polymers of HAcA and BuAcA. Vapors of the more volatile compounds present including BuOH, BuAcA, water and light ends rise into the fractionization zone situated immediately above the reaction zone of reaction vessel  1 . These vapors are forwarded through line  6  into condenser  8  from which the liquid condensate flows through  9  into separator  10  where the condensate is separated into immiscible substantially organic phase  11  and aqueous phase  12 . The organic phase condensate containing predominately BuAcA is fed through line  14  to finishing tower  15  from which high purity finished BuAcA is withdrawn as a sidestream through line  18 . The overhead vapors from tower  15  containing significant amount of BuOH and BuAcA and lesser amounts of water and light ends, are fed through line  19  to condenser  20 , and the residue from tower  15  containing primarily BuAcA is recycled through line  21  to reaction vessel  1 . The diagram indicates stream  21  forwarded to the base of vessel  1 . Those of skill in the art will appreciate that this stream may be forwarded to any location in vessel  1  as appropriate. Most of the condensate from condenser  20  is recycled through line  22  to reaction vessel  1 . However, a minor portion of the condensate is purged and fed through line  24 , to purge recovery tower  25 . The overhead vapors from purge recovery tower  25  containing significant amounts of BuOH, water, and light ends are fed through line  28  to condenser  29 . The residue of tower  25  containing significant amounts of BuAcA and BuOH and largely stripped of light ends is recycled through line  30  to reaction vessel  1 . The condensate from condenser  29  is fed through line  31  to separator  32  where it separates into organic and aqueous phases. A part of the organic phase is recycled through line  34  to purge recovery tower  25  as reflux while the remainder is withdrawn through line  35  and discarded, with the entire aqueous phase also being discarded through line  38 . The foregoing discarding of a portion of the organic and the entire aqueous phase from separator  32  act to keep the light ends of the overall system in balance. 
     The residue from vessel  1  comprising mainly low molecular weight polymers of HAcA and/or BuAcA and Michael addition products is fed through line  39  to heat treater  40  where part of the heavies are converted to HAcA and/or BuAcA. Most of the heat-treated stream from heat treater  40  containing increased amounts of HAcA and/or BuAcA is recycled through line  41  to reaction vessel  1 . However, to prevent a build up of heavy ends in the system, a slipstream from the heat treater  40  is fed through line  42  to a heavy ends removal unit  44 . The vapors from unit  44  containing a significant amount of HAcA and/or BuAcA are condensed and recycled through line  45  to reaction vessel  1  while the residue from unit  44  is withdrawn through line  46  and discarded. 
     Aqueous phase  12  from separator  10  containing a minor but significant amount of BuOH is fed through line  48  to an intermediate point of butanol recovery tower  49 . Optionally, stream  48  may be discarded. The overhead vapors comprising an azeotrope of a major portion BuOH with water is fed through line  50  to condenser receiver  51  where it separates into an organic phase comprising an increased amount of BuOH which is recycled to reaction vessel  1  through line  52 , and an aqueous phase comprising some BuOH which is either fed as reflux to butanol recovery tower  49  through line  54  or alternatively, is discarded. The residue containing predominantly water is withdrawn through line  55  and discarded.