Process for continuously synthesizing acrylic acid esters
United States Patent [191 Ohrui et al.  
 [ 1 PROCESS FOR CONTINUOUSLY SYNTHESIZING ACRYLIC ACID ESTERS [75] Inventors: Tetsuya Ohrui, Niihama; Yasuhito Sakakibara, Saijo; Yukinaga Aono; Michio Kato; Hiroshi Takao; Tsunejiro Kawaguchi, Niihama, all of Japan [73] Assignee: Sumitomo Chemical Company,  
 Limited, Osaka, Japan 221 Filed: May 21, I973 2| Appl. No.: 362,202  
 [52] U.S. Cl 260/486 R [51] Int. Cl. C07c 69/54 {58] Field of Search 260/486 R [56] References Cited UNITED STATES PATENTS 2.9l6.512 12/1959 Fisher et al. 260/486 R 3,354,199 11/1967 Lachowicz ct a1 260/486 R Primary ExaminerLorraine A. Weinberger Assistant Examiuer-Paul .l. Killos Attorney, Agent, or Firm-Stevens, Davis, Miller &amp; Mosher [451 Apr. 1, 1975 [57] ABSTRACT Acrylic acid esters such as methyl acrylate or ethyl ac rylate are continuously synthesized by esterifying one part by mole of acrylic acid with 2.0 to 3.0 parts by mole of methanol or L5 to 2.5 parts by mole of ethanol in the presence of 0.1 to 1.0 part by mole of a water-insoluble and unreactive organic solvent, such as a linear, cyclic or aromatic hydrocarbon compound having a boiling point of 100 to 160C and sulfuric acid having a concentration of 5 to 50 70 by weight in an esterification solution as a catalyst at boiling temperature, for example, 50 to 100C approximately under atmospheric pressure, while azeotropically boi!ing water formed in the esterification together with the organic solvent and distilling acrylic acid esters simultaneously, thereby keeping the concentration of sulfuric acid constant in the esterification solution; separating the resulting effluent esterification solution into an organic solvent layer and an aqueous layer by decantation; recycling the aqueous layer containing sulfuric acid to the esterification; separating and recovering the resulting ester and organic solvent contained in the organic solvent layer by steam distillation from the resulting polymers and high boiling materials; and leading the ester and organic solvent to further purification together with the resulting distillation of the esteritication.  
 11 Claims, 1 Drawing Figure PROCESS FOR CONTINUOUSLY SYNTHESIZING ACRYLIC ACID ESTERS This invention relates to an improved process for continuously synthesizing an acrylic acid ester, particularly methyl acrylate or ethyl acrylate.  
  To avoid eomplicacy of the description, the explanation will be concentrated mainly on a process for methylesterification of acrylic acid in the description which follows, but the quite same explanation is applicable to a process for ethylesterification of acrylic acid, unless otherwise especially mentioned.  
  Heretofore, it has been proposed that mcthylesterification of acrylic acid be carried out in a stirred vesseltype reactor, using sulfuric acid or sulfonic acids as a catalyst, while adding methanol to the reactor. However, in said process, the formed water leaves the reaction system, and therefore the prior art process is disadvantageous from the viewpoint of reaction equilibrium. That is, only when a large amount, for example, 5 to 7 moles of methanol, is added to one mole of acrylic acid, can an effective reaction rate and conversion be obtained. The resulting methyl acrylate azeotropically boils together with water, but the azeotropic composition is rich in methyl acrylate. Therefore, the concentration of water is considerably increased in the continuous reactor, and this is not favorable for the csterification reaction. The almost same situation prevails also in the ethylesterification, Furthermore, the residual liquid resulting from separation of methyl acrylate and methanol from the reaction solution is a contaminated dilute sulfuric acid solution containing polymers and high boiling materials, and is discarded after neutralization with an alkali. For example, it has been proposed that the residual liquid be freed from the polymers. concentrated and reused in the esterification reaction, but the polymers resulting from the esterification reaction are tarry materials and are liable to stick to vessel walls. Therefore. it is impossible to separate the polymers from the residual liquid by a proper separating operation, for example, filtration. etc. Even if the polymers are removed from the residual liquid by some means, dissolved polymers, etc. will be again deposited when the resulting residual liquid is concentrated.  
  in order to make the reaction equilibrium favorable in the methylesterification, there has been proposed a process comprising carrying out the csterification reaction in the presence of a hydrocarbon, and extracting the resulting methyl acrylate into the hydrocarbon layer (Japanese Patent Publication No. l444/65). The prior art process is advantageous in the reaction equilibrium and rate of methylesterification, and it is enough to add 1.0 to 2.0 moles of methanol to one mole of acrylic acid. However, in the prior art process, the esterification is not carried out while distilling off and separating water and methyl acrylate resulting from the csterification. As is apparent from said Japanese Patent Publication Specification, the esterification is carried out under reflux, and the resulting methyl acrylate is restricted to the process for extractively separating methyl acrylate by means of the hydrocarbon added. Therefore, the aqueous layer is a dilute solution of sulfuric acid, and thus must be discarded after neutralization in the same manner as described before. The problem of treating such waste acid is not only very important from the view point of preventing the public nuisance, but also impairs the process economy,  
  in the process for estcrifying acrylic acid in the presence of such hydrocarbons as benzene, toluene, ete., the esterification reaction is carried out with higher alcohols having three or more carbon atoms (Japanese Patent Publication No. 22858/63; U.S. Pat. No. 2,917,538). In the prior art process, the esterification is carried out, while azeotropically boiling and separating the resulting water together with the hydrocarbon added. The expensive alcohol to be added can be limited to almost equal moles of acrylic acid. and further the successive separation step can be carried out with ease.  
  Generally, benzenesulfonic acid, etc. are used as a catalyst in place of sulfuric acid, because there is no water in the reaction system, and therefore sulfuric acid will undesirably cause carbonization of reactants and products.  
  However, no art of using an organic solvent or hyd rocarbons has been developed yet to utilize the azeotropic relation with water in the methyl or ethylesterification reaction.  
  An object of the present invention is to overcome the disadvantages encountered in the prior art as described above.  
  According to the present invention, sulfuric acid as the catalyst can be not only rccyclieally used, but also the reaction equilibrium and reaction rate of methyl or ethylesterification of acrylic acid can be shifted to a very favorable direction, and thus the ester can be synthesized with ease. Furthermore, the problem of treating the waste acid is substantially completely solved, and also an improvement in consumption of sulfuric acid for the production of unit acrylic ester can be attained. Therefore, the present invention provides an economical process. Furthermore, it must be mentioned that the troubles of polymers. etc. are greatly reduced.  
  Now, the present invention will be explained in detail below:  
  According to the present invention, acrylic acid, an organic solvent and methanol or ethanol are fed, at first, to a reactor, and the esterifcation reaction is carried out in the presence of a sulfuric acid catalyst. The reaction is carried out at a boiling point temperatures, and the resulting water is distilled off azeotropieally together with methyl acrylate and the organic solvent. The rate of water distilled off is controlled by selecting the boiling point temperature so that sulfuric acid can be contained in the reaction solution at a constant sulfuric acid concentration. The reaction solution is liable to be separated into an aqueous layer and an organic solvent layer, usually in the present invention the volume ratio of the organic solvent layer to the aqueous layer being from I 9 to 7.3 in the csteriflcation reactor, and therefore, for example, stirring is carried out to obtain a good distribution and distill off vapors of azeotropic composition satisfactorily. The reaction solution withdrawn from the reactor is separated into two layers in a decanter, and the organic solvent layer is further subjected to steam distillation, etc. to recover methyl acrylate and the organic solvent, while separating polymers, high boiling materials, etc. therefrom. When the esterification reaction is carried out under said conditions where the products are distilled off while adding the organic solvent thereto at the same time, polymers, though produced in a very small proportion, are non-sticky, finely distributed solids, which are not in a tarrry state as observed in the ordinary esterifieation reaction. Therefore. these solids can be very readily separated. That is. the polymers can be readily separated from the aqueous layer containing sulfuric acid from the decanter. for example. by filtration the aqueous sulfuric acid solution thus freed from the solids including the polymers can be recyclically used in the esterification reactor.  
  In this manner the esterification catalyst can be effectively recovered. and in this case the problem of treat ing the waste acid is limited to the small amount of sulfuric acid attached to the separated solids such as poly mers. and therefore substantially solved in the present invention.  
  As is apparent from the foregoing description. it is necessary in the esterifieation reaction to distill off water by an amount corresponding to the amount of water formed by the esterification reaction and water contained in acrylic acid and/or methanol. The concentration of sulfuric acid can be kept constant thereby. Preferable the concentration of sulfuric acid as a catalyst is 5 to 50 &#34;/5 by weight. The esterification can proceed outside the range of said preferable concentration ofsulfuric acid. but the reactor capacity is increased at a lower concentration of sulfuric acid. and a higher concentration is not preferable. because coke-like polymers will sometimes form. depending upon the conditions.  
  in the present invention. esterification is carried out in the presence of an organic solvent. particularly. hydrocarbon having a boiling point of at least 100C. in an esterifieation reactor. and the water entered into the esterit&#39;ication reactor. as contained in the raw material. and the water formed by the esterification are azeotropically boiled together with the organic solvent and distilled off together with the ester formed. whereby the sulfuric acid concentration is maintained constant in the esterification reactor. Not tarry or rubbery, but granular. non-sticky polymers are formed. though in a very small amount. and distributed as small particles in the solution.  
  Therefore. in this case the hydrocarbon is effective for distilling off unnecessary water through azeotropic boiling with water. and further for converting the resulting solids of polymers. etc. to a form easy to handle.  
  Unreacted methanol and acrylic acid are much distributed in an aqueous layer. and thus contained in an aqueous sulfuric acid solution separated from the reaction solution. The solid materials can be removed by simple filtration. and the aqueous sulfuric acid solution can be recycled to the esterification reactor. Addition of water or boiling of the solution is readily conceivable in a recycle line as a mere application ofthe present invention in establishing the water balance of the entire system.  
  The organic solvent can be added to acrylic acid in the esteril&#39;ication step. but an organic solvent obtained as an organic solvent solution of acrylic acid in the estraction step of acrylic acid from an aqueous solution containing acrylic acid can be directly used in the process of the present inventionv Methyl or ethyl esterification of acrylic acid can be conveniently carried out in a tank-type reactor in the present invention. The type of the reactor is not partic ularly limited. but a vigorously agitatable tanktype reactor is preferable, because there are an oil layer and vtater layer in the reaction solution. The number of reactors is not particularly restricted. That is. one reactor vessel or a plurality of reactor vessels connected to one another in series can be used. However. two reactor vessels connected to each other in series are most preferable in view of stability and economy of operation.  
  The raw material acrylic acid is supplied to the reactor together with an organic solvent insoluble in water and nonreactive wit acrylic acid. particularly linear. cyclic or aromatic hydrocarbon. In the case of a plurality of reactor tanks connected to one another in series. it is preferable to add the total amount of the raw material acrylic acid to the first reactor tank. Furthermore. it is not necessary that the raw material acrylic acid be anhydrous acrylic acid. and it is apparent that the acrylic acid containing a small amount of water is not objectionable.  
  The organic solvent added must be separated from the resulting methyl acrylate or ethyl acrylate. and therefore must have a boiling point at least by 20C. preferebly by 35C or above. higher than those of the esters. For the purpose of azeotropically distilling off water resulting from the esterification in the reactor especially an organic solvent having a boiling point of l00C or higher is preferable. There is no upper limit to the boiling point of the organic solvent. but when steam distillation is carried out in purifying the organic solvent in the successive step. an organic solvent having a boiling point of not more than C is preferable. Concretely. for example linear. cyclic or aromatic hydrocarbons having a boiling point of l00C to 160C, preferable aromatic hydrocarbons such toluene, oxylene. p-xylene. ethylbenzene and cumerie. are used in the present invention as the organic solvent, because they are available at low costs. It is preferable to add ().l to 10 part by mole ofthe organic solvent. based on acrylic acid. to the reactor. The reaction can proceed when less than 0.l part by mole of the organic solvent is added to the reactor. but it is not preferable to add less than 0.] part by mole of the organic solvent. be-  
 cause the effect of the present invention is reduced.  
 The reaction can also proceed when more than 1.0 part by mole of the organic solvent is added. but it is not economical and not preferable to add the organic solvent in an amount over that required.  
  In the case of methyl acrylate. the proportion of methanol to be fed is not especially limited. but it is preferable to add 2.0 to 3.0 moles of methanol per mole of acrylic acid to the reactor. In the case of a plurality of reactor tanks connected to one another in serie it is preferable to add methanol to the respective react r tanks in divided portions.  
  1.1 the ease ofethyl acrylatc. it is preferable to add 1.5 to 5 moles of ethanol per mole of acrylic acid to the TCI&#39;iC fvi.  
  Flult&#39;aric acid is most effective as te catalyst. and cheap and economical. Of course, various other sultonic acids can be effectively used. but they are expensive and thus not necessary to use. When sulfuric acid is used. a portion of the organic solvent used is sulfo&#39; nated. depending upon the reaction conditions. and the resulting sulfonic acid acts not only as the catalyst. but also behaves as equally as sulfuric acid, because the sulfonir&#34; acid is soluble in water. Preferable concentration of sulfuric acid in the reactor is S to S0 be weight. preferably 5 to 30 &#39;7! by Weight.  
  Preferable reaction temperature is 50 to 100C.- particularly a boiling point of 75 to 85C. Preferablereaction pressure is approximately atmospheric.  
 Now, the present invention will be explained in detail In this case, sulfuric acid is made up through a line 5. when required.  
  In this manner. an organic solvent. which is insoluble in water and is not reactive with acrylic acid. is made by by ref n m l d i 5 present in the esterification step. and isdistilled off together with water formed from the esterlfication as well The schcmlmc l diagram Showmg one as contained in the raw materials, while keeping the of thcprcscm mvcmmn but the present water balance constant. Sulfuric acid as a catalyst can mvcmmn hnmud thcrcm&#39; be effectively utilized by recycle. Furthermore. as is Raw materials acrylic acid and OrgimlC mil/Cm are well known the esterit&#39;ication reaction itself is made adfed to the first esterification reactor through a line vumdgwus h b h i int of reaction and Sulfuric acid there) lhmugh line The equilibrium and reaction rate. In the present invention. material acrylic acid the Organic Sillvcm can be the resulting polymers can be converted to a readily Supplied thcrct through lhQ separate lines- Methanol treatable form by distilling off water from the reaction cthaml is fed through a dlvldcd 3 15 solution so as to the&#39;keep the concentration of sulfuric 4 4nd mspcctlvcly fed thfi first csmrlficmlim acid at a proper lead and making an organic solvent actor 15 and the second esterii&#39;ication reactor 16. The pmsum i h ifi tion step, and thus can be number of the esterification reactors is not especially dil fil d, Th t i sulfuric acid can be readily limited, but two vigorously agitatable reactor tanks li ll uscd. connected to each othere in series are preferable. Dis- N 1 nt invention will be explained in detail tillate vapors of water, ester. methanol or ethanol. orby way of examples. ganic solvent, etc. are withdrawn from the first esterifi- Example I cation reactor 15 through a line 9, and the reaction 50- Two 1.1 fl k were Connccicd in h other i series luliflfl i8 03d t0 the SCCOfld cslcrificilliv&#34; rcilclor 16 to set up esterification reactors as shown in the Figure. fmm thc &#34;cucwr 15 thlugh 8 y vcrflllw- Distll- 35 and a solution of 80 wt.&#34;/? acrylic acid and 20 wt./r of late vapors of water. ester. methanol or ethanol. or- Xylene mixture f i n, m-xylcne and p-xylencl ganic lven c. are wi hd f m the sewnd was supplied to the esterification reactors according to terification reactor 16 through a line H, and the reach fl di f th Fi e. Mcthylesterification tion solution is sent to a decanter 17 from the reactor was i d out t a temperature of 80C with a volume 16 through a line 10 by overflow. The organic solvent of retention of0.7 I in the first reactor and at a temperlayer is withdrawn from the decanter l7 through a line mor f 82C with a volume of retention of 0.7 l in the 13. and the residual ester and organic solvent are rcsecond reactor. The flow rate and composition at the covered from the organic solvent layer through a line respective parts are given in Table l, and methyl acry 14, for example by a steam distillation tank 20. and sent l m was d d in yield of 99.0%, an catalyst sulfuric to a purification step for the ester and organic solvent id was recovered with a percentage recovery of977z. together with the distillate vapors coming from the The recovered sulfuric acid could be continuously relines 9 and ll. Steam is directly blown into the tank 20 d by recycle,  
 Table l Flow rate (g/hr) 400.0 197.0 142.3 0.7 212.7 450.2 94.7 260.3 67.5 27.3  
 Analytical values (wtfil) Acrylic acid 800 FL} 5.9 7.0 3.3  
 X lene 20.0 l9.6 9.3 15.4 10.4 5.0 41.2  
 Methanol 100.0 100.0 5.8 16.2 18.3 47.9 24.3 3.6  
 Methyl acrylate 30.7 62.5 16.8 32.2 4.0 48.0  
 Water 2.0 l6.4 12.0 111.1 9.6 25.1 0.9  
 Sulfuric acid 98.0 10.2 23.0 31.3 2.4  
 Acrylic acid polymer 2.4 1.4  
 Temperature (C) 80 82 82 a mixture of n&#39;xylene. Ira-xylene and p-xylene through a line 2]. Polymers and high boiling materials 60 Example 2 are separated and recovered through a line 22. The aqueous layer is led to a pump 18 from the decanter 17 through a line 12. and passed through a filter 19 after increase in pressure of the aqueous layer by the pump 18. The polymers and solid materials are separated from the aqueous layer. The aqueous layer can be recovered to the outside through a line 23. or recycled to the first esterification reactor [5 through lines 6 and 7.  
  Ethyl alcohol was fed through the line 2 of the flow diagram of the Figure, and both temperatures of the first and second reactors were maintained at 85C. Ethylesterification was carried out in the same manner as in Example I. Flow rate and composition of the respective parts are given in Table 2. Ethyl acrylate was produced in yield of 99.0%. and catalyst sulfuric acid was recovered with a percentage recovery of 96.5%.  
 Table 2 l 3 4 5 8 9 10 ll l2 13 Flow rate lglhr) 400 287.2 204.6 0.8 23|.5 528.6 l0l.4 334.8 74.2 27.2  
 Analytical values (wtfii) Acrylic acid 80.0 l6.0 6 l 6.9 3.9 Xylenc* 20.0 20.0 7.2 to 3 8.9 5.5 45.7 Ethanol i000 |00.0 8.0 l9.7 21.4 54.6 26.5 7.6 Ethyl acrylate 34.8 61.0 13.1 32.l 3.7 38.9 Water 2.0 11.9 l2.l l9 5 4.4 26.3 l.2 Sulfuric acid 98.0 9.4 2| 5 28.3 2.8 Acrylic acid polymers 2.l 2.9 Temperature (C) 85 85 B5 85 a mixture of o-xylene. m-xylenc and p-xylene What is claimed is:  
  l. A process for continuously synthesizing acrylic acid esters by a sulfuric acid catalyst. which comprises: i. esterifying acrylic acid with methanol or ethanol in the presence of a water-insoluble and unreaetive organic solvent selected from the group consisting of linear. cyclic and aromatic hydrocarbons having a boiling point oft&#39;rom l00to lfi0C.. while azeotropically boiling water formed in the esterification together with the organic solvent and distilling acrylic acid esters simultaneously. thereby keeping the concentration of sulfuric acid constant in the esterification solution&#34;. 2. separating the resulting effluent esterification solution into an organic solvent layer and an aqueous layer by decantation;  
  2. A process according to claim 1, wherein the esterification (l) is carried out in a single stage.  
  3. A process according to claim 1, wherein the esterification l is carried out in a plurality of stages.  
  4. A process according to claim I, wherein the esterifieation (l is carried out at boiling temperature.  
  5. A process according to claim 4, wherein the esterification (l is carried out at 50 to C.  
  6. A process according to claim 1, wherein the esterification (l) is carried out approximately under atmospheric pressure.  
  7. A process according to claim 1, wherein the organic solvent is toluene, o-xylene, m-xylene, p-xylene, ethylbenzene or cumene.  
  8. A process according to claim I, wherein 0.] to 1.0 part by mole of the organic solvent is added to the esterification. on the basis of acrylic acid.  
  9. A process according to claim 1, wherein 2.0 to 3.0 moles of methanol per mole of acrylic acid is added to the esterification l 10. A process according to claim 1. wherein 1.5 to 2.5 moles of ethanol per mole of acrylic acid is added to the esterification l ii. A process according to claim 1. wherein the sull&#39;uric acid has a concentration of 5 to 50% by weight in the esterifieation solution.