Method for producing (meth)acrylic esters

Abstract

The subject of the present invention is a process for the synthesis of C.sub.1-C.sub.10 alkyl (meth)acrylates, by direct esterification of the (meth)acrylic acid by the corresponding alcohol, the reaction being carried out in a fixed bed membrane reactor under conditions in which the water generated by the reaction is eliminated from the reaction mixture as it is formed. The process according to the invention may operate under conditions for which the reagents are not in excess, thereby minimizing the size and energy of the equipment for separation/recycling of the streams generated during the purification of the reaction medium.

Claims

1. A process for producing an alkyl (meth)acrylate by direct esterification of (meth)acrylic acid with a linear or branched alcohol comprising from 1 to 10 carbon atoms, in the presence of an esterification catalyst, which employs a fixed bed membrane reactor in which the esterification reaction is conducted, while eliminating water generated by the reaction as water is formed, wherein the membrane reactor comprises a membrane module for dehydration coupled to a heterogeneous esterification catalyst, wherein the membrane module for dehydration is a pervaporation unit or a vapour permeation unit, and wherein the membrane module for dehydration is tubular, the esterification catalyst being located within the tubular membrane module for dehydration.

2. The process according to claim 1 chosen from the group consisting of continuous, semi-continuous, and batch type processes.

3. The process according to claim 1 wherein the membrane reactor comprises a hydrophilic membrane, or an inorganic membrane.

4. The process according to claim 1 wherein the membrane reactor comprises a membrane module for dehydration by pervaporation, based on modified silica.

5. The process according to claim 1 wherein the esterification reaction is conducted under stoichiometric conditions of the reagents.

6. The process according to claim 1 wherein the alcohol is selected from the group consisting of methanol, ethanol, butanol, 2-ethylhexanol and 2-octanol.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 schematically represents two examples of a membrane module for dehydration coupled to a heterogeneous esterification catalyst.

(2) FIG. 2 schematically represents membrane reactors which may be used in the process according to the invention.

(3) FIG. 3 schematically represents a facility for carrying out the process according to the invention, applied in particular to the synthesis of ethyl acrylate.

DETAILED DESCRIPTION OF THE INVENTION

(4) The invention is now described in greater detail and nonlimitingly in the description which follows.

(5) The terms (meth)acrylic and (meth)acrylate mean, as is customary, acrylic or methacrylic and acrylate or methacrylate, respectively.

(6) The alcohol used in the context of the invention may be linear or branched. It may be a primary alcohol or a secondary alcohol. It may comprise from 1 to 10 carbon atoms. It may be substituted or unsubstituted, and preferably it is unsubstituted. The alcohol may especially be methanol, ethanol, butanol, 2-ethylhexanol or 2-octanol. The alcohol is preferably ethanol.

(7) The corresponding esters obtained are methyl acrylate or methyl methacrylate, ethyl acrylate or ethyl methacrylate, butyl acrylate or butyl methacrylate, 2-ethylhexyl acrylate or 2-ethylhexyl methacrylate, 2-octyl acrylate or 2-octyl methacrylate.

(8) The (meth)acrylic acid is preferably acrylic acid.

(9) The ester is preferably ethyl acrylate.

(10) The reaction for esterification of the (meth)acrylic acid by the alcohol requires the presence of an esterification catalyst which is, according to the invention, a heterogeneous catalyst of solid type such as, for example, an acid cation exchange resin.

(11) As examples of acid cation exchange resins, mention may be made of the range of Amberlyst macroporous or gel resins from Dow, for example Amberlyst 15 or 131, the Lewatit range from Lanxess, for example Lewatite K1461 or the Diaion range from MCC or Dowex A.

(12) The catalyst is generally in the form of grains of a size ranging from 300 to 800 microns.

(13) According to the invention, the esterification reaction is conducted in a fixed-bed membrane reactor comprising a membrane module for dehydration coupled to the heterogeneous esterification catalyst. In this reactor, the water generated by the esterification reaction may be separated from the reaction medium as it is formed by means of the membrane module for dehydration.

(14) The membrane module for dehydration may be a pervaporation unit (feedstock in liquid phase and vaporization of the aqueous permeate on passing through the membrane), or a vapour permeation unit (feedstock in vapour phase).

(15) The membrane module for dehydration is preferably a unit for separation by pervaporation, that is to say with selective evaporation of the water through a membrane. The stream of water to be evaporated is characterized by a chemical potential difference on the two sides of the membrane module. This concentration gradient is maximized by raising the temperature and by applying a light pressure on the permeate (or vapour permeate) side. It is possible to obtain, according to this method, a permeate of high purity, greater than 95%, favouring the treatment or elimination thereof, and minimizing the loss of organic compounds of which use can be made by recycling into the process.

(16) The membranes may be hydrophilic, of either polymeric or hybrid type (polymer membrane deposited on an inorganic support). Use may be made, for example, of the Pervap 1005, Pervap 1201 or Pervap 4101 resins sold by Sulzer.

(17) As an alternative, the membranes may be inorganic in order to favour strength in acid medium. It is possible to use a ceramic membrane or a membrane based on modified silica, for example of HybSi type sold by Cramiques Techniques et Industrielles (CTI).

(18) The membranes are selected due to their performance in terms of separation selectivity (water purity of the permeate) and of the flow rate of permeate passing through the membrane. It has been observed that inorganic membranes based on modified silica not only have good strength in an acrylic acid/acrylic ester medium, but also lead to a very water-rich permeate (water content of greater than 88%) comprising less than 1% ester, with stream flow rates much greater than those generally achieved with membranes of polymer type.

(19) According to a preferred embodiment of the invention, the membrane reactor comprises a membrane module for dehydration by pervaporation based on modified silica, leading to increased productivity for the process according to the invention.

(20) According to one configuration of the invention, the membrane reactor comprises a membrane module for dehydration, at the surface of which the esterification catalyst is located. The module may be planar or tubular.

(21) According to another preferred configuration of the invention, the membrane reactor comprises a tubular membrane module for dehydration, within which an esterification catalyst is located.

(22) The membrane reactor is preferably of multitubular type.

(23) The membrane reactor is coupled to a vacuum pump in order to extract the water from the reaction medium. A vacuum compatible with industrial vacuums is used, generally of less than 100 mbar, for example of between 20 and 50 mbar.

(24) The water leaving the membrane reactor is condensed and may be sent into one of the purification columns in order to recycle any traces of organic compounds which might have crossed the membrane. As an alternative, the condensed water is sent to the biological plant for treatment of the residual organic compounds.

(25) The membrane reactor operates at a temperature ranging from 50 C. to 100 C., preferably from 55 C. to 90 C.

(26) The membrane reactor is supplied with (meth)acrylic acid and alcohol. The acid/alcohol or alcohol/acid molar ratio refers to the contents of acid and of alcohol of all the streams supplying the membrane reactor. The process according to the invention may be carried out in the presence of a stoichiometric excess of alcohol, in the presence of a stoichiometric excess of acid, or under stoichiometric conditions of the reagents.

(27) Advantageously, the process according to the invention is carried out under stoichiometric conditions of the reagents. In this mode of operation, it has been observed that the conversion is substantially equivalent to that obtained with a reaction conducted with an excess of acid or an excess of alcohol. In addition, due to the absence of a reagent in excess, the purification of the reaction medium is simplified with a lesser degree of recycling for the streams generated by the purification treatment. In particular, the energy consumption for recycling the unreacted alcohol is much lower.

(28) Generally, it is suitable to guarantee a controlled stabilization of the reaction medium by adding, to the reactor, approximately from 200 to 2000 ppm of at least one polymerization inhibitor such as, for example, hydroquinone, hydroquinone methyl ether, di-tert-butyl-para-cresol (BHT), phenothiazine, para-phenylenediamine, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or derivatives thereof, or di-tert-butylcatechol, activated by continuous injection of depleted air (7% O.sub.2). Supplementary polymerization inhibitor is generally added at the subsequent purification treatment.

(29) Examples of membrane reactors which may be used in the process according to the invention are illustrated in FIG. 1 and FIG. 2.

(30) In FIG. 1 a) a tube can be seen consisting of a membrane, around which an esterification catalyst is located. The assembly is produced such that the permeate (separated water) leaves axially relative to the tube, with the retentate comprising the reaction medium with most of the water removed leaving the module radially.

(31) In FIG. 1 b) a tube consisting of a membrane contains an esterification catalyst. According to this configuration, the assembly comprises a radial outlet for the permeate and the retentate leaves axially at the opposite end to the reagent supply.

(32) FIG. 2 a) illustrates a membrane reactor of multitubular exchanger type comprising tubes according to the configuration of FIG. 1 a).

(33) FIG. 2 b) corresponds to a multitubular exchanger in the configuration with radial outlet of the permeate. The water is eliminated through the tubes toward the shell which serves as body for this reactor. This shell is provided with tubing which enables it to be placed under vacuum. The water vapour extracted in this way from the reaction medium condenses outside of the shell via an external exchanger. The reaction medium is heated by an exchanger which pre-heats the supply.

(34) FIG. 3 represents a facility for producing alkyl (meth)acrylate according to the invention comprising a membrane reactor MR. This reactor MR is supplied by a pipe for conveying acid 1 and a pipe for conveying alcohol 2. The reactor preferentially contains an acid cation exchange resin-type catalyst contained in a tubular pervaporation module.

(35) At the outlet of the membrane reactor, the reaction mixture 3 is sent to a distillation column C which separates, at the bottom, a stream 5 comprising essentially the unreacted acid, traces of light products (boiling point lower than that of the acid), and products having a boiling point higher than the acid, and at the top, a stream 6 comprising the ester formed and products lighter than the acid (unreacted alcohol, by-products such as ethyl acetate, acetic acid in the case of the synthesis of ethyl acrylate, for example).

(36) The water eliminated through the wall of the membrane of the membrane reactor, stream 15, is condensed and sent (not shown in the diagram) into one of the purification columns to recycle any traces of organic compounds present, in particular the water which may be used for the liquid-liquid extraction phase.

(37) The stream 5 from the bottom of column C is sent to a distillation column C1 which separates a stream 4 comprising the residual acrylic acid and lighter products, this stream 4 being recycled into the reactor MR. A stream 7 consisting essentially of heavy products (adducts) is separated from column C1 and subjected to thermal cracking in the thermal cracker TC.

(38) The thermal cracking makes it possible to recycle the valuable products (starting compounds or finished product) which can potentially be recovered from the heavy products fraction. The stream 9 of acid is recycled to the column C1, the stream 8 being incinerated.

(39) The stream 6 from the top of the distillation column C is sent to a section for liquid-liquid extraction (settling tank or contactor) in order to generate, on the one hand, an aqueous phase 10 containing alcohol which is recycled to the reaction (stream 13) after distillation in a column C2 (the alcohol-depleted aqueous stream 14 possibly being recycled for the liquid-liquid extraction phase) and, on the other hand, an organic phase 11.

(40) The organic phase 11 may be subjected to one or more supplementary steps of distillation in order to give the desired purified ester 12.

(41) The process according to the invention makes it possible to make significant savings in terms of energy consumption of the facility.

(42) The following examples illustrate the present invention without aiming to limit the scope of the invention as defined by the appended claims.

EXPERIMENTAL SECTION

(43) The invention is illustrated with the reaction for synthesizing ethyl acrylate by esterification of acrylic acid with ethanol.

Example 1

(44) The chemical equilibrium constant for the synthesis of ethyl acrylate is 2. Calculations of conversion at equilibrium were carried out for different conditions: The reaction is conducted with an excess of alcohol (alcohol/acid molar ratio Rm=1.8). The reaction is conducted with an excess of acid (acid/alcohol molar ratio=2.5, i.e. alcohol/acid molar ratio Rm=0.4). The reaction is conducted under stoichiometric conditions (alcohol/acid molar ratio Rm=1). The reaction is conducted in a conventional reactor R at a temperature of between 70 and 90 C. The reaction is conducted in a membrane reactor MR at a temperature of between 70 and 90 C., and 80% of the water formed is eliminated from the reactor MR as it is formed.

(45) The results are collated in Table 1 below.

(46) TABLE-US-00001 TABLE 1 Rm alcohol/acid 1.8 1.8 1 0.4 0.5 Type of reactor R MR MR R MR Conversion at 74 90 76 81 94 equilibrium, % Ester formed, 0.74 0.9 0.76 0.81 0.94 mol Residual alcohol, 1.06 0.9 0.24 0.19 0.06 mol Residual water, 0.74 0.148 0.152 0.81 0.188 mol Residual acid, mol 0.26 0.1 0.24 1.69 1.56 Residual alcohol, 1.43 1 0.315 0.234 0.06 mol per mol of ester formed Residual acid, mol 0.35 0.09 0.315 2.09 1.66 per mol of ester formed

(47) It is observed that the conversion in a membrane reactor is higher than that in a conventional reactor, whether under conditions of excess alcohol or excess acid.

(48) The membrane reactor operating under stoichiometric conditions leads to a substantially identical conversion to that obtained with a conventional reactor operating with an excess of alcohol or to that obtained with a conventional reactor operating with an excess of acid.

(49) The membrane reactor according to the invention has this specific advantage of operating under conditions approaching stoichiometry, by leading to conversions comparable to those that exist for the conventional processes which require either an excess of alcohol or an excess of acid.

(50) The result of this is that the residual amounts of alcohol and acid are lesser, which minimizes the energy loop for recycling the alcohol and reduces the risks of fouling linked to the residence time of the acrylic acid in the process.

Example 2

(51) In an esterification reactor, the volume of esterification catalyst (or reaction volume) is determined as the product of the flow rate of reagents entering with the residence time in this reactor.

(52) Mass per unit volume of the acid: 1000 kg/m.sup.3,

(53) Mass per unit volume of the alcohol: 800 kg/m.sup.3

(54) Considering a standard residence time of one hour in the reactor, for an ester production of 100 kg/h, the reaction volumes and the corresponding supply flow rates are determined from the data in Table 1.

(55) TABLE-US-00002 TABLE 2 Rm alcohol/acid 1.8 1.8 1 0.4 0.4 Type of reactor R MR MR R MR Supply flow rate, 209 172 155 265 240 kg/h Reaction volume, m.sup.3 0.237 0.195 0.170 0.279 0.253

(56) With an equivalent ester production, the membrane reactor operating under stoichiometric conditions makes it possible to minimize the volume of catalyst to be used.

Example 3

(57) According to Example 2, a volume of 170 l of heterogeneous catalyst, for example of a resin of Amberlyst type, makes it possible to produce 100 kg/h of ethyl acrylate corresponding to the simultaneous formation of approximately 18 kg/h of water.

(58) A membrane reactor adapted to eliminate approximately 80% of the water generated, i.e. 14.4 kg/h of water, may be for example a membrane module comprising, according to a tubular exchanger configuration, 360 tubes of tubular membrane of HybSi type, 25 mm in diameter and 1 m high, filled with catalyst, which corresponds to a surface area of approximately 8.5 m.sup.2 for a membrane have a specific flow rate of aqueous permeate of 0.5 kg.Math.m.sup.2.Math.h.

Example 4

(59) An Aspen simulation was carried out for a unit for synthesizing ethyl acrylate EA comprising a membrane reactor eliminating 80% of the water formed during the reaction, and a purification assembly based on the use of a first distillation column C separating the residual acid, and a second column C2 separating the residual alcohol which is recycled to the reaction.

(60) Table 3 below summarizes the energy data obtained in comparison with the energy data for an industrial ethyl acrylate unit.

(61) It is observed that the separation of the water from the reaction generates an energy gain of the order of 25-35% over the 2 columns for recovery of the unreacted reagents.

(62) TABLE-US-00003 TABLE 3 Conventional Unit with membrane unit reactor Gain, % Reboiler column C, 4789947 3688259 kcal/h Vapour 2.5 bar 9.34 7.2 23% EA entering 13496 13741 column C, kg/h Kg vapour/kg EA 0.69 0.52 24 Reboiler column 4317096 2886301 C2, kcal/h Vapour 2.5 bar 8.42 5.63 33% EA production, 13496 13741 kg/h Kg vapour/kg EA 0.62 0.41 34% produced