Process for continuously preparing di-C1-3-alkyl succinates

Abstract

The invention relates to a process for continuously preparing di-C.sub.1-3-alkyl succinates by reacting succinic acid with an C.sub.1-3-alkanol in the presence of a fixed-bed heterogeneous acidic esterification catalyst in a tubular reactor at a temperature in the range of from 60 to 100° C., wherein a mixture, comprising succinic acid, C.sub.1-3-alkanol, mono-C.sub.1-3-alkyl succinate, di-C.sub.1-3-alkyl succinate and water, is formed in a mixing stage and fed to the entrance of the tubular reactor, and wherein 5 to 75% of the outlet flow rate of the tubular reactor are recycled directly to the mixing stage as a recycle stream, and the molar ratio of C.sub.1-3-alkanol to succinic acid added to the mixing zone, and not including the C.sub.1-3-alkanol and succinic acid at the recycle stream, being in the range of from 2.0 to 9.5. The invention furthermore relates to a process for separating the reactor effluent of an esterification of succinic acid with an C.sub.1-3-alkanol to give di-C.sub.1-3-alkyl succinates by distillation, wherein the separation is performed in a divided wall column in which C.sub.1-3-alkanol and water are removed in a top draw of the column, di-C.sub.1-3-alkyl succinate is removed in a side draw of the column, and wherein mono-C.sub.1-3-alkyl succinate and succinic acid are removed in a bottom draw of the column.

Claims

1. A process for continuously preparing di-C.sub.1-3-alkyl succinates by reacting succinic acid with an C.sub.1-3-alkanol in the presence of a fixed-bed heterogeneous acidic esterification catalyst in a tubular reactor at a temperature in a range of from 60 to 100° C., wherein a mixture, comprising succinic acid, C.sub.1-3-alkanol, mono-C.sub.1-3-alkyl succinate, di-C.sub.1-3-alkyl succinate, and water, is formed in a mixing stage and fed to an entrance of the tubular reactor, and wherein 5 to 75% of an outlet flow rate of the tubular reactor is recycled directly to the mixing stage as a recycle stream, and a molar ratio of C.sub.1-3-alkanol to succinic acid added to the mixing zone, and not including the C.sub.1-3-alkanol and succinic acid of the recycle stream, is in the range of from 2.0 to 9.5.

2. The process of claim 1, wherein in the effluent of the tubular reactor the succinic acid, C.sub.1-3-alkanol, mono-C.sub.1-3-alkyl succinate, di-C.sub.1-3-alkyl succinate and water are present in equilibrium concentrations.

3. The process of claim 1, wherein in the tubular reactor the succinic acid conversion is at least 95 wt-%.

4. The process of claim 1, wherein the molar ratio of C.sub.1-3-alkanol to succinic acid added to the mixing zone, and not including the C.sub.1-3-alkanol and succinic acid of the recycle stream, is from 3 to 7.

5. The process of claim 1, wherein 25 to 63 wt-% of the effluent of the tubular reactor is recycled to the mixing stage.

6. The process of claim 1, wherein the C.sub.1-3-alkanol is methanol and dimethyl succinate is prepared.

7. The process of claim 1, wherein the mixture formed in the mixing stage comprises at least 90 wt % of succinic acid, C.sub.1-3-alkanol, mono-C.sub.1-3-alkyl succinate, di-C.sub.1-3-alkyl succinate, and water.

8. The process of claim 1, wherein the mixture formed in the mixing stage comprises water in an amount of 10 wt-% or less.

9. The process of claim 1, wherein the tubular reactor is a plug flow reactor.

10. The process of claim 1, wherein the heterogeneous acidic esterification catalyst is an acidic ion exchange resin.

11. The process of claim 1, wherein from the effluent of the tubular reactor which is not recycled to the mixing stage, di-C.sub.1-3-alkyl succinate is separated by distillation.

12. The process of claim 11, wherein the separation is performed in a divided wall column in which C.sub.1-3-alkanol and water are removed in a top draw of the column, di-C.sub.1-3-alkyl succinate is removed in a side draw of the column, and mono-C.sub.1-3-alkyl succinate and succinic acid are removed in a bottom draw of the column.

13. The process of claim 4, wherein the molar ratio of C.sub.1-3-alkanol to succinic acid added to the mixing zone, and not including the C.sub.1-3-alkanol and succinic acid of the recycle stream, is from 3 to 4.

14. The process of claim 7, wherein the mixture formed in the mixing stage comprises at least 95 wt-% of succinic acid, C.sub.1-3-alkanol, mono-C.sub.1-3-alkyl succinate, di-C.sub.1-3-alkyl succinate, and water.

15. The process of claim 14, wherein the mixture formed in the mixing stage comprises at least 98 wt-% of succinic acid, C.sub.1-3-alkanol, mono-C.sub.1-3-alkyl succinate, di-C.sub.1-3-alkyl succinate, and water.

16. The process of claim 12, wherein the mono-C.sub.1-3-alkyl succinate and succinic acid removed in the bottom draw of the column are recycled to the mixing stage.

Description

(1) The total reaction and work-up scheme is illustrated in the FIGURE, which is a schematic representation of the process.

(2) In the FIGURE, the left part is the synthesis part of the reaction scheme, the middle part is the dimethyl succinate column (when methanol is employed), and the right hand column is the methanol recovery column.

(3) In the figure, the abbreviations have the following meanings: SYN: synthesis (esterification) DMSC: dimethylsuccinate column MRC: methanol recovery column M: methanol S: succinic acid DMS: dimethyl succinate Oligo: oligomers W: water

(4) The depicted process set-up basically comprises a mixing vessel, a plug flow reactor including the heterogeneous catalyst and an internal recycle loop, a divided wall column for final product separation via the side draw and a second column for water/alcohol separation, as well as a flash evaporator for removing oligomers.

(5) In the work-up sequence, no heterogeneous catalyst is employed. Thus, the different columns including the divided wall column are free from heterogeneous catalyst.

(6) To prevent blockage during the heterogeneously catalysed reaction of succinic acid with the C.sub.1-3-alkanol to a succinic acid ester and water in a plug flow reactor, a solution of the succinic acid in the alkanol is usually needed at the entry of the reactor. For the same reason it is preferred to achieve a homogeneous solution at the outlet of the reactor. For the ingoing solution, the minimal equivalent amount of alcohol is therefore determined by the solubility of the acid in the alcohol. The solubility of succinic acid in methanol is in the range of 20-30 wt % for a temperature range of 40-60° C. Based on the observation that some heterogeneous reaction mixtures with a lower alcohol/acid ratio than necessary to achieve a clear solution turned into clear solutions during the course of the reaction, a reaction set-up was created in which one part of the reaction mixture which ideally has reached the reaction equilibrium, is recycled back from the reactor outlet via an internal loop to the reactor entry where it is charged again with alcohol and acid in the desired ratio until the composition of the solubility point is reached. Having passed the reactor, the reaction mass added at the entry of the reactor is separated from the loop and further processed. This set-up allows the optimization of the production costs in respect of methanol recycling cost and acid/intermediate recycling costs.

(7) Since the equilibrium reaction mixture is partially recycled back to the reactor inlet, the water content in the feed is higher resulting in negligible ether formation.

(8) The side reactions in the downstream process are reduced by use of a divided wall column. In this set-up, both, the alcohol and the water leave the column as overhead product. In doing so, the monoester/organic acid catalysed back reaction is minimized. The divided wall column reduces the energy requirement and capital investment. The other advantage of a divided wall column is that light boiling components formed in the side reaction (e. g. anhydride formation leading to methanol and/or water formation) can be separated from the e.g. dimethyl succinate product stream leading to a higher purity product. Monomethyl succinate is converted to dimethyl succinate and succinic acid in the stripping section of the divided wall column which reduces the bottom temperature and supresses the oligomerisation reaction.

(9) The final product, the diester, is separated via the side draw of the divided wall column with negligible amounts of alcohol and water. By employing the divided wall column a significant product contamination with alcohol and water can be excluded since the side reactions can be suppressed.

(10) The composition of the bottom draw of the divided wall column includes predominantly monoester and in addition diester and organic acid, which are recirculated to the feed of the plug flow reactor or the mixing vessel. Due to thermal stress, by-products could be generally formed in the bottom of the column. However, since in the bottom part of the column, a reaction from monoester to either the acid or the diester is taking place, the temperature is reduced in the divided wall column, thus minimizing the formation of side products. The bottom temperature in the divided vided wall column is preferably in the range of from 150 to 165° C. If necessary or required, a flash distillation can be included to dispose possible distillation residues.

(11) The invention is further illustrated by the enclosed examples.

EXAMPLES

I. Experimental

(12) SA: Succinic Acid

(13) MMS: Monomethyl Succinate

(14) DMS: Dimethyl Succinate

(15) MeOH: Methanol

(16) 1. Determination of Kinetic Data

(17) For the determination of kinetic data a reactor set up was established in which defined solutions of succinic acid in MeOH or succinic acid, dimethyl succinate, monomethyl succinate and water in MeOH were run at 65, 80 or 100° C. through either a 60 ml or a 6 ml plug flow reactor containing a heterogeneous catalyst. Samples of the reaction mixtures were taken before entering and after leaving the reactor. All reaction mixtures used for these trials based on a succinic acid/MeOH ratio of either 1/10, 1/6 or 1/2. Residence times were calculated from the free available reactor space and the measured flow rates.

Example 1

(18) A vessel was charged with 400 g of succinic acid and 1085 g of methanol. The mixture was heated to 60° C. and the solution was transferred via HPLC pump into a heating loop where it was heated to 80° C. and from there into a 60 ml plug flow reactor, equipped with Amberlyst®36 catalyst.

(19) The mass flow was determined as 122 g/h. Samples, taken at the outlet of the reactor, determined a molar DMS:MMS:SA ratio of 87.5:12.0:0.5. The composition of the outlet flow was determined as 29.2 wt-% DMS, 3.6 wt-% MMS, 0.1 wt-% SA. 59.5 wt-% MeOH and 7.7 wt-% of water.

Example 2

(20) 1505 g of a mixture of 24.7 wt-% of succinic acid, 11.0 wt-% of DMS, 2.9 wt-% of MMS, 57.1% of MeOH and 4.3% of water was charged to a vessel and heated to 60° C. The solution was transferred via HPLC pump into a heating loop where it was heated to 65° C. and from there into a 60 ml plug flow reactor, equipped with Amberlyst®39. The mass flow was determined as 450 g/h. Samples taken at the outlet of the reactor determined a molar DMS:MMS:SA ratio of 57.6% of DMS, 30.3% of MMS and 12.1% of SA. The composition of the outlet flow was determined as 25.8 wt-% of DMS, 12.3 wt-% of MMS, 4.4 wt-% of SA, 9.2 wt-% of water and 48.2% of MeOH.

(21) 2. Decomposition of Monomethyl Succinate

Example 3

(22) 10 g of MMS (content: 93.6 wt-%) were charged to a vessel with a distillation head. The compound was heated at 180° C. for 21 h. A sample was taken from the liquid which shows a composition of 49.5 wt-% of MMS, 23.4 wt-% of MMS and 15.6 wt-% of SA.

(23) 3. Solubility

Example 4

(24) A mixture of 12.9 g of MMS, 0.68 g of DMS, 27.7 g of MeOH and 0.03 g of water was placed in a reactor, heated to 80° C. and charged with 29.8 g of SA. To the suspension were added slowly 80 ml of a solution of 2.50 g of SA, 37.1 g of MMS, 103 g of DMS, 33.8 g of MeOH and 25.1 g of water to give at 80° C. a solution containing 19 wt-% of succinic acid, 19 wt-% of MMS and 29 wt-% of DMS.

(25) These experimental data together with property data, which were derived from literature and/or experimentally determined, were included to simulate the operation of the esterification and the work-up sequence which is depicted in the FIGURE.

II. Calculation

(26) The calculation/simulation was based on the kinetic data of the heterogeneous catalyst Amberlyst® 39. The reaction temperature and pressure were set at 80° C. and 1 bar, respectively. A molar ratio of methanol to succinic acid, added to the mixing zone and not including the C.sub.1-3-alkanol and succinic acid of the recycle stream, of 3.5 was used. The recycle stream (6) was approximately 58% of the reactor effluent stream (5) or flow rate. The part of the reactor effluent which was not recycled to the mixing stage was fed to a divided wall column for further separation. A top-draw (stream 8), a side-draw (stream 9) and a bottom-draw (stream 10) were removed from this divided wall column. The resulting temperature, pressure, mass flow rate and composition of all streams are listed in the following table:

(27) TABLE-US-00001 Stream: 1 2 3 4 5 6 7 Temperature ° C. 45.0 90.6 45.0 80.0 90.0 90.0 90.0 Pressure bar 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Mass flow rate kg/h 5.51 17.82 10.12 66.52 66.52 38.58 27.94 SA Wt-% 0.00 5.76 99.96 17.41 1.14 1.14 1.14 MMS Wt-% 0.00 17.13 0.00 14.22 16.60 16.60 16.60 DMS Wt-% 0.00 13.58 0.00 32.82 50.32 50.32 50.32 MeOH Wt-% 99.90 63.47 0.00 29.09 20.84 20.84 20.84 H2O Wt-% 0.10 0.06 0.04 6.46 11.10 11.10 11.10 Stream: 8 9 10 11 12 13 14 Temperature ° C. 36.0 141.0 169.4 169.4 169.4 115.8 68.3 Pressure bar 0.2 0.2 0.2 0.2 0.2 1.7 1.7 Mass flow rate kg/h 8.94 12.50 6.50 6.50 0.00 3.11 5.81 SA Wt-% 0.00 0.00 15.80 15.80 15.80 0.00 0.00 MMS Wt-% 0.00 0.00 46.96 46.96 46.96 0.00 0.00 DMS Wt-% 0.20 99.98 37.24 37.24 37.24 0.57 0.00 MeOH Wt-% 65.13 0.00 0.00 0.00 0.00 0.01 99.90 H2O Wt-% 34.67 0.02 0.00 0.00 0.00 99.42 0.10