Process for the separation of formaldehyde from crude acrylic acid

Abstract

A process for purification of a crude product stream recovered from the production of acrylic acid by an aldolisation reaction is disclosed. The product stream comprises acrylic acid, formaldehyde, water, non-condensable vapours and optionally heavy by-products. The process comprises: providing the crude product stream in the vapour phase to a first separation column operated at a temperature and pressure to form an intermediate overhead stream comprising water, formaldehyde and methanol; and passing said intermediate overhead stream to a formaldehyde separation column operated at a temperature and pressure to enable a stream having a higher formaldehyde concentration than the formaldehyde concentration in the intermediate overhead stream to be formed and recovered from at or near the bottom of the formaldehyde separation column as a formaldehyde enriched stream.

Claims

1. A process for purification of a crude product stream recovered from the production of acrylic acid by an aldolisation reaction, said product stream comprising acrylic acid, formaldehyde, water, non-condensable vapours and optionally heavy by-products; said process comprising: (a) providing the crude product stream in the vapour phase to a first separation column operated at a temperature of about 110° C. to about 150° C. and at a pressure of about 1.0 bara to about 1.5 bara to form an intermediate overhead stream comprising water, formaldehyde and methanol; and (b) passing said intermediate overhead stream to a formaldehyde separation column operated at a temperature at a temperature of from about 60° C. to about 80° C. and at a pressure of from about 0.2 bara to about 0.8 bara to enable a stream having a higher formaldehyde concentration than the formaldehyde concentration in the intermediate overhead stream to be formed and recovered from at or near the bottom of the formaldehyde separation column as a formaldehyde enriched stream.

2. The process according to claim 1 wherein a water stream recovered from the formaldehyde separation column is passed to a water stripping column.

3. The process according to claim 2 wherein the water stripping column is operated at a pressure of from about 3 to about 6 bara.

4. The process according to claim 2 wherein the water stripping column is operated at a temperature of from about 120° C. to about 150° C.

5. The process according to claim 2 wherein heat integration is provided between the water stripping column and the formaldehyde separation column.

6. The process according to claim 5 wherein an overhead from the water stripping column is used to provide heat to a reboiler on the formaldehyde recovery column.

Description

(1) The present invention will now be described, by way of example, with reference to the accompanying figures in which:

(2) FIG. 1 is a schematic diagram of one aspect of the process of the present invention; and

(3) FIG. 2 is a schematic diagram of the process of a second aspect of the present invention.

(4) It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

(5) As shown in FIG. 1 feed is fed in line 1 to an aldol condensation reactor 2 where reaction occurs. The product stream is cooled in the cooling equipment (not shown) of the reactor 2 before being fed in line 3 to the first separation column 4. The feed which will be in the vapour phase will comprise formaldehyde, water, acetic acid, and acrylic acid. Heavy by-products, nitrogen and other non-condensable vapours, and methanol will also generally be present.

(6) The first separation column 4 does not include a reboiler. However, an overhead condenser 5 is present. The first separation column 4 uses the nitrogen and other non-condensibles to provide the stripping medium. An overhead stream 6 from the first separation column 4 having been passed through the condenser 5 may be passed to a vapour/liquid separator 7 where gases can be vented in line 8. Separated liquid can be returned in line 9 to the first separation column 4 as reflux.

(7) The first separation column 4 is operated such that three streams are formed. An intermediate product stream 10 is recovered from at or near the bottom of the first separation column 4. An intermediate side draw stream 11 is recovered optionally from a point above the point at which the feed to the first separation column 4 is added. This side draw stream will generally comprise the acetic acid, some acrylic acid and water. An intermediate overhead stream 12 comprising formaldehyde and water is also recovered. Acetic acid and methanol may also be present.

(8) The intermediate product stream 10 is passed to the second separation column 13. In this example the second separation column 13 includes both a reboiler 14 and a condenser 15. A vapour/liquid separator 16 may be present after the condenser such that any gas present can be separated and vented in line 17. A polymerisation inhibitor may be added to this separation column 13 at any suitable point, not shown. The product acrylic acid is recovered as a side draw 18. A bottom stream 19 may be recovered from the second separation column 13. This will comprise any heavies including byproducts and oligomers.

(9) An overhead stream 20 from the second separation column 13 will comprise acetic acid and water. This is passed to the third separation zone which comprises an acid separation zone which is column 21 and an entrainer stripping zone which is column 22. The acid separation column 21 is provided with reboiler 23 and condenser 24. The entrainer stripping column 22 is provided with reboiler 25 and condenser 26.

(10) The intermediate side draw 11 from the first separation column 4 may also be fed to the acid separation column 21.

(11) An entrainer is supplied to the acid separation column 21 in stream 31 where it is combined with recycled entrainer before being supplied to the column 21 in stream 32.

(12) Any acrylic acid which has been fed to the acid separation column 21 is separated as columns bottom stream 27 which may be recycled to the second separation column 13.

(13) Acetic acid separated in acid separation column 21 is removed as a side draw in stream 28 and returned to the aldol condensation reactor 2 via mixer 59. Acid separation column 21 overhead 29 is cooled in condenser 24 before being passed to decanter 30. Any non-condensed gases are removed in vent 33. Since the entrainer is immiscible with water, two liquid phases are formed and these can be separated in decanter 30. An entrainer-rich stream 34 is returned to column 21 as reflux.

(14) A water-rich stream recovered from the decanter 30 is passed in stream 35 to the entrainer stripping column 22. The overhead stream 36 from column 22 is passed to condenser 26 and then to decanter 37. Any non-condensed gases are removed in vent 38. In decanter 37 two liquid phases are separated with an entrainer rich stream being returned in line 39 to the acid separation column 21. The other liquid phase is returned to the entrainer stripping column 22 as reflux.

(15) The intermediate overhead stream 12 which comprises water, formaldehyde, acetic acid and methanol is fed to the aldehyde recovery column 40. The aldehyde recovery column 40 is provided with a reboiler 41 and a condenser 42. Formaldehyde recovered from the bottom of the aldehyde recovery column 40 is recycled in stream 43 to the aldol condensation reactor 2 via mixer 59.

(16) Overhead stream 44 having been passed through condenser 42 is passed to vapour/liquid separator 45. Any gases can be separated and removed in vent 46. A portion of the liquid is returned in line 47 to the aldehyde recovery column 40 as reflux. The remainder is passed in stream 48 to a water stripping column 49. The bottom stream from entrainer stripping column 22 may also be fed to this water stripping column in line 53. In this column the water is separated from any residual formaldehyde. The water stripping column includes a reboiler 50 and a condenser 51. Water is recovered from the bottom of the column in stream 52. The separated formaldehyde is removed as overhead in stream 54. It is then cooled in condenser 51 and then passed to vapour/liquid separator 55. Any separated gas can be removed in vent 56. A portion of the liquid can be returned to the water stripping column in line 57 as reflux. The remainder is returned to the aldehyde recovery column in line 58.

(17) Modifications of the process illustrated in FIG. 1 are illustrated in FIG. 2. Whilst these modifications are discussed in combination, it will be understood that one or more of them may be used.

(18) In general the process of FIG. 1 is the same as that of FIG. 2 and the same items are given the same reference numerals. As illustrated in FIG. 2, the overhead stream 36 from the entrainer stripping column 22, having been passed through condenser 26, rather than being passed to decanter 37 of FIG. 1, is passed to decanter 30 in line 60 where it is combined with the overhead stream from the acid separation column 21.

(19) In the arrangement of FIG. 2, there is heat integration between the water stripping column 49 and the aldehyde recovery column 40. In this arrangement, the overhead stream 61 from the water stripping column 49 is used to provide heat in reboiler 62 to the aldehyde recovery column 40. Having been passed through reboiler 62, the stream is passed to a decanter 63 which may include a vent 64. Reflux is provided to the water stripping 49 via line 65. Methanol may be recovered in line 66. Recovered formaldehyde may be returned to the formaldehyde recovery column 40 in line 58.

(20) The addition of the entrainer in line 67 to the acid separation zone 21 is illustrated in FIG. 2.

(21) The present invention will now be described by way of example with reference to the following examples.

EXAMPLE 1

(22) The following example demonstrates the advantages associated with the pressure swing in the formaldehyde recovery zone.

(23) In this example, the desired end product from the formaldehyde recovery zone is a recyclable stream with a fixed formaldehyde to water ratio of 0.9:1 by mass. The other target product from the formaldehyde recovery zone is a stream containing 99.9% by mole water, suitable for waste treatment. This is to be achieved from two streams containing water, acetic acid and formaldehyde.

(24) TABLE-US-00001 Stream 1 Stream 2 Water 0.769 0.971 Formaldehyde (equivalent) 0.189 0.027 Acetic 0.041 0.002 Dissolved lights 0.001 —

(25) The columns are simulated in Aspen Plus 8.4, using UNIFAC group methods with the Hayden-O'Connell vapour phase correction, and apparent components using equilibrium chemistry to account for the formation of multiple formaldehyde-derived components.

COMPARATIVE EXAMPLE 1

(26) In this comparative example, a single column separation, where the column has 90 theoretical stages, a reboil duty of 20 MW, and a condenser pressure of 5 bar absolute is used. The aim is to recover a formaldehyde-rich product is recovered as an overhead stream. The residue stream is only 95.3% water by mole, the remainder comprising primarily acetic with some formaldehyde. Formaldehyde recovery of this column is 97.8%

COMPARATIVE EXAMPLE 2

(27) In this comparative example, a further single column separation, where the column has 90 theoretical stages, and a condenser pressure of 0.2 bar absolute. The column achieves a formaldehyde recovery of 99.7%. Furthermore, the water stream is 99.7% pure, with the remainder formaldehyde. The acetic content of the feed is recovered to the formaldehyde rich product, in which it can be recycled to the reactor. However, the reboil duty required to achieve this is 27 MW. The practical design of this column would be challenging, due to the low pressure drop requirements.

EXAMPLE 3

(28) The present invention overcomes the disadvantages with the prior art. In this example, two columns are used. The first column has 50 theoretical stages and stream 1 is fed to it. The first column also has a condenser pressure of 0.2 bar absolute. The second column has 40 theoretical stages and stream 2 is fed to it. The second column has a condenser pressure of 5 bar absolute. In this case, 99.7% recovery of formaldehyde is again achieved. Similarly, the acetic acid is recovered for recycle in the formaldehyde-rich stream. However, the purity of the water stream is 99.9% by mole. Furthermore, the duty of the first column is 14.3 MW and the duty of the second column is 5.4 MW, giving an overall duty of 19.7 MW, which is lower than the two comparative examples. Lastly, the condenser temperature of the second column is sufficient to provide duty to the first column, giving a net duty requirement after heat integration as low as 10 MW.

EXAMPLES 4-5

(29) The following examples demonstrate the use of water as an entrainer in the acetic acid and water separation zone, specifically the acetic acid/acrylic acid separation step.

(30) For both examples, a column of 48 theoretical stages is modelled in Aspen Plus 8.4. The condenser pressure is 0.4 bar absolute. The column removes acrylic product as a side-draw at 99% by mol purity and 97% recovery.

COMPARATIVE EXAMPLE 4

(31) The feed stream composition is as follows:

(32) TABLE-US-00002 Mol fractions Stream 3 Water — Formaldehyde 0.009 Acrylic Acid 0.572 Acetic Acid 0.415 Maleic Acid 0.004

(33) In this example, the reboil duty needed to achieve the specified separation is 2972 kJ/kg of acrylic product.

EXAMPLE 5

(34) The feed stream composition is as follows:

(35) TABLE-US-00003 Mol fractions Stream 3 Water 0.150 Formaldehyde 0.007 Acrylic Acid 0.486 Acetic Acid 0.353 Maleic Acid 0.004

(36) In this example, the reboil duty needed to achieve the specified separation is 2704 kJ/kg of feed.

(37) Thus the reboil duty required where water is allowed to slip into an intermediate product stream is substantially reduced when compared to the arrangement where it is removed.