GB1564680A

GB1564680A - Energy conversation in a butadiene recovery process

Info

Publication number: GB1564680A
Application number: GB2244278A
Authority: GB
Original assignee: Polysar Ltd
Current assignee: Polysar Ltd
Priority date: 1977-08-02
Filing date: 1978-05-25
Publication date: 1980-04-10
Also published as: DE2823901A1; TR20161A; CA1104517A; FR2399397A1; JPS5427503A; FR2399397B1

Description

(54) ENERGY CONSERVATION IN A BUTADIENE RECOVERY PROCESS (71) We, POLYSAR LIMITED, a company organized under the laws of Canada, of Sarnia, Ontario, Canada, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention is directed to a means of conserving energy, especially heat energy, in a butadiene recovery and purification process.

It is well known in the prior art that butadiene is obtainable by cracking of higher molecular weight hydrocarbons or by dehydrogenation of n-butane or b-butenes and that the butadiene so produced is mixed with other hydrocarbons, typically including other C4 hydrocarbons. The mixture containing butadiene is not amenable to simple distillation processes for the recovery of pure butadiene therefrom because of the similar vapor pressures of many of the components in the mixture.

Thus, it has proved necessary to obtain pure butadiene by methods other than simple distillation. One method that has achieved considerable commercial acceptance is the use of an extractive distillation of the mixture in the presence of particular polar solvents, which polar solvents alter the volatility of certain components of the mixture such that a distillation process may be used. Suitable polar solvents include acetonitrile, dimethyl formamide, N-methyl pyrrolidone, furfural, dimethyl acetamide, suipholane and others.

By the selection of suitable polar solvents for an extractive distillation, the mixture of butadiene and other C4 hydrocarbons may be distilled to yield a butadiene rich stream because the presence of the polar solvent causes the butanes and butenes present in the mixture to exhibit a higher volatility permitting their separation by distillation from the less volatile butadiene and acetylenes. By such an extractive distillation process, the butanes and butenes are removed as an overhead stream and the butadiene and acetylenes are removed, together with the polar solvent, as a bottoms stream. This bottoms stream is then subjected to a stripping step whereby the polar solvent is separated from the butadiene yielding a vapor rich in butadiene and containing some acetylenes as impurities.

This vapor rich in butadiene is then condensed and fed to a final butadiene finishing column where it is distilled and butadiene having a purity of at least 97 weight per cent is obtained as an overhead vapor stream. This final butadiene finishing column requires a supply of heat for its operation.

The polar solvent separated from the butadiene and acetylenes in the stripping step is subjected to purification for re-use in the extractive distillation process. This purification may comprise a number of steps including removal of residual hydrocarbons contained therein, washing with e.g. water and finally a distillation. The distillation of the polar solvent yields a product suitable for recycling to the extractive distillation stage of the process.

For operation of the final butadiene finishing column, heat is supplied by one or more reboilers attached to the bottom of the column. For the polar solvent distillation, the majority of the heat is supplied by direct addition of steam to the distillation column.

Because of the need to be able to separate all the components, as described above, and to be able to condense, where necessary, various hydrocarbon streams using ambient temperature process water rather than special refrigeration units, the temperatures and pressures within the whole process have to be carefully balanced-if the pressure in a hydrocarbon stream is too low it would be difficult to readily condense it. The whole process is fairly energy intensive and with today's energy costs any reduction of energy used in the process is highly desirable.

We have now discovered a method of conserving heat energy in a butadiene recovery and purification process wherein the heat obtained by the condensation of a hot vapor stream to a liquid stream is used to at least partly supply the heat necessary to operate a butadiene finishing column.

This invention provides an improved process for the recovery of butadiene from mixtures thereof with other C4 hydrocarbons, wherein, after separation of the butadiene from the other C4 hydrocarbons by extractive distillation in the presence of a polar solvent and subsequent separation of the polar solvent to yield a butadiene enriched stream, the butadiene enriched stream is fed to a final butadiene finishing column operated to cause the distillation therein and separation of a stream containing at least 97% butadiene-1,3, the improvement being the supply of at least a portion of the heat necessary for the operation of said final butadiene finishing column by heat exchange of the contents of said finishing column with a hot vapor stream comprising said polar solvent, said heat being essentially that obtained by the condensation of said hot vapor stream to a liquid stream.

Wherever used herein, butadiene means butadiene-1,3.

Reference is made to the accompanying drawing, which represents a schematic flow diagram for a process of the prior art and of the present invention.

In the Figure, 1 is a final butadiene finishing column, the impure butadiene being fed to it through line 2. The purified butadiene vapor is removed via line 3 and condensed (not shown). Part of the liquified butadiene is removed as high purity butadiene and the remainder is refluxed back by line 4 into column 1. The heat to operate the finishing column 1 is supplied by circulating the liquid from the bottom of the column through two heat exchangers. In the prior art, the liquid was fed by lines 6 and 6A through heat exchangers 7 and 7A, being supplied with steam through lines 8 and 8A, and the heated liquid was returned to the column by lines 9 and 9A.

The distillation tower 25 is used to purify the polar solvent used elsewhere in the process for the extractive distillation of butadiene (not shown). The impure polar solvent stream is fed to tower 25 by line 26.

Steam is supplied to the tower through line 27. The liquid bottoms of the tower are removed through line 28 and are recycled to other parts of the process. The overhead vapor stream is removed from the tower through line 23. As in the prior art, when valves 14 and 16 are closed and valve 17 is open, the vapor is passed through the heat exchanger 18 and cooled by ambient temperature water fed through line 19. The liquid stream passes from the exchanger 18 by line 20 and is then fed partly through line 22 back into the distillation tower 25 for reflux control purposes and partly through line 21 to the process for the extractive distillation of butadiene (not shown).

In the process of the present invention, heat is supplied to column 1 by circulating the liquid through line 10 to heat exchanger 11 and back to the column by line 12.

Additional heat, as required, can be supplied by one or both of heat exchangers 7 and 7A being operated with either no steam to one of 7 or 7A or reduced steam flow to one of or both of 7 or 7A. The heat to exchanger 11 is supplied by closing valve 17 and opening valves 14 and 16 so that the hot polar solvent vapor passes through line 13 to heat exchanger 11 and back through line 15 as a hot liquid. Depending on the conditions required in the process, the hot liquid stream in line 15 may or may not be further cooled in heat exchanger 18, cooling being controlled by whether or not ambient temperature water is fed through exchanger 18.

Thus, the present invention can be clearly seen as utilizing the heat available from the condensation of the hot polar solvent vapor to the liquid state. Heat available from further cooling of the liquid polar solvent is very small compared to that for the change of state from vapor to liquid.

In an example of the process of the prior art, and with reference to the Figure, approximately 18,000 pounds per hour of impure butadiene is fed by line 2 to the final butadiene finishing column 1. The butadiene stream is at a temperature of about 115 F, which temperature may be varied from about 110 to about 120"F, and contains about 85 weight per cent of butadiene, the balance being a mixture of cis- and trans - butene - 2 (approximately 14 weight per cent), butadiene-1,2 (about 1 weight per cent) and acetylenes (nearly 1 weight per cent). The butadiene content of the stream in line 2 may vary from about 85 to about 95 weight per cent, the cis- and trans - butene - 2 may vary from about 2 to about 14 weight per cent, the butadiene-1,2 may range from almost zero to about 1 weight per cent and the acetylenes may range from almost zero to about 1 weight per cent. Steam at about 15 psi is fed to the two heat exchangers 7 and 7A by lines 8 and 8A at a total rate of about 16,000 pounds per hour. The temperature of the material in lines 9 and 9A is thereby increased to about 128"F, although it may be varied from about 125 to about 130"F. A liquid bottom stream is removed from column 1 through line 5 at a rate of about 4,000 pounds per hour and comprises butadiene-1,3, cis- and trans - butene - 2, some butadiene-1,2 and some acetylenes. The overhead from the column is removed through line 3 at a rate of about 130,000 pounds per hour. This stream of purified butadiene contains at least 97, and preferably at least 98.5 weight per cent of butadiene-1,3, up to about 2, and preferably I, weight per cent of butene-2, and traces of butadiene-1,2 and acetylenes.

After condensation to the liquid state (not shown), approximately 14,000 pounds per hour of this stream is removed as pure liquid butadiene and approximately 116,000 pounds per hour is recycled throuth line 4 back to the column for purposes of reflux control.

Also in a process of the prior art and with reference to the Figure, a polar solvent stream is fed through line 26 to distillation tower 25. This stream contains about 10 weight per cent of acetonitrile and about 90 weight per cent of water, although the composition may range from about 5 to about 12 weight per cent of acetonitrile, the balance being water. The temperature of the stream in line 26 is about 160"F, although this can be varied from about 152" to about 164"F. Steam is fed into the tower through line 27 (150 psi steam, at a flow rate of about 10,000 pounds per hour). The flow from the bottom of the tower through line 28 is about 40,000 pounds per hour. The overhead vapour stream is removed through line 23. This overhead stream contains about 70 weight per cent of acetonitrile and about 30 weight per cent of water, although the composition may be varied from about 60 to about 80 weight per cent acetonitrile and from about 20 to about 40 weight per cent of water. The temperature of the vapor stream in this Example is about 185"F, although this temperature may be varied from about 180 to about 190"F. The rate of flow in line 23 is about 14,000 pounds per hour, although the flow rate may be varied from about 11,000 to about 17,000 pounds per hour. Valves 14 and 16 are closed and valve 17 is open so that the vapor stream is passed directly to the heat exchanger 18 for cooling, the cooled liquid product in line 20 being at a temperature of about 120"F, although this temperature may be varied from about 115 to about 130"F. From line 20, about 5,000 pounds per hour of liquid acetonitrile-water are removed through line 21 for supply to the extractive distillation process and about 9,000 pounds per hour are returned through line 22 to the tower, for purposes of reflux control.

When operating the process according to the present invention and with reference to the Figure, the hot vapor in line 23 is passed to the heat exchanger 11 to supply heat for the operation of column 1. This is achieved by closing valve 17 and opening valves 14 and 16, thereby causing the vapor in line 23 to pass by line 13 to exchanger 11 and back through line 15 to exchanger 18 for any further cooling necessary before the liquid stream joins the remainder of the process.

At a flow rate of about 14,000 pounds per hour of vapor at a temperature of about 185"F, condensation of the vapor in exchanger 11 to a liquid at a temperature of about 170"F is equivalent to supplying about 8,000 pounds per hour of steam. Thus, of the approximately 16,000 pounds per hour of 15 psi steam necessary to heat the contents of column 1, about 8,000 pounds per hour can be replaced by the condensation of the polar solvent vapor to liquid in exchange 11. Remaining heat necessary for the operation of tower 1 can be supplied by steam supplied to either or both or exchangers 7 and 7A, as in the prior art.

Thus, by the process according to the present invention, at least 50% of the heat energy requirements for the operation of column 1 are obtained by condensation of a hot vapor stream from the operation of tower 25. This energy saving corresponds to between 5 and 10fez of the total steam comsumption of the butadiene recovery and purification facility.

WHAT WE CLAIM IS: 1. A process for the recovery of butadiene from mixtures thereof with other C4 hydrocarbons, wherein, after separation of the butadiene from the other C4 hydrocarbons by extractive distillation in the presence of a polar solvent and subsequent separation of the polar solvent to yield a butadiene-enriched stream, the butadiene enriched stream is fed to a final butadiene finishing column operated to cause the distillation therein and separation of a stream containing at least 97% butadiene-1,3, characterized in that at least a portion of the heat necessary for the operation of said final butadiene finishing column is supplied by heat exchange of the contents of said finishing column with a hot vapor stream comprising said polar solvent, said heat being essentially that obtained by the condensation of said hot vapor stream to a liquid stream.

2. T'ne process of Claim 1 wherein the hot vapor stream comprising said polar solvent is at a temperature of 1800 to 1900 F.

3. The process of Claim 1 or 2 wherein the temperature for the operation of the final butadiene finishing column is from 125 to 130"F.

4. The process of Claims 1, 2 or 3 wherein the butadiene enriched stream contains from 85 to 95 weight per cent of butadiene.

5. The process of any one of the preceding claims wherein at least 50 /O of the heat necessary for the operation of said final

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. some acetylenes. The overhead from the column is removed through line 3 at a rate of about 130,000 pounds per hour. This stream of purified butadiene contains at least 97, and preferably at least 98.5 weight per cent of butadiene-1,3, up to about 2, and preferably I, weight per cent of butene-2, and traces of butadiene-1,2 and acetylenes. After condensation to the liquid state (not shown), approximately 14,000 pounds per hour of this stream is removed as pure liquid butadiene and approximately 116,000 pounds per hour is recycled throuth line 4 back to the column for purposes of reflux control. Also in a process of the prior art and with reference to the Figure, a polar solvent stream is fed through line 26 to distillation tower 25. This stream contains about 10 weight per cent of acetonitrile and about 90 weight per cent of water, although the composition may range from about 5 to about 12 weight per cent of acetonitrile, the balance being water. The temperature of the stream in line 26 is about 160"F, although this can be varied from about 152" to about 164"F. Steam is fed into the tower through line 27 (150 psi steam, at a flow rate of about 10,000 pounds per hour). The flow from the bottom of the tower through line 28 is about 40,000 pounds per hour. The overhead vapour stream is removed through line 23. This overhead stream contains about 70 weight per cent of acetonitrile and about 30 weight per cent of water, although the composition may be varied from about 60 to about 80 weight per cent acetonitrile and from about 20 to about 40 weight per cent of water. The temperature of the vapor stream in this Example is about 185"F, although this temperature may be varied from about 180 to about 190"F. The rate of flow in line 23 is about 14,000 pounds per hour, although the flow rate may be varied from about 11,000 to about 17,000 pounds per hour. Valves 14 and 16 are closed and valve 17 is open so that the vapor stream is passed directly to the heat exchanger 18 for cooling, the cooled liquid product in line 20 being at a temperature of about 120"F, although this temperature may be varied from about 115 to about 130"F. From line 20, about 5,000 pounds per hour of liquid acetonitrile-water are removed through line 21 for supply to the extractive distillation process and about 9,000 pounds per hour are returned through line 22 to the tower, for purposes of reflux control. When operating the process according to the present invention and with reference to the Figure, the hot vapor in line 23 is passed to the heat exchanger 11 to supply heat for the operation of column 1. This is achieved by closing valve 17 and opening valves 14 and 16, thereby causing the vapor in line 23 to pass by line 13 to exchanger 11 and back through line 15 to exchanger 18 for any further cooling necessary before the liquid stream joins the remainder of the process. At a flow rate of about 14,000 pounds per hour of vapor at a temperature of about 185"F, condensation of the vapor in exchanger 11 to a liquid at a temperature of about 170"F is equivalent to supplying about 8,000 pounds per hour of steam. Thus, of the approximately 16,000 pounds per hour of 15 psi steam necessary to heat the contents of column 1, about 8,000 pounds per hour can be replaced by the condensation of the polar solvent vapor to liquid in exchange 11. Remaining heat necessary for the operation of tower 1 can be supplied by steam supplied to either or both or exchangers 7 and 7A, as in the prior art. Thus, by the process according to the present invention, at least 50% of the heat energy requirements for the operation of column 1 are obtained by condensation of a hot vapor stream from the operation of tower 25. This energy saving corresponds to between 5 and 10fez of the total steam comsumption of the butadiene recovery and purification facility. WHAT WE CLAIM IS:

1. A process for the recovery of butadiene from mixtures thereof with other C4 hydrocarbons, wherein, after separation of the butadiene from the other C4 hydrocarbons by extractive distillation in the presence of a polar solvent and subsequent separation of the polar solvent to yield a butadiene-enriched stream, the butadiene enriched stream is fed to a final butadiene finishing column operated to cause the distillation therein and separation of a stream containing at least 97% butadiene-1,3, characterized in that at least a portion of the heat necessary for the operation of said final butadiene finishing column is supplied by heat exchange of the contents of said finishing column with a hot vapor stream comprising said polar solvent, said heat being essentially that obtained by the condensation of said hot vapor stream to a liquid stream.

butadiene finishing column is supplied by the condensation of said hot vapor stream to a liquid stream.

6. The process of any one of the preceding claims wherein said hot vapor stream contains from 60 to 80 weight per cent of acetonitrile and from 40 to 20 weight per cent of water.

7. The process of Claim 1, substantially as hereinbefore described with reference to the accompanying drawing.

8. Butadiene whenever recovered by a process in accordance with any one of Claims I to 7.