WO2000017134A1

WO2000017134A1 - Separation of hydrocarbons using adsorption technics

Info

Publication number: WO2000017134A1
Application number: PCT/GB1999/002927
Authority: WO
Inventors: Peter John Herbert Carnell; Mark Sean Doran; Peter Edward James Abbott
Original assignee: Imperial Chemical Industries Plc
Priority date: 1998-09-24
Filing date: 1999-09-03
Publication date: 2000-03-30

Abstract

Recovery of higher hydrocarbons, e.g. aromatics, from a hydrogen-containing off-gas comprising passing the off-gas through a bed of a higher hydrocarbons adsorbent, periodically regenerating that bed by passing a stream of regeneration gas at a higher temperature through said bed, cooling said regeneration gas to condense the higher hydrocarbons and separating the condensed higher hydrocarbons.

Description

SEPARATION OF HYDROCARBONS USING ADSORPTION TECHNICS

This invention relates to hydrocarbons and in particular to the production of higher hydrocarbons, by which term we mean aromatic and non-aromatic hydrocarbons containing 5 or more carbon atoms. Aromatic hydrocarbons are often prepared by reforming a predominantly non- aromatic hydrocarbon feedstock, such as naphtha, at an elevated temperature. In the ~ catalytic reforming process various reactions occur, depending on the feedstock and conditions and catalysts, if any, employed, and often involve dehydrogenation, cyclisation and aromatisation reactions. These reactions generate hydrogen as a by-product. The reaction products are generally separated from the hydrogen and light gases by cooling and separation of the liquid and gaseous phases. However in many processes, the separated hydrogen-containing phase may contain some residual higher hydrocarbons. In some processes part of the hydrogen is recycled, generally to minimise coke formation, while the remainder is discharged as an off-gas, which is used as fuel or for other purposes. These higher hydrocarbons which remain in the hydrogen stream represent several disadvantages. Firstly they represent a loss of valuable gasoline-grade product. Secondly they reduce the purity of the product hydrogen stream. Finally they are often more difficult to treat downstream (for instance in steam reforming). Hydrogen off-gases containing small amounts of such higher hydrocarbons are also produced in other hydrocarbon processing operations, e.g. hydro-treating.

In the present invention higher hydrocarbons in such hydrogen off-gases are recovered.

Accordingly the present invention provides a process for the recovery of higher hydrocarbons from a hydrogen-containing off-gas comprising passing the off-gas, at a first, adsorption, temperature, through a bed of a regenerable higher hydrocarbon adsorbent whereby residual higher hydrocarbons are adsorbed from said off-gas, and periodically regenerating that bed by passing a stream of regeneration gas at a regeneration temperature greater than said adsorption temperature through said bed, whereby said adsorbed higher hydrocarbons are desorbed into said regeneration gas stream, and thereafter cooling said regeneration gas to condense the higher hydrocarbons and separating the condensed higher hydrocarbons from the regeneration gas.

The process is preferably operated cyclically using two or more beds of hydrocarbon adsorbent with at least one bed on adsorption duty at any one time. In such an arrangement the regeneration gas is preferably part of the off-gas that has passed through one of beds on adsorption duty. Thus a part stream of the off-gas that has had the residual higher hydrocarbons adsorbed by a bed on adsorption duty is heated and passed, preferably in a direction counter-current to the flow of gas when on adsorption duty, through the bed undergoing regeneration. The regeneration gas is then cooled and the higher hydrocarbons condensed and separated. The regeneration gas can then be recombined with the off-gas leaving the bed or beds on adsorption duty.

Regenerable hydrocarbon adsorbents that may be employed include silica gels and molecular sieve materials, e.g. zeolites, including silicalites.

The adsorption stage is conveniently effected at a temperature between 10°C and - 50°C, generally at between 30°C and 45°C. Regeneration is preferably effected at a temperature at least 100°C, particularly at least 150°C, above the temperature employed in the adsorption step. Where the off-gas fed to the bed on adsorption duty also contains some water vapour, this will also be adsorbed by the bed unless the adsorbent is hydrophobic, e.g. a silicalite. Where water is adsorbed by the regenerable hydrocarbon adsorbent, the desorption temperature should be sufficient to effect desorption of adsorbed water, e.g. by heating to a temperature in the range 200-300°C.

Regeneration is preferably effected by heating the regeneration gas to a suitable temperature and passing the heated gas through the bed undergoing regeneration. The heating of the regeneration gas is then stopped, before the whole of the bed has been fully heated, while continuing to pass the regeneration gas through the bed. In this way a pulse of heated gas may be passed through the bed to effect desorption and the bed is then cooled, by the continuing flow of regeneration gas, back to the temperature required for adsorption duty.

The amount of gas required to effect desorption of the higher hydrocarbons from the bed is only a fraction, e.g. 10 to 30%, of the off-gas that passes through the bed during the adsorption stage. Hence the regeneration gas contains the desorbed higher hydrocarbons in a much greater concentration than in the gas undergoing adsorption. Thus in order to recover the desorbed higher hydrocarbons, not only is the amount of gas that has to be cooled to effect condensation of the higher hydrocarbons relatively small, but also the temperature to which the regeneration gas need be cooled in order to recover the desorbed higher hydrocarbons is significantly greater than that which would be required to recover the same amount of higher hydrocarbons from the unreacted gas in the absence of the adsorption/desorption process. Hence the cooling requirements are significantly decreased. The hydrogen off-gas often contains a small amount of hydrogen chloride, especially where the off-has results from the catalytic reforming of hydrocarbons since organic chlorine compounds are often incorporated into the catalytic reformer feed in order to create acidic sites on the reforming catalyst. The presence of hydrogen chloride in the hydrogen off-gas is often undesirable. Thus where the hydrogen-off-gas is used as a source of hydrogen for a catalytic process, e.g. hydrogenation, the presence of chlorides may poison the catalysts used in that process. Also it may lead to corrosion problems and/or to the deposition of salts.

It is therefore often desirable to subject the hydrogen off-gas to a chloride absorption stage. This is conventionally effected by passage of the hydrogen off-gas through a bed of an absorbent for hydrogen chloride, such as a basic absorbent, for example an alkali-containing alumina composition. Examples of such materials are - described in US 3943226, EP 0746409 and WO 99/39819. It has been found that when using such chloride absorbents, the effectiveness of the absorbent is significantly decreased if the hydrogen off-gas contains small amounts of hydrocarbons. Thus entrained liquid hydrocarbons or a higher hydrocarbons content such that condensation thereof is liable to occur on the absorbent particles or in the pores thereof can significantly decrease the effectiveness of the chloride absorbent. While the hydrogen off-gas is often passed through a de-mister prior to passage through the absorbent, such de-misters are only partially effective in removing entrained liquid hydrocarbons. Even if the hydrogen off-gas at source is free from entrained liquid hydrocarbons, condensation to give significant amounts of entrained liquid hydrocarbons may readily occur if there is any significant heat loss from the pipeline transporting the hydrogen-off gas: often it is desirable to deploy the chloride absorbent a significant distance from the hydrogen off-gas source and lagging and/or steam heating the pipeline is not economic. Furthermore, even if the conditions are such that bulk condensation of the hydrocarbons is unlikely, there is a risk of capillary condensation in the pores of the absorbent. In addition there is a risk that the effectiveness of the absorbent as a chloride absorbent can be decreased by adsorption of hydrocarbons by the chloride absorbent even if fouling by entrained liquid hydrocarbons or hydrocarbon condensation is avoided. Since the chloride absorbent may also adsorb higher hydrocarbons which can be desorbed by means of a heated regeneration gas, in some cases the chloride absorbent may also serve as the regenerable hydrocarbon adsorbent. However in a preferred form of the invention, a regenerable hydrocarbon adsorbent, which is preferably not a chloride absorbent, is employed upstream of a chloride absorbent as the risk of condensation of higher hydrocarbons on, or in the pores of, the chloride absorbent can be significantly decreased. Indeed this reason alone may provide sufficient incentive to adopt the present invention: the other benefits such as recovery of the higher hydrocarbons may be an ancillary benefit.

Thus by employing the process of the invention, using a regenerable hydrocarbon adsorbent that is preferably not an absorbent for hydrogen chloride, upstream of a chloride absorbent, the higher hydrocarbons content can be decreased to a level such that the risk of condensation of higher hydrocarbons on, or in, the chloride absorbent can be minimised. In a preferred arrangement, the chloride absorption is integrated with the process of the present invention by providing each bed of regenerable hydrocarbon adsorbent with a bed of chloride absorbent downstream of the regenerable hydrocarbon absorbent and using part of the hydrogen off-gas that has passed through a chloride absorbent bed as the regeneration gas. In a preferred arrangement, the regeneration gas is taken from the hydrogen off-gas stream that has passed through the regenerable hydrocarbon adsorbent - bed, but not through the chloride absorbent bed associated with that hydrocarbon adsorbent bed, the regeneration gas is heated and then passed through the chloride absorbent bed associated with the hydrocarbon adsorbent bed undergoing regeneration and then through the hydrocarbon adsorbent bed to be regenerated. Since the regeneration gas is heated, the chloride therein is more readily absorbed by the chloride absorbent: this has the effect of increasing the chloride absorption capacity of the chloride absorbent. Also any hydrocarbon that has not been adsorbed by the hydrocarbon adsorbent bed but has been deposited on, or in the pores of, the chloride absorbent will be removed from the chloride absorbent bed during the regeneration process.

The invention is illustrated by the accompanying drawings in which Figure 1 is a diagrammatic flow sheet of a process in accordance with the invention, and in Figure 2 part of Figure 1 is shown to illustrate a modified arrangement. Figure 3 is a diagrammatic flow sheet of a process employing chloride absorbent beds downstream of a regenerable hydrocarbon adsorbent.

Referring to Figure 1 , hydrogen off-gas, containing residual aromatic hydrocarbons, from a gasoline reforming process is fed, at a temperature typically in the range 25°C to 45°C, via line 10, valve 11 and line 12, to a bed 13 of an adsorbent, e.g. silica gel, capable of adsorbing higher hydrocarbons. The higher hydrocarbons are adsorbed and the treated off-gas leaves the bed via line 14, valve 15 and line 16.

During part of the time that bed 13 is on adsorption duty, a second bed 17 that has previously been on adsorption duty undergoes regeneration. Regeneration is effected by taking a part stream of the treated off-gas from line 16 via line 18, heating the part stream in a heat exchanger or fired heater 19 and passing the heated part stream, via line 20, valve 21 and line 22, counter-currently through the bed 17. The adsorbed higher hydrocarbons are desorbed and the part stream carrying the desorbed higher hydrocarbons leaves bed 17 via line 23 and is fed, via valve 24 and line 25, to a heat exchanger 26 wherein the part stream is cooled. The cooled part stream is then fed to a separator 27 wherein the condensed higher hydrocarbons are separated as stream 28. The cooled part stream from which the condensed higher hydrocarbons have been separated leaves separator 27 via line 29 and is added to the remainder of the treated off-gas from line 16 to form an effluent off-gas stream 30. A valve 31 is provided in line 16 after line 18, so that when partially closed, sufficient pressure is provided to force the regeneration gas to flow via line 18 through bed 17.

The part stream taken via iine 18 is only heated for part of the time that the bed 17 is undergoing regeneration so that a "pulse" of heat passes through the bed 17. Thus, after the heating has stopped, the continued passage of the regeneration gas through bed 17 effects cooling of the heated bed to return the bed to readiness for adsorption duty. -

After bed 17 has been regenerated and cooled, valves 21 and 24 are closed. When it is desired to bring regenerated bed 17 on to adsorption duty, valves 32 and 33 are opened so that the off-gas to be treated flows from line 10, via valve 32 and line 23 into bed 17 and the treated off-gas leaves bed 17 via line 22, valve 33 and line 16. At this stage both beds 13 and 17 are on adsorption duty. Bed 13 is then taken off adsorption duty by closing valves 11 and 15 and regeneration effected by opening valves 34 and 35, so that the regeneration gas can flow from line 20, via valve 34 and line 14, into bed 13 and the regeneration gas containing the desorbed aromatic compounds leaves bed 13 via line 12, valve 35 and line 25.

Where the off-gas also contains water, since the latter will tend to be more readily adsorbed by the adsorbent than the higher hydrocarbons, the water will be adsorbed at the inlet end of the bed, and little, if any, water will be adsorbed in the lower portions of the bed. To desorb the water, higher temperature regeneration gas is generally necessary. Hence it is seen that the lower part of the bed need not be heated to such an extent as the upper part of the bed. Thus, during regeneration, that end of the bed that is the inlet end when on adsorption duty is heated to a higher temperature than the outlet end. In order to minimise the heat requirement for regeneration the arrangement of Figure 2 may be employed.

In Figure 2 only the bed undergoing regeneration is shown. In this modification the adsorbent is split into upper and lower beds 17a and 17b. A sparger line 36 is provided to introduce regeneration gas to between the beds 17a and 17b. During regeneration a valve 37 in iine 22 is initially open, and a valve 38 in the sparge line 36 is initially closed so that the regeneration gas flows from line 22 through the lower and upper beds and leaves the upper bed 17b through line 23. This serves to effect desorption of the higher hydrocarbons from the lower bed 17a, while the water largely remains adsorbed in the upper bed 17b. Thereafter the regeneration gas is heated more strongly and valve 38 opened and valve 37 closed. The hotter regeneration gas thus passes from valve 21 , via line 39, valve 38 and sparge line 36, only through the upper bed 17b to effect desorption of water therefrom. After passage of the hot regeneration gas through the upper bed 17b, heating of the regeneration gas is stopped, valve 37 is opened and valve 38 closed. The cooling regeneration gas thus passes through both beds to effect cooling thereof. It will be appreciated that the hot regeneration gas supplied to the sparge line 36 may be heated separately from that fed to the lower bed 17a via line 22 and/or may be further heated by means of a heat exchanger (not shown) interposed in line 38. Also it will be appreciated that it is not necessary that the beds 17a and 17b be separated by an adsorbent-free zone. Thus a continuous bed of adsorbent may be employed with means to inject the regeneration gas from the sparge line 36 into the adsorbent bed at a suitable - location. Where separate beds are employed, the adsorbent of the upper and lower beds may be of dissimilar materials. In this arrangement, it may be desirable to subject both the upper and lower beds to the high temperature regeneration periodically, e.g. every 10 adsorption/desorption cycles, to desorb any water that has been adsorbed in the lower bed. As an example of the invention, the hydrogen off-gas from a gasoline reforming plant typically containing 1.3% by volume of higher hydrocarbons (0.5% pentanes, 0.3% hexanes, 0.3% benzene and 0.2% toluene) is fed at a pressure of 20 bar abs. and at 40°C through a suitable adsorbent wherein essentially all these hydrocarbons are adsorbed. If the amount of regeneration gas required to effect desorption of the bed is 20% of the volume of gas treated in the adsorption stage, assuming the desorption is effected with gas at the same pressure as employed in the adsorption step, the regeneration gas leaving the bed will have an average higher hydrocarbons content of about 6.5%. Cooling this regeneration gas to 40°C effects condensation of 2.6 moles of higher hydrocarbons per 100 moles of regeneration gas employed. Thus about 80% of the higher hydrocarbons in the off-gas can be recovered.

Where the hydrogen containing off-gas also contains water and the arrangement of Figure 2 is employed, it will be appreciated that the regeneration gas leaving the beds during the initial stages of regeneration will contain the bulk of the higher hydrocarbons and little water, while in the subsequent stage when the hotter regeneration gas is employed, the regeneration gas leaving the beds will contain water but little higher hydrocarbons. Consequently the composition of the liquid separated as the stream 28 may vary with time and so it may be desirable to recover, e.g. collect and add to the higher hydrocarbons separated in the reforming plant, stream 28 only during the period when little water is being desorbed.

In the embodiment of Figure 3, the vessels containing the regenerable hydrocarbon adsorbent (which is not a chloride absorbent) are of the type shown in Figure 2 but with beds 40 and 41 of a chloride absorbent disposed above, i.e. downstream when the beds are on adsorption duty, the beds 13 and 17 respectively of regenerable hydrocarbon adsorbent. In this embodiment, while bed 13 is on adsorption duty, part, e.g. about 20% of the gas that has passed through bed 13, is taken via sparge line 42 and valve 43 and fed, via line 44, as the regeneration gas to heater 19. The remainder of the gas that has passed through bed 13 passes through bed 40 where chloride is absorbed and then passes, via line 14 and valve 15, to line 16.

After heating in heater 19, the heated regeneration gas then flows, via line 20, valve 21 and line 22, into the vessel undergoing regeneration and containing beds 41 and 17. The chloride in the regeneration gas is absorbed by bed 41 and at the same time any hydrocarbon deposited on, or in the pores of, the absorbent of bed 41 is desorbed - together with the hydrocarbons desorbed from bed 17. The regeneration gas then passes, via line 23, valve 24 and line 25, to cooler 26 and thence to separator 27 wherein condensed hydrocarbons are separated and recovered via line 28. The cooled regeneration gas from which the hydrocarbons have been separated is then passed via line 29 and united with the hydrogen off-gas in line 16 to form a product stream 30 from which both chloride has been absorbed and higher hydrocarbons recovered.

When it is desired to regenerate bed 13, valves 11 , 43, 21 , and 24 are closed and valves 32, 45 (in the sparge line 46 associated with the vessel containing adsorbent bed 17), 33, 34, and 35 are opened, so that beds 17 and 41 are on adsorption/absorption duty and part of the hydrogen off-gas that has passed through bed 17 is used as the regeneration gas.

As in the embodiment of Figure 1 , the heating during regeneration need not be continuous.

Claims

Claims.

1. A process for the recovery of higher hydrocarbons from a hydrogen-containing off-gas comprising passing the off-gas, at a first, adsorption, temperature, through a bed of a regenerable higher hydrocarbon adsorbent whereby residual higher hydrocarbons are adsorbed from said off-gas, and periodically regenerating that bed by passing a stream of regeneration gas at a regeneration temperature greater than said adsorption temperature through said bed, whereby said adsorbed higher hydrocarbons are desorbed into said regeneration gas stream, and thereafter cooling said regeneration gas to condense the higher hydrocarbons and separating the condensed higher hydrocarbons from the regeneration gas.

2. A process according to claim 1 wherein during regeneration of the bed, the regeneration gas passes through the bed in a direction counter to the flow of hydrogen off-gas through that bed when that bed is on adsorption duty.

3. A process according to claim 1 or claim 2 operated cyclically using two or more beds of hydrocarbon adsorbent with at least one bed on adsorption duty at any one time.

4. A process according to claim 3 wherein the regeneration gas is part of the off-gas that has passed through one of beds on adsorption duty.

5. A process according to any one of claims 1 to 4 wherein the regeneration gas is heated to said regeneration temperature greater than said adsorption temperature for only part of the period when the bed is undergoing regeneration

6. A process according to claim 5 wherein the heating of the regeneration gas is stopped, before the whole of the bed has been fully heated, and passage of the regeneration gas through the bed is continued whereby a pulse of heated gas passes through the bed to effect the desorption and then cooling of the bed is effected by the continued passage of the regeneration gas.

7. A process according to any one of claims 1 to 6 wherein each portion of the bed undergoing regeneration is heated to a temperature at least 100°C above the adsorption temperature.

8. A process according to any one of claims 1 to 7 wherein, during regeneration, that end of the bed that is the inlet end when on adsorption duty is heated to a higher temperature than the outlet end.

9. A process according to any one of claims 1 to 8 wherein the regenerable hydrocarbon adsorbent is also capable of absorbing hydrogen chloride. -

10. A process according to any one of claims 1 to 8 wherein at least one bed of a chloride absorbent is disposed downstream of the regenerable hydrocarbon adsorbent bed.

11. A process according to claim 10 wherein each bed of regenerable hydrocarbon adsorbent is provided with a bed of chloride absorbent downstream of the regenerable hydrocarbon absorbent bed and the regeneration gas comprises part of the hydrogen off-gas that has passed through a chloride absorbent bed.

12. A process according to claim 11 wherein the regeneration gas is taken from the hydrogen off-gas stream that has passed through the regenerable hydrocarbon adsorbent bed, but not through the chloride absorbent bed associated with that hydrocarbon adsorbent bed, the regeneration gas is heated and then passed through the chloride absorbent bed associated with the hydrocarbon adsorbent bed undergoing regeneration and then through the hydrocarbon adsorbent bed to be regenerated.

AMENDED CLAIMS

[received by the International Bureau on 8 February 2000 (08.02.00); original claim 10 cancelled; original claims 1, 9 and 11 amended; remaining claims unchanged (2 pages)]

1. A process for the recovery of higher hydrocarbons and removal of chloride from a hydrogen-containing off-gas comprising passing the off-gas, at a first, adsorption, temperature, through a bed of a regenerable higher hydrocarbon adsorbent whereby residual higher hydrocarbons are adsorbed from said off-gas, and then through a bed of a chloride absorbent whereby chloride is absorbed from said off-gas, periodically regenerating the higher hydrocarbon adsorbent bed by passing a stream of regeneration gas at a regeneration temperature greater than said adsorption temperature through the higher hydrocarbon adsorbent bed, whereby said adsorbed higher hydrocarbons are desorbed into said regeneration gas stream, and thereafter cooling said regeneration gas to condense the higher hydrocarbons and separating the condensed higher hydrocarbons from the regeneration gas.

5. A process according to any one of claims 1 to 4 wherein the regeneration gas is heated to said regeneration temperature greater than said adsorption temperature for only part of the period when the bed is undergoing regeneration.

9. A process according to any one of claims 1 to 8 wherein the regenerable hydrocarbon adsorbent is not a chloride absorbent.

11. A process according to any one of claims 1 to 9 wherein each bed of regenerable hydrocarbon adsorbent is provided with a bed of chloride absorbent downstream of the regenerable hydrocarbon absorbent bed and the regeneration gas comprises part of the hydrogen off-gas that has passed through a chloride absorbent bed.