WO1999056856A1

WO1999056856A1 - Process for treating a fluid using two beds containing dissimilar particulate material

Info

Publication number: WO1999056856A1
Application number: PCT/GB1999/001343
Authority: WO
Inventors: Peter Edward James Abbott; Peter John Herbert Carnell
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1998-05-01
Filing date: 1999-04-29
Publication date: 1999-11-11
Anticipated expiration: 2000-11-01
Also published as: AU3621499A

Abstract

A fluid is treated, e.g. purified, by passing the fluid successively through a first bed (13) of a first particulate material and a second bed (14) of a second, dissimilar, particulate material. The beds are disposed within a single vessel provided with primary ports (16, 12) for the inlet and outlet of the fluid stream and with a secondary port (15, 23) connecting with a region between said first and second beds. Periodically a dissimilar fluid stream, or the fluid stream at a different temperature, is passed through one of said beds by passage from one of the primary ports to the secondary port or vice versa, e.g. to effect regeneration of that one bed.

Description

1

PROCESS FOR TREAΗNG A FLUID USING TWO BEDS CONTAINING DISSIMILAR PARTICULATE MATERIAL

This invention relates to a fluid processing and in particular to a process wherein a process fluid is contacted successively with beds of dissimilar particulate material, e g catalysts or sorbents In the processing of fluid streams it is often desirable to subject the process fluid stream to a 5 plurality of processing steps in which the stream is successively contacted with beds particulate materials For example it may be desirable to contact the process fluid with a particulate sorbent material, such as a molecular sieve, to adsorb impurity components from the process fluid stream, and then to contact the so-treated process fluid stream with a different particulate material which performs a similar or different function

10 In many cases periodically it is desirable to treat one of the beds with a different fluid, or with the fluid under different conditions of e g temperature Contact of the material in the other bed with the dissimilar fluid and/or under the dissimilar conditions may be undesirable For example, after adsorbing impurities from a process fluid stream using a bed of a molecular sieve material, it may be desirable to regenerate the molecular sieve by subjecting the latter to an increased temperature

15 and/or by passing a different fluid stream through the molecular sieve Subjecting another bed to such increased temperature and/or contact with the different fluid might have an adverse effect upon the material of that other bed In order to avoid such problems the beds are often disposed in separate vessels

Particularly when operating on a large scale at high pressures, particularly at above about

20 50 bar abs , the cost of individual vessels is very high In the present invention the dissimilar beds are located within the same vessel and so capital cost savings may be achieved

Accordingly the present invention provides a process for the treatment of a process fluid comprising passing the process fluid successively through a first bed of a first particulate material and a second bed of a second, dissimilar, particulate material, wherein said beds are disposed

25 within a single vessel provided with primary ports for the inlet and outlet of tne uid stream and with a secondary port connecting with a region between said first and second beαs id periodically passing a dissimilar fluid stream, or a fluid stream at a different temperature, through one of said beds by passage from one of said primary ports to said secondary port or vice versa

The vessel thus has a primary port associated with, and forming an inlet to, the first bed, a

30 secondary port communicating with a region between the beds, and a second primary port associated with, and forming an outlet from, the second bed During normal operation of the process, the process fluid enters the vessel through the primary port associated with the first bed, passes through the beds and then leaves the vessel via the primary port associated with the second bed During the periodic treatment, the flow of fluid through one of the primary ports is stopped, e g

35 by means of a suitable isolating valve, and the fluid employed in the periodic treatment enters the vessel through the primary port associated with the bed requiring treatment and leaves the vessel via the secondary port Alternatively the fluid employed in the periodic treatment enters the vessel 2

through the secondary port and leaves the vessel via the primary port associated with the bed requiring treatment

One application of the invention is the treatment of a gas or liquid, particularly a hydrocarbon, process stream at elevated pressure by passage through a particulate bed of a molecular sieve material to adsorb a contaminant, such as water or sulphur compounds, from the process fluid stream, and then through a different particulate sorbent to remove a different contaminant or to remove a further amount of said contaminant, e g as a polishing step For example a sorbent containing copper and/or zinc compounds, often in association with alumina, or elemental copper in association with other compounds such as zinc compounds and/or alumina, may be employed to remove residual sulphur compounds, or a sorbent such as copper sulphide may be employed to remove contaminants such as mercury and/or arsenic compounds such as arsines Periodically it is necessary to regenerate the molecular sieve This may be effected by passing a heated fluid through the molecular sieve to desorb the adsorbed components therefrom Heating of the different particulate sorbent may be undesirable In the present invention the regeneration fluid is passed through the molecular sieve bed, but not through the sorbent bed, by feeding the regeneration fluid to the primary port forming the inlet to the molecular sieve bed and discharging the regeneration fluid through the secondary port communicating with a region between the molecular sieve and sorbent beds Alternatively the regeneration fluid may be fed to the secondary port and passes through the molecular sieve bed and leaves the latter through the primary port associated with the molecular sieve bed During the periodic regeneration, passage of fluid through the sorbent bed is prevented by closing an isolating valve in the process fluid line from the primary port associated with the sorbent bed

Depending on the treatments involved, the bed requiring the periodic treatment may be the first or the second bed Where the periodic treatment involves the use of a treating fluid at an elevated temperature compared to the temperature prevailing during normal operation, the bed that is periodically treated is preferably disposed above the other bed so that heating of the latter through convection is minimised

The present invention allows different fluids to be used for regeneration Thus a liquid, such as dry methanol, could be used to regenerate one bed, e g a molecular sieve adsorbent, and a different liquid, e g hexane could be used to remove waxes from the adsorbent of another bed Similarly the beds could be regenerated with a different stripping gases such as hydrogen and nitrogen which could give process advantages in some cases

The invention is illustrated by reference to the accompanying drawings in which Figure 1 is a diagrammatic cross section of a vessel containing two beds, Figure 2 is a section alonfg the line l-l of Figure 1 ,

Figure 3 is a view similar to Figure 1 showing an alternative embodiment, Figure 4 is a diagrammatic flow sheet of the process of the invention 3

In Figures 1 and 2 there is shown a preferred form of vessel for containing two beds The vessel has an outer shell 10 and is provided with two primary ports, an inlet port 11 at the upper end and an outlet port 12 at the lower end The first bed 13 is disposed in the upper part of the vessel and the second bed 14 is disposed in the lower part of the vessel Disposed across the interior of the shell 10 and passing out through the shell is a hollow header 15 forming a secondary port

Extending laterally from header 15 are a plurality of pipes 16 These pipes are closed at their outer ends but at their inner ends communicate with the interior of header 15 Pipes 16 have a plurality of perforations (not shown in Figures 1 or 2) therethrough Surrounding each lateral pipe 16 is a mesh cage 17 In use, the vessel is charged with the particulate materials through a manhole 18 at the upper end of the shell 10 The particulate material below header 15 and lateral pipes 16 forms the second bed 14 while the particulate material above header 15 and pipes 16 forms the first bed 13 The mesh cages 17 serve to prevent the particles from blocking the perforations in pipes 16 A manhole 19 is provided to permit the particulate materials forming the beds to be discharged A perforate grid 20 is provided adjacent the outlet port 12 to prevent the particulate material from entering the outlet port 12

If desired, a particle restraining grid can be disposed above header 15 to maintain the first and second beds separate In that case a free space may be provided below the particle restraining grid and the particles in the lower, second, bed 14 In this case the header 15 and its associated lateral pipes 16 and cages 17 may be replaced simply be a port through the wall of vessel 10

In another embodiment, as shown in Figure 3, a particle restraining grid 21 is provided to define the lower end of the upper bed 13 A partition plate 22 is provided below this grid 21 and the secondary port 15 is simply a port extending through the shell 10 A further secondary port 23 is provided through the shell 10 communicating with the space below partiction plate 22 Secondary ports 15 and 23 are connected external to the shell 10 by a conduit 24 provided with an isolating valve 25 Further conduits 26, 27, provided with isolating valves 28, 29, communicate with secondary ports 15, 23 respectively

In normal operation, isolating valves 28 and 29 are closed and valve 25 is open so that the process fluid passes from inlet 11 , through bed 13 out of the shell 10 through secondary port 15, through connecting conduit 24, back into the vessel through secondary port 23, and thence through bed 14 to outlet port 12 When it is desired to give the relevant bed its periodic treatment, the isolating valve 25 is closed if bed 13 is to be treated, the appropriate treatment fluid is fed via conduit 26 into secondary port 15, through bed 13, and out through primary port 11 Alternatively the treatment fluid is fed via primary port 11 , through the bed 13, and then out of the shell via secondary port 15 Bed 14 can be treated analogously thus treatment fluid may be fed via conduit 27 into secondary port 23, through bed 14, and out through primary port 12 Alternatively 4

the treatment fluid is fed via primary port 12, through the bed 14, and then out of the shell via secondary port 23

With the arrangement of Figure 3, contact of one bed with the fluid used for the periodic treratment of the other bed can be avoided Also it is possible to subject each bed to separate periodic treatments if desired

In Figure 4 a diagrammatic flowsheet is shown employing two vessels, each having two beds of particulate material with one vessel "on-line" and one bed of the other vessel undergoing periodic treatment

Thus the fluid undergoing processing is fed from a supply line 30 through an open valve 31a to the inlet port 11 a of the first vessel The fluid flows through the beds 13a, 14a in the first vessel and then through the outlet port 12a via an open valve 32a to a product collection line 33 The second vessel is provided with corresponding valves 31b and 32b which are closed during the processing of the fluid in the first vessel Meanwhile bed 13b of the second vesel is undergoing the periodic treatment by feeding the treatment fluid from a supply line 34 through an open valve 35a into the second vessel via its secondary port 15b The treatment fluid passes up through bed 13b of the second vessel and thence out of the second vessel through port 11 b and to a spent treatment fluid collection line 36 via an open valve 37b The first vessel is provided with corresponding valves 35a and 37a which are closed during the processing of the fluid from supply 30 When the periodic treatment of bed 13b has been completed, valves 35b and 37b are closed, thereby isolating the second vessel When it is desired to subject the bed 13a of the first vessel to the periodic treatment, the second vessel is brought on line by opening valves 31 b and 32b and the first vessel is taken off line by closing valves 31 a and 32a The bed 13a can then be given the periodic treatment by opening valves 35a and 37a

It will be appreciated that in some cases, during normal operation only part of the process fluid need pass through the second bed Thus all of the process fluid passes through the first bed, part is discharged from the vessel through the secondary port, and then the remainder of the process fluid passes through the second bed Such an arrangement is desirable where it is desired to reduce the content of one contaminant in the process fluid to a very low level but a very low level of a second contaminant is not necessary For example it may be desirable to remove essentially all of the mercury in a natural gas stream, but it is only necessary to decrease the sulphur content to a given level Passage of all of the process fluid through a first bed comprising a mercury absorbent will be effective to decrease the mercury level to the desired level and passage of part of the thus purified process fluid through a second bed comprising a sulphur absorbent is effective to remove essentially all of the sulphur compounds from the part stream passing through the second bed By combining the part stream that has passed through the second bed with the remainder of the process fluid that has left the vessel through the secondary port, a product stream having the desired content of sulphur compounds may be achieved Another example is where the first bed is 5

a bed of a molecular sieve employed to effect drying and the second bed is a sorbent for sulphur compounds a dry process fluid having a sulphur content meeting a desired specification can thus be achieved

In the embodiments of Figures 1 to 4, the beds are shown as axial flow beds However it will be appreciated that in some cases, particularly where it is desired that the fluid only undergoes a small pressure drop during passage through the bed, it may be desirable to employ a radial flow configuration for one or both beds In particular, it may be desirable to employ a radial flow configuration for the first bed in the aforementioned instance where only part of the process fluid passes through the second bed The invention is of particular utility in the following applications a) Drying a hydrocarbon fluid, such as natural gas, particularly in the dense phase, typically at a pressure above 100 bar abs , by passage through a bed of a particulate molecular sieve, and then passing the dried fluid through a bed of a particulate absorbent for mercury, especially copper sulphide The periodic treatment involves regeneration of the molecular sieve by passing therethrough a stream of a heated fluid, particularly a hydrocarbon b) Purification of a fluid stream, such as a hydrocarbon, containing contaminants such as waxes by passage through a bed of a particulate high surface area adsorbent such as activated alumina or activated carbon, and then removing other contaminants such as sulphur compounds by passage through a second bed of a particulate composition comprising zinc and/or copper oxides, carbonates or basic carbonates Regeneration of the high surface area adsorbent bed to remove the deposited waxes is effected periodically with a stream of a gas at an elevated temperature If the waxes were not removed, the pores of the material of the second bed would tend to become blocked by the waxes rendering the second bed ineffective c) Removal of traces of sulphur compounds such as carbonyl sulphide and/or hydrogen sulphide from a gas stream such as propylene by passage through a bed of a particulate material effective to remove such sulphur compounds, for example copper oxide or basic copper carbonate, possibly containing alumina where the gas contains carbonyl sulphide The copper compound absorbs the hydrogen sulphide producing water as a byproduct CuO + H₂S > CuS + H₂0 Some of the water may effect hydrolysis of carbonyl sulphide

COS + H₂0 > C0₂ + H₂S

The residual water and/or carbon dioxide may be removed by passage through a second bed of a suitable particulate absorbent, e g a molecular sieve, which requires periodic regeneration by means of e g a gas stream at an elevated temperature d) Hydrodesulphunsing a feedstock such as a hydrocarbon, e g natural gas, by passage of the feedstock in admixture with hydrogen, at an elevated temperature, through a bed of a particulate hydrodesulphuπsation catalyst, such as a sulphided cobalt-, or nickel-, molybdate 6

composition The hydrodesulphuπsed feedstock is then passed through a second bed of an absorbent for sulphur compounds, e g zinc and/or copper oxides, carbonates, or basic carbonates To render the hydrodesulphunsation catalysts effective it is necessary to pre-sulphide it This may be done by passing a suitable sulphur-containing fluid through the hydrodesulphunsation catalyst bed Clearly passage of the fluid used for pre-sulphiding the hydrodesulphunsation catalyst through the subsequent sulphur absorbent bed would be undesirable and so by using the present invention, the hydrodesulphunsation catalyst and the sulphur absorbent may be situated in the same vessel and the hydrodesulphunsation catalyst sulphided in situ e) Applications where regeneration of the beds needs to be effected at different temperatures For example a first bed of an adsorbent such as activated carbon or silica gel may be used to adsorb heavy aromatic hydrocarbons from a feedstock, and then a second bed of a molecular sieve adsorbent is employed to adsorb water from the feedstock Regeneration is effected by passing regeneration fluid through both beds to heat the beds to a first temperature, e g 120-200°C, to remove part of the adsorbed water and the materials adsorbed on the first bed Then regeneration fluid at a higher temperature is passed through the second bed and out through the secondary port between the first and second beds so that the second bed is heated to a temperature, e g 250-300°C, that is higher than the first temperature to complete the regeneration of the second bed This is of use where the adsorbent of the first bed is unable to withstand the higher regeneration temperature required to complete the regeneration of the second bed f) It is common to use, as a drier, a first bed of an alumina adsorbent and a second bed of a molecular sieve adsorbent Although alumina has a higher capacity for water than molecular sieves, it does not fully remove the water as the latter is less strongly bound to the alumina adsorbent Hence an energy saving arrangement would be to regenerate both beds by heating to a first temperature, for example a temperature in the range 150-220°C and then heat only the molecular sieve to a second, higher, temperature, for example in the range 250-300°C This would save energy and shorten the regeneration time as there is less adsorbent to heat and to cool

Claims

A process for the treatment of a process fluid comprising passing the process fluid successively through a first bed of a first particulate material and a second bed of a second, dissimilar, particulate material, wherein said beds are disposed within a single vessel provided with primary ports for the inlet and outlet of the fluid stream and with a secondary port connecting with a region between said first and second beds, and periodically passing a dissimilar fluid stream, or a fluid stream at a different temperature, through one of said beds by passage from one of said primary ports to said secondary port or vice versa

A process according to claim 1 wherein all of the process fluid is passed through one bed, part is discharged through the secondary port and the remainder is passed through the second bed, and then the process fluid that has passed through the second bed is combined with the process fluid that has been discharged through the secondary port

A process according to claim 1 or claim 2 wherein the beds are separated from one another by a partition and a first secondary port is disposed at the outlet side of the first bed and a second secondary port is disposed at the inlet side of the second bed and said first and second secondary ports are connected outside said vessel.

4. A process according to any one of claims 1 to 3 wherein one bed is a bed of a molecular sieve and the other bed is a bed of a particulate absorbent and the periodic treatment comprises regeneration of the molecular sieve by passing a heated fluid stream through the molecular sieve bed

5 A process according to any one of claims 1 to 3 wherein the periodic treatment comprises a regeneration step wherein a regeneration fluid through at least one of the beds and for at least part of the regeneration step, the regeneration fluid is passed through only one of the beds

6 A process according to claim 5, wherein the regeneration step comprises a) passing a regeneration fluid at a first temperature through both beds and then b) passing the regeneration fluid at a second, higher, temperature through only one of the beds