US20080237090A1

US20080237090A1 - Process and system for redcuing the olefin content of a hydrocarbon feed gas and production of a hydrogen-enriched gas therefrom

Info

Publication number: US20080237090A1
Application number: US11/694,309
Authority: US
Inventors: Nicholas Musich; Raju S. Natarajan; Harald Klein
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2007-03-30
Filing date: 2007-03-30
Publication date: 2008-10-02

Abstract

Olefins can impose deleterious effect on hydrocarbon reforming processes used to generate hydrogen-enriched gas, and thus are converted into saturated compounds. Since the hydrogenation process to convert the olefins into saturated compound, difficulties arise in attempting to regulate the temperature of the hydrogenation. To facilitate temperature regulation, the hydrogenation reaction is carried out in a shell-tube reactor, containing catalyst-filled tubes, which is operated under isothermal or essentially isothermal conditions. Preferably, the heat exchange medium introduced into the shell side of the reactor to regulate the hydrogenation temperature is boiling water.

Description

FIELD OF THE INVENTION

This invention relates to processes and systems for producing enriched hydrogen gas from hydrocarbon fuel sources, such as natural gas. In particular, the invention relates to processes and systems for producing enriched hydrogen gas from hydrocarbon fuel sources involving reducing the olefin content of the olefin hydrocarbon fuel and then subjecting the resultant hydrocarbon fuel with reduced olefin content to hydrocarbon conversion such as steam reforming.

BACKGROUND OF THE INVENTION

Hydrogen gas is produced for a variety of chemical and industrial processes. For example, hydrogen is used as a raw material in ammonia synthesis, methanol synthesis, and hydrogen chloride synthesis. Additionally, hydrogen is used to manufacture hydrogen peroxide and is used in the production of oleochemicals. Further, hydrogen is used to remove sulfur from hydrocarbon fuels such as gasoline and diesel.
One use of hydrogen gas that is of increasing importance is a fuel for is electrochemical fuel cells. An electrochemical fuel cell coverts hydrogen, using oxygen as an oxidant, to produce electricity. Being a clean form of energy, it is expected that the use of hydrogen gas as a fuel source will continue to grow, and thus demand for hydrogen gas will continue to increase.
Processes for production of hydrogen gas by conversion of hydrocarbons are well known in the art such as catalytic steam reforming, partial oxidation reforming and autothermal reforming processes. In such processes, the presence of olefins in the feed stream can be detrimental to the operation of the reformers. The operating conditions of a reformer reactor and/or an upstream hydrotreater can, depending on the amount of hydrogen present and the temperature, lead to saturation of the olefins which in turn will result in the generation of heat. Thus, steps must be taken to control heat generation due to saturation of olefins.
Traditionally procedures for controlling heat generation in a hydrotreater include controlled addition of hydrogen combined with cooling, and removing a portion of the product gas, cooling it, compressing it, and recycling it back to the hydrotreater. However, such procedures are expensive and not very energy efficient. Additionally, the use of heat exchangers and compressors involve additional capital expense, and can be less reliable due to the use of rotating equipment that can break down.
One can also control the temperature of the hydrotreater by controlling the amount of hydrogen present. In a hydrotreatment step, hydrogen is, of course, a key reactant. If one can control the amount of hydrogen available for the reaction, by increasing or decreasing the amount of the hydrogen, one can control the reaction. In turn, one can control the temperature rise due to the reaction by controlling the amount of hydrogen available for the reaction.
However, keeping the hydrogen content low to reduce the temperature rise can result in incomplete olefin hydrotreatment. To address this possibility, the gas exiting the hydrotreater can be cooled, combined with more hydrogen, and passed through a second hydrotreatment bed. This multiple stage arrangement, however, can be expensive. Moreover, this multiple stage procedure can not be used in situation where the hydrogen content in the hydrocarbon feed gas is not controllable or not easily controllable.
Thus, a need exists for improved processes for hydrotreatment of hydrocarbons such as performed as part of a hydrogen production process.

SUMMARY OF THE INVENTION

In accordance with the invention, prior to reforming, the olefin content of a hydrocarbon feed stream is reduced by subjecting the hydrocarbon feed to isothermal hydrotreatment, in the presence of hydrogen, in a shell and tube reactor, the tubes being filled with catalyst. This arrangement allows for better control of the temperature of the resultant dehydrogenated hydrocarbon stream. The dehydrogenated hydrocarbon stream can then be subjected to desulfurization and subsequent reforming, e.g., steam reforming, to produce a hydrogen-enriched gas.
Additionally, this isothermal arrangement requires less capital expenditure then above-mentioned multiple stage arrangement. Moreover, the inventive isothermal arrangement can be used in situations where the hydrogen content of the feed gas is not easily controllable, whereas the above-mentioned multiple stage arrangement can not.
While the process is generally described below with respect to a hydrogen production plant, the hydrotreatment procedure of the invention can be used in other facilities such as carbon monoxide plants, methanol synthesis plants, and synthesis gas plants.
Therefore, in accordance with the invention, there is provided a process for subjecting an olefin containing gas to a hydrogen treatment wherein olefin compounds are converted into saturated compounds, the process comprising:
introducing hydrogen and a hydrocarbon feed gas comprising olefins, alkanes, hydrogen, CO₂, and sulfur compounds such as mercaptanes, thiophenes, and H₂S into a hydrotreater wherein olefins are converted into saturated compounds and organic sulfur compounds such as mercaptanes and thiophenes are converted into H₂S;
wherein the hydrotreater is a shell and tube catalytic reactor containing catalyst filled tubes, the hydrogen and hydrocarbon feed gas being introduced into the catalyst filled tubes, and the shell having at least one inlet for introduction of a heat exchange medium to cool the catalyst filled tubes, and at least one outlet for removal of the heat exchange medium, and wherein the hydrocarbon feed gas is introduced into the catalyst filled tubes at an inlet temperature of about 400 to 525° F.; and
removing the hydrocarbon feed gas from the catalyst filled tubes of the hydrotreater at a temperature that differs from the inlet temperature by no more than 30° F. (preferably no more than 20° F., especially no more than 15° F., in particular no more than 10° F., for example, not more 5° F.).
According to a further aspect of the invention there is provided a process for treating a hydrocarbon gas to reduce the olefin content and organic sulfur compound content thereof, the process comprising:
introducing hydrogen and a hydrocarbon feed gas comprising olefins, alkanes, hydrogen, CO₂, and sulfur compounds such as mercaptanes, thiophenes and H₂S, into a hydrotreater wherein olefins are converted into saturated compounds and organic sulfur compounds such as mercaptanes and thiophenes are converted into H₂S;
wherein the hydrotreater is a shell and tube catalytic reactor containing catalyst filled tubes, the hydrogen and hydrocarbon feed gas being introduced into the catalyst filled tubes, and the shell having at least one inlet for introduction of a heat exchange medium to cool the catalyst filled tubes, and at least one outlet for removal of the heat exchange medium, and wherein the hydrocarbon feed gas is introduced into the catalyst filled tubes at an inlet temperature of about 400 to 525° F.;
removing the hydrocarbon feed gas from the catalyst filled tubes of the hydrotreater at a temperature that differs from the inlet temperature by no more than 30° F. (preferably no more than 20° F., especially no more than 15° F., in particular no more than 10° F., for example, not more 5° F.); and
subjecting the hydrocarbon feed removed from the hydrotreater to a desulfurization step for removal of H₂S.
According to a further aspect of the invention, following desulfurization, the desulfurized hydrocarbon feed stream is delivered to a hydrocarbon cracking procedure, such as a steam reforming, to produce a hydrogen-enriched synthesis gas.
According to a further aspect of the invention, the hydrocarbon cracking procedure is steam reforming wherein the desulfurized hydrocarbon feed stream is converted in the presence of steam to produce a hydrogen-enriched synthesis gas containing, inter alia, H₂and CO.
According to a further aspect of the invention, the product gas stream from the steam reforming procedure is treated in a shift reactor wherein CO is reacted with water to form hydrogen and CO₂, thereby increasing hydrogen production.
According to a further aspect of the invention, the hydrocarbon feed stream is preheated to about 500° F. before being introduced into the hydrotreater.
According to a further aspect of the invention, the heat exchange medium used in the hydrotreater is boiling water whereby the temperature of the hydrocarbon feed stream within the catalyst tubes can be controlled by controlling the pressure of the steam generated in the shell side of the reactor during heat exchange.
In accordance with another aspect of the invention, there is provided a system for preparing a hydrogen-enriched gas from a hydrocarbon feed stream, the system comprising:
a catalytic hydrotreater comprising a shell and tube reactor wherein the tubes contain a catalyst suitable for hydrogenation of olefins and organic sulfur compounds,
the shell and tube reactor further having a first inlet for introducing a hydrocarbon feed, or a mixture of hydrocarbons and hydrogen, into the tube-side of the reactor, and a first outlet for removing a hydrocarbon stream having reduced the olefin content and organic sulfur compound from the tube-side of the reactor,
the shell and tube reactor further having a second inlet for introducing a heat exchange medium to the shell-side of the reactor, and a second outlet for removing the heat exchange medium from the shell-side of the reactor,
an adsorber, the adsorber having an adsorbent bed containing an adsorbent for removal of hydrogen sulfide (H₂S) from a hydrocarbon gas stream, the adsorber further comprising an inlet for introduction of a hydrocarbon gas stream containing sulfur compounds, and an outlet for discharging a desulfurized hydrocarbon gas stream,
a first conduit in fluid communication with said first outlet of said reactor and in fluid communication with said inlet of said adsorber for introducing at least a portion of the hydrocarbon stream having reduced the olefin and organic sulfur compounds content to said adsorber,
a hydrocarbon reformer, such as a steam reforming, comprising an inlet for introducing a desulfurized hydrocarbon gas stream and an outlet for discharging a hydrogen-enriched synthesis gas, and
a second conduit in fluid communication with said outlet of said adsorber and in fluid communication with said inlet of said reformer for introducing at least a portion of the desulfurized hydrocarbon gas stream to said reformer.
Upon further study of the specification and appended claims, further aspects and advantages of this invention will become apparent to those skilled in the art.
In accordance with the invention a hydrocarbon feed containing olefins is subjected to an isothermal (or essentially isothermal) catalytic hydrotreatment within a shell and tube reactor, the tubes containing hydrotreatment catalyst. The hydrocarbon feed can be obtained from a variety of sources such as natural gas, refinery waste gas, waste gas from petrochemical plants, LPG, naphtha, etc. Generally, the components of the hydrocarbon feed will, of course, vary depending on the type of feedstock selected. Alkanes that may be present in the feedstock include methane, ethane, propane, and butane. Olefins that may be present in the feedstock include ethylene, propylene, and butylene. The process of the invention is suitable for treating hydrocarbon feed containing, for example, 5 to 50 mol % olefins, preferably 5 to 30 mol % olefins, especially 5 to 20 mol % olefins.
The hydrocarbon feed may contain additional components such H₂, CO, CO₂, sulfur compounds, and nitrogen compounds. In the hydrotreatment stage, organic sulfur compounds within the hydrocarbon feed are also subjected to hydrogenation wherein organic sulfur compounds are converted into hydrogen sulfide (H₂S). Prior to being introduced into the isothermal hydrotreater, the hydrocarbon feed is preferably combined with a hydrogen stream prior to introduction into the hydrotreater, in order to enhance mixing. However, it is also possible to introduce the hydrocarbon feed and hydrogen stream separately into the hydrotreater via separate inlets.
The amount of hydrogen delivered to the hydrotreater will depend on the amount of olefins/organic sulfur compounds present in the hydrocarbon feed. Preferably, the amount of hydrogen is an excess of the amount of olefins/organic sulfur compounds present. In general, it is preferred that the molar ratio of hydrogen to olefins/organic sulfur compounds introduced into the hydrotreater be, for example, 1.25:1 to 3:1. Thus, for example, the combined hydrocarbon/hydrogen feed which is preheated to a temperature of about 400 to 525° F., particularly about 450 to 500° F., preferably has a molar ratio of hydrogen to olefins/organic sulfur compounds of 1.25:1 to 3:1.
The hydrocarbon feed and hydrogen may also be preheated prior to introduction into the hydrotreater. Preheating can be performed by in a heat exchanger by indirect heat exchange against any suitable heating medium such as steam or a process gas. Preferably, the hydrocarbon feed and hydrogen are preheated to a temperature of about 400 to 525° F., particularly about 450 to 500° F.
Other optional pretreatments include compression with subsequent condensate removal to remove at least some of the heavier hydrocarbons.
Hydrocarbon feed and hydrogen are introduced into a distribution zone of the hydrotreater. From distribution zone, the hydrocarbon feed/hydrogen mixture is distributed into a plurality of tubes (e.g., 50 to 500 or 100 to 250 tubes) wherein each tube contains a hydrogenation catalyst.
Suitable hydrogenation catalysts include those comprising at least one Group VI metal or compound thereof (such as oxides and sulfides) and at least one Group VIII metal or compound thereof (such as oxides and sulfides) on a suitable support such alumina or silica alumina. For example, the catalyst can be a cobalt-molybdenum or nicke-molybdenum catalyst on an alumina or silica support. A nickel-molybdenum catalyst is preferred.
Hydrogenation conditions within the hydrotreater are preferably a temperature of about 400 to 525° F., particularly about 450 to 500° F., and preferably a pressure of about 150 to 500 psig, particularly about 250 to 400 psig.
In order to remove the heat generated in the catalysts tubes as a result of the exothermic hydrogenation reaction, a cooling heat exchange medium is introduced to the shell side of the tube and shell hydrotreater reactor. As noted above, the hydrotreater is intended to operate under isothermal or essentially isothermal conditions. Thus, the temperature of the hydrocarbon stream removed from the outlets of the catalyst filled tubes differs from the temperature of the hydrocarbon feed/hydrogen mixture introduced into the inlets of the catalyst filled tubes temperature by no more than 30° F. (preferably no more than 20° F., especially no more than 15° F., in particular no more than 10° F., for example, not more 5° F.). To facilitate isothermal operation and to enhance the effectiveness of the heat exchange, the heat exchange medium used in the shell side of the reactor is preferably an evaporating liquid. In other words, the heat exchange medium is preferably a liquid medium that, during the course of the heat exchange, undergoes a phase conversion to a gas. In particular, the liquid heat exchange medium is water that converts to steam during while removing the heat from the catalytic hydrogenation reaction. Boiling water has a high heat transfer coefficient which will aid in keeping the reaction gas temperature from significantly increasing. Further, the use of a water/steam heat exchange medium facilities control of the temperature since the temperature can be controlled by regulating the steam pressure in the shell side of the reactor. Thus, the use of a water/steam heat exchange medium provides better control of the reaction temperature, and also generates a process stream (i.e., steam) that can be used for other processes, such as in a downstream stream reformer.
Upon exiting the outlets of the catalyst-filled tubes, the hydrotreated gas is introduced into a lower header and then discharged let from the hydrotreater via an out. The temperature of the discharged hydrotreated gas is generally about 405 to 540° F., particularly about 455 to 515° F.
The discharge temperature is sufficient to permit desulfurization. Therefore, the hydrotreated gas discharged from the hydrotreater is in introduced into one or more adsorbers or absorbers for the removal of hydrogen sulfide. As noted above, during the hydrotreatment, organic sulfur compounds are converted into hydrogen sulfide by hydrogenation. The adsorbent used in the beds of the adsorbers can be, for example, activated carbon, molecular sieves, or oxides of certain metals, such as alumina, iron oxide and zinc oxide. Zinc oxide is preferred. Alternatively, the hydrotreated gas can be desulfurized in an absorber that employs a liquid absorbent such as an aqueous solution of an alkanolamine, for example, monoethanolamine.
The adsorbers/absorbers can be arranged in series or in parallel. If some of the adsorbers/absorbers are arranged in parallel, this will allow some of the adsorbers/absorbers to be operating while others are being subjected to maintenance.
The hydrotreated/desulfurized gas removed from the adsorbers/absorbers is then delivered to a hydrocarbon reforming process, such as steam reforming. In the reforming process, the hydrocarbons are converted to provide a hydrogen-enriched gas that further contains, for example, CO, CO₂, methane, nitrogen.
The entire disclosures of all applications, patents and publications, cited above and below, are hereby incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:

FIG. 1 illustrates a flow chart of the process according to the invention; and

FIG. 2 shows a more detailed view of the hydrotreatment reactor of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, a hydrocarbon feed (1) such as a natural gas (1 a), a refinery gas (1 b), or both are combined with a hydrogen stream (2). The resultant mixture is then preheated by indirect heat exchange against process gas in heat exchanger (3). The hydrocarbon/hydrogen mixture removed from heat exchanger (3) is introduced into reactor (4) at a temperature of 500° F. and a pressure of 350 psig.
Reactor (4) is a shell and tube reactor having a plurality of tubes containing Ni-Mo catalyst on a support. The hydrocarbon/hydrogen mixture passes through the catalyst filled tubes whereby olefins contained in the hydrocarbon/hydrogen mixture undergo hydrogenation to form paraffins. Additionally, organic sulfur compounds present within the hydrocarbon/hydrogen mixture undergo hydrogenation to form hydrogen sulfide.
To operate the reactor under isothermal conditions, boiling water (5) is introduced into the sell side of reactor (4) to absorb the heat of the exothermic hydrogenation reactor. During the heat exchange, at least a portion of the water is converted into steam. This allows the reaction temperature to be controlled by controlling the steam pressure within the shell side of reactor (4). The resultant water/steam is removed from the shell side of the reactor via line 6.
The hydrotreated gas discharged from the catalyst-filled tubes is introduced to adsorbers (7) and (8), connected in series. Each of adsorbers (7) and (8) contains a bed of zinc oxide particles which remove hydrogen sulfide from the hydrotreated gas. After discharge from adsorbers (7) and (8), the desulfurized/hydrotreated gas (9) is sent to a hydrocarbon reforming stage (10), such as a steam reformer, to generate a hydrogen-enriched product gas.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Claims

1. A process for subjecting an olefin containing gas to a hydrogen treatment wherein olefin compounds are converted into saturated compounds, said process comprising:

introducing hydrogen and a hydrocarbon feed gas comprising olefins, alkanes, hydrogen, CO₂, and sulfur compounds into a hydrotreater wherein olefins are converted into saturated compounds and organic sulfur compounds are converted into H₂S;

wherein the hydrotreater is a shell and tube catalytic reactor containing catalyst filled tubes, the hydrogen and hydrocarbon feed gas being introduced into the catalyst filled tubes, and the shell having at least one inlet for introduction of a heat exchange medium to cool the catalyst filled tubes, and at least one outlet for removal of the heat exchange medium, and wherein the hydrocarbon feed gas is introduced into the catalyst filled tubes at an inlet temperature of about 400 to 525° F.; and

removing the hydrocarbon feed gas from the catalyst filled tubes of the hydrotreater at a temperature that differs from the inlet temperature by no more than 30° F.

2. A process according to claim 1, wherein the temperature of the hydrocarbon feed gas removed from the catalyst filled tubes of the hydrotreater differs from the inlet temperature by no more than 15° F.

3. A process according to claim 1, wherein the temperature of the hydrocarbon feed gas removed from the catalyst filled tubes of the hydrotreater differs from the inlet temperature by no more than 10° F.

4. A process for treating a hydrocarbon gas to reduce the olefin content and organic sulfur compound content thereof, the process comprising:

introducing hydrogen and a hydrocarbon feed gas comprising olefins, alkanes, hydrogen, CO₂, and sulfur compounds, into a hydrotreater wherein olefins are converted into saturated compounds and organic sulfur compounds are converted into H₂S;

wherein the hydrotreater is a shell and tube catalytic reactor containing catalyst filled tubes, the hydrogen and hydrocarbon feed gas being introduced into the catalyst filled tubes, and the shell having at least one inlet for introduction of a heat exchange medium to cool the catalyst filled tubes, and at least one outlet for removal of the heat exchange medium, and wherein the hydrocarbon feed gas is introduced into the catalyst filled tubes at an inlet temperature of about 400 to 525° F.;

removing the hydrocarbon feed gas from the catalyst filled tubes of the hydrotreater at a temperature that differs from the inlet temperature by no more than 30° F.; and

subjecting the hydrocarbon feed removed from the hydrotreater to a desulfurization step.

5. A process according to claim 4, wherein the temperature of the hydrocarbon feed gas removed from the catalyst filled tubes of the hydrotreater differs from the inlet temperature by no more than 15° F.

6. A process according to claim 5, wherein the temperature of the hydrocarbon feed gas removed from the catalyst filled tubes of the hydrotreater differs from the inlet temperature by no more than 10° F.

7. A process according to claim 4, wherein desulfurization is performed by introducing the hydrotreated gas into one or more adsorbers containing at least one adsorbent comprising zinc oxide.

8. A process according to claim 4, further comprising, following desulfurization, delivering the desulfurized hydrocarbon feed stream to a hydrocarbon cracking procedure to produce a hydrogen-enriched synthesis gas.

9. A process according to claim 8, wherein said hydrocarbon cracking procedure is steam reforming wherein the desulfurized hydrocarbon feed stream is converted in the presence of steam to produce a hydrogen-enriched synthesis gas containing H₂and CO.

10. A process according to claim 9, further comprising, following steam reforming, delivering the product gas stream from the steam reforming to a shift reactor wherein CO is reacted with water to form hydrogen and CO₂.

11. A process according to claim 1, wherein the heat exchange medium used in the hydrotreater is boiling water, and the temperature of the hydrocarbon feed stream within the catalyst tubes can be controlled by controlling the pressure of the steam generated in the shell side of the reactor during heat exchange.

12. A process according to claim 1, wherein the hydrocarbon feed introduced into the hydrotreater contains 5 to 50 mol % olefins.

13. A process according to claim 1, wherein the molar ratio of hydrogen to olefins and organic sulfur compounds introduced into the hydrotreater is 1.25:1 to 3:1.

14. A process according to claim 1, wherein the catalyst in the tunes of the hydrotreater comprise at least one Group VI metal or compound thereof and at least one Group VIII metal or compound thereof.

15. A process according to claim 14, wherein said catalyst is a nickel-molybdenum catalyst.

16. A process according to claim 1, wherein said hydrotreater is operated at a temperature of 450 to 500° F., and a pressure of 250 to 400 psig.

17. A system for preparing a hydrogen-enriched gas from a hydrocarbon feed stream, the system comprising:

a catalytic hydrotreater comprising a shell and tube reactor wherein the tubes contain a catalyst for hydrogenation of olefins,

the shell and tube reactor further having a first inlet for introducing a hydrocarbon feed, or a mixture of hydrocarbons and hydrogen, into the tube-side of the reactor, and a first outlet for removing a hydrocarbon stream having reduced olefin content from the tube-side of the reactor,

the shell and tube reactor further having a second inlet for introducing a heat exchange medium to the shell-side of the reactor, and a second outlet for removing the heat exchange medium from the shell-side of the reactor,

an adsorber having an adsorbent bed containing an adsorbent for removal of hydrogen sulfide from a hydrocarbon gas stream, the adsorber further comprising an inlet for introduction of a hydrocarbon gas stream containing sulfur compounds, and an outlet for discharging a desulfurized hydrocarbon gas stream,

a first conduit in fluid communication with said first outlet of said reactor and in fluid communication with said inlet of said adsorber for introducing at least a portion of the hydrocarbon stream having reduced olefin content to said adsorber,

a hydrocarbon reformer comprising an inlet for introducing a desulfurized hydrocarbon gas stream and an outlet for discharging a hydrogen-enriched synthesis gas, and a second conduit in fluid communication with said outlet of said adsorber and in fluid communication with said inlet of said reformer for introducing at least a portion of the desulfurized hydrocarbon gas stream to said reformer.