PATENT
Document Type and Number:United States Patent Application 20120141367
Inventors:
Wynn, Nicholas P. (Redwood CIty, CA, US)
Gottschlich, Douglas E. (Redwood City, CA, US)
Lin, Haiqing (Mountain View, CA, US)
Application Number: 13/370942
Publication Date: 06/07/2012
Assignee: MEMBRANE TECHNOLOGY AND RESEARCH, INC (Menlo Park, CA, US)
Abstract:
FIELD OF THE INVENTION
The invention relates to a gas separation process for the production of hydrogen by autothermal reforming of natural gas, where the hydrogen removal step is followed by a membrane-based carbon dioxide separation step. Residual gas from the membrane separation step is recycled back to the autothermal reformer.
BACKGROUND OF THE INVENTION
Methane reforming is a chemical process that results in the production of pure hydrogen from natural gas. There are two basic types of natural gas reforming technologies: steam methane reforming (SMR) and autothermal reforming (ATR). Both methods work by exposing natural gas and steam to a catalyst (typically nickel) at high temperature.
Conventional steam methane reforming (SMR) uses an external source of hot gas to heated tubes in which a catalytic reaction takes place that converts steam and lighter hydrocarbons—such as natural gas or refinery feedstock—into hydrogen and carbon oxides (i.e., syngas).
During a conventional SMR process, the following two reactions take place concurrently:
CH4+2H2O→CO2+4H2
CH4+H2O→CO+3H2
A water-gas shift reaction is then performed using steam to convert carbon monoxide to carbon dioxide and generate additional hydrogen. This is typically followed by a pressure-swing adsorption (PSA) step to purify the hydrogen.
Because the above two reactions are both endothermic, residual, unreacted tail gas from the PSA process is not wasted, but finds a ready use to heat the reformer to drive the reaction.
In autothermal reforming (ATR), oxygen is added to the process, resulting in the additional reactions:
CH4+O2→CO2+2H2
CH4+½O2→CO+2H2
The above reactions are exothermic and—provided that enough oxygen is added to balance the two endothermic reactions out—can result in an overall adiabatic process. Therefore, no additional heat source is needed. Although requiring oxygen feed, the ATR has the advantage that it is more compact and less expensive to build than the SMR, because the heat transfer surface required is considerably less with an adiabatic process.
A basic process schematic for a conventional ATR process is shown in FIG. 1. According to this process, natural gas, 101, steam, 102, and oxygen, 103, are introduced into an autothermal reformer, 104. The resulting gas, 105, comprising carbon dioxide, carbon monoxide, hydrogen, unreacted natural gas and excess steam, is cooled and sent to a shift reactor, 107. The shift reaction is performed through the addition of steam, 106, to convert the carbon monoxide to carbon dioxide. The resulting gas mixture, 108, is cooled in step, 109, to condense out water, 110. The resultant gas mixture, 111, is passed to a hydrogen separation unit, 112 (typically, a pressure swing adsorption unit) to separate hydrogen, 113. Tail gas, 114, contains mostly carbon dioxide, together with some carbon monoxide, hydrogen, inerts, and unreacted methane.
Tail gas from the ATR process described above is not pure carbon dioxide, so if it is desirable to recover and/or capture carbon dioxide for sequestration or some other purpose, the tail gas stream needs to be purified. Not only does the carbon dioxide have to be purified, but the impurities should be recovered in fairly concentrated form to minimize carbon dioxide loss, i.e., maximize carbon dioxide recovery.
The process of the subject invention achieves this objective in a different, more efficient way. By using the process of the invention, substantially all of the carbon dioxide is recovered; losses of methane and carbon monoxide are minimized; and steam consumption in the reformer is drastically reduced.
SUMMARY OF THE INVENTION
The invention is a gas separation process for the production of hydrogen by autothermal reforming of natural gas, where the hydrogen removal step is followed by or preceded by a membrane-based carbon dioxide separation step. Residual gas from these separation steps is recycled back to the autothermal reformer.
A basic embodiment of the process of the invention comprises the following steps:
(a) introducing natural gas, steam, and oxygen into an autothermal reformer, to produce a gas mixture comprising carbon dioxide, carbon monoxide, hydrogen, water vapor, and residual natural gas;
(b) cooling the gas mixture to condense out water;
(c) treating the resultant gas mixture to remove hydrogen, to produce a hydrogen product stream and a hydrogen-depleted gas mixture;
(d) providing a membrane having a feed side and a permeate side, wherein the membrane is selective to carbon dioxide over hydrogen;
(e) passing the hydrogen-depleted gas mixture across the feed side;
(f) withdrawing from the permeate side a permeate stream that is enriched in carbon dioxide relative to the hydrogen-depleted gas mixture;
(g) withdrawing from the feed side a residue stream that is depleted in carbon dioxide relative to the hydrogen-depleted gas mixture; and
(h) passing the residue stream as a recycle stream back to the autothermal reformer.
The residue stream is typically treated to remove nitrogen, argon, and other inerts prior to being passed as a recycle stream back to the reformer in step (h).
The process produces a hydrogen product stream as well as a hydrogen-depleted gas mixture. The hydrogen-depleted gas mixture may be compressed and condensed to produce high-purity carbon dioxide prior to being passed across the feed side of the membrane. Alternatively, the carbon dioxide-enriched permeate stream may be compressed and condensed to produce high-purity carbon dioxide.
In an alternative embodiment of the inventive process, carbon dioxide is removed from the gas mixture first, followed by a hydrogen removal step. A basic embodiment of this process comprises the following steps:
(a) introducing natural gas, steam, and oxygen into an autothermal reformer, to produce a gas mixture comprising carbon dioxide, carbon monoxide, hydrogen, water vapor, and residual natural gas;
(b) cooling the gas mixture to condense out water;
(c) providing a membrane having a reed side and a permeate side, wherein the membrane is selective to carbon dioxide over hydrogen;
(d) passing the gas mixture across the feed side;
(e) withdrawing from the penneate side a permeate stream that is enriched in carbon dioxide relative to the gas mixture;
(f) withdrawing from the feed side a residue stream that is depleted in carbon dioxide relative to the gas mixture;
(g) treating the residue stream to remove hydrogen, to produce a hydrogen product stream and a hydrogen-depleted gas stream; and
(h) passing the hydrogen-depleted gas stream as a recycle stream back to the autothermal reformer.
The hydrogen-depleted gas stream is typically treated to remove nitrogen, argon, and other inerts prior to being passed as a recycle stream back to the reformer in step (h).
The gas mixture may be compressed and condensed to produce high-purity carbon dioxide prior to being passed across the feed side of the membrane. Alternatively, the carbon dioxide-enriched permeate stream may be compressed and condensed to produce high-purity carbon dioxide.
In either of the above process embodiments, the hydrogen removal step is typically a pressure swing adsorption (PSA) step, but a hydrogen-permeable membrane with good hydrogen/carbon dioxide selectivity can also be used, as may any other technology capable of adequate separation of hydrogen from carbon dioxide.
The membrane to recover carbon dioxide typically has a selectivity to carbon dioxide over hydrogen of at least 6 and a carbon dioxide permeance of at least about 200 gpu. Preferably, the membrane has a selectivity to carbon dioxide over hydrogen of at least 8, more preferably, at least 10, and a carbon dioxide permeance of at least about 400 gpu.
Condensation of carbon dioxide may be combined with membrane separation to enhance the purity of the recovered carbon dioxide.
The process of the invention results in the production of at least 20% more hydrogen than a conventional autothermal reforming process.
The invention is applicable to any process that produces hydrogen from hydrocarbon/carbon feedstock using steam and oxygen and where carbon dioxide recovery is desired. The inventive process has a number of advantages over conventional ATR processes, including:
Because residual carbon dioxide is not lost to the process, no shift reactor, and consequently, no steam supply, are necessary.
Because residual methane is not lost to the process, not all natural gas need be reacted per pass;
as such, the autothermal reactor can be operated with less steam and can be smaller.
Because residual carbon dioxide is not lost to the process, not all carbon dioxide nee& to be recovered per pass; as such, carbon dioxide recovery can be performed at higher carbon dioxide partial pressure, reducing membrane area and maximizing carbon dioxide purity by minimizing co-permeation of other components.
Because residual hydrogen is not lost to the process, hydrogen yield is maximized.
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