Patent Application
20110085966
Hydrogen is most often manufactured using natural gas as a feedstock through the steam methane reforming process. With the current high prices of natural gas it is preferred to manufacture hydrogen from lower cost sources of fuel such as residual fuels. Residual fuels from refineries consist of petroleum coke, visbreaker tar, pitch from deasphalting processes, vacuum residues, atmospheric residues and similar fuels. Coal is also a desirable low cost fuel that can be used to produce hydrogen.
The typical method of producing hydrogen from residual fuels or coal is to gasify it by partially oxidizing it by contact with oxygen and steam or water at elevated temperatures to form a syngas. The syngas consists of hydrogen, carbon monoxide, methane and carbon dioxide. Higher quantities of hydrogen are usually produced by further reacting the syngas with steam over a catalyst to promote the water gas shift reaction of carbon monoxide and steam to hydrogen and carbon dioxide.
After the removal of acid gases such as hydrogen sulfide and carbon dioxide in processes such as amine contactors, Selexol or Rectisol units, the hydrogen still needs to be purified. Hydrogen can be further purified to remove residual amounts of Carbon monoxide through a catalytic reaction to form methane (methanation) and water. This will produce a final product hydrogen stream with about 97% purity. The remaining composition is methane, nitrogen and argon. If higher purity hydrogen (>99%) is desired, the hydrogen is further processed through a Pressure Swing Adsorption (PSA). Due to the limits of PSA technology, the typical hydrogen recovery is about 87-90%.
The present invention provides a process for producing pure hydrogen with higher recovery.
The present invention is a method of high hydrogen recovery from syngas from a gasifier. This method includes providing a syngas stream from the gasifier to first hydrogen separation device, wherein said first hydrogen separation device is a pressure swing adsorption device, thereby producing a first high purity hydrogen stream and a tailgas stream. This method also includes increasing the pressure of said tailgas stream in a first compressor; thereby producing a pressurized tailgas stream. This method also includes providing said pressurized tailgas stream to a second hydrogen separation device, thereby producing a second high purity hydrogen stream and a residue stream. This method also includes directing said residue stream to said gasifier, and combining said first high purity hydrogen product stream and said second high purity hydrogen product stream, thereby producing a high purity hydrogen product stream.
Turning now to
The raw syngas stream is then contacted with a catalyst in a shift reactor 306 to promote the production of hydrogen from CO and H2O, thereby producing a shifted syngas stream. An indirect contact heat exchanger 307 then transfers heat indirectly between the BFW or other process streams 321 and the hot shifted syngas stream thereby producing a cooled, shifted syngas stream and steam or other elevated temperature process streams 308. Following the shift reaction (and heat transfer), the acid gases, H2S and CO2, are removed from the cooled shifted syngas stream by contacting with a solvent in an acid gas removal process 309. While the preferred solvent is methanol, any appropriate solvent known in the art may be utilized. The acid gas removal process 309 results in streams primarily comprising CO2 310, H2S 311, and H2O 312. Following the syngas acid gas removal, purified syngas stream typically has hydrogen with a purity of about 90-99%. The hydrogen is first purified in a first hydrogen separation device 314. This first hydrogen separation device 314 may be a PSA with about 87-90% recovery 318.
The acid gas removal unit 309 may be split into two parts, first part, removing H2S and other sulfur compounds being placed upstream of the shift reactor 307. Those familiar with the art know trade offs of removing sulfur compounds from downstream to upstream of the shift reactor.
The tail gas from the first hydrogen separation device is further recovered in a second hydrogen separation device 316 after compression in a first compressor 315. The second hydrogen separation 316 device may be a PSA. The second hydrogen separation device 316 will have a slightly lower recovery (typically about 85-88%) than the first one due to the lower hydrogen content in its feed gas. The resulting overall recovery is improved to 98%, while maintaining a purity of over 99.9% H2. The tail gas from the second hydrogen separation device 316, is then used as fuel 320. The high purity hydrogen stream 317 is then combined with the hydrogen stream 318 from the first hydrogen separation device 314, and exported as product 319.
Now turning to
Residual argon and nitrogen coming in with the oxygen will also be present in the raw syngas 405. The raw syngas is then contacted with a catalyst in a shift reactor 406 to promote the production of hydrogen from CO and H2O, and thereby producing a shifted syngas stream. An indirect contact heat exchanger 407 then transfers heat indirectly between the BFW or other process streams 423 and the hot shifted syngas stream, thereby producing a cooled, shifted syngas stream and steam or other elevated temperature process streams 408. Following the shift reaction (and heat transfer), the acid gases, H2S and CO2, are removed from the cooled shifted syngas by contacting with a solvent, preferably methanol in an acid gas removal process 409. This acid gas removal process 409 results in streams primarily comprising CO2 410, H2S 411, and H2O 412. Following the syngas acid gas removal, purified syngas stream typically has hydrogen with a purity of about 90-99%. The hydrogen is first purified in a first hydrogen separation device 414. The first hydrogen separation device 414 may be a PSA, with about 87-90% recovery 418.
The tail gas from the first hydrogen separation device 414 is further recovered in a second hydrogen separation device 416 after compression 415. The second hydrogen separation device 416 may be a membrane unit. The permeate of the membrane 417 is hydrogen of purity 80-99%. All or part of the permeate 418 is then combined with the hydrogen stream 418 from the first PSA 414, and exported as product 419. All or part of the permeate 418 may be recycled as stream 430 to PSA 414. The high pressure residue from the membrane 420 is sent to the gasifier 421 for conversion of methane and CO to hydrogen. A smaller fraction of the residue is purged off as a fuel stream 422. The amount of purge will depend upon the amount of inerts N2 and Ar brought in with oxygen for gasification.
