1. Field of this Invention
This invention relates to a process for the separation of a gas mixture containing hydrogen, carbon monoxide and methane to obtain product streams of substantially pure hydrogen and carbon monoxide.
2. Description of the Prior Art
The prior art has commonly employed cryogenic processes for the separation of synthesis gas to yield hydrogen and carbon monoxide as recovered products. Such processes typically involve at least a partial liquifaction of the feed gas mixture and require the efficient use of vapor-liquid contacting and separation equipment for overall economic operation.
When manufactured for the production of carbon monoxide by primary steam reforming of natural gas or by partial oxidation of higher hydrocarbon fossil fuels, the synthesis gas mixture contains residual methane as well as the hydrogen and carbon monoxide common to all synthesis gas streams. The cryogenic processes employed for the separation of such synthesis gas mixtures are designed to reject methane and produce carbon monoxide and hydrogen at a purity consistent with the end use requirement. These designs are intended to minimize the carbon monoxide content of the rejected hydrogen and methane streams in order to maximize carbon monoxide recovery. Characteristically the gas mixture will contain approximately 50 to 70 mol % hydrogen, 15 to 45 mol % carbon monoxide and 2 to 6 mol % methane, together with minor impurities, as for example trace amounts of nitrogen.
Since essentially three primary components are present in the above-described synthesis gas mixture--hydrogen, carbon monoxide and methane--the prior art has commonly employed two serial multiple-plate column liquid-vapor contactors to carry out the synthesis gas separation. In one conventional process arrangement employing such liquid-vapor contactors, the synthesis gas feed stream is provided at elevated pressure and cooled by heat exchange to form a vapor-liquid mixture which is introduced to the first contacting column. In the first column, the introduced feed is contacted with a chilled methane wash liquid for absorption of the carbon monoxide in the methane wash liquid. Hydrogen is obtained from the first column as carbon monoxide-free overhead product and bottoms liquid is recovered comprising methane and the absorbed carbon monoxide. The recovered bottoms liquid is then throttled to reduced pressure and fractionated in the second contacting column. From the second column, carbon monoxide is recovered as overhead and methane is recovered as bottoms. The methane bottoms are chilled and recycled as the aforementioned methane wash liquid for the first contacting column.
Although the above separation system entails a comparatively simple apparatus arrangement, the carbon monoxide product recovered by the process is unsatisfactory for use in most chemical synthesis applications by virtue of its relatively high hydrogen content. Accordingly, the prior art has attempted to obtain improvement in purity of the carbon monoxide product by removal of the hydrogen contaminant upstream of the second contacting column. In one such improvement scheme, the synthesis gas is cooled by heat exchange, as before, and introduced as a vapor to the first contacting column. The bottoms liquid from the first contacting column is throttled to lower pressure and passed to a flash drum for vapor-liquid separation. In the flash drum an equilibrium vapor-liquid separation is achieved to reject the bulk of the hydrogen which would otherwise be contained in the feed to the second contacting column. The liquid from the flash drum thus freed from the hydrogen contaminant is then throttled to still lower pressure prior to its introduction to the second contacting column.
By the above-described improvement modifications, a carbon monoxide overhead product from the second contacting column can be obtained with hydrogen contaminant concentrations of less than 5000 parts per million (p.p.m.). Nonetheless the product recovery attainable in such modified systems is extremely sensitive to product purity. As a result high losses are encountered in the provision of product carbon monoxide containing hydrogen contaminant at concentration levels of less than 5000 p.p.m. Such losses occur by flash-off of carbon monoxide with the hydrogen in the equilibrium flash drum and consequent removal of the flashed carbon monoxide with the hydrogen withdrawn from the drum. Inasmuch as end use specifications for the carbon monoxide product in many applications, as for example for acrylic and polyurethane resin production, require a hydrogen content of less than about 3000 p.p.m., it has been necessary to operate the prior art process with comparatively low recovery levels, with a maximum recovery of about 90%, as based on the content of carbon monoxide in the synthesis gas feed mixture, to meet such end use carbon monoxide product specifications.
In the prior art, the refrigeration content of the reduced pressure, low temperature product streams has been utilized to cool the synthesis gas feed mixture prior to its introduction to the absorber column. Nonetheless, refrigeration is generally required to provide reflux for the second contacting column and to cool the feed gas mixture and the methane wash liquid for the first contacting column. Under such conditions, the minimum pressure at which the final contacting column can be economically operated is about 20 psia. Such minimum pressure constraint is imposed by the requirement of providing sufficient pressure to overcome the flow resistance associated with the product transfer lines. Since the process involves two substantial reductions in main stream pressure in the aforementioned throttling steps, considerable compression energy must be expended in initial pressurization of the synthesis gas feed mixture for the process.
Accordingly, it is an object of the present invention to provide an improved process for the separation of a synthesis gas mixture containing hydrogen, carbon monoxide and methane to provide a high purity (carbon monoxide-free) hydrogen product and a high purity (hydrogen-free) carbon monoxide product.
It is another object of the invention to provide an improved process of the above type wherein high recovery of carbon monoxide is achieved.
It is still another object of the invention to provide an improved process of the above type characterized by low process energy requirements.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.