Optimization of the hydrogen separation membrane unit of an autothermal reforming plant
Agbabiaka, Olusda Abraham
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This research work has been carried out to enhance the performance of the membrane unit of an Autothermal Reforming (ATR) plant. The plant in question refers to the ATR plant of one of the leading petrochemical industries in South Africa. The membrane is used to separate hydrogen from a feed of hydrogen-carbon monoxide mixture (reformed gas) containing gases such as carbon dioxide and small amounts of methane, nitrogen, oxygen, argon, water, ammonia and hydrogen cyanide. The exit streams from the membrane are permeate (hydrogen gas) and non-permeate (synthesis gas, a mixture of hydrogen and carbon monoxide) with trace quantities of the other aforementioned gases. This dissertation investigated into the causes of decreasing membrane hydrogen (permeate) purity and insufficient membrane capacity to achieve the required synthesis gas (non-permeate) ratio. High differential pressure across the membranes as well the presence of ammonia and water in the feed gas to the membrane fibres were some of the causes of the membranes' low performance. Others include high speed of the membrane feed gas, the presence of strongly adsorbed gas - methane and insufficient heating of the feed gas by the preheaters. These led to broken and twisted membrane fibres. As a result, carbon monoxide and other constituents of the feed slipped into the permeate stream. From the research work, ways of enhancing the performance of the membrane unit were proffered. They include integrating a Pressure Swing Adsorption unit (upstream) with the membrane unit, recycling the permeate back to the feed stream, increasing the temperature of the feed gas and lowering the differential pressure across the membranes. Furthermore, the condensate should be prevented from getting to the membrane fibres by installing a drying unit, a demister unit or drains in the membrane feed gas header. In addition, periodic maintenance and backwashing should be carried out on the membranes. However, recommendations were made on further study regarding the cost impacts of implementing the ways of optimizing the membrane unit and simulation of the membrane unit's operation. The latter was suggested in order to obtain the process variables and parameters for improving the performance of the unit in question.
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