Hydrogen PSA process product purity control method and controller
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Hydrogen PSA process product purity control method and controller Marian Simo • Andrew C. Rosinski Paul W. Belanger • Safdar Baksh
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Received: 16 May 2013 / Accepted: 13 September 2013 / Published online: 22 September 2013 Ó Springer Science+Business Media New York 2013
Abstract It is well known in the industry that a primary means for controlling the pressure swing adsorption (PSA) process product gas purity is the adjustment of PSA feed time or adsorption time. If the product impurity is too high, the feed time is shortened and if the impurity level is below the target the feed time is increased. Conventionally, the plant operator monitors the product purity and manually adjusts the feed time. Several control methodologies such as classical feedback and feedforward systems were suggested to automate this task with limited success. A novel control methodology based on the measurement of impurity fronts within the adsorber bed was developed by the Praxair Adsorption R&D team. The response of the concentration measurements inside the adsorber vessel to the process upsets and changes in feed time is more rapid than in the product stream. Consequently, closed loop control performance can be made much more effective and the operating impurity set points for product gas can be more aggressive resulting in longer PSA feed times, higher bed utilization and thus higher hydrogen recovery. The control methodology will be discussed in greater detail along with the advantages it has to offer such as improved process performance, disturbance rejection capability and improved process robustness. The control methodology will be illustrated using the hydrogen PSA process as an example. Keywords
Hydrogen PSA Product purity control
M. Simo (&) A. C. Rosinski P. W. Belanger S. Baksh Praxair Technology Center, 175 E. Park Dr., Tonawanda, NY 14150, USA e-mail: [email protected]
1 Introduction Adsorption processes today are well established in the chemical industry and they are often the method of choice for difficult gas separations. Adsorption processes can be distinguished by the process cycle employed, for example, pressure swing adsorption (PSA), temperature swing adsorption (TSA) or vacuum swing adsorption (VSA). Multiple adsorber vessels can be used to increase the production capacity and separation efficiency usually expressed as the main component yield or recovery. The impurities are removed from the feed stream utilizing one or more adsorbents contained within the vessel to produce the product stream. In case of hydrogen PSA process, the adsorber bed typically contains an initial layer of alumina to adsorb water vapor, a layer of activated carbon to adsorb carbon dioxide and hydrocarbons and a final layer of zeolite adsorbent to adsorb carbon monoxide and nitrogen. The time an adsorber bed spends in the feed or adsorption step is referred to as the feed time. As the feed time is increased the impurity front(s) are moving towards the product end of the bed and eventually the impurity breakthrough is observe
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