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Chemical and Process Engineering Resources

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Category: Equipment Design
Question: What are some design criteria to keep in mind when designing and operating a gas adsorber using solid adsorbents?
Keywords: adsorption,adsorber,design,operation,attrition,velocity,criteria
Answer: Expected good results with an allowance for physical access space and volume above and below the bed(s). Sometimes you may want to use several separated beds for varying the adsorbent, allowing for sampling, distributing weight and volume load, avoiding channeling, etc.You do need space and volume to inspect, install, and remove such internals as:Adsorbent;Bed support(s);Bed holddowns;distributors; andinert fillers (such as ceramic spheres).Depending on your adsorber's diameter, you would normally employ about 12" to 24" of internal height as non-adsorbent volume below the bed as well as above it. With downward adsorption flow, s superficial velocity of 29 cm/sec (1 ft/sec) should be good for most applications. Try to keep the adsorption velocities down below this whenever possible as long as there's no risk of getting a preferential channeling effect by the process gas.A shock load imposed on an adsorption bed is a physical force, in the form of increased, spontaneous gas flow, such that the solid-particle bed is made to move or "lift". Sometimes these shocks can be of a magnitude that crushes or severely reduces some of the particles to dust. The source(s) of shock loads are:* Process upsets upstream of the adsorption unit;* Quick, or abrupt opening of inlet or outlet valves when putting a bed into adsorption service;* Quick, or abrupt opening of inlet or outlet valves when putting a bed into regeneration service;* Sudden internal pressure changes within the bed, such as Pressure Relief Valve activation while on stream;* Sudden or rapid de-pressurization of a bed.All of the above represents un-needed and undesired bed movement. Ideally, bed movement should be reduced to zero to maximize adsorbent life and effectiveness.The inherent cyclic nature of solid bed adsorption systems make them susceptible to required pressure changes. However, one can markedly reduce the potential danger of pressure shocks by "ramping" up and/or down with the inlet or outlet valves. Another way to control this is to design a pressurization/de-pressurization scheme whereby process gas is bled into the bed at a controlled rate (usually with a resistance orifice) prior to the bed being put back into adsorption service. With today's instrumentation hardware and software the above techniques should not be a difficult problem for process engineers. Reducing the source of bed movement is one positive design step you should incorporate. Another is arresting or neutralizing the effects of any potential force that may cause bed movement. This is done by fixing or "anchoring" the adsorbent bed. The idea here is to mechanically design the internals such that an external force will find it difficult to move or lift the bed. One process technique is to use the direction of flow to work in your behalf. That is why downward flow is used in your case. As long as the bed support is strong enough, any incoming force will keep the bed pinned against the bottom support and movement should be nil.The use of "filler" inert material (such as ceramic spheres) is quite common in helping to support the bed in the bottom section and also retaining the bed in the upper section. A fixed bottom support is usually used and a strong, flexible, "floating" screen is employed between the top of the adsorbent bed and the inert spheres in the top section. The goal here is to avoid void spots within the adsorber vessel, thereby curtailing or eliminating internal space available for bed movement or migration. I have also used spring-loaded screens in the top section to assure bed hold down. Don't forget that your internal material will expand and contract with regeneration heat and cool down.Liquid water can damage some adsorbents and should be removed prior to the adsorption unit with efficient separators.One last item. Make sure that your regeneration flowrate is always controlled and never "shocked" into your bed. Since you are regenerating in the upflow direction, the bed will have a tendency to "lift" if the regen flow is excessive, even if for only an instantaneous moment.Source: Cheresources Message Board, Mr. Art Montemayor