Regeneration cycle

High boiling organics (normal bp >150 180° C) preferentially adsorbed extremely difficult to remove during regeneration cycle. 

The Catofin process, which was formerly the property of Air Products (Houdry Division), uses a proprietary chromium catalyst in a fixed-bed reactor operating under vacuum. There are actually multiple reactors operating in cycHc fashion. In sequence, these reactors process feed for about nine minutes and are then regenerated for nine minutes. The chromium catalyst is reduced from Cr to Cr during the regeneration cycle. 

Liquid Sorption. If a moist gas is passed through sprays of a liquid sorbent, such as lithium chloride or an ethylene glycol solution, moisture is removed from the air at a rate depending on the vapor pressure difference. This is a function of the absorbent concentration and is maintained at the required level by a regeneration cycle. The regeneration process is continuous and is achieved by allowing a percentage of the chemical into the exhaust-heated air. 

Molecular sieve beds do not suffer any chemical degradation and can be regenerated indefinitely. Care should be taken to minimize mechanical damage to the solid crystals as this may decrease the bed s effectivenes . The main causes of mechanical damage are sudden pressure and/or temperature changes when switching from absorption to regeneration cycles. 

The chemical and solvent processes previously discussed remove acid ga from the gas stream but result in a release of H2S and CO2 when the solvent is regenerated. The release of H2S to the atmosphere may be limited by environmental regulations. The acid gases could be routed to an incinerator or flare, w hich would convert the HiS to SO2. The allowable rate of SO2 release to the atmosphere may also be limited by environmental regulations. For example, currently the Texas Air Control Board generally limits H2S emissions to 4 Ib/hr (17.5 tons/year) and SO2 emissions to 25 tons/year. There are many specific restrictions on these limits, and the allowable limits are revised periodically. In any case, env ironmental regulations severely restrict the amount of H S that can be vented or flared in the regeneration cycle. 

Multiple desiccant beds are used in cyclic operation to dry the gas on a continuous basis. The number and arrangement of the desiccant beds may vary from two towers, adsorbing alternately, to many towers. Three separate functions or cycles must alternately be performed in each dehydrator. They are an adsorbing or gas drying cycle, a heating or regeneration cycle, and a cooling cycle. 

Figure 16-9. The regeneration cycle of o gas turbine uses recovered exhaust heat to preheat the compressed air prior to combustion.

The run and regeneration cycle on small to medium-size units is normally governed by a multi-port valve, which is the only moving part required. There have been many cases where this has failed and brine was injected directly into the boiler. A conductivity meter on the make-up line would guard against this. 

The dryer unit has an air inlet, an air outlet, a waste air outlet, two heater coils, and two 4-way reversing valves. Although the illustration shows the two tanks in a functioning mode, they are universal in operation. By this, we mean that this changes the tanks over when the active tank becomes totally saturated, and the tank that was on the regeneration cycle then becomes the active unit. 

Check the softener for adequate salt and run it through a regeneration cycle to ensure that the correct level of brine is drawn into the resin bed. The resin may lack adequate capacity due to fouling or cracking, or it may have been partially lost down the drain. Alternatively, the softener may be undersized. 

When using fixed beds, there are various arrangements used to switch between adsorption and regeneration. Cycles involving two, three and four beds are used. Clearly, the more beds that are used, the more complex is the cycle of switching between the beds, but the more effective is the overall system. 

Adsorption/desorption kinetics the time of the adsorption-regeneration cycle greatly depends on the kinetics of the C02 adsorption-desorption profile, which is measured in breakthrough experiments. Sorbents that adsorb and desorb C02 in a shorter time are preferred as these reduce the cycle time as well as the amount of sorbent required, and ultimately the cost of C02 separation. 

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