Fixed Bed Adsorption Operations

In an adsorption operation the movement of the solid phase gives rise to seri­ous problems (solid phase displacement devices, mechanical resistance of the adsorbent, etc.). As a result, the adsorbent is generally implemented in a fixed bed.

The adsorption-desorption operation is carried out batch-wise as shown in the simplified flowsheet in Figure 1.11.

Fixed Bed Adsorption Operations

Fixed Bed Adsorption Operations

The unit includes at least two beds, with one running in the adsorption phase, while the other is in desorption and switchover by a set of valves. For example if bed C1 is operating in the adsorption phase, valves Vu and V13 are open whereas valves V12 and V14are closed. In the case of a mixture of two components A and B, where component B is retained selectively by the adsor­bent, it is possible during the adsorption phase to get component A pure at the exit of bed CT with component B fixed on the bed. During the desorption stage which is carried out at the same time on bed C2 a desorption agent D is pumped in by keeping valves V24 and V22 open whereas valves V14 and V12 are closed. During the desorption phase, component B with desorption agent D is obtained at the exit of bed C2.

In practice, the operating procedure is often more complex and may include a purge step designed to keep from polluting the effluent from the des­orption stage by a large amount of fluid phase remaining in the fixed bed after the adsorption stage. As a result, the unit may have three or four beds oper­ating simultaneously.

This type of process is well suited whenever the adsorbent has high selec­tivity versus the component to be separated out. Fixed bed adsorption is used in particular to dehydrate refinery gases and to separate normal paraffins from isoparaffins in light gasolines in order to impi’ove the octane number and send only isoparaffins to the gasoline pool.

When selectivity is low, fixed bed adsorption is not so well suited and it is important to approach counter-flow conditions. Here a counter-flow contact can be simulated without moving the solid phase by changing the inlet and exit points of the feed, the desorption agent and the effluents. This is the solu­tion used in particular to separate paraxylene by means of X or Y zeolites.

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