Bed lifting

Flow Sheet. Most purge-swing appHcations use two fixed-bed adsorbers to provide a continuous flow of feed and product (Fig. 16). Single beds are used when the flow to be treated is intermittent or cycHc. Because the purge flow is invariably greater than that of adsorption, purge is carried out in the down-flow direction to prevent bed lifting, and adsorption is up-flow. 

They are used with metal and plastic packings to prevent the bed lifting, or the entrainment of individual pieces of packing from being carried out of the tower. These packings usually do not break, and as long as the bed temperature is below the softening or deflection point... 

Group C powders are of small particle size, cohesive by nature and hence, difficult to fluidize. Either the fluidized bed lifts as a solid plug of material or forms stable channels of air flow, which allows the fluidizing gas to escape. The latter phenomenon is referred to simply as channelling . It may be possible to fluidize such powders by mechanical agitation (e.g., stirring, vibration). 

Flow direction considerations for liquid systems are somewhat different than those for vapor flow, In liquid or dense-phase flow the buoyancy force of the liquid must be considered as well as the pressure drop (see below). Thus, for upfiow adsorption the flow velocity must not cause bed lifting, As the Dow rate exceeds lifting velocity, the pressure drop increases only very slowly with increasing velocity. Because of this, sometimes liquid systems are designed with some bed en puns ion wben it is desirable to limit pressure drop. Since too much expansion wjtl cause the adsorbent io become well mixed, with a concomitant drop in removal efficiency, expension is usually limited to about 10%. Higher velocities also tend to create loo much particle turbulence, abrasion, attrition, and erosion. Upflow ndsorption is a preferred direction if the... 

When an adsorbent bed is to be filled with a liquid, there must be sufficient tima for any gas that may be trapped in the pores (o ontgas. Otherwise the vapor may later contaminate a product or, in the case of upflow udsorption, the effective bulk density may be lowered enough to cause excessive bed lifting or flow channeling. 

The critical parameter for upllow in pecked beds of adsorbent at high Flow rates is the potential for particle movement or bed lifting,6 This occurs as the conditions approach those for the onset of fluidization. At thet point, the pressure drop is given by... 

In liquid or dense-phese flow, the buoyancy force of the liquid must be considered as well as the pressure drop. Thus, for upflow adsorption the maximum upward flow velocity without bed lifting is thel where the sum of the pressure drop and the buoyancy just equals the foree due to gravity that is. the pressure drop equation is the same as (hat used above to test for bed lifting. 

Most of the flow directions in a pressure-swing process are fixed relative to one another. The most common configuration is for adsorption to take place upflow, thus the name blowdown for counteicunem depressurization. Therefore, some precautions must be taken to assure that no bed lifting or crashing occurs (see below). Pressure-swing stqis are more diflicuh to assess because many steps such as depressurization, blowdown, pressurization, and equalization b in at a high flow rale and decrease to low or no flow. 

In the case of the thermally efficient CO2 circulation developed by Distillers (Carbon Dioxide) Ltd., these two factors are interrelated. The flow rate is determined at the design stage by the evaporative capacity of the main heat exchanger (the condenser-evaporator in Figure 4.4). Resistance to flow occurs in the bed of hop powder through which the CO2 flows and at the filters, principally those at the tops of the extractor vessels. Flow is in the upward direction at up to 5 cm/s, which is sufficient to fluidise the bed, lifting the finer particles to the top and thence to the filters. The design and installation of these filters is thus critical to the achievement of CO2 flow. 

For liquid phase applications the buoyancy forces need to be considered as well. The flow velocity in the upwards direction should normally be sufficiently low to prevent bed lifting. However, in some applications it is desirable to allow some bed expansion to occur and so limit the pressure drop. As the minimum velocity to cause lifting is exceeded, the pressure drop increases only slightly with further increases in velocity. Too much expansion, however, can cause the bed to become well mixed. If this were to occur within a fixed bed then it would resemble the batch process and create the risk of reduced purity in the product. Other problems caused by high velocities include abrasion, attrition and erosion. When desirable, expansion is accordingly limited normally to about 10%. 

It is preferable in a liquid phase application to refill an adsorbent bed in the upwards direction because it is easier to sweep out pockets of gas or vapour and so prevent maldistribution in the proceeding adsorption step. Consideration must be given to the time required to ensure that the gas and vapour pockets have been removed completely otherwise there is a risk that they will contaminate the product in the adsorption step and cause excessive bed lifting if flow is upwards during the adsorption step. 

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