Evaporation of water from air-fluidized porous particles

The drying rate of wetted porous silica-alumina particles., FCC catalyst, was measured in a 15 cm diameter air-fluidized bed with a cloth distributor, containing about 1.5 kg catalyst. The particles tolerated the addition of remarkably large quantities of liquid water, up to 50% of the dry weight, without upsetting the fluidization characteristics: evidently the water was accommodated within the porous particle structure. The drying rate was measured in two ways: (1) by mounting the whole fluidized bed on a balance and (2) by measuring the inlet and outlet air humidities by wet and dry bulb thermometers and measuring also the air flow rate. The chan-e in weight from method (I) agreed well with results from (2) using a material balance. Tests with two methods of injecting the water, locally and evenly distributed, gave the same drying rate after an initial transient, showing that water readily distributes itself amongst the catalyst particles. The wet catalyst exhibited a constant drying rate followed by a falling rate. At the transition between these rate laws, the water content is consistent with the N-2 BET area of the catalyst being covered by a monolayer of water; this could be of practical value to designers and operators of dryers. During the constant-rate period, the exit air had a relative humidity which was constant at 60-70%. Calculations show that there is excellent air-solids contact, so the exit air appears to be in equilibrium with the wet catalyst. During the falling-rate period, it appears that the area of the monolayer diminishes, as also does its equilibrium vapour pressure. These characteristics make FCC catalyst an admirable material for a hospital fluidized bed to support a patient with certain injuries, e.g., serious bums, for which dry conditions and an ability to absorb liquid are important. (C) 2001 Elsevier Science Ltd. All rights reserved.