Column Developments

These have centred mainly on a better understanding of the fluid dynamics within the column and specifically across the trays (the first book on this topic was by Lockett in 1986), and also on improved prediction methods for determining plate and overall column efficiencies. A better knowledge of the interrelation of these two aspects is beginning to emerge from several research groups, e.g. Biddulph and co-workers, and their studies of the relationship between Marangoni surface tension effects and plate efficiencies (American Institute of Chemical Engineers Journal 37(8): 1261-1264,1991). A development since the 1970s is the preference for sieve trays, rather than bubble or valve trays which were prevalent up to that time. The AIChE Bubble Tray Design Manual (1958) is still used (with modifications) and quoted, even for the calculation of sieve tray efficiencies for which it was not intended and for which it provides rather poor results. However, this older method has provided a starting point for recent studies which examine the significance of fluid-related variables (such as surface tension) and how this knowledge can be used to design more efficient tray columns.

Many papers have been published in the more industry-orientated journals (e.g. Chemical Engineering (NY), Chemical Engineering Progress and Hydrocarbon Processing) concerning column operation and performance problems. These papers also discuss particular aspects of internal column design and use of the newer structured packings, now available as alternatives to tray systems, in relation to column performance and operation. The use of high efficiency packings requires a better understanding of the hy-drodynamic conditions and the mass transfer processes that occur in the packing. Fouling and plugging within distillation equipment can become serious problems and they are areas in need of better understanding. High surface area packings are popular because they promote efficiency, however they are also more prone to fouling problems.

If solids are present in a stream then design solutions generally act to keep the solids moving. Therefore any liquid maldistribution within the column or tray channelling (due to initial vapour maldistribution) must be corrected to avoid plugging problems. Plugging and fouling have varying effects depending upon the actual in-service conditions, and hence it is difficult to devise generic strategies or solutions. General advice tends to focus upon the need for good distributor design, good wetting (and wettability) of the mass transfer surface, and good distribution of the flow streams within the equipment.

Papers describing column operational problems, such as product draw limitations especially for column revamps (Chemical Engineering Progress 94(6), 63-77, 1998), are particularly useful for designers and process operators.

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Solar Panel Basics

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