Ion exchange and similar devices, almost without exception, operate with fixed or semi-continuous beds. The standard design for a fixed-bed is a vertical cylindrical tank equipped with a resin support-liquid collection system at the bottom and a distribution system above the resin level. The distribution and collection systems are critical design features since the liquid must be delivered uniformly over the total surface of the resin bed without downward jetting which will disturb the level of the bed whereas the liquid leaving the bottom layer of resin must be collected uniformly across the bed.
Although fixed-bed systems are the most widely used, they suffer from several significant disadvantages. One is that the ion exchange reaction occurs only in the volume of resin in the exchange zone. Thus, resin above the exchange zone is exhausted and inert whereas that below the exchange zone is not even in use. Not surprisingly therefore, the total resin inventory in a fixed-bed column is of necessity considerably larger than needed at any one time. Additionally, since the resin in the column must be periodically cleaned and regenerated, there is a considerable amount of wasted down-time. Finally, since the fluid with which the process is carried out will have progressively decreased concentrations of the ion to be exchanged with the resin as it proceeds down the column, the concentration gradient likewise decreases and the reaction becomes less efficient.
Although such fixed-bed ion exchange processes may be carried out continuously by connecting several fixed-bed columns in parallel, with regeneration being carried out in one column while the reaction proceeds in one or more additional columns, such systems have not proven themselves completely satisfactory. More specifically, a group of fixed-bed columns still suffer from many of the disadvantages inherent in a single fixed column such as the limited volume of the actual exchange zone and the presence of a much larger amount of resin than is actually required at any one time. Additionally, even though the problem of diminished concentration gradient could be alleviated to some extent by arranging two or more fixed-bed columns in series, with fresh ion exchange fluid being fed into each of those columns, there is nonetheless still a greatly diminished reaction driving force within each individual column due to the aforementioned necessity of including far greater amounts of resin in the column than are needed at any given time.
Another significant disadvantage of conventional fixed-bed resin exchange systems is the difficulty of adapting such systems to more complicated ion exchange processes. More specifically, the addition of a single column to an existing group of exchange columns requires the installation of relatively complex valving arrangements which can perform the proper mixing of feed materials which might consist of fresh materials from outside the system, the effluent from one or more of the columns already in the system, a mixture of fresh and recycled materials or even no feed at all. Likewise, valves must be provided at the discharge end of the column which are capable of purging the column effluent from the system, recycling it directly to one or more of the other columns, mixing it with one or more other feed or recycled streams for entry into another column, or a combination of any of the above operations. One skilled in the art will quickly come to the realization that a system comprised of ten columns, each of which has a specific feed and discharge relationship with respect to the remaining columns, will require an extremely intricate system of valves. It will also be appreciated that even a relatively minor change in any of the operating conditions in the sample 10 column process might require a significant manipulation of the valving arrangement.
Continuous (or in fact semi-continuous) contactors have been developed which solve some of the above-identified problems. In such systems, the resin and the solution pass countercurrent to one another and steady state zones can be set up as required for exhaustion, regeneration and the required intermediate rinses. The volume of resin can thereby be reduced to the sum of the working volumes plus the interconnecting resin transfer system. Even with this improved approach, however, the flows must periodically be interrupted, so truly continuous feed and discharge is not obtained, i.e., a true steady-state profile is not achieved.
Continuous contactors generally take the form of either pulsed columns or fluidized beds. In pulsed columns, resin is moved up or down through the contacting zones by periodic application of pressure or a vacuum. Solutions flow through the resins between pulses. In fluidized bed systems, the exchanges take place with non-compacted resin. The resin may fall down through a baffled column against the upflow stream or the exchange may take place in stirred compartments or troughs where the resin is forwarded mechanically against the flow of solution.
Continuous contactors offer many advantages over the fixed-bed type, one of the more significant of which is the more efficient utilization of resin. Additionally, far greater volumes of very concentrated solutions can be treated in continuous contactors than would be possible using a fixed-bed reactor. The necessity of maintaining a uniform distribution of fixed solutions into and collection of product solutions from continuous contactors is one potential major drawback of such systems. Mechanical breakage of resins as well as the need for a large number of mechanical valves are often problems when pulsed bed systems are employed.
Although more efficient utilization of resin can be achieved through continuous contacting systems, a substantial degree of inefficiency nonetheless exists. More specifically, since the increments in the pulsed bed, for example, are necessarily of a finite volume, the reaction equilibrium existing in any one of those increments will not always be very favorable. Such a loss in chemical efficiency is due in part to the partial reclassification of resin which occurs during each pulse. Of course, it would be impractical to make the increments too small.