Other Types of Separators

Separations that take place in a chromatographic column are very similar to other types of separation, and it is for this reason that we will cover several of the more important separation techniques. A chromatographic column may be likened to a distillation column in many respects in fact, some of the terminology in chromatography is taken from distillation theory. 

Process synthesis heuristics fall into several categories. Many of these were proposed origiually for multicomponent distillation but have since been applied 10 other types of separation. Some heuristics have bean verified through calculation and experimentation while others are described best as common senes, in any case, they serve as effective guidelines in the selection and sequencing of separation processes. 

Several other types of separators are now available for GC-MS, each with advantages and disadvantages. Updegrove and Haug (6) give details on the various separators, as well as a discussion of many problems experienced in GC-MS analysis. 

Which operating system should be used for a particular separation problem Should the stem be operated in the gas or liquid phase Should adsorption, partition, size exclusion, or km exchange be used These questions ultimately revert to economics. At the current state of the art, exact answers are not available. However, some qu itative comparisons and general rules can be iltawn which should be helpful in selecting the process. The important question of whether a chromatographic or other type of separation should be used is not addressed. 

By definition the membrane liquid phase of a liquid membrane must be immiscible with the solutions in contact with it. This allows the chemical composition of all three liquid phases (including the membrane liquid phase) to be altered independently of one another. For this reason liquid membranes offer a higher degree of control over the membrane separation process compared to other types of separation membranes. 

Plate evaporators may be used as a heating element in a forced circulation system in which boiling is deliberately suppressed. Bolling may also occur within the gasketed-plate evaporator with the mixture of liquid and vapor discharged into a cyclone or other type of separator. 

Centrifugal separators cannot be sized with the F factor approach used with other types of separators because the maximum allowable velocity is a weaker function of gas density than the square root dependence in the F factor correlation. Maximum velocities for centrifugal separators depend upon the design and are different for each of the several manufacturer units. Pressure drop varies from 1 to 10 velocity heads depending upon the manufacturer. Velocity head should be based on inlet (outlet) nozzle velocity. 

Then, there are some general applications, such as studies of porous polymer gels, important in gel permeation chromatography and other types of separations. The preceding lists are obviously incomplete. It is the hope of the author that you, the student, will discover yet unknown applications. 

The principle of the cyclone is to feed the slurry tangentially and force it to rotate. By centrifugal forces, the coarser particles sink to the bottom of the cone while the finer particles float to the top. Both streams separate. The underflow, which consists of coarse material, and the overflow, which consists of fines, are then directed to other circuits. The underflow is fed back to the ball mills and the overflow is directed to the flotation circuit or other types of separators. 

Obviously, if pressure drops on the order of 6 to 8 kPa are acceptable from a process point of view, an 8 or 13-unit multicyclone (or some number in between) could prove to be a viable option for the task at hand. If not, other types of separation equipment may have to be considered such as a baghouse, an electrostatic precipitator, or a wet scrubber. Nevertheless, even if the pressure drop across the cyclones were acceptable, one would still need to consider the long-term wear implications associated with operating the cyclones at velocities in the range of 35 to 40 m/s. If the solids being processed are not especially abrasive and/or if they are sufficiently flne in size, it may be possible to operate at these velocities. If not, erosion-protective liners may need to be installed. Some bare-metal multicyclone systems have been observed to operate for many years at velocities of 70 to 85 m/s while processing several tons per day of rather abrasive sand-like particles that were under about 25 fim in size. 

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