Scrubbers separation number

The predominant separation mechanism in scrubbing is impact due to the inertia of a particle with density relatively higher than that of the liquid. In such a way, the relative velocity between the particle and the liquid droplet is an important factor and the process may depend on the acceleration of, both, the particle and the droplet in relation to the point of impact. The process may be controlled by varying the droplet velocity, in order for the particle to collapse with it. The efficiency of the scrubbers is normally expressed as the fraction of particles collected from a volume of gas, determined by the projected area of the receiving body To achieve this, a separation number Se (Maas, 1979), may be defined as... 

Efficiency of separation versus separation number for spherical droplets in scrubbers. 

As pointed out in Volume 1, Chapter 3, a compressor may have any number of stages. Each stage normally contains a suction scrubber to separate any liquids that carry over or condense in the gas line prior to the compressor cylinder (or case for centrifugal compressors). When gas... 

Equipment Selection Criteria and Guidelines A number of factors should be considered to determine when to select which type ofvapor/gas/solid-liquid separator (blowdown drum or cyclone separator) to handle a multiphase stream from a relief device, and which final control or destruction equipment (emergency flare, emergency scrubber, or quench pool/catch tank) should be selected. These factors include the plot plan space available, the operating limitations of each type, and the physicochemical properties of the stream. 

A wet scrubber that will handle fume as well as dust is the Feld illustrated in Fig. 33. This consists of a number of separate washing chambers, one above the other. 

In distillation, the liquid reflux returned to the top of the column plays, in a sense, very much the role of a solvent. In the course of its downward flow, it dissolves residues of the heavy component contained in the vapor phase and thereby contributes to its enrichment in the lighter, more volatile component. A reduction of reflux, or of the reflux ratio, may therefore be expected to result in an increase in the required number of stages in much the same way as happens in the case of the gas scrubber. Ultimately, when the operating lines simultaneously intersect the equilibrium curve, a pinch results and the number of stages goes to infinity. This is shown in Figure 7.19a. The reflux ratio at which this occurs can be read from the intercept Xpi/ R +1) and represents the minimum required to achieve the desired separation. 

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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