Contacting devices

External Fluid Film Resistance. A particle immersed ia a fluid is always surrounded by a laminar fluid film or boundary layer through which an adsorbiag or desorbiag molecule must diffuse. The thickness of this layer, and therefore the mass transfer resistance, depends on the hydrodynamic conditions. Mass transfer ia packed beds and other common contacting devices has been widely studied. The rate data are normally expressed ia terms of a simple linear rate expression of the form... 

Distillation has been practiced in one form or another for centuries. It was of fundamental importance to the alchemists and was in use weU before the time of Christ. The historical development of distillation has been pubHshed (1) as has the history of vapor—Hquid contacting devices (2). 

Three principal vapor—Hquid contacting devices are used in current crossflow plate design the sieve plate, the valve plate, and the bubble cap plate. These devices provide the needed intimate contacting of vapor and Hquid, requisite to maximizing transfer of mass across the interfacial boundary. 

Information on the liquid- and gas-handling capacity of the contacting device chosen for the pariicular separation problem. Such information includes pressure drop charac teristics of the device, in order that an optimum balance between capaital cost (column cross section) and energy requirements might be achieved. Capacity and pressure drop charac teristics of the available devices are covered later in this Sec. 14. 

Plate Typ es Plate columns utihzed for hquid-gas contacting may be classified according to mode of flow in their internal contacting devices ... 

Extraparticle Transport and Dispersion Mechanisms Extraparticle mechanisms are affecded by the design of the contacting device and depend on the hydrodynamic conditions outside the particles. 

Rate equations are used to describe interphase mass transfer in batch systems, packed beds, and other contacting devices for sorptive processes and are formulated in terms of fundamental transport properties of adsorbent and adsorbate. 

Contactor design is important in order to maximize the ozone-transfer efficiency and to minimize the net cost for treatment. The three major obstacles to efficient ozone utilization are ozone s relatively low solubility in water, the low concentrations and amounts of ozone produced from ozone generators, and the instability of ozone. Several contacting devices are currently in use including positive-pressure injectors, diffusers, and venturi units. Specific contact systems must be designed for each different application of ozone to wastewater. Further development in this area of gas-liquid contacting needs to be done despite its importance in waste treatment applications. In order to define the appropriate contactor, the following should be specified ... 

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

Gerster [176] presents the results of studies on the tray efficiencies of both tray and packing contacting devices. Note that Gerster compares his work to the AIChE Manual [2]. 

This suggests that caution must be exercised when establishing a tray efficiency for any type contacting device by (1) using actual test data if available for some similar system or (2) comparing several methods of predicting efficiency, and (3) possible use of a more conservative efficiency than calculated to avoid the possibility of ending up with a complete column with too few actual trays—a disastrous situation if not discovered prior to start-up operations. 

The contacting devices of (1) above may actually be missing, i.e., blown off one or more trays, so all that is existing is a rain-deck tray with no liquid-vapor contacting. 

Chen [133] highlights the long-term growth of the technically popular use of bubble cap trays, valve and sieve trays, followed by the increased popularity of packed columns accompanied by the development of random and structured packings. There are some applications in chemical/ petrochemical/petroleum/gas treating processes where one type of contacting device performs better and is more economical than others. Chen [133] points out ... 

Because the packed tower is a continuous contacting device as compared to the step-wise plate tower, performance capacity is expressed as the number of transfer units, N, the height of the transfer unit, H.T.U., and mass transfer coefficients K a and Kj a. Figure 9-68 identifies the key symbols and constant flow material balance. 

Packed towers are used in some distillation operations in preference to plate towers. Usually the selection requires an understanding of the fouling characteristics of fluids of the system. These towers have been used even in polymer forming operations. However, other contacting devices can be cleaned easier. For some processes the packed tower is much more effective as well as cheaper than a tray tower. 

Figure 10-173. Direct contact tray column for heat transfer. This could be a baffle tray, sieve type tray, bubble or other contact device, or open spray or random packed column. (Symbols only used by permission Smith, J. H. Hydrocarbon Processing, Jan. 1979, p. 147. Gulf Publishing Company. All rights reserved.)...

E. Baffle Tray Column (or, Termed Shower Deck, No Holes, Caps, or Other Contact Devices)... 

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

In the extraction process, the LPG from the prewash tower enters the bottom of an extractor column. The extractor is a liquid/liquid contactor in which the LPG is counter-currently contacted by a caustic solution. Another option is the use of a fiber film contacting device. The mercaptans dissolve in the caustic (Equation 1-14). The treated LPG leaves the top of the extractor and goes on to a settler, where entrained caustic is separated. 

Figure 11-10. Culcululcd values of ihc injection currenl components and the total device current as a function of bias for the Pi (upper panel), Cu (central panel), and Al (lower panel) hole injecting contact devices.

Biomedical Applications Due to their excellent blood compatibility (low interaction with plasma proteins) and high oxygen and moisture permeabilities, siloxane containing copolymers and networks have been extensively evaluated and used in the construction of blood contacting devices and contact lenses 376). Depending on the actual use, the desired mechanical properties of these materials are usually achieved by careful design and selection of the organic component in the copolymers. 

Typical Values. Table 11.3 shows typical parameter values for mechanically agitated tanks and other gas-liquid contacting devices. Not shown are values for kgAj since these are usually so large that they have no influence on the mass transfer rate. 

TABLE 11.3 Typical Operating Ranges for Gas-Liquid Contacting Devices... 

The main process variables in differential contacting devices vary continuously with respect to distance. Dynamic simulations therefore involve variations with respect to both time and position. Thus two independent variables, time and position, are now involved. Although the basic principles remain the same, the mathematical formulation, for the dynamic system, now results in the form of partial differential equations. As most digital simulation languages permit the use of only one independent variable, the second independent variable, either time or distance is normally eliminated by the use of a finite-differencing procedure. In this chapter, the approach is based very largely on that of Franks (1967), and the distance coordinate is treated by finite differencing. 

Select the type of contacting device plates or packing. 

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