Plate hydraulic design

The basic requirements of a plate contacting stage are that it should  

Provide sufficient liquid hold-up for good mass transfer (high efficiency). 

Have sufficient area and spacing to keep the entrainment and pressure drop within acceptable limits. 

Have sufficient downcomer area for the liquid to flow freely from plate to plate. 

Plate design, like most engineering design, is a combination theory and practice. The design methods use semi-empirical correlations derived from fundamental research work combined with practical experience obtained from the operation of commercial columns. Proven layouts are used, and the plate dimensions are kept within the range of values known to give satisfactory performance. 

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This efficiency term (C) is potentially very comprehensive as it accounts for the key factors in plate hydraulic design, e.g. hole diameter, liquid depth, vapour velocity and open area. 

Marine and hydraulic-oil coolers use these characteristics to enhance the coefficient of otheiwise poorly performing fluids. The higher metallurgies in marine applications like 90/10 Cu-Ni afford the higher cost of plate-fin design to be offset by the less amount of alloy material being used. On small hydrauhc coolers, these fins usually allow one to two size smaller coolers for the package and save sldd space and initial cost. 

It is important to understand whether there will be two-liquid phases present in the column. If two-liquid phases form in a large part of the column, it can make the column difficult to operate. The formation of two-liquid phases also affects the hydraulic design and mass transfer in the distillation (and hence stage efficiency). If it is possible to avoid the formation of two-liquid phases inside the column, then such behavior should be avoided. Unfortunately, there will be many instances when two-liquid phases on some plates cannot be avoided. The formation of two-liquid phases can also be sensitive to changes in the reflux ratio. 

Michalik (M9), 1957 Study of hydraulics of laminar film flow of Newtonian fluid in tubes and on vertical plates. Optimum design parameters for wetted-wall columns derived. 

Sieve trays are used throughout the absorption column, however two distinct hydraulic designs are required. The first sieve plate design is required for trays below the weak-acid feed point. Above the weak-acid feed point, the downcoming liquid flowrate is diminished. The vapour flowrate essentially remains constant throughout the column. Different vapour to liquid ratios above and below the weak-add feed point require a second hydraulic design to be considered. 

For bubble-cap plates, hydraulic-gradient must be given serious consideration. It is a function of cap size, shape, and density on the plate. Methods for analyzing bubble-cap gradient may be found in the chapter by BoUes (Smith, Design of Equilibrium Stage Processes, Chap. 14, McGraw-Hill, New York, 1963) or in previous edition of this handbook. 

NORMAL OPERATION OF SIEVE PLATE. A sieve plate is designed to bring a rising stream of vapor into intimate contact with a descending stream of liquid. The liquid flows across the plate and passes over a weir to a downcomer leading to the plate below. The flow pattern on each plate is therefore crossflow rather than countercurrent flow, but the column as a whole is still considered to have countercurrent flow of liquid and vapor. The fact that there is crossflow of liquid on the plate is important in analyzing the hydraulic behavior of the column and in predicting the plate efficiency. 

A manifold nozzle (see Figure 4.39a) is equipped with a hydraulic cylinder installed in the clamping plate and designed to operate at a maximum pressure of 5 MPa (closure force approximately 2,800 Pa). The pin runs in the manifold, giving the flow channel a rectilinear shape. One drawback, though, is the displacement of the pin away from the nozzle and cylinder axis (flexure) when there is a change in manifold temperature. The nozzle is closed by a cylindrical pin. 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

Figure 11 shows a t pical liquid collector plate for a column that uses one side downcomer to withdraw the liquid. The maximum diameter for such a design is about 12 ft, which is limited by the hydraulic gradient necessary for such a liquid flow-path length, For larger diameter columns, two opposite side downcomers or a center downcomer normally is used unless the total amount of liquid collected is relatively small. 

The plate structure must be designed to support the hydraulic loads on the plate during operation, and the loads imposed during construction and maintenance. Typical design values used for these loads are ... 

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