Plates liquid flow

Trays with drilling or slits in the base plate. Liquid flow controlled across the base plate or drops downward through the base plate openings where the vapor flows counterflow upward, e.g., sieve tray, turbogrid tray)... 

Figure 5.26. Schematic distillation column with sieve plates. Liquid flows from a downcomer and across the sieve plate, into the next downcomer. Vapor flows through the holes in the sieve plate, creating a froth above the liquid.

The cross-flow plate (Fig. 14-16 ) utilizes a hquid downcomer and is more generally used than the counterflow plate (Fig. 14-16Z ) because of transfer-efficiency advantages and greater operating range. The liquid-flow pattern on a cross-flow plate can be controlled by placement... 

FIG. 14-17 Common liquid-flow patterns, cross-flow plates. (Smith, Design of Eqiiihhriiim Stage Processes, McGraw-Hill, New York, 1963. )... 

Typically the plate is half-moon in shape and is sloped slightly in the direction of liquid flow. Gas contacts the liquid as it showers from the plate, and a serrated lip or weir at the edge of the plate can be used to improve the distribution of liquid in the shower. 

Plate-Column Capacity The maximum allowable capacity of a plate for handling gas and liquid flow is of primaiy importance because it fixes the minimum possible diameter of the column. For a constant hquid rate, increasing the gas rate results eventually in excessive entrainment and flooding. At the flood point it is difficult to obtain net downward flow of hquid, and any liquid fed to the column is carried out with the overheaa gas. Furthermore, the column inven-toiy of hquid increases, pressure drop across the column becomes quite large, and control becomes difficult. Rational design caUs for operation at a safe margin below this maximum aUowable condition. 

FIG. 14-29 Pressure-drop contributions for cross-flow plates, = pressure drop through cap or sieve, equivalent height of plate liquid = height of weir ... 

Ejfects of Gas and Liquid Mixing As noted previously, it is necessary in most instances to convert point efficiency E g to Murphree plate efficiency E, ,. This is true because of incomplete mixing only in small laboratoiy or pilot-plant columns, under special conditions, is the assumption E g = E, , likely to be valid. For a crossflow plate with no hquid mixing there is plug flow of hquid. For this condition of liquid flow, Lewis [Ind. Eng. Chem., 28, 399 (1936)] analyzed effects of gas mixing on efficiency. He considered three cases ... 

Gas unmixed liquid flows in the same direction on successive plates. 

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. 

However, in certain cases, the impingement plate design is preferred over packed-tower columns when either internal cooling is desired, or where low liquid flow rates would inadequately wet the packing. 

PM Control - An impingement-plate scrubber is a vertical chamber with plates mounted horizontally inside a hollow shell. Impingement-plate scrubbers operate as eountercurrent PM collection devices. The scrubbing liquid flows down the tower while the gas stream flows upward. Contact between the liquid and the particle-laden gas occurs on the plates. The plates are equipped with openings that allow the gas to pass through. Some plates are perforated or slotted, while more complex plates have valve-like openings. 

The characteristics of the pump relate the applied pressure on the cake to the flowrate at the exit face of the filter medium. The cake resistance determines the pressure drop. During filtration, liquid flows through the porous filter cake in the direction of decreasing hydraulic pressure gradient. The porosity (e) is at a minimum at the point of contact between the cake and filter plate (i.e., where x = 0) and at a maximum at the cake surface (x = L) where sludge enters. A schematic definition of this system is illustrated in Figure 2. 

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. 

Collected liquids flow down the pipes and drain to a collection sump. Collected solids are washed off the plates with water or other liquid. Sometimes the dust/solids can be removed by mechanically vibrating or knocking on the plates while the particles are dry. The electrical power of the precipitator is applied only... 

Caps should be arranged on the plate in 60° equilateral layout, with the liquid flowing into Ae apex of the triangle rather than parallel to the base. The liquid flows normal to each row of caps. 

Cross flaw Liquid flowing across a plate (rather than straight down through the holes) so that it falls to the plate below through a channel at one side of the plate. 

Sutherland, S.,Jr., Characterisdcs of Countercurrent Vapor 99. Liquid Flow at a Perforated Plate. M. S. Thesis, Jan. 1958. 

Plates are pressed from stainless steel, titanium, HasteT loy or any material ductile enough to be formed into a pressing. The specious design of the trough pattern strengthens the plates, increases the effective heat transfer area and produces turbulence in the liquid flow between plates. Plates are pressed in materials between 0.5 and... 

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