Plate pressure drop

The pressure drop over the plates is an important design consideration. There are two main sources of pressure loss that due to vapour flow through the holes (an orifice loss), and that due to the static head of liquid on the plate. 

A simple additive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hj) the head of clear liquid on the plate (hw + how) and a term to account for other, minor, sources of pressure loss, the so-called residual loss hr. The residual loss is the difference between the observed experimental pressure drop and the simple sum of the dry-plate drop and the clear-liquid height. It accounts for the two effects the energy to form the vapour bubbles and the fact that on an operating plate the liquid head will not be clear liquid but a head of aerated liquid froth, and the froth density and height will be different from that of the clear liquid. 

It is convenient to express the pressure drops in terms of millimetres of liquid. In pressure units  

The pressure drop through the dry plate can be estimated using expressions derived for flow through orifices. 

Methods have been proposed for estimating the residual head as a function of liquid surface tension, froth density and froth height. However, as this correction term is small the use of an elaborate method for its estimation is not justified, and the simple equation 

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

For valve plates, values of K and Ko depend on whether the valves are fully open. They also depend on the shape and weight of the valves. Vendors of valve plates make K and Ko data (or their equivalent) readily available. An analysis of valve plate pressure drop has been reported by Bolles [Chem. Eng. Pmgr. 72(9), 43 (1976)], and typical dry head loss data, shown in Fig. 14-31, are taken from that worlc. 

Pressure drop. The pressure drop over the plates can be an important design consideration, particularly for vacuum columns. The plate pressure drop will depend on the detailed design of the plate but, in general, sieve plates give the lowest pressure drop, followed by valves, with bubble-caps giving the highest. 

Check the plate pressure drop (Section 11.13.14), if too high return to step 6. 

The height of the weir determines the volume of liquid on the plate and is an important factor in determining the plate efficiency (see Section 11.10.4). A high weir will increase the plate efficiency but at the expense of a higher plate pressure drop. For columns operating above atmospheric pressure the weir heights will normally be between 40 mm to 90 mm (1.5 to 3.5 in.) 40 to 50 mm is recommended. For vacuum operation lower weir heights are used to reduce the pressure drop 6 to 12 mm ( to in.) is recommended. 

Active holes Blanking area 1935 Plate pressure drop 140 mm liquid = 1.3 kPa... 

The plate pressure drop can be estimated using a form of the equation for flow in a conduit equation 12.18. 

Figure 7.4c shows a AP transmitter used with an orifice plate as a flow transmitter. The pressure drop over the orifice plate (the sensor) is converted into a control signal. Suppose the orifice plate is sized to give a pressure drop of 100 in H2O at a process flow rate of 2000 kg/h. The AP transmitter converts inches of HjO into milliamperes, and its gain is 16 mA/100 in HiO. However, we really want flow rate, not orifice-plate pressure drop. Since AP is proportional to the square of the flow rate, there is a nonlinear relationship between flow rate F and the transmitter output signal ... 

For example, suppose we want to control the mass flow rate of a gas. Controlling the pressure drop over the orifice plate gives only an approximate mass flow rate because gas density varies with temperature and pressure in the line. By measuring temperature, pressure, and orifice-plate pressure drop, and feeding these signals into a mass-flow-rate computer, the mass flow rate can be controlled as sketched in Fig. 8.3a. 

The indicated flow of acetic acid is 9000 liters per day. The instrument technician checks the flowmeter to see if it has drifted, by opening valve B, with A and C closed (see Fig. 6.7). It should go back to zero— but a reading of 2000 liters per day is noted. The full range on the flowmeter is 10,000 liters per day. What is the real flow rate of the acetic acid The answer is not 2000 liters. Why Because flow varies with the square root of the orifice plate pressure drop. To calculate the correct acetic acid flow ... 

C0 = orificecoefficient (C, = 0.775for3/16 inch holediameters). The dry-plate pressure drop is found to be 13 mm (0.51 in.) of liquid. The total pressure drop is given by the equation ... 

Turn-down Plate material Downcomer Plate spacing Plate thickness Plate pressure drop... 

APt = total plate pressure drop. Pa (N/m ) hf = total plate pressure drop, mm liquid. 

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