Plate Efficiency and HETP

First compute the relative volatility between propane and n-butane at 245 psia and the average temperature of the two feeds of (170 + 230)/2 = 200°F. The respective average T-values by the SRK equation of state are 1.76 and 0.84, giving aLK,HK = 1.76/0.84 = 2.10. Applying the Fenske equation [Eq. (14.1)] between the two feeds, using the key component feed flow rates, gives  

Therefore, N = 2(1.93) = 3.86. If this is rounded up to a value of 4, then four equilibrium stages should be placed between the two feed stages.  

For modem random packings with low-viscosity liquids  

For stmctured packings at low-to-moderate pressures and low-viscosity liquids  

For high-pressure service with stmctured packings  

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Plate efficiencies and HETP values are complex functions of measurable physical properties temperature, pressure, composition, density, viscosity, diflusivity, and surface tension measurable hydrodynamic factors pressure drop and liquid and vapor flow rates plus factors that cannot be predicted or measured accurately foaming tendency, liquid and gas turbulence, bubble and droplet sizes, flow oscillations, emulsification, contact time, froth formation, and others. Values for plate efficiency, HETP, or HTU, particularly those that purport to compare various devices, are usually taken over a limited range of concentration and liquid-to-vapor ratios. The crossovers in Fig. 2.5 and the rather strange behavior of the ethyl alcohol-water system, Fig. 2.6, demonstrate the critical need for test data under expected operating conditions. ... 

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