Plate hydraulics

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, De.sign of Equilibrium Stage Proce.s.se.s, Chap. 14, McGraw-Hill, New York, 1963) or in previous edition of this handbook. 

Assuming constant liquid holdup on the plate, which is equivalent to neglecting the plate hydraulic variations, a component balance for the feed plate is given by... 

A, free diameter of the connection to the chamber B, cylindrical opening by the movement of D C, opening between condenser wall and valve plate D, valve plate, hydraulic valve drive E, condensation surface of the refrigerated coils F, in- and outlet of the refrigerant G, tube connection to the vacuum pump H, water drain during defrosting of the condenser pch and pm pressure in the chamber and in the condenser, respectively. 

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. 

C, opening between condenser wall and valve plate D, valve plate, hydraulic valve drive ... 

The simulation of actual start-up operation is very difficult unless there is a detailed rigorous model including plate hydraulics. However, the simulation of start-up operation from step 5 (section 3.3.1) is fairly easy and can be done without considering a detailed hydraulic model. To do this the filling of the holdup in steps 3 and 4 (section 3.3.1) can be achieved in different ways ... 

The RD model consists of sets of algebraic and differential equations, which are obtained from the mass, energy and momentum balances performed on each tray, reboiler, condenser, reflux drum and PI controller instances. Additionally, algebraic expressions are included to account for constitutive relations and to estimate physical properties of the components, plate hydraulics and column sizing. Moreover, initial values are included for each state variable. A detailed description of the mathematical model can be found in appendix A. The model is implemented in gPROMS /gOPT and solved using for the DAE a variable time step/variable order Backward Differentiation Formulae (BDF). 

The comparison of efficiencies indicates that efficiency does not depend strongly upon the type of plate. Since the efficiency of perforated and bubble cap plates is much the same, the choice of plate depends more strongly on the plate hydraulic characteristics. 

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