Tray columns preliminary design

For preliminary design, liquid entrainment is usually used as a reference. To prevent entrainment, the vapor velocity for tray columns is usually in the range 1.5 to 3.5 ms-1. However, the entrainment of liquid droplets can be predicted using Equation 8.3 to calculate the settling velocity. To apply Equation 8.3 requires the parameter KT to be specified. For distillation using tray columns, KT is correlated in terms of a liquid-vapor flow parameter FLV, defined by ... 

Overall column or section efficiencies are complicated functions of tray design, fluid properties, and operating conditions. Some empirical correlations of overall column efficiency have been developed but are capable only of rough estimates of efficiency that may at best be useful in preliminary design studies (Lockett, 1986). 

Equipment design, as discussed in the preceding section, requires preliminary evaluation of column efficiency. There is a close conphng between geometry and efficiency. Several different efficiency terms will be nsed for tray columns, bnt the one nltimately needed for design is overall column efficiency ... 

For preliminary design, column operating pressure and type condenser can be established by the procedure shown in Fig. 12.4, which is formulated to achieve, if possible, reflux drum pressures Pp between 0 and 415 psia (2.86 MPa) at a minimum temperature of 120°F (49°C) (corresponding to the use of water as the coolant in the overhead condenser). The pressure and temperature limits are representative only and depend on economic factors. Both column and condenser pressure drops of 5 psia are assumed. However, when column tray requirements are known, more refined computations should allow at least 0.1 psi/tray for atmospheric or superatmospheric column operation and 0.05 psi/tray pressure drop for vacuum column operation together with a 5 to 2 psia condenser pressure drop. Column bottom temperature must not result in bottoms decomposition or correspond to a near-critical condition. A total condenser is used for reflux drum pressures to 215 psia. A partial condenser is used from 215 psia to 365 psia. A refrigerant is used for overhead condenser coolant if pressure tends to exceed 365 psia. 

After selection of the column internal diameter (the fundamental column specification), the sieve plates must then be designed. This involves a trial and error approach. A preliminary plate design is proposed based upon typical tray configurations, then the hydraulic... 

Tray layout. The preliminary tray and downcomer layout is prepared in the column-sizing phase and refined during the hydraulic design phase. In addition to the parameters previously set, such parameters as hole diameter or the type of valve unit are determined. 

The hardware design proceeds In two phases primary (basic) and secondary (detailed layout). The primary phase sets column diameter, type of tray, and split of tray area Into bubbling and downcomer areas. This phase also provides a preliminary (and usually close) estimate of tray spacing, number of passes, and other features of tray and downcomer layout such as weir height, fractional hole area, hole diameter, and clearance under the downcomer. These estimates are later firmed up in the secondary phase. 

As stated in the introduction to Chapter 6, detailed tray layout is performed in the sscondary phase of tray design. A preliminary layout, however, is needed at the primary phase, because the layout influences column size. In the present example, the preliminary layout will be based on tray layout guidelines detailed in a companion book (1). 

SLOT VELOCITIES AND RELATIVE DIMENSIONS. After the design of the cap has been established, the next step is to determine the number of bubble caps to be used per tray. This number is set by the allowable gas velocity through the slots. If the velocity is too high, pressure drop may be excessive and the liquid may be blown away from the cap, thus resulting in poor efficiency as shown by Fig. 16-8. On the other hand, if the velocity is too low, the gas bubbles will have little opportunity to disperse through the liquid, and the efficiency of the tray will be low. Davies has recommended the following equations for use in preliminary estimates of slot velocities in rectification columns ... 

In this chapter, methods are presented for determining capacity (or column diameter) and efficiency of some commonly used devices for vapor-liquid and liquid-liquid contacting. Emphasis is placed on approximate methods that are suitable for preliminary process design. Tray selection, sizing, and cost estimation of separation equipment are usually finalized after discussions with equipment vendors. 

The downcomer layout is listed in Table 12.6. Until now, we have conducted a preliminary column design, which provides the colunm geometry and tray layout. 

---