Plate distillation

Example 9 Loading/Flooding of a Distillation Plate An available sieve plate column of 2.5-m diameter is being considered for an etbylben-zene/styrene separation. An evaluation of loading at the top plate will be made. Key dimensions of the single-crossflow plate are ... 

Eduljee, H. E. Entrainment From Bubble-Cap Distillation Plates, British Chem. Engr, p. 474, Sept. (1958). 

EDULJEE, H. E. (1958) Brit. Chem. Eng. 53, 14. Design of sieve-type distillation plates. 

A5. H.E. Eduljee, Design of Sieve Tray Type Distillation Plates , British Chemical Engineering, 53, pp. 14-1 7 (1958). 

Recent work (Sec. 6.4.1) has shown that this model is an oversimplification of the processes occurring on a distillation plate. Nevertheless, many of the modern design procedures are based on this model and are expressed in terms of this model. 

For example, when we consider the design of specialty chemical, polymer, biological, electronic materials, etc. processes, the separation units are usually described by transport-limited models, rather than the thermodynamically limited models encountered in petrochemical processes (flash drums, plate distillations, plate absorbers, extractions, etc.). Thus, from a design perspective, we need to estimate vapor-liquid-solid equilibria, as well as transport coefficients. Similarly, we need to estimate reaction kinetic models for all kinds of reactors, for example, chemical, polymer, biological, and electronic materials reactors, as well as crystallization kinetics, based on the molecular structures of the components present. Furthermore, it will be necessary to estimate constitutive equations for the complex materials we will encounter in new processes. 

A schematic of the distillation plate is given in Figure 12.4. In many texts the terms plate and tray are used interchangeably, but we will use the convention that the plate consists of a tray , which holds the liquid, and a vapour space , which holds the vapour. The ith plate receives a liquid flow Li. via the downcomer attached to the tray of the (i -f /)th plate above. It also receives a vapour flow , i from the (i — l)th plate below it through bubble-caps or valve caps. E is used to denote evaporation rate - the letter V for vapour has already been claimed for volume.) The boiloff from the liquid on the ith tray into the plate s vapour space is denoted B,-, while the vapour flow from... 

Figure 12.4 Distillation plate, showing liquid and vapour flows.

We will now derive the equations necessary to describe the dynamic behaviour of a distillation plate. The modelling procedure for multi-component boiling has strong similarities to the situation of single component boiling condensing already described in Section 12.2. We shall wish to know the masses of all the components in the liquid phase and in the vapour phase, the temperature of the plate, the pressure in the plate, the specific volume of the liquid components and the specific volume of the vapour mixture as a whole. This information is sufficient to allow us to calculate the mole fractions in the vapour and the liquid and the inter-plate flows. 

The several flows into and out of the distillation plate make the form of the differential equations rather busy , but it is possible to simplify the algebra without any loss of information by grouping together the expressions that are independent of the boiloff rates. Hence we may rewrite equation (12.58) as ... 

Mu total liquid mass in distillation plate 1 kmol Pd... 

Qcfh volume flow in cubic feet per contents of distillation plate t J... 

A water distillation plant to produce heavy water containing 99.8 a/o D from natural water containing 0.015 percent D is designed as an ideal cascade without stripping with a separation factor of 1.03. The plant s inventory of water is effectively all in the liquid phase. The depth of liquid on each distillation plate is 30 cm. The plate efficiency is 100 percent. The liquid downflow rate is 1 cm /s per cm of column cross section. Using Eq. (12.204), what would be an upper bound for the equilibrium time ... 

DISTILLATION PLATE EFFICIENCY. The two-film theory can be applied to mass transfer on a sieve tray to help correlate and extend data for tray effieiency. The bubbles formed at the holes are assumed to rise through a pool of liquid that is vertically mixed and has the local composition x. The bubbles change in composition as they rise, and there is assumed to be no mixing of the gas phase in the vertical direction. For a unit plate area with a superficial velocity the moles transferred in a thin slice dz are... 

The overall distillation plate efficiency, E , may be correlated in terms of the following variables liquid density, vapor density, liquid viscosity, vapor viscosity, liquid diffusivity, surface tension, pressure, temperature, pressure drop, liquid flow rate, vapor flow rate, bubble size, contact time. 

In industrial continuous distillations plate columns are preferably used [34]. Recent developments of laboratory and semi-technical apparatus have been thoroughly discussed by Stage who has also dealt with the question of systematic tests for efficiency- [35]. The advantages of bubble-cap columns in continuous operation have already been mentioned (cf. chap. 5.2.2.3 Fig. 167). Cyclic operation (Fig. 164) yields efficiencies >150%. 

Bolles, W. L., "Multipass Flow Distribution and Mass Transfer Efficiency for Distillation Plates, AIChE J. 22(1), 1976, p. 153. 

Lim, C. T., K. E. Porter, and M. J. Lockett, "The Effect of Liquid Channelling on Two-Pass Distillation Plate Efficiency, Trans. Inst. Chem. Engs. (London) 52,... 

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