Description of Distillation Process
To begin with, let’s consider a schematic diagram of a distillation column with a condenser and a reboiler (Fig. 2.1a). The column feed, in the common case, is vapor–liquid mixture of flow rate F and with concentrations of components zi F , where i is the component number, the vapor feed of flow rate VF with concentrations of components yi F , and the liquid feed of flow rate LF with composition xi F .
The reboiler serves to make a vapor flow, which goes upward along the column length; the condenser serves to make a liquid flow, which flows down from tray to tray from top to bottom. On tray j, the liquid flow from tray j + 1 meets the vapor flow from tray j - 1. These flows are not equilibrium and, therefore, a mass exchange takes place on the tray; a part of the lightest components converts from liquid into the vapor phase, and a part of the heaviest components (of higher boiling temperatures) converts from vapor into the liquid phase. Although on real trays complete equilibrium between liquid and vapor is never reached, in the world practice a model of a theoretical tray (
this equilibrium is achieved (1, 1÷2 real trays correspond to one theoretical tray) is accepted. Due to the mass exchange between liquid and vapor, the composition on the trays varies along the column length – on the overhead trays, there is a high concentration of light components and, on the bottom trays, there is a high concentration of heavy components. Thus, a separation of the initial mixture occurs. The vapor rising from the column is condensed in the condenser. One part of the formed liquid is removed as an overhead product (distillate is the amount D of composition xiD), and the other part comes back into the column (reflux in the amount LN of the same composition xiD). Such a condenser, which condenses all vapor from the column, is called a total condenser. Sometimes a partial condenser is applied,
Basic Concepts of Distillation
when the distillate is removed in form of vapor; and sometimes a mixed condenser is applied, when one part of the distillate is liquid and the other part is vapor. Liquid from the bottom of the column goes to the reboiler where it is partially evaporated (reboilers of this kind are called partial). The vapor in amount V1, of composition yi1 comes back to the column, and the remaining liquid in amount B of composition xi B is removed as bottom product.
Ratio R = LN/Dis called the reflux ratio and ratio S = V1/Bis called the reboil ratio. In the reboiler, there is a input of heat in amount QR, and, in the condenser, there is a removal of heat in amount Qcon. Thus, distillation is a two-phase (liquid–vapor) multistage counterflow potentially equilibrium process (in some cases – in cases of heteroazeotropic distillation – three phases may occur on the trays: two liquid phases and one vapor phase).