University Taster

Chemical Engineering – University Taster

  1. 1. Prerequisites
  2. 2. Chemistry & Thermodynamics
  3. 3. Fluid Dynamics
  4. 4. Separation Processes
  5. 5. Biochemical Engineering
  6. 6. Sustainable Chemical Engineering
  7. 7. Next Steps
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In reality, distillation columns used in industry have many components. Oil refineries, for example, used fractional distillation columns to separate crude oil into 7 fractions which will be used for different applications. This lesson will discuss how a column can be designed and some operational considerations.

Column Design

The main features of a distillation column are the number of plates, the spacing of trays, and the column diameter.

When designing a distillation process, a mass balance can be carried out on the sections above and below the feed stream, as per Figure 1. The upper section is known as the rectifying section, where below the feed is called the stripping section. 

Figure 1. Sections of distillation column above and below feed. \(F\) is a flow rate of the feed, \(x\) is the mole fraction of a component in each flow, \(y\) is the mole fraction of the other component in each flow, \(V\) is the vapour flow at a plate, \(n\) and \(m\) are arbitrary plate numbers, \(L\) is the liquid flow at a plate, \(W\) is the bottoms flow rate, \(D\) is the tops flow rate.

Equation 1 shows equations for the top operating line, for the rectifying section, and the bottom operating line, for the stripping section, both of which are used in conjunction to determine the number of stages required in the distillation column.

\(\text{Top \: operating \: line:} \: y_n = \frac{L_{n+1}}{V_n} x_{n+1} + Dx_D\)

\(\text{Bottom \: operating \: line:} \: y_m = \frac{L_{m}}{V_m} x_{m+1} - \frac{W}{V_m} x_W\)

Equation 1. Equations for the top (\(y_n\)) and bottom (\(y_m\)) operating lines. \(L\) is the liquid flow rate at a specific plate, \(x\) is the mole fraction of a compound at a specific plate, \(D\) is the tops flow rate, \(W\) is the bottoms flow, \(V\) is the vapour flow rate, and \(n\) and \(m\) are plate numbers in the column.

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