Multi Component Distillation

The separation of the chemicals by differentiating them on how easily they can get vaporized is referred to as multi component distillation. Precisely, multi component distillation is just the separation of the mixture of the chemicals. An appropriate example for the multi component distillation is petroleum refining.

Crude oil

Crude oil is very complex in nature. It is a complex mixture of the hydrocarbons which have got numerous molecules. Almost all commercial distillation is referred to as multi component distillation. By nature, the theory and the practice of the multi component distillation tends to be complex.

Components of multi-component distillation

Usually, in multi-component column distillation there are two components which exist in significant qualities in both the top and the bottom products. The names of the two components are the heavy non-key components and the light non-key components. Usually the heavy key components will all end up in the bottom product whereas the light non-key components will end in the top product.

Solving a problem in the design or in the simulation involves solving a variety of coupled algebraic equations approximately two for each plate or each component. A multivariable Newton’s method is used to solve these equations by using the powerful computers today. However, a complete simultaneous solution method might need a considerable computer time since it is often prone to convergence problems. An optimal ordering of the equations is involved in the simplified methods such that they can be solved with one equation at time sequentially and analytically in most cases. By this the computer programming can be simplified to the needed extent. For various algebraic equations that are solved here simultaneously, there is no such numerical method which will guarantee a solution which gives a clue that solving smaller subsets of the equation if possible one equation at a time would be more beneficial.

Design problem

One should consider the design problem for some special cases where there are no heavy non-keys or light non-keys. An overall material balance should be employed to decide the bottom composition to a high degree since the product specification determines the distribution of the keys and as a result all heavy non-keys can be alleged to finish in the bottom product. On the contrary, the top composition of the second case is determined using the overall material balance since all the light non-keys and the product specification are assumed to finish in top product.

The bubble point calculation is used in the Multi component Distillation in order to determine the vapour composition rising from the reboiler or if relative volatility is known it can be simply determined using:

y_i_p  =   alpha_i  * x_i_p / [sum_j (alpha_j * x_i_p) ]

Questions:

• What is the significance of bubble point calculation?
• What is multi component column distillation?