Batch Distillation
In the continuous operation, after the column has been operated for a certain period of time, the system reaches a steady state
At steady state, the properties of the system, such as
• The feed flow rate
• The flow rates of, e.g., the distillate and the bottom
• The feed composition
• The compositions of the distillate and the bottom
• Reflux ratio (L0/D)
system's pressure are constant.
With these characteristics, a continuous distillation is the thermodynamically and economically efficient method for producing large amounts of material of constant composition
However, when small amounts of products of varying compositions are required, a batch dis tillation provides several advantages over the continuous distillation (the details of the batch distillation will be discussed later in this chapter)
Batch distillation is versatile and commonly employed for producing biochemical, biomedical, and/or pharmaceutical products, in which the production amounts are small but a very high purity and/or an ultra clean product is needed
The equipment for batch distillation can be arranged in a wide variety of configurations
In a simple batch distillation (Figure 1), va pour (i.e. the product) is withdrawn from the top of the re-boiler (which is also called the "still pot") continuously, and by doing so, the liquid level in the still pot is decreasing continuously
Figure 1: A simple batch distillation (from "Separation Process Engineering" by Wankat, 2007)
Note that the distillation system shown in Figure 1 is similar to the flash distillation.
However, there are a number of differences between the batch distillation (e.g., Figure 1) and the flash distillation: i.e. in the flash distillation, feed is continuous ly fed into the column, whereas there is no continuous feed input into the still pot for the batch distillation
• In the flash distillation, the products (i.e. vapour and liquid products) are withdrawn continuously from the system, where as, for the batch distillation, the remaining liquid in the still pot is drained out of the pot (or the re-boiler) only at the end of the distillation.
Another configuration of batch distillation is a constant-level batch distillation, which is similar to the simple batch distillation, as illus trated in Figure 1; however, in this configuration, the liquid (i.e. the feed) is continuously fed into the still pot (or the re-boiler) to keep the liquid level in the pot constant
The more complex batch distillation (than the simple and the constant-level batch distillation) is the multi-stage batch distillation
In this distillation system, a staged or packed distillation column is placed on top of the re boiler (or the still pot), as shown in Figure 2
Figure 2: A multi-stage batch distillation
(from "Separation Process Engineering" by Wankat, 2007)
In the usual operation of the multi-stage dis tillation system, the distillate is withdrawn con tinuously from the system, until the distillation is ended
Another way of operating the multi-stage batch distillation is that the system is operated such that there is no distillate withdrawn from the column (or system), thus resulting in a con tinuous change in the concentration or com position of liquid in the pot (or the re-boiler)
Additionally, when a pure bottom product is required, an inverted batch distillation is em ployed
In this technique (i.e. the inverted batch dis tillation), the bottom product or the liquid in the re-boiler is withdrawn continuously while the distillate is withdrawn only at the end of the distillation
7.1 Binary-mixture Batch Distillation: Rayleigh Equation
The material balances for the batch distillation are different from those for continuous distillation
In the batch distillation, the main focus is at the total amounts of input(s) [i.e. feed(s)] and outputs (e.g., distillate or bottom) collected at the end of the distillation, rather than the rates of such inputs and outputs
The material balances around the batch dis tillation system for the entire operating time are as follows
Overall:
F = the total amount of feed fed into the distillation column for the entire operating period
Wfinal = the final amount of liquid in the re-boiler (the notation W is used because the remaining liquid in the still pot is normally a waste)
Dtotal = the total amount of the distillate withdrawn from the distillation column (in some textbooks, the notation D final may be used)
Species balance (for a more volatile compo nent: MVC):
xf = mole fraction of a more volatile species in the feed
xw final = the mole fraction of an MVC of the remaining liquid in the re-boiler
xd avg = an average concentration of an
MVC in the distillate (in some textbooks the notation D, final may be used)
Normally, F and xf , are specified (or given in the problem statement), and the value of either x w final , x D avg is also specified (or given)
Thus, there are 3 unknowns for the binary mixture batch distillation system:
• W final
•D total
• either x D,avg or x w, final
Problematically, however, by just performing material balances, we have only 2 equations (i.e. Eqs. 1 and 2)
Hence, another or additional equation is required.
