Optimize Batch Distillation

Proper design depends upon an understanding of key relationships.

By John E. Edwards, P & I Design

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Chemical processors rely on batch distillation for a variety of tasks, including eliminating impurities/unwanted components from reaction mixtures, removing water and drying, changing solvent between reaction stages of multistage syntheses, concentrating prior to crystallization, controlling temperature of strong exothermic reactions at reflux, recovering solvent, and fractionating complex mixtures.

The simplest form of batch distillation involves a single flash separation and is used where a large difference in volatility exists between the components. Such a distillation doesn’t need a fractionating column between the batch still, normally a stirred jacketed reactor, and the condenser. Simple batch distillation, also referred to as pot-to-pot, provides one theoretical plate of separation.

Figure 1 depicts a typical batch distillation arrangement utilizing a stirred batch reactor as the reboiler (still), packed column, overhead condenser, rundown cooler and accumulator (receiver), together with nomenclature related to these components.

When the difference in volatility between components is small or when operating over narrow composition ranges, a rectification section is necessary between the still and the condenser. Overhead facilities are required to provide control of reflux ratio and layer separation when handling heterogeneous azeotropes.

In operation, the system is brought to steady state under total reflux. Overheads are continuously withdrawn in accordance with the reflux control strategy. Cuts (fractions) are taken by switching to different accumulators (receivers), following a time, temperature or overhead composition strategy.

Batch distillations can be run several ways:

• maintaining a constant reflux ratio, giving varying overhead composition; (The distillation is continued until the still or distillate receiver achieves the desired composition.)

• varying the reflux ratio, giving constant overhead composition; (As the distillation proceeds, the still is depleted of the lighter component with the reflux ratio continually increasing. The distillation is terminated at a maximum economic reflux ratio, when the desired still composition is achieved or when still heat input can’t sustain the reflux ratio.)

• using repetitive total reflux; (The unit is operated at total reflux until equilibrium is established; then distillate is withdrawn as total drawoff for a short period of time before returning to total reflux. This technique is useful for separating a low-boiling trace component.) or,

• minimizing time by varying reflux ratio. (This provides the most-cost-effective mode of operation consistent with achieving the desired separation.)

Distillations normally are performed at atmospheric pressure. However, reduced-pressure operation sometimes is required for achieving a desired separation, improving economics by decreasing heat input, or processing temperature-sensitive materials.

Multipurpose operation demands care when selecting column internals to achieve acceptable loadings and operational turndown.

OPERATION AND CONTROL

The mass and energy balances constrain operation and control of a batch distillation [1]. If conditions are fixed at the still, then conditions at the top of the column can be varied and vice versa.

A common strategy is to control the heat input at the still to maximize the distillate draw rate and sustain the desired reflux ratio. The still bottoms temperature will increase as the distillation proceeds, reducing the temperature driving force. If the heating medium is steam, flow rate or pressure can control the heat input; if it’s a heat transfer fluid, increasing its temperature can maintain the heat input. Alternatively if the heating medium conditions are fixed, reducing the distillation pressure will raise the temperature driving force; if necessary, this can be done in incremental steps throughout the distillation.

At constant pressure, the top and bottom temperatures provide an indication of composition. If the product is the still residues, the distillation is continued until the bottom temperature reaches a target limit. If the product is the distillate, the column top is maintained at a targeted temperature by increasing the reflux ratio as the distillation proceeds. At some point the still heat input won’t be able to sustain the increasing reflux ratio, at which point the column top temperature will start to rise and the distillation should be stopped.

The operating instructions can determine the distillation by specifying temperature cutoff values or by providing time steps for an established separation. When carrying out repetitive batch distillations, it may be desirable to shorten the distillation time by using intermediate fractions between the component fractions.

Basic instrumentation and control required includes still-bottoms and column-top temperatures, still pressure (possibly, including its control), reflux and distillate flow control as well as level measurements appropriate for the plant configuration. The heating medium used and jacket configuration will determine still heat input control [2]. To start the distillation, apply heat to the still and continue until stable conditions at total reflux have been achieved. Then begin the distillate draw at fixed reflux ratio for bottoms composition or variable reflux for top composition. Multiple distillate cuts require separate receivers for each.

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