Solvent Recovery

Many plants, such as pharmaceutical, printing, explosives, electronic and chemical, generate waste solvents that must be either shipped away for disposal or recovered.

There are many parameters to be addressed to determine the feasibility of solvent recovery. The most important parameters are:

■ Prices of solvents to be recovered.

■ Costs of disposal of solvents if not recovered.

■ Capital and operating costs of a solvent recovery system.

■ Achievable purity of recovered solvents.

There are also less tangible benefits to recovery. For example, when the solvents are recovered, there is no potential liability for solvents shipped out for disposal. Also, the recovery reduces the vulnerability to shortages and price increases.

A number of separation techniques can be used for recovery, depending on the composition of the waste. If the solvent has only to be recovered from a solid, then the recovery can be performed by evaporation. If the solvent is in an air or gas stream, then the solvent can be recovered by refrigeration and/or by carbon adsorption. When solvents are mixed and need to be recovered and purified, the process becomes quite complicated. The most important technique for this recovery is distillation. Other techniques are generally only used when a separation by distillation is either difficult or impossible.

Solvents can be recovered by continuous or batch distillation. The selection is dependent on the complexity of the mixture and the volumes to be processed.

If the separation is fairly simple, such as a ternary or binary mixture, and the volume to be recovered is quite high, it may be best to use continuous distillation. This type of distillation minimizes energy. Also for large duties, energy can be saved by operating in multi-effect mode. A two-effect isopropyl alcohol recovery system is shown in Figure 12.

The principle operation of continuous solvent distillation is the same as described under steam stripping, which is one type of distillation process. A typical

Multi Effect Distillation Images
Figure 12. Two-effect isopropyl alcohol recovery system.

continuous distillation column has two sections. One section below the feed is referred to as the stripping section. This is where the light components in the feed are stripped out of the heavy components to produce a bottoms product with small quantities of the light components. The section above the feed is the rectification section, where the light components are concentrated. Many steam strippers do not include a rectification section, since with partially miscible components, an effective concentration can be obtained merely by decanting.

For solvent recovery, batch distillation is still the most common technique used for the purification of solvents. Although in the process industries, most distillation systems are continuous, batch systems are preferred for the distillation of relatively small quantities of solvents. Also, to separate a multi-component mixture of n components by continuous distillation, a minimum of n-1 separate columns are required, which involves a significantly higher capital cost. A batch system can often separate many components in one column, albeit with a premium on utilities.

The design of a batch distillation system is usually extremely complicated and best left to experts in the field. The multicomponent nature of the feed, coupled with the added parameter of time, (which is not a factor with continuous distillation), results in complex calculations. While there are a number of hand calculation techniques, a far easier and more accurate technique is to use one of the process simulator computer programs that are available from Simulation Sciences (BatchSim™).

While it is not proposed here to detail the theory of batch distillation, it is important to look at some of the general parameters involved.

Images Batch Disstillation
Figure 13. Batch distillation system.

For a mixture of 2, 3, or even 4 solvents, batch distillation will enable the user to recover solvents at high purity, providing there are no azeotropes present. For mixtures containing many components, it will usually only be economical to recover the dominant and/or the most expensive components. For example, a solution containing over 5 components in relatively equal proportions may not be worth processing.

A flow schematic of a typical batch distillation system is shown in Figure 14. Waste solvents from the feed tank are pumped into the batch tank. When the tank is about 80% full, the feed is stopped and the contents of the batch tank are heated to boiling by the heating medium in the reboiler. Once the mixture starts to boil, vapor is carried up the column and is condensed in the overhead condenser. The condensate flows either to a reflux drum or to a decanter (as shown). Reflux is then pumped back to the top of the column. At start up, the system is operated at total reflux until the required purity of the most volatile component is achieved. At this point the product is withdrawn at a rate controlled by reflux ratio. The reflux ratio is set according to data from an on-line analyzer or temperature profile in the column. When the reflux ratio becomes too high (typically 15 or 30 to 1), then it is no longer economical to continue to produce a top product. The flow is diverted to a slop out tank, and the reflux ratio is reduced. Eventually the most

Figure 14. Flow schematic of a typical batch distillation system for solvent extraction.

Figure 14. Flow schematic of a typical batch distillation system for solvent extraction.

Batch Distillation

MAIN CONDENSER

DECANTER

1

/ V

)

volatile component will be completely driven off. The steps can be repeated for each volatile component required to be recovered. The system illustrated shows a reversible decanter so that either the heavy or light phase can be refluxed, or the decanter can be used merely as a reflux tank.

The advantage of batch distillation is the added dimension of time, which allows multiple cuts to be taken from the top of the column. Thus, the components can be taken off as products in order of their volatility. In addition, the process can be stopped at any time to allow for the addition of a further component for use as an extraction agent. The main disadvantage of batch distillation is that it essentially has only one theoretical stripping stage. It is a rectification process. Therefore, this is an inefficient process when it is required to recover the least volatile component at high purity. However, this is not usually the case with VOC recovery, since bottoms water contaminated with small quantities of solvent can be recycled back to the steam stripper.

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  • miniya
    What is a pumped reflux distillation system?
    9 months ago

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