Equipment for the LLE process

3.4.1 Criteria

There are various requirements for designing and building an extractor. For best performance, extractors are expected to have the capabilities described below. In practice, however, meeting all the criteria is technically challenging. The criteria are listed here from the most important to least important for most cases. However, the sequence of importance may be reordered in specific conditions.

High volumetric efficiencyis also known as theproduct-specific extract ratio per unit area. High volu-metric efficiency is usually achieved with a high number of theoretical stages per unit and thus less volume required for the extractor column.

Avoiding surface-active impuritiesis important for columns with a long life. Surface-active impurities can reduce column capacity by more than 20% and efficiency by 60%. The efficiency of a rotating-disk contactor reduced faster than that of a small-diameter agitated column when trace surface-active contaminants were added [18].

Production capacityis determined according to the products. Some products need small extraction oper-ations because of low throughput demand whereas others need large operoper-ations to make this process economically possible. It is necessary to understand the scale of operation that is required. A common

strategy is to gain familiarity with the extractor on a small scale, and then extract the product on a large scale industrially.

Flexibility and the ease of scaling up refer to the ability to successfully follow the LLE process from small scale to large scale. In some cases, researchers can apply one extractor to several LLE processes;

thus, this flexibility in terms of the extractor can create cost savings.

Tolerance for fouling, and ease of cleaning are important characteristics for any industrial equipment.

The LLE extractor aspect of this process is the most serious concern because of the frequent handling of this chemical solution. Tolerance for fouling makes it possible to maintain a long operation cycle. Ease of cleaning saves break time in the process for maintenance.

Easy operationfor the extractor can simplify the operation protocol and reduce the operator requirements.

This type of operation can be more easily transferred to another plant or new plant and also facilitates troubleshooting during the years of operation.

Theavailability of the extractoris the last criterion listed here but it can still be a very important one. If the local extractor dealer can provide timely instructions, maintenance, and a troubleshooting service, this will show advantages during the long-term operation of the extractor.

For the industrial operation of LLE, better process control is needed for a large-scale system. Solvent loss during the process is an important problem. The reasons for solvent loss are solubility, vaporization, stable emulsions, entrainment of solvent, and sampling during the process. The equipment is another factor leading to solvent loss. Better operation with less solvent loss is more environmental friendly, especially for a large-scale operation.

3.4.2 Types of extractors

Several types of extractors were designed and are currently used in large-scale industrial production. These extractor types can be categorized into three groups: the static extraction column, the rotary or agitated column, and the centrifugal extractor.

Thestatic extraction columnis an extractor without any stirring mechanism to separate the extractant from diluents. Two types of liquid solvent are mixed and separated in the column after the solutes are transferred. Liquid solvents are moved into the static extraction column, and separated after the extraction, due to their different densities. As shown in Figure 3.6, three kinds of static extraction column have been designed, to give better performance. First, the spray column is the basic extractor without any facilities in the column. Next, the packed column is filled with metal, plastic, or ceramic materials to improve the separation efficiency. Last, the sieve-plate column disperses solvent into small drops using a plate with holes. The solvent dispersal enhances mass transfer and surface interaction.

Agitated columns were developed in order to control the efficiency of LLE. Three types of major agitated columns are illustrated in Figure 3.7: the rotary-impeller column, the reciprocation plate column, and the rotating-disk column.

The centrifugal extractor is an advanced design to separate extractant from diluents using a rapid centrifuge. Figure 3.8 shows the basic structure of a centrifugal extractor (Costner Industries—CINC). A hollow rotor is rotated inside the column. The light and heavy phases are separated by the centrifugal rotor. These phases run out as the mixed phase is fed in. With the development of LLE equipment, many commercial derivatives are possible [19].

Light phase out

Heave phase out Light phase in

Heave phase in

(a)

Light phase out

Heave phase out Light phase in

Heave phase in

(b)

Light phase out

Heave phase out Light phase in

Heave phase in

(c)

Figure 3.6 Three kinds of static columns: (a) spray column; (b) packed column; (c) sieve plate column

Light phase out

Heave phase out Light phase in

Heave phase in

(a)

Light phase out

Heave phase out Light phase in

Heave phase in

(b)

Light phase out

Heave phase out Light phase in

Heave phase in

(c)

Figure 3.7 Three kinds of rotary or agitated columns: (a) rotary-impeller column; (b) reciprocating-plate column;

(c) rotating-disk column

1 2

3 4

Heavy phase out Light phase out

Heavy/Mixed phase in

Figure 3.8 Costner Industries centrifugal extractor. Area 1 and 2: light phase, Area 3 and 4: heavy phase

3.4.3 Issues with current extractors

Some difficulties for LLE equipment to overcome are noted here. For static extraction columns, the process offers the advantages of a large diameter, simple construction and operation, only a single operating interface, and small footprint compared to mixer-settler equipment. However, some parameters still need to be improved, such as the interfacial area, drop size, and drop velocity. Another issue of concern is mass-transfer efficiency, which should be improved. An important development in static column extractor is redistribution in the column. The minimization of packing size and drop size increases the interface of solvent. An agitated extraction column is an improvement over the static column. Several kinds of agitation were reported recently, such as the SCHEIBEL column, the Kuhni column, and the KARR reciprocating column [19]. Centrifugal extractors also improve the LLE process because they reduce diffusion path lengths and increase the driving force for separation. The typical application of this design is the one-stage or multistage centrifugal extractor applied for penicillin extraction [20].