Pulsed Columns

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General

Liquid/liquid extraction is a process for separating the components of a mixture and is closely related to distillation. It may be used as a complementary or alternative technique when distillation is difficult such as in the separation of thermally sensitive materials or when materials have similar boiling points.

The use of liquid/liquid extraction process for material separation continues to gain importance both technically and economically. Low power requirements and the possibility provided for selective separation of components means that the fields of application for liquid/liquid extraction processes are continually being extended.

Garg has gained considerable experience in the construction of glass extraction columns. On the basis of this know-how, a new range of production units have been developed.

Pulsed columns are a part of this range.

Operation

Simple packed and sieve plate columns used for extraction processes do not often achieve the high efficiency that would be expected from a comparable distillation system. This is caused partly by the low counter phase velocity and partly by the distribution of droplets in the extraction column. Further difficulties are often experienced in scaling up the process.

In order to overcome these problems, successful attempt have been made to add mechanical energy to the extraction column by pulsation. This improves the dispersion in the column and leads to an increase in mass transfer and throughput.

With approximately equal counter current throughputs of the two phases, a pulsating sieve plate column provides ideal flow and uniform dispersion of small same-size droplets.

Construction

Pulsed extraction columns fall into two main categories and both are very similar in construction. The only difference is that the first type incorporates packed column sections fitted with sieve plate assemblies. In both cases, there are separator vessels at the top and bottom of the column.

The two phases are fed into the column by means of a twin-headed dosing pump which can be adjusted to set the exact throughput required for the particular process. A pulsing unit is connected to the bottom of the column and pulses the head of liquid through column.

To meet the requirements of the processes involved in liquid/liquid extraction, a wide range of packings are available in various sizes and in different materials of construction.

The stainless steel sieve plate inserts normally have a plate spacing of 100 mm and they are pre-assembled to form complete units before being fitted into the column sections. if the process requires plates constructed from other materials or the free-plate areas greater than the approximate 20 % normally used, the assemblies can be easily exchanged.

When temperature control of the extraction process is required, heating jackets can be provided for the column and immersion heaters for the separator vessels.

To complete the extraction column, the following ancillary equipment is recommended :

i) Dosing pumps or metering pumps.

ii) Pulsation pumps.

iii) Interface detector using the principle of capacitance change.

Performance

Pulsed columns are suitable for liquid extraction processes where the specific gravity (S.G) difference between light and heavy phases is in the range from 0.05 to 0.6

The following examples are selected from the specific gravity (S.G) difference and other system variables such as interfacial tension and viscosity.

The following examples are selected from the many possible extraction applications for which pulsed columns can be used.

In the first case, using a pulsed sieve plate extraction column, a plate efficiency of not less than 90% is obtained whilst in the second case , the plate efficiency is only 30%

Example 1 :

Process : carrier Solvent water (continuous phase) Solute :acetic acid Extraction Solvent : Methyl-isobutyl-ketone (dispersed phase)

S.G Difference : 0.2

Feed Ratio : 1:1 (carrier solvent : extraction solvent)

Throughput : glass reaching rings (6 x 6 mm) 3m3/m2h porcelain saddles(15 x 15 mm) 20m3/m2h sieve plates (20 % free area ) 36 m3/m2h plate efficiency achieved - 90%

Example 2:

Process : carrier Solvent water (continuous phase) Solute :acetic acid Extraction Solvent : Toluene (dispersed phase)

S.G Difference : 0.13

Feed Ratio : 1:1.5 (carrier solvent : extraction solvent)

Throughput : glass reaching rings (6 x 6 mm) 3m3/m2h porcelain saddles(15 x 15 mm) 8 to 10m3/m2h sieve plates (20 % free area ) 22.5 m3/m2h plate efficiency achieved - 20%

Design Procedure :

The complexity of liquid / liquid extraction processes makes it practically impossible to theoretically calculate, within the accuracy required, the design of an effective production scale plant. For this reason, extensive pilot plant tests are carried out to establish the maximum throughput and separation effect of extraction columns.