GRANULAR CARBONS.

As mentioned before, the only difference between a granular and a powdered carbon is the size of the particle.

Accordingly, the time needed to get the same result is much longer when using granular carbon packed in a column through which the liquid flows. This kind of operation is advisable for continuous processes to achieve a stable quality of the liquor that is being purified and when working with large flows. It has an advantage; the carbon can be regenerated and therefore the consumption is lower; but the needed investment is high and is not always justified.

The column operation has the same principle as the countercurrent contact; moreover, it could be considered a multiple stage contact. Between the influent and the effluent, there is an impurity concentration gradient and the incoming carbon is exhausted faster that the outgoing carbon.

At the beginning of the operation there is a point in the column where the impurity concentration is the same as that of the outlet. This zone is known as the mass transference zone (MTZ). The rest of the carbon is still virgin. As the operation continues, part of the carbon is exhausted and the mass transference zone begins moving towards the outlet. Finally, there comes a time when the MTZ reaches the outlet. A moment later, the concentration of impurities in the effluent will begin to increase and it can be considered that the column is exhausted. How long this takes depends on several factors:

a) Liquor flow (the larger the flow the larger the MTZ)

b) Size of the carbon particle; the smallest carbon
gives smaller MTZ's, but a higher-pressure drop.

c) Temperature; due to the reduction of viscosity, an increase in temperature generally reduces the MTZ height.

d) Characteristics of the carbon being used (pore size) and of the product that is being purified (diffusion coefficient).

It should be emphasized that the capacity of a Granular Carbon is the same as that of a Powdered Carbon, the way to determine the easiness to adsorb a certain impurity is grinding the carbon and running Freundlich isotherms in the laboratory.

In order to get the design parameters for a column (diameter, carbon height, flow, etc.) a pilot test must be carried out. A simple way to do so is using several 3 ½" or 40" columns in a series.

As a general rule, we can say that the ratio of carbon within the diameter of the columns varies from 2 to 1; and 5 to 1 and the average flow measured as the number of volumes of liquor beds that flow through the carbon in an hour (VCH) ranges between:

• Discoloration 0.2 - 0.6
• Deodorization 1.0 - 2.0
• Treatment 1.0 - 4.0

There are two system variations for granular carbon:

• Mobile bed system
• Fixed bed system

MOBILE BED SYSTEM

In the mobile bed system, the liquor flows upwards and part of the carbon is extracted periodically. Such is replaced by virgin or regenerated carbon through the upper part of the column.

This is an efficient arrangement that provides a continuous operation, but it has some disadvantages, for example:

The liquor must be free of any suspended solids. If there are any, the carbon bed will act as a filter and create pressure.

The flow must be accurately controlled. If there are any important variations, the carbon bed might liquefy and the liquor might drag some carbon.

FIXED BED SYSTEM

In this system the liquor flows downwards, the carbon slowly wears out from top to bottom and when the outgoing concentration is higher than the allowed maximum, the column is no longer operated and the carbon if fully regenerated. This arrangement is less efficient because when the column is no longer operated, part of the carbon still has a certain degree of activity, yet, it is more versatile. In the event the liquor had suspended solids that were caught by the carbon bed, the operation is stopped and it is backwashed easily.

In addition, if the flow increases considerably there would be no difficulties. A variation for this arrangement to render it more efficient and to better exhaust the existing carbon is to use two columns in a series. When the first column is exhausted, No. 2 column becomes No. 1, and a recently regenerated column enters the system as column No. 2.

In the fixed bed systems, it is always necessary to have an empty 40 - 50% space. Such space must be available to backwash the carbon bed. When backwashing, a flow that allows from 20% to 30% bed expansion must be used.

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