SEPERATION PROCESS IN CHEMICAL INDUSTRY

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Separation Process

Separation processes are basically of two types:
1.      Component separation
2.      Phase separation

                        Here we are concerned with a phase separation process. In phase separation, two or more phases are separated because the force acting on one phase differs from the force acting on the other phase or because one of phase has an impact on a solid barrier.

The reasons for using vapor-liquid separator include
1.      To recover valuable product.
2.      To improve product purity.

Types of Gas-Liquid Separators

  There are three types of gas-liquid separators:

 1. Vertical Gas-Liquid Separators:
                   In vertical gas-liquid separators the gas-liquid mixture is separated by gravity and impaction. The mixture enters the separator about midway where a splash plate deflects the stream downward. Major portion of the liquid flows downward, and the vapor, containing liquid drops, flows upward. As the vapor rises, large drops settle to the bottom of the separator by gravity.

                        In a separator, there is not a single drop size, but a distribution of drop sizes. To prevent all drops from being carried out by a gas stream would require an uneconomically large separator. Thus, a maximum gas velocity is specified so that all but the very small drops are recovered.

2. Horizontal Gas-Liquid Separators:
                        The vertical and horizontal vapor liquid separators have the same design equations and have similar design calculations. Therefore the only parameter on which one can be selected for any required separation is for checking the value of L/D ratio. If L/D ratio is greater than 5, then horizontal separators are used.

3. Knock-Out Drums:
                        These are used when the liquid content of the incoming stream is very low. An example is the use with compressors.

 Selection of Separator:
                        Here the purpose of the separator is to separate 3 different phases from each other. These are the vapor hydrocarbon phase that is mainly hydrogen, the liquid hydrocarbon phase that contains all the unconverted VGO and the liquid products produced by hydrocracking in the first-stage reactor, and the sour-water phase that contains ammonium bisulfide.

                        The usual residence time for a vertical separator is 3-5 minutes, which is sufficient for vapor-liquid separation. For the separation of the two immiscible liquid phases, an additional residence time of 5 minutes must be provided. Hence a total residence time of about 10 minutes is required. The range for horizontal separators is 7-10 minutes. Therefore, a horizontal separator is selected.

Significance in the Process:
                        The requirement of separator in our process is:

1.      To recover hydrogen gas that is recycled and used for quenching in the reactor.
2.      To remove H2S and send to scrubber
3.      To separate hydrocarbons from sour water.
4.      To control the pressure of the reactor using back pressure.

 Phenomenon Occurring in the Equipment:
                        The principle of the separator can be easily explained with the help of figure 13 showing the internal construction of the equipment. The vapor-liquid mixture enters at the side of the separator and strikes the impingement plate, placed vertically. Liquid flows downward and gas flows across sides of the plate. Maximum vapor-liquid separation takes place here. The vapors carrying liquid droplets come in contact with coalescer and agglomerate the liquid droplets. The size enlargement makes the settling down easy.


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