DISTILLATION COLUMN IN CHEMICAL INDUSTRY

In process industries, it is often desired to separate the components of a liquid mixture. The easiest way is to carry out this separation is by distillation. This unit operation makes use of the difference in the boiling point or the relative volatilities of the components. Distillation is considered to be the preferred separation technique if the relative volatility difference between the two key components (that are required to be separated from each other) is greater than 1.2.

Distillation is carried out by the use of ESA (energy as separating agent). The addition of heat to the system raises the temperature of the system and causes the more volatile component to go to the vapor state preferably as compared to the less volatile component. On the removal of heat, the condensation occurs and the less volatile component is preferentially condensed. As the addition and subsequent removal of heat continues, the vapor phase become rich in the more volatile component and the liquid phase becomes rich in the less volatile component. The two separated phases are the required products.Distillation finds very wide applications in petroleum refining and subsequent processing. The hydrocarbons are highly volatile (i.e., they have high vapor pressures) and have very wide range of boiling points. Therefore, it is easier to fractionate a given petroleum feedstock by distillation. These fractions are not true compounds, but they are mixtures themselves. However, they have the properties that are desired for their use as fuel

Selection of the Distillation Column

Batch and Continuous Columns:

                        Distillation columns may be batch or continuous, based on the way feed is introduced. In batch columns, a batch of feed is charged and operation is carried out till the desired degree of separation is achieved. Then the material is removed and the next batch is charged. These columns are suitable for very low throughputs and for systems where very high purity is required. Continuous columns process continuous feed streams. They are widely used in industries for high throughputs. Here we are concerned with the later type of operation, i.e., continuous operation.

Plate and Packed Columns:

                        The performance of a distillation column depends upon the intimate contact of liquid and vapor streams. Two configurations are widely used in this respect. These are the plate and the packed columns. A general comparison of the two configurations is made below.

1.      Packed columns are continuous contacting units. On the other hand, vapor-liquid contact in a tray column occurs only at discrete locations, i.e., on the trays.

2.      The liquid and vapor streams never reach equilibrium in a packed column. In a tray column, the streams leaving any stage are assumed to be in equilibrium.

3.      Packed columns can operate over a relatively wide range of vapor flow rates. On the other hand, plate columns offer wide operating range with respect to the liquid flow rates.

4.      Packed columns cannot work efficiently under stressed conditions of temperature and pressure.

5.      Liquid distribution can be a problem in packed columns, and cause channeling.

6.      For diameters less than about 0.6 m, a plate column cannot be constructed. Therefore, a packed column has to be used.

7.      Design information for plate columns is more readily available and more reliable.

8.      If the system contains solid contents, a plate column is preferred. The solids can accumulate in the voids of the packing and choke them.

9.      For large column heights, weight of the plate column is much smaller as compared to that of a packed column.

10. For cleaning of packed columns, packing must be removed.

In petroleum refining, the distillation columns are plate columns. This selection has the following aspects.

1.      The distillation columns in petroleum refining are complex columns. Actually, they are fractionators, and multiple products are obtained from a single column as side streams. The side streams are often steam-stripped to meet the flash point specification and the overlapping standards of two fractions. The resultant vapor streams are again introduced into the column. Hence, a distillation column in a petroleum refinery has a number of streams going into and out of the system. These inlets and outlets are easy to locate at discrete points in a tray column, but difficult for a packed column.

2.      Because of the multiple inlet and outlet streams, the liquid and vapor flow rates widely vary inside the column. A packed column is not suitable for such a condition.

3.      To avoid problems like flooding and entrainment because of the variable liquid and vapor flow rates, inter-stage cooling or heating is often required. Sometimes, pump-arounds are also used for this purpose. This provision can only be made in a plate column.

                        Keeping in view the above factors, the column selected for the current design is a plate column.

Tray Selection:

                        Three basic types of trays are used in plate columns. These are sieve trays, bubble-cap trays, and the valve trays. The trays selected for the column are sieve trays. The supporting factors are:

1.      They are light-weight and the cheapest available, and easiest to fabricate and install.

2.      They have higher capacity and lower pressure drop than other tray types.

3.      Sufficient design data is available.

4.      The maintenance cost is lower because of the ease of cleaning.

Components of Distillation System

 In general, a distillation system is composed of four basic components.

1.      The distillation column itself, as a tray or packed column where the vapor and liquid streams are brought in contact with each other, and the separation occurs.

2.      A reboiler to provide the heat required for vaporization.

3.      A condenser to remove the heat from the system and condenses the vapors leaving from the top of the column.

4.      A reflux drum to hold the condensed liquid after the separator and provide continuous liquid reflux to the column.

In this particular case, the column has no reboiler, and the heat of vaporization is provided by direct steam injection. This steam also serves the purpose of reducing the boiling point of the hydrocarbons by contributing towards the total pressure of the system. The advantage of using steam is that water is immiscible with hydrocarbons and can be efficiently separated. The schematic of the distillation column for the system under consideration is shown in figure 17.

                        Another important difference here is the side stripper. A side stream is taken from the middle of the distillation column and introduced at the top plate of the stripper. Actually this stream is mainly the desired diesel product, but it also contains lighter fractions accumulated with it. These light hydrocarbons tend to reduce the flash point of the diesel. To meet the specification and to minimize the overlap between diesel and kerosene products, the side stream is stripped with steam to remove the light components. The stripper is also a tray column with few trays. Diesel is drawn as the product from the bottom of the stripper. The vapor leaving from the top of the stripper contain the stripping steam and the stripped light components. This vapor stream is again introduced to the main distillation column.

Design of Distillation Column
The design steps for a column design are

1.      Calculation of the minimum reflux ratio

2.      Calculation of the optimum reflux ratio

3.      Calculation of the theoretical number of stages

4.      Calculation of the actual number of stages

5.      Calculation of column diameter

6.      Calculation of weeping point

7.      Calculation of pressure drop

8.      Calculation of shell thickness

Chashma Distillery Plant DI Khan