Distillation Column Design

• For a specification of not more than 1 mol of the light key in the bottom product and not more than 1mol of the heavy key in the top product, and a reflux ratio of 2.5, make a stage-bystage calculation to determine the product composition and number of stages required.
• The precise location of the feed point will affect the number of stages required for a specified separation and the subsequent operation of the column. As a general rule, the feed should enter the column at the point that gives the best match between the feed composition (vapor and liquid if two phases) and the vapor and liquid streams in the column.
Selection of Trays:
DESIGNING STEPS OF DISTILLATION COLUMN
Calculation of Minimum number of stages, Nmin Calculation of Minimum Reflux Ratio Rm. Calculation of Actual Reflux Ratio. Calculation of theoretical number of stages. Calculation of actual number of stages. Calculation of diameter of the column. Calculation of weeping point, entrainment. Calculation of pressure drop. Calculation of the height of the column.
• To estimate the dew and bubble points, assume that nothing heavier than the heavy key appears in the tops, and nothing lighter than the light key in the bottoms.
Reflux considerations
The reflux ratio, R, is normally defined as: R = (flow returned as reflux)/(flow of top product taken off) The number of stages required for a given separation will be dependent on the reflux ratio used.
Flooding and Weeping
Weeping Flooding
Flow weeps through the holes at low vapor velocity
Liquid cannot get down the column at high vapor velocity
Feed-point location
Dew points and bubble points
• Dew points and bubble points can be calculated from a knowledge of the vapor-liquid equilibrium for the system. In terms of equilibrium constants, the bubble point is defined by the equation:
Theoretical nBiblioteka Baidu. of Plates:
0.566 R Rmin N N min 0.751 N 1 R 1
Problem
• A butane-pentane splitter is to operate at 8.3 bar with the following feed composition:
The design of a distillation column can be divided into the following steps: Specify the degree of separation required: set product specifications. Select the operating conditions: batch or continuous; operating pressure. Select the type of contacting device: plates or packing. Determine the stage and reflux requirements: the number of equilibrium stages. Size the column: diameter, number of real stages. Design the column internals: plates, distributors, packing supports. Mechanical design: vessel and internal fittings.
Total reflux
Total reflux is the condition when all the condensate is returned to the column as reflux: no product is taken off and there is no feed. At total reflux the number of stages required for a given separation is the minimum at which it is theoretically possible to achieve the separation.
Calculation of Minimum no. of Plates:
The minimum no. of stages Nmin is obtained from Fenske equation which is, Nmin = LN[(xLK/xHK)D(xHK /xLK)B] LN (αLK/HK) average
The separation of liquid mixtures by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the enrichment, or more commonly, the rectifying section.
Distillation Column Design
Syed Zaheer Abbas
Distillation:
• Process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat”
Optimum reflux ratio
Practical reflux ratios will lie somewhere between the minimum for the specified separation and total reflux. For many systems the optimum will lie between 1.2 to 1.5 times the minimum reflux ratio.
Choice b/w Tray & Packed Column
Plate column are designed to handle wide range of liquid flow rates without flooding. For large column heights, weight of the packed column is more than plate column. Man holes will be provided for cleaning in tray Columns. In packed columns packing must be removed before cleaning. When large temperature changes are involved as in the distillation operations tray column are often preferred. Random-Packed Column generally not designed with the diameter larger than 1.5 m and diameters of commercial tray column is seldom less than 0.67m
Minimum reflux
As the reflux ratio is reduced a pinch point will occur at which the separation can only be achieved with an infinite number of stages. This sets the minimum possible reflux ratio for the specified separation.