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BINARY VLE

Separation of hydrocarbon mixtures is carried out in vertical distillation columns. In a vertical tray distillation column, various trays are placed on top of each other with some tray spacing with a condenser placed at top of the column and reboiler placed at bottom of the column. Liquid is vaporized in reboiler resulting in increase in temperature and pressure of reboiler. Because of pressure differential created by vaporization of liquid in reboiler, the vapors move upwards in the column. These vapors are condensed in the condenser by decreasing the temperature when they reach the top of the column and the condensed liquid at lower temperature is fed back into the column. This liquid travels down the column due to gravity. This way apart from a pressure differential between reboiler and condenser, a temperature differential is created between reboiler and condenser. This temperature differential which gets adjusted because of lower temperature liquid flowing to the tray from top and higher temperature vapors bubbling through the liquid on the tray from below results in different temperatures at different trays in the column. Similarly pressure differential within the column adjusts to a low value because of presence of liquid head on each of the trays. The pressure differential in a distillation column is a low value implying that distillation column operates at almost the same constant pressure throughout its height. A hydrocarbon mixture has a different liquid composition and vapour composition under equilibrium at a given temperature and pressure. As both these liquid and vapour compositions vary with temperature and pressure so because of the pressure differential and temperature differential exiting throughout the height of distillation column, different compositions of liquid and vapors exist on different trays of the distillation column. On one of the intermediate trays feed stream enters the column. A part of the vapors entering top condenser after condensing is taken out from condenser as distillate stream whereas a part of liquid entering bottom reboiler is taken out from reboiler as residue stream. At steady state amount of feed and the amount of various feed components entering the column is equal to total amount of distillate and residue and total amount of various components in distillate and residue. Distillate stream and residue stream has different compositions depending on the temperature and pressure of the condenser and reboiler. These compositions are different from feed composition implying that separation has been achieved in the distillation column.

This is the general theory based on which all kind of distillation operations including continuous, batch and semi-batch distillations are carried out in a vertical tray distillation column. Both binary and multicomponent distillations are carried out in a distillation column based on the above given theoretical background.

For multicomponent distillation, separate detailed tray by tray calculations performed iteratively may be required for optimized design of the distillation column but simpler calculation procedures exist for binary distillation. Distillation columns for binary mixtures can be designed by very simple McCabe-Thiele method and slightly more complex and more accurate Ponchan-Savarit method. Both McCabe-Thiele and Ponchon-Savarit methods require Concentration diagrams (xy plot) and Temperature-Concentration diagrams (Txy plot) at the operating pressure of the distillation column. Ponchan-Savarit method requires enthalpy-concentration diagram (Hxy plot) in addition to these two diagrams.

EQ-COMP can be used to design multicomponent distillation columns by calculating VLE properties at each of the tray conditions separately step by step. EQ-COMP can accurately find out various VLE properties at low as well as high pressures and can further post-process the data generated by running various modules of EQ-COMP number of times to generate Concentration diagrams (xy plot), Temperature-Concentration diagrams (Txy plots) and Enthalpy-Concentration diagram (Hxy plots) for any binary mixture comprising of non-polar or mildly polar hydrocarbons at any pressure. These diagrams can be used in McCabe- Thiele method or Ponchan-Savarit method to find out various design parameters like theoretical number of trays, feed tray location, reflux ratio etc. for distillation columns for binary mixtures.

Concentration diagrams (xy plot), Temperature-Concentration diagrams (Txy plots) and Enthalpy-Concentration diagram (Hxy plots) for Methane-Ethane mixture at 10 bars pressure generated by EQ-COMP are given below.

x-y plot for methane-ethane mixture at 10 bars pressure (generated by EQ-COMP)

xy plot for methane-ethane mixture at 10 bars pressure (generated by EQ-COMP)

T-x-y plot for methane-ethane mixture at 10 bars pressure (generated by EQ-COMP)

Txy plot for methane-ethane mixture at 10 bars pressure (generated by EQ-COMP)

H-x-y plot for methane-ethane mixture at 10 bars pressure (generated by EQ-COMP)

Hxy plot for methane-ethane mixture at 10 bars pressure (generated by EQ-COMP)

In total, EQ-COMP provides the following information as part of the same binary VLE calculation module:

1. Concentration diagram (xy plot) and concentration diagram (xy plot) data.

2. Temperature-concentration diagram (Txy plot) and Temperature-concentration diagram (Txy plot) data.

3. Enthalpy-Concentration diagram (Hxy plot) and Enthalpy-Concentration diagram (Hxy plot) data.

4. Phase envelope data for binary mixtures of same compounds with 9 different compositions starting from 0 % of one compound to 100 % of that compound with increments of 12.5 %.

5. Critical point data for binary mixtures of same compounds with 9 different compositions starting from 0 % of one compound to 100 % of that compound with increments of 12.5 %.

6. Critical point profile based on critical point data of binary mixtures of same compounds with 9 different compositions starting from 0 % of one compound to 100 % of that compound with increments of 12.5 %.

 

EQ-COMP can generate complete VLE analysis of any binary mixture comprising of any of the hydrocarbons given in the list of included hydrocarbons providing the information given above. Customization of EQ-COMP for inclusion of additional non-polar or mildly polar compounds in the binary mixture is also possible. The VLE analysis of a binary mixture providing information given above can be provided by EQ-COMP as a single calculation.


Critical point profile for methane-ethane mixture is given below:

Critical point profile for methane-ethane mixture (generated by EQ-COMP)

Critical point profile for methane-ethane mixture (generated by EQ-COMP)