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Modeling of Vacuum Distillation

Distillation

It is a unit operation in which vaporization of a liquid mixture yields a vapor phase which consists of more than one component and it is desired that one or more component should be recovered in nearly pure state. Separation by distillation mainly depends on the difference in volatility of the respective components of a given liquid mixture or we can say that it primarily depends on relative volatility of the components. For obtaining desired product it is required that composition of vapor should be different from that of liquid with which it is in equilibrium. Vacuum distillation is a method of distillation, in which the pressure above the liquid mixture which is to be distilled is reduced to less than the vapor pressure of mixture (usually less than atmospheric pressure) causing the evaporation of the most volatile components (i.e. those having the lowest boiling points).

Variables

Optimum Pressure

Vacuum operation: Use of a vacuum should be considered for heat sensitive compounds or polymerizable materials. Vacuum is usually not used unless required, e.g., a low bottom temperature is needed to avoid thermal decomposition.

Optimum Temperature

Differences in Re-boilers and Condensers-

1. Re-boiler temperatures should be kept low enough to avoid bottoms degradation and/or fouling.

2. Common temperature differences used for heat exchange across re-boilers and condensers are shown in the following table:

I) Determine Process Operation Variables F×XF=XD×D+XB×B                                 …     (1)
 * Assumed feed rate, composition, purity of distillate and bottoms, and the quality of the feed are known.
 * Perform overall material and component balances to determine the compositions of the distillate and bottoms.

F=D+B                                                    …     (2)

where

F= Feed rate of input stream

XF =Composition of light component in feed

XD =Mole Fraction of light in distillate

XB =Mole Fraction of light in Bottom

D =Total distillate amount

B =Total bottom amount

*If more than two components, these values

'''Step 2. Determine the Minimum Re-flux Ratio'''

This graphical approach is determined using the McCabe-Thiele Method for binary mixtures. The ratio of re-flux flow to distillate flow is called the re-flux ratio.

Assumptions

The following assumptions are implied when using McCabe Thiele Method:
 * Constant Molal Overflow. The molar flow rates of the vapor and liquid are nearly constant in each section of the column. This also ensures the operating lines are straight lines.


 * Heat Effects are negligible. For example, heat losses to and from the column are small and    neglected.


 * For every mole of vapor condensed, another mole of liquid is vaporized.

Procedure
 * The liquid and vapor leaving the tray is in equilibrium with the vapor and liquid entering the tray.

If an equilibrium curve is not given, draw a y-x diagram (y representing the vapor phase and x the liquid). The equilibrium curve can be obtained by relating the relative volatility to the composition of the liquid:

y = a×x/ (1+x×(a-1))                                …     (3)

This shows the bubble-point and dew point of a binary mixture at constant pressure. An equilibrium line describes the compositions of the liquid and vapor in equilibrium at a fixed pressure. The equilibrium line crossing the forty-five degree line is an indication of an azeotropic mixture. 1. Draw the diagonal line connecting the points (0.0, 0.0) to (1.0, 1.0).
 * Forty Five Degree Line

This is your forty-five degree line. 2. The feed line can be constructed by locating the point on the forty five degree line that corresponds to the feed composition. This point can be extended with a slope of q/(q-1) where q is the feed quality. The feed line can be directly plotted through the following equation:
 * Feed Line (q-Line)

y=q/(q-1)X - XF/(q-1)                                 …    (4) 3. Draw the operating line for the enriching section. First,  find the desired top product composition (on the x-axis) and locate the corresponding point on the forty-five degree line. Connect this point to the point where the equilibrium cure and the feed line intersect. This is the upper operating line. The y intercept of this line is equal to XD/(R+1). The following equation can be used to determine the minimum re-flux:
 * Upper Operating Line

R(min)=(XD/y-intercept ) – 1                     …    (5) 4. Draw the operating line for the stripping section. First, find the desired bottom product composition (on the x-axis) and locate the corresponding point on the forty-five degree line. Draw a line from this point to the intersection of the equilibrium curve and the feed line. This is your lower operating line. The slope of this line is equal to (Vb(min)+1)/Vb(min) where Vb is the boil-up ratio. The boil-up ratio is the fractional amount of liquid that is boiled back into the column to the amount of liquid leaving. [3]
 * Lower Operating Line

Factor effected by Vacuum Distillation

Relative Volatility: Vacuum distillation increases the relative volatility of the key components in many applications. The higher the relative volatility, the more separable are the two components.

Example:

Petroleum Refining: Crude oil is first refined in an Atmospheric Distillation Column. Fractions of crude oil such as lighter gases (C1-C4), gasoline, naphtha, kerosene, fuel oil, diesel etc. are separated in the atmospheric distillation column. Then after taking out these lighter hydrocarbon cuts, heavy residue remaining at the bottom of the atmospheric distillation column needs to be refined. These heavy hydrocarbon residues are sent to a Vacuum Distillation Column for further separation of hydrocarbons under reduced pressure.

Advantages of Vacuum Distillation:

Industrial-scale vacuum distillation has several advantages- Reference:
 * 1) Close boiling mixtures require- many equilibrium stages to separate the key components, due to which operating costs of the process operation increases. So to reduce the number of stages or to reduce the operating cost one can use vacuum distillation
 * 2) For many systems, the product degrade or polymerize at elevated temperatures so for to acquire reduced temperature at lower pressure we can use vacuum distillation