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distillation column design
Distillation is a common unit operation in the chemical industry for separating components in a mixture. The process involves heating a mixture to vaporize its components, and then condensing the vapor to obtain a purified liquid. A distillation column is the primary equipment used in this process. In this article, we will discuss the step-by-step design of a distillation column along with examples and the required formulas.
11 steps for distillation column design
Step 1: Define the Problem:
The first step in designing a distillation column is to define the problem. This involves specifying the feed composition, desired product purity, and the operating conditions such as pressure and temperature. The feed composition determines the number of trays or packing required, while the desired product purity determines the reflux ratio.
Step 2: Determine the Operating Pressure:
The second step is to determine the operating pressure of the distillation column. This is done by evaluating the vapor-liquid equilibrium (VLE) data for the components of the mixture. The pressure at which the components of the mixture have different boiling points is the operating pressure. The VLE data can be obtained from published literature or by conducting experiments.
Step 3: Determine the Number of Trays:
The third step in distillation column design is to determine the number of trays required for the distillation column. The number of trays is based on the desired product purity and the feed composition. The minimum number of trays required is calculated using the Fenske equation, which is given by:
Nmin = (log (D/F)) / (log (α – 1))
where Nmin is the minimum number of trays, D is the number of theoretical plates required for the separation, F is the reflux ratio, and α is the relative volatility of the components.
Step 4: Tray Efficiency:
The tray efficiency is the ratio of the actual separation achieved on a tray to the theoretical separation. The tray efficiency is a critical parameter as it affects the number of trays required for the column. The tray efficiency can be determined experimentally or estimated using correlations. The most commonly used correlation is the Murphree tray efficiency, which is given by:
Em = (Hmin – Hl) / (Hv – Hl)
where Em is the Murphree tray efficiency, Hmin is the minimum reflux ratio, Hl is the enthalpy of liquid leaving the tray, and Hv is the enthalpy of vapor entering the tray.
Step 5: Column Diameter:
The column diameter is determined based on the number of trays required, the vapor flow rate, and the tray spacing. The diameter can be calculated using the following equation:
Dc = (4 * F * V / (π * ρ * (1 – ε))) ^ 0.5
where Dc is the column diameter, F is the reflux ratio, V is the vapor flow rate, ρ is the density of the vapor, and ε is the fraction of the column occupied by trays.
Step 6: Liquid Flow Rate:
The liquid flow rate is determined based on the tray spacing and the desired liquid residence time on each tray. The liquid flow rate can be calculated using the following equation:
L = V / (ε * (1 + K))
where L is the liquid flow rate, V is the vapor flow rate, ε is the fraction of the column occupied by trays, and K is the vapor-liquid equilibrium ratio.
Step 7: Reboiler Duty:
The reboiler duty is the amount of heat required to vaporize the liquid feed and maintain the column at the desired operating temperature.
The reboiler duty can be calculated using the following equation:
Q = L * (Hv – Hf)
where Q is the reboiler duty,
L is the liquid flow rate,
Hv is the enthalpy of vapor entering the calculation, the enthalpy of vapor entering is the same as the enthalpy of liquid leaving the tray, and
Step 8: Condenser Duty:
The condenser duty is the amount of heat that must be removed from the column to condense the vapor and obtain the desired product purity.
how to calculate condenser duty?
The condenser duty can be calculated using the following equation:
Qc = V * (Hf – Hv)
or second formula is, we use some of latent heat and sensible heat.
where Qc is the condenser duty, V is the vapor flow rate, Hf is the enthalpy of the feed, and Hv is the enthalpy of vapor entering the tray.
Step 9: Reflux Ratio:
The reflux ratio is the ratio of the liquid returned to the column to the liquid product. The reflux ratio is determined by the desired product purity and the number of trays required ( distillation column design procedure). The reflux ratio can be calculated using the following equation:
F = L / (D – 1)
where F is the reflux ratio, L is the liquid flow rate, and D is the number of theoretical plates required for the separation.
Step 10: Column Height:
The column height is determined based on the number of trays required, the tray spacing, and the diameter of the column. The height can be calculated using the following equation:
Hc = (N * Htray) + Hv + Hf + Hcond
where Hc is the column height, N is the number of trays, Htray is the tray spacing, Hv is the height of the vapor disengagement space, Hf is the height of the feed plate, and Hcond is the height of the condenser.
Conclusion
In this article, we discussed the step-by-step design of a distillation column along with examples and the required formulas. The design of a distillation column involves defining the problem, determining the operating pressure, determining the number of trays required, tray efficiency, column diameter, liquid flow rate, reboiler duty, condenser duty, reflux ratio, and column height. The design process can be complex, and the use of software programs can assist in the calculation of some of the parameters.
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