Selection of Distillation Column
The selection of a distillation column depends on several factors, including:
- Feed composition: The composition of the feed stream, including the number of components, their concentrations, and their physical properties.
- Separation requirements: The desired separation of the components, including the purity of the products and the recovery of the valuable components.
- Operating conditions: The operating conditions, including the temperature, pressure, and reflux ratio.
- Equipment constraints: The constraints of the equipment, including the column diameter, height, and tray spacing.
Sizing of Distillation Column
The sizing of a distillation column involves calculating the column diameter, height, and tray spacing. The following equations are used for sizing:
Column diameter: The column diameter is calculated using the following equation:
D = √(4 * Q * ρ / (π * ΔP * η))
where:
- D = column diameter (m)
- Q = vapor flow rate (m³/s)
- ρ = vapor density (kg/m³)
- ΔP = pressure drop (Pa)
- η = tray efficiency
Column height: The column height is calculated using the following equation:
H = N * TS
where:
- H = column height (m)
- N = number of trays
- TS = tray spacing (m)
Tray spacing: The tray spacing is calculated using the following equation:
TS = (D * ΔP) / (4 * ρ * g)
where:
- TS = tray spacing (m)
- D = column diameter (m)
- ΔP = pressure drop (Pa)
- ρ = vapor density (kg/m³)
- g = acceleration due to gravity (m/s²)
Mathematical Theory Behind Sizing
The mathematical theory behind sizing a distillation column is based on the following principles:
Mass transfer: The mass transfer between the vapor and liquid phases is described by the following equation:
N_A = k * A * (C_Ai - C_A)
where:
- N_A = mass transfer rate of component A (mol/s)
- k = mass transfer coefficient (m/s)
- A = interfacial area (m²)
- C_Ai = concentration of component A in the vapor phase (mol/m³)
- C_A = concentration of component A in the liquid phase (mol/m³)
Heat transfer: The heat transfer between the vapor and liquid phases is described by the following equation:
Q = U * A * (T_v - T_l)
where:
- Q = heat transfer rate (W)
- U = overall heat transfer coefficient (W/m²K)
- A = interfacial area (m²)
- T_v = temperature of the vapor phase (K)
- T_l = temperature of the liquid phase (K)
Optimization Rules for Selection
The optimization rules for selecting a distillation column are based on the following principles:
- Minimize energy consumption: Minimize the energy consumption by optimizing the reflux ratio, column pressure, and heat exchanger design.
- Maximize separation efficiency: Maximize the separation efficiency by optimizing the column design, tray spacing, and mass transfer coefficient.
- Minimize capital cost: Minimize the capital cost by optimizing the column diameter, height, and material selection.
Feedback Stock Composition and Range of Mixture
The feedback stock composition and range of mixture are important factors in selecting and sizing a distillation column. The following considerations should be taken into account:
- Feed composition: The feed composition should be analyzed to determine the number of components, their concentrations, and their physical properties.
- Range of mixture: The range of mixture should be determined to ensure that the column can handle the expected variations in feed composition and operating conditions.
- Product specifications: The product specifications should be defined to ensure that the column can produce the desired products with the required purity and recovery
To design a distillation column and its selection as well as sizing needs ideas of vapor and liquid phase thermodynamics because it helps to calculate the minimum number of equilibrium stages on which the needed separation takes place. In addition, a continuous column's minimum reflux ratio factor depends on the VLE data. As a part of the introduction, brief requirements and tools used for sizing this separation equipment were discussed.
For the learner, the Fenske-Underwood equation is enough to understand about minimum reflux ratio and the minimum number of stages. At last, safety and economic factors shape the drawing and mechanical parts of the column.
Distillation word refers to the separation operation in chemical and petroleum industries. Only the concept of boiling point variation draws attention to the idea of distillation. Fractional distillation or fractionation word is awarded for this purpose only. 50% of a plant’s energy consumption falls under the distillation account. Also, its promising separation makes it still alive in the latest chemical industries also.
New adaptable technologies of solvent extraction, adsorption, membrane, and reactive distillation turn the old methods into a hybrid system. Distillation process types like extractive distillation and reactive distillation are now in demand.
Batch distillation and continuous distillation operation mode depend on the factor of time only and works similarly in theory.
Column internals:
Any column is equipped with an internal setup that has the only purpose of providing mass and heat transfer. The internal structures directly affect the vapor and liquid phase mass and heat transfer simultaneously. Parts like trays, re-distributors, packings, distributors, and baffles are some of them. Each part ultimately should provide the purpose of enough contact between the phases. Column height and diameter are calculated finally by the inclusion of these internal parts. In immediate need of design, two internal parts are enough in design calculations that would be trays and packing. Either of these two parts is the main internal structures that determine the separation, efficiency, and capacity of the column.
During the selection of the tray, sieve, bubble cap and valve trays are common in use. Packing is of random and structured by packing materials like saddles and rings.
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Comparison of Distillation Column Trays |