Definition
In the chemical industry, various gaseous materials and products are essential, necessitating the separation of gas mixtures into their individual components for effective processing There are three primary methods employed for this separation of gas mixtures.
Suction separation method Physicochemical method Chemical method
This report focuses solely on the suction separation method.
The suction separation method involves the transfer of one substance into another at their phase separation interface This process is termed absorption when a liquid is used to take in the substance, while it is referred to as adsorption when a porous solid is employed.
Figure 1 The difference of adsoption and absoption
Absorption is the process where a mixture of gases intimately contacts a liquid, allowing some constituents of the gas to dissolve in the liquid This interaction typically occurs in packed or plate columns In this context, the gas being absorbed is known as the absorbent, the liquid performing the absorption is called the solvent, and any gas that remains unabsorbed is referred to as the inert gas.
The absorption process is used to
Recovery of precious components Clean air.
Separate the gas mixture into components.
In gas absorption operations, selecting the right solvent is crucial, with water often being the preferred choice due to its low cost and abundance However, it is essential to consider additional properties of the solvent to ensure optimal performance.
High gas solubility is essential as it enhances the absorption rate and reduces the amount of solvent required Typically, using a solvent with a chemical nature similar to that of the solute being absorbed results in optimal solubility.
2 Volatility - a low solvent vapor pressure is desired since the gas leaving an absorption unit is ordinarily saturated with the solvent and much will therefore be lost.
4 Cost (particularly for solvents other than water).
5 Viscosity - low viscosity is preferred for reasons of rapid absorption rates, improved flooding characteristics, lower pressure drops, and good heat transfer characteristics.
6 Chemical stability - the solvent should be chemically stable and, if possible, nonflammable.
8 Low freezing point - if possible, a low freezing point is favored since any solidification of the solvent in the column could prove disastrous.
Once the solvent is specified, the choice (and design) of the absorption system may be determined.
The general design procedure consists of a number of steps that have to be taken into consideration details of which follow shortly.
3 Estimation of operating data (usually obtained from a mass and energy balance, where the energy balance determines whether the absorption process can be considered isothermal or adiabatic).
When column selection is not clearly defined, it is essential to perform calculations for various column types, ultimately making a decision based on economic factors.
When calculating the diameter of a column, packed columns typically rely on flooding conditions, while plate columns are determined by the optimal gas velocity or the liquid handling capacity of the plate.
To estimate the height of a column, whether packed or plate, it is essential to calculate the number of transfer units or theoretical plates For packed columns, the height is determined by multiplying the number of transfer units—derived from equilibrium and operating data—by the height of a transfer unit In contrast, for plate columns, the number of theoretical plates, often obtained from plotting equilibrium and operating lines, is divided by the estimated overall efficiency to determine the actual number of plates, which then allows for the estimation of column height based on the spacing between plates.
To determine the pressure drop in packed columns, it is essential to utilize correlations that consider the type of packing, operational data of the column, and the physical properties of the involved constituents In the case of plate columns, the pressure drop is calculated for each plate and then multiplied by the total number of plates to obtain the overall pressure drop.
The principle types of gas absorption equipment may be classified as follows:
In this part, our group will focus on introducing packed column and plate column.
Packed columns are vertical structures filled with high surface area packing material In these columns, liquid is evenly distributed and trickles down through the packed bed, maximizing the surface area for gas contact The counter-current packed column is the most widely used design for gas removal or recovery processes.
The packed column operates by allowing a gas stream to flow upward through a packed bed, countercurrent to an absorbing or reacting liquid introduced at the top This design maximizes efficiency by ensuring that as the gas rises, the solute concentration decreases, providing fresh solvent for optimal contact Consequently, this setup maintains the maximum average driving force for mass transfer throughout the packed bed, enhancing the overall absorption or reaction process.
Figure 2 Typical counter-current packed column
The packing is crucial for the functionality of this equipment, requiring a thorough understanding of its operational characteristics and how different types impact performance due to their significant physical differences Examples of packing include various materials and configurations tailored to specific applications.
The main point to be considered in choosing the column packing include:
- Durability and corrosion resistance (the packing should be chemically inert to the fluids being processed)
The free space per unit volume in a packed column significantly influences both the liquor holdup and the pressure drop across the system It is essential that the fractional void volume, which represents the fraction of free space within the packed bed, remains sufficiently large to ensure optimal performance.
The wetted surface area per unit volume of packed space is a crucial factor, as it influences the interfacial surface between liquid and gas This measurement often differs from the actual geometric surface due to the incomplete wetting of the packing material by the fluid.
- Resistance to the flow of gas (this effects the pressure drop over the column)
- Packing stability and structural strength to permit easy handling and installation
- Weight per unit volume of packed space
- Cost per unit area of packed space
Cross-flow packed columns operate with air moving horizontally through the packed bed while scrubbing liquid flows vertically down, offering low water consumption and high air flow capacity at minimal pressure drop They are particularly advantageous for recovering highly soluble gases, as they maintain a lower pressure drop compared to counter-current scrubbers at equivalent flow rates Additionally, the cross-flow design reduces the size of pumps and fan motors, minimizes piping, and decreases the risk of plugging from solids at the packing support plate This configuration allows for higher gas and liquid rates due to the extremely low pressure drop However, it is important to note that these systems tend to have higher liquid entrainment, necessitating the use of mist eliminators downstream.
Figure 5 Cross-flow operation in a packed column
Packed columns are characterized by a number of features to which their widespread popularity may be attributed.
- Minimum structure — the packed column usually needs only a packing support and liquid distributor approximately every 10 feet along its height
- Versatility — the packing material can be changed by simply discarding it and replacing it with a type providing better efficiency
Equipment
Packed column
Packed columns are vertical structures filled with high surface area packing material, allowing liquid to trickle down through the packed bed and facilitating extensive gas contact The counter-current packed column is the most widely used unit for gas removal or recovery processes.
In a packed column, the gas stream ascends through a packed bed while interacting with an absorbing or reacting solvent introduced from the top This configuration maximizes efficiency, as the solute concentration in the gas decreases during its ascent, ensuring a continuous supply of fresh solvent for optimal contact Consequently, this setup maintains the highest average driving force for mass transfer throughout the packed bed.
Figure 2 Typical counter-current packed column
The selection of packing is crucial for the optimal performance of equipment, requiring a thorough understanding of its operational characteristics and the impact of physical differences among various types Examples of packing types include different materials and designs tailored to specific applications.
The main point to be considered in choosing the column packing include:
- Durability and corrosion resistance (the packing should be chemically inert to the fluids being processed)
The fractional void volume in a packed bed is crucial as it influences both the liquor holdup and the pressure drop within the column To ensure optimal performance, this fraction of free space per unit volume should be maximized.
The wetted surface area per unit volume of packed space is crucial as it defines the interfacial surface between liquid and gas This measurement is often not equal to the actual geometric surface, primarily because the packing is typically not fully wetted by the fluid.
- Resistance to the flow of gas (this effects the pressure drop over the column)
- Packing stability and structural strength to permit easy handling and installation
- Weight per unit volume of packed space
- Cost per unit area of packed space
Cross-flow packed columns operate with horizontal air movement through the packed bed, while the scrubbing liquid flows vertically downwards, resulting in low water consumption and high air flow capacity with minimal pressure drop This design is particularly advantageous for recovering highly soluble gases, as it offers lower pressure drops compared to counter-current scrubbers when using the same mass flow rates of liquid and gas Additionally, the cross-flow principle leads to reduced pump and fan motor sizes, less piping, and decreased plugging from solids at the packing support plate, allowing for higher gas and liquid rates However, these systems tend to have higher liquid entrainment, necessitating the use of mist eliminators downstream.
Figure 5 Cross-flow operation in a packed column
Packed columns are characterized by a number of features to which their widespread popularity may be attributed.
- Minimum structure — the packed column usually needs only a packing support and liquid distributor approximately every 10 feet along its height
- Versatility — the packing material can be changed by simply discarding it and replacing it with a type providing better efficiency
Ceramic packing is an ideal choice for handling corrosive fluids due to its superior corrosion resistance compared to metal or plastic options Its quick and easy replacement when deteriorated makes it a practical solution, especially for applications involving hot combustion gases.
The pressure drop in packed columns is generally low, particularly under standard operating conditions However, at very high liquid flow rates, the liquid can become the continuous phase, leading to thicker flowing films that merge, which may increase the pressure drop per linear foot of packed height.
- Range of operation — although efficiency varies with gas and liquid feed rates, the range of operation is relatively broad
Plate columns
Plate columns, often known as tray columns, are vertical cylinders designed for the staged operation of liquid and gas contact on plates or traps.
The principle operation involves liquid entering from the top and descending through gravity, flowing across each plate and through downspouts to the plate below Simultaneously, gas rises through openings in the plates, bubbling through the liquid to create froth, which then disengages and moves to the next plate above.
The overall effect is a multiple counter-current contact of gas and liquid.
Figure 6 Typical bubble-cap plate column.
Each plate in a column represents a distinct stage where fluids interact closely, facilitating interface diffusion and separation The required number of theoretical plates is influenced by the complexity of the separation process and is calculated based on material balances and equilibrium principles Additionally, the diameter of the column plays a crucial role in the overall efficiency of the separation.
Packed vs plate tower comparison
Packed columns and plate columns are the two most commonly utilized gas absorption devices, each with unique applications While packed columns are more frequently employed, both types offer distinct advantages and disadvantages that should be evaluated for optimal use in specific scenarios.
- The pressure drop of the gas passing through the packed column is smaller.
The plate column is capable of accommodating very low liquid feed rates while allowing for higher gas feed compared to the packed column Additionally, it can be engineered to manage liquid rates that would typically cause flooding in a packed column.
- If the liquid deposits a sediment, the plate column is more advisable.
Incorporating manholes into the plate column design facilitates the removal of accumulated sediment, preventing clogs that could compromise packing materials and lead to expensive maintenance Additionally, packed columns are at risk of plugging when exposed to gas containing particulate contaminants.
In mass transfer processes involving significant heat effects, plate columns facilitate easier cooling or heating of liquids This is achieved by installing a system of pipes within the liquid on the plates, allowing for direct heat transfer through the pipe walls to the processing area In contrast, addressing the same issue in packed columns requires segmenting the process into multiple sections, where cooling or heating occurs between these segments.
- The total weight of the plate column is usually less than the packed column designed for the same capacity.
- A well-installed plate column avoids serious channeling difficulties insuring good, continuous contact between the gas and liquid throughout the column.
- Temperature changes are apt to do more damage to the packed column than to the plate column.
- In highly corrosive atmospheres, the packed column is simpler and cheaper to construct.
- The liquid holdup in the packed column is considerably less than in the plate column.
Plate columns offer significant benefits for absorption processes involving chemical reactions, especially when these reactions occur at a slower rate The efficiency of the process is enhanced by allowing a longer residence time for the liquid within the column, which also facilitates better control over the reaction dynamics.
- Packed columns are preferred for liquids with high foaming tendencies.
When evaluating the advantages of plate columns versus packed columns, the decision typically hinges on a detailed cost analysis for each type Generally, packed columns are more cost-effective for smaller diameters, typically up to 2 or 3 feet Conversely, for larger sizes, plate columns often prove to be the more economical choice.
Calculation
Design and performance equations - Packed columns
For design the packed columns, calculations generally involve the determination of three unknown system variables: the liquid rate, the column diameter, the column height (and Pressure Drop).
Counter-current flow procedures are commonly used in absorption or stripping operations, where gas is introduced at the bottom of the column and liquid solvent is added at the top.
Figure 7 Mole balance, counter-current flow (Figure 10.7 - Theodore & Ricci, 2011[2])
The overall material balance for the counter-current absorption process is:
G m1 is gas rate of Feed gas (kmol/h)
G m2 is gas rate of Treated gas (kmol/h)
L m1 is liquid flow rate of Lean solution (kmol/h)
L m2 is liquid flow rate of Rich solution (kmol/h)
For component A, the mass (or mole) balance becomes:
In gas phase separation, the mol fraction of component A in the feed gas is represented as y A1, while the mol fraction of A in the treated gas is denoted as y A2 In the liquid phase, the lean solution's mol fraction of A is indicated by x A1, and the rich solution's mol fraction of A is represented as x A2.
Assuming G m1 = G m2 = G m and L m1 = L m2 = L m (suitable for many applications where solute concentrations are reasonably small)
This equation is a straight line, which is known as the operational line It passes through the points (x A1 , y A1 ) and (x A2 , y A2 ) and has a slope of
L m /G m on x, y coordinates, as shown in below Figure:
Figure 8 Operating and equilibrium lines (Figure 10.8 - Theodore &
The column operation is frequently specified as some factor of the minimum liquid–to–gas ratio For instance, (L m /G m ) ac t is 1.5(L m /G m ) min is a typical situation frequently encountered.
Below is an example for find liquid rate:
Example 4.1 (Illustrative Example 10.2 - Theodore & Ricci, 2011[2]) Given the following information for a packed counter-current gas scurbber, determine the liquid rate in lbmol/h.ft 2
The mol fractions of the solute in the inlet and outlet gas are 0.08 and 0.002, respectively.
The mol fractions of the solute in the inlet and outlet liquid are 0.001 and 0.05, respectively.
From the provided data, the parameters can be summarized as shown below:
Hence, the liquid rate for the packed counter-current gas scurbber is 27 lbmol/h.ft 2
Column diameter is often calculated using flooding considerations.
Flooding occurs when high gas rates hinder liquid flow in a column, resulting in liquid accumulation and blockage This condition disrupts the effective mixing of gas and liquid within the packing and leads to increased pressure drop The flooding velocity is defined as the superficial gas velocity at which this phenomenon happens Typically, 50–75% of the flooding rate is used to determine the appropriate column diameter for optimal operation.
The procedure to determine the column diameter is as follows:
Step 1: Calculate the abscissa (mass basis for all terms)
L = lb/s (Liquid rate in mass basis)
G = lb/s (Gas rate in mas basis) ρ G = lb/ft 3 (Density of gas) ρ L Step
Find by looking up in below figure:
Figure 9 Generalized pressure drip correlation to estimate column diameter. (Figure 10.11- (Theodore & Ricci, 2011[2])
Step 3: Solve the ordinate equation for G f at flooding.
Step 4: Calculate the column cross-sectional area, S, for the fraction of flooding velocity chosen for operation, f, by the equation:
W (m) is the mass flow rate of the gas in lb/s; S is the area in ft 2 Step 5: Calculate the column diameter
A packed column is used to absorb a toxic pollutant from a gas stream. From the data given below, calculate the column diameter.
Density of gas (air) = 0.075 lb/ft 3
Density of water = 62.4 lb/ft 3
Gas mass flow rate = 3500 lb/h;
Liquid mass flow rate = 3600 lb/h,
Packing type 1-inch Raschig rings; packing factor F = 160
For the fraction of flooding velocity chosen for operation: f= 0.5
(Note: Example is based on Illustrative Example 10.4 - Theodore & Ricci,
Step 1: Calculate the abscissa (mass basis for all terms)
Step 2: Proceed to the flooding line and read the ordinate (design parameter):
=>Look up in the Figure 4.4 , we can see that:
Figure 10 Generalized pressure drip correlation to estimate column diameter (Figure 10.11- (Theodore & Ricci, 2011) [2])
Step 3: Solve the ordinate equation for G f at flooding:
Step 4: Calculate the column cross-sectional area, S
Step 5: Calculate the column diameter
Hence, column diameter is 2.43 ft.
The column height might be calculated by equation:
N OG is the number of overall transfer units
H OG is the height of a single transfer unit and Z is the height of the column packing
For the height of a single transfer units (H OG ), it is usually determined experimentally for the system under consideration or obtained from the manufacturer.
For the number of overall transfer units (N OG ), it is calculated based on different condition:
In many operations, the constituent to be absorbed (e.g., HCl) is in the very dilute range:
If operating line and equilibrium line are both parallel and straight:
If the operating line and equilibrium line are just straight (and not necessarily parallel):
If Henry’s law applies, the number of transfer units is given by
If the gas is highly soluble in the liquid and/or reacts with the liquid,Theodore has shown that:
If the operating line and/or equilibrium line are curved:
Example 4.3 (Illustrative Example 10.5 - Theodore & Ricci, 2011 [2])
When a gas is highly soluble, the number of overall gass transfer units
N OG in a packed tower is given by:
Calculate N OG if y 1 = 200 ppm and y 2 = 0.5 ppm
If we have: H OG = 1.25 ft
Pressure drop is the difference in total pressure between two points in a fluid-carrying network[4].
When designing a packed column for liquid and gas flows, it is crucial to consider the economic implications, as the flow rates of both phases significantly affect the pressure drop across various random packings.
As liquid throughput increases while maintaining a constant gas rate, the pressure drop rises until it reaches the liquid flooding rate Once flooding occurs, any additional liquid that cannot pass through accumulates on top of the packing, resulting in an increasingly deeper pressure drop.
In a constant liquid downflow scenario, an increase in gas flow leads to a rising pressure drop until the flooding rate is achieved Consequently, even a minor increase in gas flow results in a reduction of the allowable liquid throughput This causes liquid to accumulate on top of the packing, further escalating the lead pressure drop.
For precise pressure drop data, it is best to obtain information directly from the manufacturer, especially with specific packing However, for estimation purposes, the "Generalized Pressure Drop Correlation for Column Diameter" (Figure 4.4) can be utilized to yield satisfactory results.
Design and performance equations - Plate columns
When designing plate columns, key factors include determining the column diameter, selecting the type and number of plates (commonly bubble-cap or sieve plates), and establishing the plate layout, physical design, and spacing, all of which influence the overall column height While this chapter will not delve deeply into these aspects, as they were covered in Chapter 9, it will provide a brief overview of effective absorber design strategies that can serve as a reliable basis for estimation purposes.
The diameter of the column and its cross section must be sufficiently large to manage the gas and liquid flow rates, preventing flooding and excessive entrainment The superficial gas velocity at which flooding occurs for a specific type of plate is defined by a particular relationship.
In the context of plate columns, the gas velocity (VF) is measured in cubic feet per second per square foot, while densities are expressed in pounds per cubic foot, and CF is an empirical coefficient influenced by plate type and operating conditions The net cross section of a column is defined as the difference between the total column cross section and the area occupied by downcomers For nonfoaming liquids, 80-85% of VF is typically utilized, whereas for foamy liquids, it is 75% or less, depending on an analysis of entrainment and pressure drop characteristics When designing a column, the gas flow rate is crucial for calculating diameter, as indicated by Equation (10.16) Furthermore, evaluating the liquid handling capacity of the plates is essential to avoid flooding or gas maldistribution, particularly when the liquid-to-gas ratio is high and the column diameter is large A reasonable design assumption for plate handling capacity is 30 gallons per minute per foot of diameter, though a well-designed single-pass cross-flow plate can manage up to 60 gallons per minute without excessive liquid gradient Additionally, low gas flow rates may lead to weeping, where liquid flows through the plate's perforations instead of over them.
The height of a column is determined by the product of the number of real plates (calculated by dividing theoretical plates by overall plate efficiency) and the chosen plate spacing In this context, a theoretical plate represents a unit of separation where two dissimilar phases come into contact before separation During this interaction, various components of the mixture diffuse between the phases An equilibrium stage is achieved when the phases are sufficiently mixed to reach equilibrium, resulting in no further net changes in phase composition under specific operating conditions To calculate the number of theoretical plates, one can utilize an operating diagram that includes both an operating line and an equilibrium curve.
In previous discussions of equilibrium stages, it was assumed that the phases leaving the stage were in balance; however, achieving the necessary residence time and intimacy of contact for true equilibrium in counter-current multistage equipment is impractical Consequently, the concentration changes in a given stage are less than what equilibrium suggests, leading to the use of stage efficiencies to describe this phenomenon Overall stage efficiency, often referred to as plate efficiency, measures the ratio of theoretical contacts needed for a specific separation to the actual contacts required While accurate information on this efficiency is valuable, numerous variables complicate the determination of reliable overall stage efficiency values, which are typically obtained through experimental or field test data, or provided by vendors.
The number of theoretical plates can be directly determined without graphical methods when both the operating line and equilibrium curve are straight, typically in dilute solutions This scenario is common with relatively dilute gases, as found in air pollution control, and liquid solutions where Henry’s law applies Due to the small quantity of gas absorbed, the total liquid and gas flows entering and leaving the column remain nearly constant, resulting in a predominantly straight operating line In these cases, the Kremser–Brown–Sounders equation is utilized to calculate the number of theoretical plates, Np.
In gas-liquid equilibrium, the natural logarithm (ln) can be used instead of the logarithm (log) in both the numerator and denominator The term mx 0 represents the gas composition in equilibrium with the incoming liquid, where m is Henry’s law constant, indicating the slope of the equilibrium curve If the incoming liquid is devoid of solute gas, then x 0 equals zero, allowing for further simplification of Equation (10.17) This context is essential for understanding the solute concentrations present in the gas stream.
Inlet and outlet conditions are represented by +1 and y1, while L and V denote the total mole rates of liquid and gas flow per unit time and cross-sectional area of the column Small fluctuations in L and V can be approximately balanced by employing the geometric average values from the top and bottom of the column For practical application, Equation (10.17) is illustrated in Figure 10.16, facilitating its solution Chen derived a simplified algebraic equation to estimate the theoretical plates, n, applicable to both absorbers and strippers, ultimately presenting it in a specific notation.
The absorption factor, denoted as A, plays a crucial role in determining the gas mole fractions at different stages of a separation process Specifically, y t represents the mole fraction at the top plate, while y b indicates the mole fraction at the bottom plate Additionally, the equilibrium line is defined by the equation y n = B + mx t, where y n signifies the gas mole fraction at plate n.
The number of actual trays in countercurrent absorption columns is influenced by tray efficiency, mechanical design, and operational conditions When both the equilibrium curve and operating lines are linear, the overall tray efficiency (E0) can be calculated, allowing for the analytical determination of the actual number of trays required.
Murphree efficiency (EMGE), adjusted for entrainment, is detailed in Chapter 9, with empirical data for standard tray designs accessible in the literature These data, illustrated in Figure 10.17, provide accurate estimates for bubble-cap trays and can serve as rough approximations for sieve and valve trays Once the overall efficiency of the tower is established, the actual number of trays can be calculated accordingly.
Figure 12 Overall tray efficiencies of bubble-cap tray absorbers. The general procedure to follow in sizing a plate tower is given below [7].
1 Calculate the number of theoretical stages, N, using Figure 10.16 or Equation
2 Estimate the efficiency of separation, E This may be determined at the local
(across plate), plate (between plates), or overall (across column) level The overall efficiency, E 0 , is generally employed.
3 Calculate the actual number of plates:
4 Obtain the height between plates, h This is usually in the 12- to 36- inch range.
Many towers use a 24-inch plate spacing.
5 The tower height, Z, is then
6 The diameter may be calculated directly from Equation (10.16).
7 The plate or overall pressure drop is difficult to quantify accurately It is usually in the 2- to 6-inch H 2 O per plate range for most columns with the lower and upper values applying to small and large diameters, respectively.