SUBJECT MASS TRANSFER AND EQUIPMENTS REPORT ABSORPTION

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY AND EDUCATION FACULTY OF CHEMICAL AND FOOD TECHNOLOGY SUBJECT: MASS TRANSFER AND EQUIPMENTS REPORT ABSORPTION Instructor: M.Eng Phạm Văn Hưng Student’s name: Nguyễn Thị Quỳnh Trân Trần Tiểu Phụng Nguyễn Trần Hoàng Anh Nguyễn Hoàng Hồng Thắm HO CHI MINH CITY, MAY 2022 Content I Definition II Process III Equipment 3.1 Packed column 3.2 Plate columns 11 3.3 Packed vs plate tower comparison 12 IV Calculation 14 4.1 Design and performance equations - Packed columns 14 4.1.1 Liquid Rate 14 4.1.2 The column diameter 18 4.1.3 The column height 22 4.1.4 Pressure drop 25 4.2 Design and performance equations - Plate columns 25 REFERENCES 32 I Definition [1] Many gaseous materials are used in the chemical industry, as are many products obtained in gaseous form As a result, if we want to keep processing gas mixtures, we must separate them into their constituents There are methods to separate the gas mixture Suction separation method Physicochemical method Chemical method This report focuses solely on the suction separation method The suction separation method is understood as the reception of one substance into another through their phase separation surface We call it absorption if we use a liquid to absorb it, and adsorption if we use a porous solid Figure The difference of adsoption and absoption The process of absorption conventionally refers to the intimate contacting of a mixture of gases with a liquid so that part of one or more of the constituents of the gas will dissolve in the liquid This contact usually takes place in some type of packed or plate column In this case, the gas that is absorbed is referred to as the absorbent, the liquid that absorbs is referred to as the solvent, and the gas that is not absorbed 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 AI Process [2] In gas absorption operations, the choice of a particular solvent is also important Frequently, water is used as it is inexpensive and plentiful, but the following properties must also be considered Gas solubility - a high gas solubility is desired since this increases the absorption rate and minimizes the quantity of solvent necessary; generally, a solvent of a chemical nature similar to that of the solute to be absorbed will provide good solubility 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 Corrosiveness Cost (particularly for solvents other than water) Viscosity - low viscosity is preferred for reasons of rapid absorption rates, improved flooding characteristics, lower pressure drops, and good heat transfer characteristics Chemical stability - the solvent should be chemically stable and, if possible, nonflammable Toxicity 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 Solvent selection Equilibrium data evaluation 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) Column selection (should the column selection not be obvious or specified, calculations must be carried out for the different types of columns, and the final selection based on economic considerations) Calculation of column diameter (for packed columns this is usually based on flooding conditions, and for plate columns is based on the optimum gas velocity or the liquid handling capacity of the plate) Estimation of the column height or the number of plates (for a packed column, the column height is obtained by multiplying the number of transfer units, obtained from a knowledge of equilibrium and operating data by the height of a transfer unit; for plate columns, the number of theoretical plates, often determined from the plot of equilibrium and operating lines, is divided by the estimated overall efficiency to give the number of actual plates, which in turn allows the column height to be estimated from the plate spacing) Determination of pressure drop through the column (for packed columns, correlations dependent on packing type, column operating data, and physical properties of the constituents involved need to be available to estimate the pressure drop through the packing; for plate columns, the pressure drop per plate is obtained and multiplied by the number of plates) III Equipment [2] The principle types of gas absorption equipment may be classified as follows: - Packed columns (continuous operation) - Plate columns (stage operation) - Miscellaneous In this part, our group will focus on introducing packed column and plate column 3.1 Packed column Packed columns are usually vertical columns that have been filled with packing or material of large surface area The liquid is distributed over and trickles down through the packed bed, thus exposing a large surface area to contact the gas The counter-current packed column (see Fig.2) is the most common unit encountered in gaseous removal or recovery Principle operation of packed column: The gas stream moves upward through the packed bed against an absorbing or reacting liquor (solventscrubbing solution), which is introduced at the top of the packing This results in the highest possible efficiency Since the solute concentration in the gas stream decreases as it rises through the column, there is fresh solvent constantly available for contact This provides the maximum average driving force for the mass transfer process throughout the packed bed Figure Typical counter-current packed column Figure Absorption column The packing is the heart of this type of equipment Its proper selection entails an understanding of packing operational characteristics and the effect of performance of the points of significant physical difference between the various types Some examples of packing include: Figure Some typical packings 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) - Free space per unit volume of packed space (this controls the liquor holdup in the column as well as the pressure drop across it; ordinarily, the fractional void volume, or fraction of free space, in the packed bed should be large) - Wetted surface area per unit volume of packed space (This is very important since it determines the interfacial surface between liquid and gas; it is rarely equal to the actual geometric surface since the packing is usually not completely 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 Packed columns may also operate in a cross-flow mode (see Fig 5) where the air stream moves horizontally through the packed bed and is irrigated by the scrubbing liquid which flows vertically down through the packing Crossflow designs are characterized by low water consumption and fairly high air flow capacity at a low pressure drop Where highly soluble gases are to be recovered, the cross-flow packed scrubber has several advantages over the counter-current scrubber For example, when operating with the same liquid and gas mass flow rates, a cross-flow scrubber has a lower pressure drop Besides reducing water consumption drastically, the cross-flow principle also reduces pump and fan motor sizes Other advantages include less piping, less plugging from solids dropout at the packing support plate, and the possible use of higher gas and liquid rates because of the extremely low pressure drop On the other hand, liquid entrainment from these systems is rather high and mist eliminators are usually required downstream Figure 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 - Corrosive-fluids handling — ceramic packing is used and may be preferable to metal or plastic because of its corrosion resistance When packing does deteriorate, it is quickly and easily replaced; it is also preferred when handling hot combustion gases - Low pressure drop — unless operated at very high liquid rates where the liquid becomes the continuous phase as the flowing films thicken and merge, the pressure drop per lineal foot of packed height is relatively low - Range of operation — although efficiency varies with gas and liquid feed rates, the range of operation is relatively broad 10 �� 0.05 − 0.001 = 18(0.08-0.002) => �� = 27 lbmol/h.ft2 Hence, the liquid rate for the packed counter-current gas scurbber is 27 lbmol/h.ft2 4.1.2 The column diameter Column diameter is often calculated using flooding considerations Flooding is an undesirable condition when the gas rate is so high that it prevents the liquid from flowing freely down the column, leading to liquid accumulating and the blockage of the entire cross section for flow This prevents the packing from mixing the gas and liquid effectively and increases the pressure drop The superficial gas velocity at which it occurs is called the flooding velocity 50–75% of the flooding rate is the usual operating range being taken to calculate the column diameter 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) ρG = lb/ft3 (Density of gas) ρL = Step (design parameter): 18 Find by looking up in below figure: Figure Generalized pressure drip correlation to estimate column diameter (Figure 10.11- (Theodore & Ricci, 2011[2]) Step 3: Solve the ordinate equation for Gf 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 Step 5: Calculate the column diameter 19 Example 4.2 A packed column is used to absorb a toxic pollutant from a gas stream From the data given below, calculate the column diameter (Lm /Gm)act = 1.306; Density of gas (air) = 0.075 lb/ft3 Density of water = 62.4 lb/ft3 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, 2011 [2]) Solution of Example 4.2 Step 1: Calculate the abscissa (mass basis for all terms) Step 2: Proceed to the flooding line and read the ordinate (design parameter): 20 We have = 0.0281 =>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 Gf at flooding: 21 Step 4: Calculate the column cross-sectional area, S 3500/3600 � � = ��� = (0.5)(0.419) = 4.64 ��2 Step 5: Calculate the column diameter 0.5 � = ( �) � = 1.13� 0.5 = 1.13 4.64 Hence, column diameter is 2.43 ft 4.1.3 The column height The column height might be calculated by equation: � = ������ Where: NOG is the number of overall transfer units HOG 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 (HOG), it is usually determined experimentally for the system under consideration or obtained from the manufacturer For the number of overall transfer units (NOG), it is calculated based on different condition: 22 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 Coburn’s equation: If the gas is highly soluble in the liquid and/or reacts with the liquid, Theodore has shown that: 23 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 NOG in a packed tower is given by: Calculate NOG if y1 = 200 ppm and y2 = 0.5 ppm Solution of Example 4.3: We have: NOG =ln 200 = ln 400 = 5.99 0.5 If we have: HOG= 1.25 ft => The packing height is: Z = HOGNOG= 5.99 × 1.25 = 7.49 ft 24 4.1.4 Pressure drop Pressure drop is the difference in total pressure between two points in a fluid-carrying network[4] It is an important economic consideration in the design of a packed column for any combination of liquid and gas flows in the operable range The gas and liquid flow rates influences the pressure drop for most random packings[2]: With an increase in liquid throughput at a constant gas rate, pressure drop increases until the liquid flooding rate is reached At the point of flooding, any slight liquid excess, which cannot pass through, remains atop the packing and builds up a deeper and deeper head (or pressure drop) At constant liquid downflow, an increase in the gas flow is accompanied by an increasing pressure drop until the flooding rate is reached Therefore, the slightest gas increase will cause a decrease in permissible liquid throughput As a result, the liquid again accumulates atop the packing such that the lead pressure drop again continues to increase With a particular packing, the data for the most accurate pressure drop will be available directly from the manufacturer Nevertheless, for the purposes of estimation, the figure of "Generalized pressure drip correlation to estimate column diameter" (Figure 4.4) can be used to provide reasonable results 4.2 Design and performance equations - Plate columns [2] The determination of the column diameter, type, and number of plates to be used (typically either bubble-cap or sieve plates), actual plate layout and physical design, and plate spacing are the most essential design issues for plate columns; they, in turn, determine column height It is beyond the scope of this chapter to analyze each of these in any depth, especially since Chapter dealt 25 with it As a result, the discussion that follows will be a succinct overview of some of the generic absorber design strategies that will yield sufficient results for estimating purposes [6] The column diameter, and hence its cross section, must be big enough to handle the gas and liquid at speeds that not result in floods or excessive entrainment At flooding, the superficial gas velocity for a certain type of plate is given by the relation [6] �� = �� � −� � � 0.5 (10.16) �� where VF (the notation usually employed for plate columns) is the gas velocity through the net column cross sectional area for gas flow, ft 3/s.ft2, the densities are in lb/ft3, and CF is an empirical coefficient that depends on the type of plate and operating conditions The difference between the column cross section and the area taken up by downcomers called the net cross section In practice, a certain percentage of V F is employed for nonfoaming liquids (80– 85%) and 75 percent or less for foamy liquids [2] Naturally, the value is contingent on a thorough examination of entrainment and pressure drop characteristics The gas flow rate is assumed to be the determining factor in the computation of column diameter based on Equation (10.16) Following the assumption of a plate arrangement, the plate's liquid handling capabilities must be checked The check will tell whether the column will show a propensity toward flooding or gas maldistribution on the plate if the liquid–to–gas ratio is high, and the column diameter is big If this is the case, the liquid rate is the determining factor in determining the column diameter, and a plate-handling capacity of 30 gal/min of liquid per foot of diameter is a reasonable design assumption [7] A well-designed single-pass cross-flow plate, on the other hand, can normally handle up to 60 gal/min of liquid per foot of diameter without causing an excessive liquid gradient It's also worth noting that 26 low gas rates might cause weeping, which occurs when liquid flows down through the perforations in the plate rather than over it The product of the number of real plates (theoretical plates divided by overall plate efficiency) and the plate spacing chosen determines the column height In plate column calculations, the theoretical plate (or stage) is the theoretical separation unit It is defined as a plate on which two dissimilar phases are brought into proximity before being physically separated Various diffusing components of the mixture spread themselves between the phases during contact [2] In an equilibrium stage, the two phases are well mixed for a long enough period to allow the phases to achieve equilibrium before exiting the stage At equilibrium, no additional net changes in phase composition are conceivable for a given set of operating conditions The operating diagram, which consists of an operating line and an equilibrium curve, can be used to calculate the number of theoretical plates The phases leaving the stage were believed to be in equilibrium in the previous discussion of equilibrium stages It is not practical to offer the combination of residence duration and intimacy of touch required to achieve balance in genuine counter-current multistage equipment [2] As a result, the concentration changes for a given stage is lower than what equilibrium considerations imply To describe this situation, stage efficiencies are used The overall stage (plate efficiency) is a commonly used efficiency phrase that refers to the ratio of theoretical contacts necessary for a particular separation to the actual number of contacts required for the same activity While having solid information on such an efficiency is desirable and useful, there are so many variables at play that finding truly dependable values for the overall stage efficiency is difficult This value is usually derived through experiment or field test data, although it could also be given by the vendor The number of theoretical plates may be determined directly without recourse to graphical techniques for cases where both the operating line and the 27 equilibrium curve may be considered straight (dilute solutions) This will frequently be the case for a relatively dilute gas (as usually encountered in air pollution control) and liquid solutions where, often, Henry’s law is usually applicable Since the quantity of gas absorbed is small, the total flows of liquid and gas entering and leaving the column again essentially remain constant Hence, the operating line will be substantially straight For such cases, the Kremser–Brown–Sounders [8][9] equation applies for determining the number of theoretical plates, Np: (10.17) �� = log Note that ln may be employed rather than log in both the numerator and denominator Here mx0 is the gas composition in equilibrium with the entering liquid (m is Henry’s law constant = slope of the equilibrium curve) If the entering liquid contains no solute gas, then x = and Equation (10.17) can be simplified further The solute concentrations in the gas stream, ���+1 and y1 represent inlet and outlet conditions, and L and V (that appear in A) the total mole rates of liquid and gas flow per unit time per unit column cross-sectional area [2] Small variations in L and V may be roughly compensated for by using the geometric average value of each taken at the top and bottom of the column Equation (10.17) has been plotted in Figure 10.16 for convenience and may be used for the solution to this equation Chen [10] derived a simplified algebraic equation that could be used to estimate the theoretical plates, n, in either an absorber or stripper The final equation took the form (retaining Chen’s notation): �� = � +∅ � (10.18) ��+∅ where ∅ = � −�(�+�� ) � � �−1 (10.19) 28 and A is the absorption factor, yt is the top plate gas mole fraction, yb is the bottom plate gas mole fraction and y n = B + mxt (equilibrium line) is the gas mole fraction at plate n Figure 11 Number of theoretical stages for countercurrent absorption columns The number of actual trays, which is based on the tray efficiency, is determined by the mechanical design and conditions of operation For the case where the equilibrium curve and operating lines are straight, the overall tray efficiency E can be computed, and the number of actual trays determined analytically [2]: �0 = where EMGE = Murphree efficiency, as noted in Chapter 9, corrected for entrainment (values available in the literature) Empirical data for standard tray 29 designs within standard ranges of liquid and gas rates are available [2] These data, as shown in Figure 10.17, are accurate for bubble-cap trays and can be used as rough estimates for sieve and valve trays After the overall efficiency of the tower is determined, the number of actual trays is calculated using: ���� = � �0 = �� (10.21) �0 Figure 12 Overall tray efficiencies of bubble-cap tray absorbers The general procedure to follow in sizing a plate tower is given below [7] Calculate the number of theoretical stages, N, using Figure 10.16 or Equation (10.17) 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, E0, is generally employed Calculate the actual number of plates: � � ��� = �0 (10.21) 30 Obtain the height between plates, h This is usually in the 12- to 36- inch range Many towers use a 24-inch plate spacing The tower height, Z, is then �= ����ℎ (10.22) The diameter may be calculated directly from Equation (10.16) The plate or overall pressure drop is difficult to quantify accurately It is usually in the 2- to 6-inch H 2O per plate range for most columns with the lower and upper values applying to small and large diameters, respectively 31 REFERENCES [1] Nguyễn, B (n.d.) Các trình, thiết bị cơng nghệ hóa chất thực phẩm (pp 151–152) Nhà xuất khoa học kỹ thuật Hà Nội [2] Theodore, L., & Ricci, F (2011) Mass Transfer Operations for the Practicing Engineer John Wiley & Sons [3] Adsorption vs Absorption - Differences and Examples (2021, July 15) Science Notes and Projects https://sciencenotes.org/adsorptionvs-absorption-differences-and-examples/ [3]“Absorption and Stripping.” https://www.cpp.edu/~tknguyen/che313/pdf/chap5-1.pdf (accessed May 12, 2022) [4]“What is Pressure Drop?,” Central States Industrial https://www.csidesigns.com/blog/articles/what-is-pressure-drop-andhow-does-it-affect-your-processing-system (accessed May 13, 2022) [5] R TREYBAL, “Mass Transfer Operations,” 2nd edition, McGraw- Hill, New York City, NY, 1967 [6] E HENLEY and H STAFFIN, “Stage Process Design,” John Wiley & Sons, Hoboken, NY, 1963 [7] H SAWISTOWSKI and W SMITH, “Mass Transfer Process Calculations,” Interscience, New York City, NY, 1963 [8] A KREMSER, Nat’l Petrol News, Washington, DC, 22(21), 42, 1930 [9] M SOUDERS and G BROWN, Ind Eng Chem., New York City, NY, 24, 519, 1932 [10] N CHEN, “Calculating Theoretical Plates on Absorbers or Strippers,” Chem Eng New York City, NY, May 11, 1964 32 ... solute gas, then x = and Equation (10.17) can be simplified further The solute concentrations in the gas stream, ���+1 and y1 represent inlet and outlet conditions, and L and V (that appear in... & Ricci, F (2011) Mass Transfer Operations for the Practicing Engineer John Wiley & Sons [3] Adsorption vs Absorption - Differences and Examples (2021, July 15) Science Notes and Projects https://sciencenotes.org/adsorptionvs -absorption- differences -and- examples/... support plate, and the possible use of higher gas and liquid rates because of the extremely low pressure drop On the other hand, liquid entrainment from these systems is rather high and mist eliminators