In tower design, when calculating the tower diameter, how is the gas velocity in an empty tower determined? Is the gas velocity in an empty tower related to the type of material?
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In tower design, when calculating the tower diameter, how is the gas velocity in an empty tower determined? Is the gas velocity in an empty tower related to the type of material?
For example, ethanolamine tends to cause foaming; empirically, the linear velocity of gas in the empty tower, whether in a desulfurization tower or a regeneration tower, should not exceed 0.7 m/s.
4.1.3 Calculation of the process dimensions of a packed tower The calculation of the process dimensions of a packed tower includes determining the tower diameter, the height of the packing layer, and the segmentation. 4.1.3.1 Calculation of tower diameter The diameter of the packed tower is still calculated using Equation 4-1; that is, in Equation (4-1), the gas volume flow rate Vs is given by the design requirements. As can be seen from the above equation, the key issue in calculating the tower diameter is determining the gas velocity in an empty tower, u. (1) Determination of the empty tower gas velocity ① Bubble point velocity method: The bubble point velocity represents the upper limit for the operating gas velocity in a packed tower. The operating empty tower gas velocity must be lower than the bubble point velocity; the ratio of the operating empty tower gas velocity to the bubble point velocity is known as the bubble point ratio. For bulk packing, the empirical value of the flooding ratio is u/uF=0.5~0.85; for structured packing, it is u/uF=0.6~0.95. The selection of the flooding ratio takes into account two main factors: the operating pressure of the packed tower and the degree of foaming of the fluid being processed. In the design, for towers that are operated under pressure, a higher flooding ratio should be adopted ； For columns operating under reduced pressure, a lower flooding point ratio should be adopted ； For systems prone to foaming, the bubble formation rate should be set at a low value ； For foam-free systems, a higher spreading rate can be adopted. The bulk gas velocity can be calculated using empirical equations or obtained from correlation charts. a. The Bain-Hougen relationship: The critical gas velocity for fillers can be calculated using the Bain-Hougen relationship, namely equation (4-2), where uF represents the critical gas velocity in m/s, and g is the acceleration due to gravity, 9.81 m/s2 ； at — total specific surface area of the filler, m2/m3 ； ε —— porosity of the packing layer, m3/m3 ； ρV, ρL — densities of the gas and liquid phases, kg/m3 ； μL — liquid viscosity, mPa·s ； wL, wV — mass flow rates in the liquid and gas phases, kg/h ； A, K — correlation constants. The constants A and K are related to the shape and material of the filler, and the values of A and K for different types of fillers are listed in Table 4-3. The universal point gas velocity calculated from Equation 4-2 has an error within 15%. Table 4-3 Values of A and K in Equation 3-34
Bulk packing type A K
Regular packing type A K
Plastic Bauler rings 0.0942 1.75
Metal wire corrugated packing 0.30 1.75
Metal Bauler rings 0.1 1.75
Plastic wire corrugated packing 0.4201 1.75
Plastic stepped rings 0.204 1.75
Metal mesh corrugated packing 0.155 1.47
Metal stepped rings 0.106 1.75
Metal perforated plate corrugated packing 0.291 1.75
Ceramic rectangular saddles 0.176 1.75
Plastic perforated plate corrugated packing 0.291 1.563
Metal ring rectangular saddles 0.06225 1.75
b. Eckert’s general correlation chart
The critical gas velocity for bulk packing can be calculated using Eckert’s correlation chart, as shown in Figure 4-5. During calculation, the value of the horizontal coordinate is first determined using the gas-liquid phase loads and relevant property data; then a vertical line is drawn to intersect the corresponding contour line, and a horizontal line is drawn through that intersection point to intersect the vertical coordinate, thereby determining the value of the vertical coordinate. The u corresponding to this value is the universal point gas velocity uF. It should be noted that when calculating the critical gas velocity using Eckert’s general correlation diagram, the packing factor required is the wet packing factor at the flooding condition, known as the critical packing factor, denoted by ΦF. The general packing factor ΦF is related to the liquid spray density; for the convenience of engineering calculations, an average value of the general packing factor that is independent of the liquid spray density is often used. Table 4-4 lists the average spreading factor values for some bulk fillers, which can be used as a reference in design. Figures 4-5 General relationship diagram between bubble point and pressure drop in packed columns. In the figure, u0 represents the gas velocity in an empty column, in m/s ； φ —— wet packing factor, also known as packing factor, 1/m ； ψ——the ratio of the density of water to the density of the liquid ； g — gravitational acceleration, m/s2 ； ρV, ρL —— are the densities of the gas and liquid, respectively, in kg/m3 ； wV, wL —— are the mass flow rates of gas and liquid, respectively, in kg/s. This graph is applicable to randomly packed granular packing materials such as Raschig rings, arc-saddle packing, rectangular-saddle packing, and Pall rings; it also shows the flooding curves for two types of regularly packed materials: neatly arranged Raschig rings and string grid packing. For other fillers, there is no reliable data on filler factors. Table 4-4 Average Packing Factor for Bulk Packing Materials. Packing Type, Packing Factor, 1/m: DN16, DN25, DN38, DN50, DN76. Metal Baffle Rings: 410 –, 117, 160 –; Metal Annular Saddle Rings: –, 170, 150, 135, 120; Metal Stepped Rings: –, –, 160, 140 –; Plastic Baffle Rings: 550, 280, 184, 140, 92; Plastic Stepped Rings: –, 260, 170, 127 –; Ceramic Annular Saddle Rings: 1100, 550, 200, 226 –; Ceramic Lassell Rings: 1300, 832, 600, 410 –. ② Gas Kinetic Energy Factor (F-factor) Method: The gas kinetic energy factor is abbreviated as the F-factor, and it is defined as in equation (4-3). The gas kinetic energy factor method is commonly used to determine the empty-tower gas velocity for structured packing materials. During calculation, first find the F factor of the packing under the operating conditions from a manual or chart, and then the operating empty tower gas velocity u can be calculated using equation 4-3. The appropriate operating gas kinetic energy factor for common regular packings can be found in relevant charts. It should be noted that the use of the gas kinetic energy factor method to calculate the appropriate empty tower velocity is generally applied in low-pressure operations (pressures below 0.2 MPa). ③The gas-phase load factor (Cs factor) method: The gas-phase load factor, abbreviated as the Cs factor, is defined as in equation (4-4). This method is often used to determine the empty-tower gas velocity for structured packing. During calculation, first determine the maximum gas-phase load factor Cs,max; then calculate Cs using the relationship Cs=0.8Cs.max (4-5), and subsequently determine the operating empty tower gas velocity u using equation 4-4. The calculation of Cs.max for commonly used structured packings can be found in the relevant packing manuals, or it can be obtained from the Cs.max curve shown in Figure 4-6. The abscissa ψ in the figure is called the flow parameter, defined as in (4-6). The curve in Figure 4-4 applies to plate corrugated packing. Based on the 250Y type plate corrugated packing, a correction factor C must be applied to other types of plate corrugated packing; its values are shown in Table 4-5. Table 4-5 Maximum load correction coefficients for other types of corrugated packing Packing type Model Correction coefficient Plate corrugated packing 250Y 1.0 Mesh corrugated packing BX 1.0 Mesh corrugated packing CY 0.65 Ceramic corrugated packing BX 0.8 (2) Calculation and rounding of tower diameter Once the empty tower gas velocity u is determined using the method above, the tower diameter D can be calculated using equation 4-1. It should be noted that after the tower diameter D is calculated using Equation 4-1, it must also be rounded in accordance with the standards for tower diameter series. Common standard tower diameters include: 400, 500, 600, 700, 800, 1000, 1200, 1400, 1600, 2000, 2200 mm, etc. After rounding, calculate the operating empty tower gas velocity u and the flooding rate. (3) Verification of liquid spray density The liquid spray density in a packed tower refers to the amount of liquid sprayed per unit time and per unit area of the tower cross-section. Its calculation formula is given in equation (4-5), where U represents the liquid spray density, in m3/(m2·h) ； Lh——liquid spray volume, m3/h ； D — Diameter of the packed tower, in m. To ensure good wetting of the packing, the liquid spray rate in the tower must be at least a certain threshold value; this threshold value is known as the minimum spray density, denoted as Umin. For bulk packing, the minimum spraying density is typically calculated using the following formula: Umin = (LW) / minat (Equation 4-6). Here, Umin represents the minimum spraying density, in m3/(m2·h) ； (LW) min — minimum wetting rate, m3/(m·h) ； at — total specific surface area of the filler, m2/m3. The minimum wetting rate refers to the minimum volumetric flow rate of liquid per unit length around the packing at the cross-section of the tower. Its value can be calculated using empirical formulas (see the filler manual), or some empirical values can be used. For bulk packing with a diameter not exceeding 75 mm, the minimum wetting rate (LW) min can be taken as 0.08 m3/(m·h) ； For bulk packing with a diameter greater than 75 mm, take (LW) min = 0.12 m3/(m·h). For structured packing, the minimum spray density can be found in relevant packing manuals; in design, Umin is usually taken as 0.2. The liquid spray density used in actual operation should be greater than the minimum spray density. If the liquid spray density is less than the minimum spray density, adjustments are required, and the tower diameter must be recalculated.
Bulk packing type A K
Regular packing type A K
Plastic Bauler rings 0.0942 1.75
Metal wire corrugated packing 0.30 1.75
Metal Bauler rings 0.1 1.75
Plastic wire corrugated packing 0.4201 1.75
Plastic stepped rings 0.204 1.75
Metal mesh corrugated packing 0.155 1.47
Metal stepped rings 0.106 1.75
Metal perforated plate corrugated packing 0.291 1.75
Ceramic rectangular saddles 0.176 1.75
Plastic perforated plate corrugated packing 0.291 1.563
Metal ring rectangular saddles 0.06225 1.75
b. Eckert’s general correlation chart
The critical gas velocity for bulk packing can be calculated using Eckert’s correlation chart, as shown in Figure 4-5. During calculation, the value of the horizontal coordinate is first determined using the gas-liquid phase loads and relevant property data; then a vertical line is drawn to intersect the corresponding contour line, and a horizontal line is drawn through that intersection point to intersect the vertical coordinate, thereby determining the value of the vertical coordinate. The u corresponding to this value is the universal point gas velocity uF. It should be noted that when calculating the critical gas velocity using Eckert’s general correlation diagram, the packing factor required is the wet packing factor at the flooding condition, known as the critical packing factor, denoted by ΦF. The general packing factor ΦF is related to the liquid spray density; for the convenience of engineering calculations, an average value of the general packing factor that is independent of the liquid spray density is often used. Table 4-4 lists the average spreading factor values for some bulk fillers, which can be used as a reference in design. Figures 4-5 General relationship diagram between bubble point and pressure drop in packed columns. In the figure, u0 represents the gas velocity in an empty column, in m/s ； φ —— wet packing factor, also known as packing factor, 1/m ； ψ——the ratio of the density of water to the density of the liquid ； g — gravitational acceleration, m/s2 ； ρV, ρL —— are the densities of the gas and liquid, respectively, in kg/m3 ； wV, wL —— are the mass flow rates of gas and liquid, respectively, in kg/s. This graph is applicable to randomly packed granular packing materials such as Raschig rings, arc-saddle packing, rectangular-saddle packing, and Pall rings; it also shows the flooding curves for two types of regularly packed materials: neatly arranged Raschig rings and string grid packing. For other fillers, there is no reliable data on filler factors. Table 4-4 Average Packing Factor for Bulk Packing Materials. Packing Type, Packing Factor, 1/m: DN16, DN25, DN38, DN50, DN76. Metal Baffle Rings: 410 –, 117, 160 –; Metal Annular Saddle Rings: –, 170, 150, 135, 120; Metal Stepped Rings: –, –, 160, 140 –; Plastic Baffle Rings: 550, 280, 184, 140, 92; Plastic Stepped Rings: –, 260, 170, 127 –; Ceramic Annular Saddle Rings: 1100, 550, 200, 226 –; Ceramic Lassell Rings: 1300, 832, 600, 410 –. ② Gas Kinetic Energy Factor (F-factor) Method: The gas kinetic energy factor is abbreviated as the F-factor, and it is defined as in equation (4-3). The gas kinetic energy factor method is commonly used to determine the empty-tower gas velocity for structured packing materials. During calculation, first find the F factor of the packing under the operating conditions from a manual or chart, and then the operating empty tower gas velocity u can be calculated using equation 4-3. The appropriate operating gas kinetic energy factor for common regular packings can be found in relevant charts. It should be noted that the use of the gas kinetic energy factor method to calculate the appropriate empty tower velocity is generally applied in low-pressure operations (pressures below 0.2 MPa). ③The gas-phase load factor (Cs factor) method: The gas-phase load factor, abbreviated as the Cs factor, is defined as in equation (4-4). This method is often used to determine the empty-tower gas velocity for structured packing. During calculation, first determine the maximum gas-phase load factor Cs,max; then calculate Cs using the relationship Cs=0.8Cs.max (4-5), and subsequently determine the operating empty tower gas velocity u using equation 4-4. The calculation of Cs.max for commonly used structured packings can be found in the relevant packing manuals, or it can be obtained from the Cs.max curve shown in Figure 4-6. The abscissa ψ in the figure is called the flow parameter, defined as in (4-6). The curve in Figure 4-4 applies to plate corrugated packing. Based on the 250Y type plate corrugated packing, a correction factor C must be applied to other types of plate corrugated packing; its values are shown in Table 4-5. Table 4-5 Maximum load correction coefficients for other types of corrugated packing Packing type Model Correction coefficient Plate corrugated packing 250Y 1.0 Mesh corrugated packing BX 1.0 Mesh corrugated packing CY 0.65 Ceramic corrugated packing BX 0.8 (2) Calculation and rounding of tower diameter Once the empty tower gas velocity u is determined using the method above, the tower diameter D can be calculated using equation 4-1. It should be noted that after the tower diameter D is calculated using Equation 4-1, it must also be rounded in accordance with the standards for tower diameter series. Common standard tower diameters include: 400, 500, 600, 700, 800, 1000, 1200, 1400, 1600, 2000, 2200 mm, etc. After rounding, calculate the operating empty tower gas velocity u and the flooding rate. (3) Verification of liquid spray density The liquid spray density in a packed tower refers to the amount of liquid sprayed per unit time and per unit area of the tower cross-section. Its calculation formula is given in equation (4-5), where U represents the liquid spray density, in m3/(m2·h) ； Lh——liquid spray volume, m3/h ； D — Diameter of the packed tower, in m. To ensure good wetting of the packing, the liquid spray rate in the tower must be at least a certain threshold value; this threshold value is known as the minimum spray density, denoted as Umin. For bulk packing, the minimum spraying density is typically calculated using the following formula: Umin = (LW) / minat (Equation 4-6). Here, Umin represents the minimum spraying density, in m3/(m2·h) ； (LW) min — minimum wetting rate, m3/(m·h) ； at — total specific surface area of the filler, m2/m3. The minimum wetting rate refers to the minimum volumetric flow rate of liquid per unit length around the packing at the cross-section of the tower. Its value can be calculated using empirical formulas (see the filler manual), or some empirical values can be used. For bulk packing with a diameter not exceeding 75 mm, the minimum wetting rate (LW) min can be taken as 0.08 m3/(m·h) ； For bulk packing with a diameter greater than 75 mm, take (LW) min = 0.12 m3/(m·h). For structured packing, the minimum spray density can be found in relevant packing manuals; in design, Umin is usually taken as 0.2. The liquid spray density used in actual operation should be greater than the minimum spray density. If the liquid spray density is less than the minimum spray density, adjustments are required, and the tower diameter must be recalculated.
Excuse me, sir, which book is this from?
Which material describes it? None of the things you copied contain formulas
Sinopec’s “Tower Equipment” or Guidelines for Modern Packing Tower Technology