4.3.1 Quench Tower Design [PDF]

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4.3.1 Quench tower design



The mainly purpose of the quencher is to cool the flue gases. There is no accumulation and consumption in the quencher, so the mass balance will be based on input equal output. Cooling by liquid quenching is essentially accomplished simply by spraying water at the top of the tower a hot gasestravel upwards through the tower. When the water evaporates, the energy necessary to vaporize the water is obtained at the expense of the hot gases, resulting in a reduction in the gas temperature (4). The basic design parameters for spray tower are the diameter and height.



4.3.1.1 Design procedure The diameter of a spray tower may be calculated from the superficial velocity ( ) (m/sec), which is given below: Eq. (1) Where: : Density of liquid and gas, respectively The area (m2 ) of the tower is obtained by following equation: Eq. (2)



Where: : Volumetric flow rate of the gas. The diameter (m) is then given by: Eq. (3) In order to calculate the tower height (Z) (ft), a volumetric heat transfer



coefficient (U) (Btu/hr-ft3 .oF) is employed. This coefficient is then used to establish the required contact volume (V) as the following equation: Eq.(4) Where: V (ft3): tower volume q



: heat rate required to vaporized the water to the gas discharge temperature : log-mean temperature difference driving force across the quencher



If the liquid-side resistance to heat transfer is neglected, U is given by the hot gas volumetric film heat transfer coefficient (h), as the following: Eq. (5)



Where: h



: gas-side volumetric heat transfer coefficient. Gas superficial mass velocity at the bottom of tower. Liquid superficial mass velocity at the bottom of tower.



The superficial mass velocity of a stream is calculated by dividing the mass flow rate (at or near the tower bottom) by the cross-sectional area. The mass flow rates are specified at the bottom of the tower because these quantities change as the streams move through the tower (due to the exchange of material) and reach their highest values at the bottom (4). The tower volume is given by: V = A Z Eq. (6) Equations (4) through (6) may be solved simultaneously for Z.



4.3.1.2 Design calculation The mainly purpose of the quencher is to cool the flue gases. There is no accumulation and consumption in the quencher. The mass and energy balance were shown on figure 4.1 as the following: F2 = 3963254 mol/hr T2= 46 oC



Water F1w = 45898 mol/hr T1w = 44 oC



Q - 101



Flue gases F1= 4488023 mol/hr T1= 170 oC



F2w = 570776 mol/hr T2w= 67 oC



Figure 4.1: Quench spray tower



The following table 4.1 shows the composition of the flue gases that enter the tower: Table 4.1: The molecular weight and the composition of each component Component Mwt Composition CO2



44



0.086



N2



28



0.692



O2



32



0.015



H2 O



18



0.207



The diameter of the spray tower is calculated from the superficial velocity (Eq (1)):



The volumetric flow rate of the gas is calculated as below:



The required area (A) and diameter (D) are then calculated by using Eqs. (2&3):



Equations 4, 5& 6 were used in order to calculate the tower height (Z) as the folllowing: The heat duty was found from energy balance:



The log-mean temperature difference across the quench unit is:



In equation (5), which is :



The terms G and L are superficial mass velocities for the gas and liquid, respectivily, and they were determind as the following:



Equation (5) is rearranged to solve for hZ0.5:



The design equation for the spray tower volume (V) is given by Eq. (6):



Note: The volumetric heat transfer coefficient (U or h) is



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