Special Topic 6 of the Registered Chemical Engineer Professional Examination: Gas Absorption

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The last edit to this post was made by zhanghp30 on 2018-10-13 at 23:02. 1. When the operating gas ratio for a certain absorption process, the inlet temperature of the absorbent, the outlet concentration of the gas, the concentration of the absorbent entering the tower, and the operating pressure are all kept constant, and it is assumed that a low-concentration binary gas mixture is being absorbed, then if a packing material with better performance is selected during design, the following statements are incorrect (BCD). A The number of mass transfer units required remains unchanged; the height of the packing layer decreases； The number of mass transfer units required for B remains unchanged, and the height of the packing layer remains unchanged ； The number of mass transfer units required for C decreases, and the height of the packing layer is reduced ； The number of mass transfer units required for D remains unchanged, and the height of the packing layer remains unchanged ； The number of BCD mass transfer units is related to phase equilibrium and inlet/outlet concentrations; if these remain unchanged, the number of mass transfer units remains unchanged ； By using a good filler, the height of the mass transfer unit decreases; since the number of mass transfer units remains unchanged, the height of the filler layer also decreases. Note that the incorrect choice should be selected as per the requirements of the question! 2. In a packed tower, clean water is used to absorb ammonia from the gas mixture. When the water pump’s capacity to draw water decreases, the number of gas-phase mass transfer units will ( A ) A increase B decrease C remain unchanged D it cannot be determined. A S=m*G/L; as L decreases, S increases, so the absorption driving force becomes smaller while the mass transfer driving force increases. 3. In the packed tower, pure solvent is used for countercurrent absorption of the soluble component A in the mixture. It is known that the initial composition of A in the gas mixture is 0.06 (mole fraction, the same below). The outlet composition of the gas after absorption is 0.005, and the composition of the solution after absorption is 0.095. Under the operating conditions, the gas-liquid phase equilibrium relationship is y=0.6x. The mass transfer coefficients in the liquid and gas phases are proportional to Ky*a, which is proportional to G^0.7, and to Kx*a, which is proportional to L^0.7. The absorption process is governed by the gas film effect, with HOG = 1 m. Determine: (1) The gas phase composition at the tower cross-section when the height of the packing layer is reduced by half ； (2) When the liquid phase flow rate increases, with the gas phase flow rate and the inlet compositions of the gas and liquid remaining unchanged, how does the amount of solute A absorbed change? ( ) A. Increase B. Decrease C. Remain unchanged D. Unable to determine. 4. A well-known environmental engineering company in Shanghai was commissioned by a prominent foreign company to remove the soluble component A from a gas mixture. After preliminary investigations conducted on-site by process engineers, it was found that the initial composition of gas A was 0.045. Pure solvent was used in a packed tower for counter-current absorption; the outlet composition of the gas was 0.018, while the outlet composition of the solution was 0.088. Under the operating conditions, the gas-liquid equilibrium relationship was y = 0.5x. It is known that this absorption process is controlled by the gas film effect. Determine: (1) The number of gas transfer units, NOG ； (2) The client requires that, with the composition at the gas phase and gas-liquid inlet remaining unchanged, when the liquid flow rate is doubled, the outlet concentration after gas absorption must not exceed 0.006. Process engineers perform process calculations, ultimately arriving at the indicators required by the client. A: Can be achieved; B: Cannot be achieved; C: Conditions are lacking; D: Unable to determine. 5. At a certain point in the absorption tower, the concentration of the gas phase is y = 0.025, the concentration of the liquid phase is x = 0.01, the gas-phase mass transfer coefficient is ky = 2 kmol/m2·h, the overall gas-phase mass transfer coefficient is Ky = 1.5 kmol/m2·h, and the equilibrium relationship is y = 0.5x. What is then the concentration of the gas phase at the gas-liquid interface at that point? ye = 0.5 * 0.01 = 0.005. NA = Ky(y – ye) = ky(y – yi). Therefore, 1.5 * (0.025 – 0.005) = 2 * (0.025 – yi), so yi = 0.01. 6. The packing height of a certain absorption tower is 4.5 m. Clean water is used for counter-current absorption of the harmful component A present in the exhaust gas. The equilibrium relationship is y = 1.5x, the concentration of the liquid phase at the outlet is 0.008, while the concentrations of the gas phase at the inlet and outlet are 0.02 and 0.004 respectively. (1) NOG; (2) By how many times is the operating liquid-gas ratio greater than the minimum liquid-gas ratio? (3) The local environmental protection agency requires that the emission concentration be less than 0.003. What measures can be taken to meet this standard? (1) V/L = 0.0008 / (0.02 – 0.004) = 0.5. S = mV/L = 1.5 * 0.5 = 0.75. NOG = 1 / (1 – S) * Ln = 2.77. (2) (L/V)min = (y1 – y2) / (x1* – x2) = 1.2. (L/V) / (L/V)min = 2 / 1.2 = 1.667. (3) This issue is generally resolved by keeping the liquid-gas ratio constant and increasing the packing height. Thus, NOG’ = 1 / (1 – S) * Ln. Here, y2’ = 0.003, so NOG’ = 3.53. The increase in packing height is deltaH = HOG(NOG’ – NOG) = (4.5 / 2.77) * (3.53 – 2.77) = 1.23 m