Case Study on Registered Chemicals in 2011, Question 15 in the Afternoon
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15. Use a pure absorbent to absorb benzene from a mixed gas; it is known that the molar fraction of benzene in the mixed gas is 0.05, the gas flow rate is 65 kmol/h, and the equilibrium relationship between benzene in the two phases is Y=25X. The liquid-to-gas ratio used in the operation is 1.4 times the minimum possible liquid-to-gas ratio. When a absorption rate of 90% is required, what is the amount of absorbent used (in kmol/h)? (A) 1890 (B) 1940 (C) 2050 (D) 2320 Is the answer B or C? Thank you
V1 = G*(1-y1) = 65*(1-0.05) = 61.75 kmol/h. The liquid-to-gas ratio per minute is given by (Y1-Y2)/(X1*-X2) = Y1-Y2/(Y1/m-0); thus, it equals m*Za (absorption rate) = 25*0.9 = 22.5. The actual liquid-to-gas ratio is L/V = 1.4; therefore, (L/V)min = 1.4*22.5 = 31.5. Hence, L = 31.5*61.75 = 1945 kmol/h. The closest value among the options is (B).
For questions like this, only experts can say what the right choice is. It is clear that the phrase “when a absorption rate of 90% is required” is causing an issue; the answer is a fixed value, and the calculation result should be 1945 km/h. For a value of Xiaoyu, the recovery rate is less than 90%; if the value is higher than that, the recovery rate is greater than 90%. It’s definitely confusing as to how to choose.
I have some doubts regarding the gas flow rate: why is it not 65, but rather V1=G*(1-y1)=65*(1-0.05)=61.75 kmol/h?
In the liquid-to-gas ratio L/V, V refers to the amount of inert gas, while 65 represents the amount of the mixed gas; the flow rate of the inert gas is calculated using the formula V = G(1-y1)
I wonder which answer to choose during the exam; how do you all decide on that?
Strictly speaking, L is the flow rate of the absorbent and V is the flow rate of the inert gas. However, when the concentration at the gas inlet is low, it can be assumed that L represents the flow rate of the absorbing solution and V represents the total gas flow rate. As for what constitutes a low gas inlet concentration, chemical engineering principles state that it refers to an inlet molar concentration of less than 0.1, while the textbooks from Tianjin University specify a value of less than 0.01; it’s up to individuals to decide in such cases; if necessary, it’s always better to carry out precise calculations
I checked Chen Minheng’s textbook on chemical engineering principles; the gas flow rate referred to is the total flow rate.