传质与分离习题(英文)钟理

Problem for Mass Transfer and Separation Processes

Absorption

1 At 20℃, the ammonia solubility in water is l0kgNH3／1000kgH2O, gas phase equilibrium molar fraction y* is 0.008, try to calculate following coefficients, Henry coefficient E, equilibrium constant m when the following conditions are

(1) Total pressure above the ammonia is 101.3kPa (absolute atmosphere); (2) Total pressure above the ammonia is 301.9kPa (absolute atmosphere);

2 The ammonia–air mixture containing 9% ammonia (molar fraction) is contact with the ammonia-water liquid containing 5% ammonia (molar fraction). Under this operating condition, the equilibrium relationship is y*=0.97x. When the above two phases are contact, what will happen, absorption or stripping?

3 When the temperature is 10 oC and the overall pressure is 101.3KPa , the solubility of oxygen in water can be represented by equation p=3.27?104x, where p (atm) and x refer to the partial pressure of oxygen in the vapor phase and the mole fraction of oxygen in the liquid phase, respectively. Assume that water is fully contact with the air under that condition, calculate how much oxygen can be dissolved in per cubic meter of water?

4 An acetone-air mixture containing 0.02 molar fraction of acetone is absorbed by water in a

packed tower in countercurrent flow. 99％ of acetone is removed when mixed gas molar flow rate is 0.03kmol·s-1m-2 and practice absorbent flow rate L is 1.4 times as much as the min amount required. Under the operating condition, the equilibrium relationship is y*=1.75x, volume total absorption coefficient is Kya=0.022 kmol·s-1m-2y-1.. What is the molar flow rate of the absorbent and how height of packing will be required?

5 The mixed gas from an oil distillation tower contains H2S=0.04(molar fraction). Triethanolamine (三乙醇胺) is used as the absorbent to remove 99% H2S in the packing tower, the equilibrium relationship is y*=1.95x, the molar flux rate of the mixed gas is 0.02kmol·m-2·s-1，volume total

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absorption coefficient is Kya=0.05 kmol·s1m-2, The solvent free of H2S enters the tower and it contains 70% of the H2S saturation concentration when leaving the tower. Try to calculate: (a) the number of mass transfer units Noy, and (b) the height of packing layer needed, Z.

6 Ammonia is removed from ammonia–air mixture by countercurrent scrubbing with water in a packed tower at an atmospheric pressure. Given: the height of the packing layer Z is 6 m, the mixed gas entering the tower contains 0.03 ammonia (molar fraction, all are the same below), the gas out of the tower contains ammonia 0.003; the NH3 concentration of liquid out of the tower is 80% of its saturation concentration, and the equilibrium relation is y*=1.2x. Find: (1)the practical liquid—gas ratio. (2) the number of overall mass transfer units.

(3) if the molar fraction of the ammonia out of the tower will be reduced to 0.002 and the other operating conditions keep unchanged, is the tower suitable?

7 Pure water is used in an absorption tower with the height of the packed layer 3m to absorb ammonia in an air stream. The absorptivity is 99 percent. The operating conditions of absorber are 101.3kpa and 20oC, respectively. The flux of gas V is 580kg/(m.h), and 6 percent (volume %) of

2ammonia is contained in the gas mixture. The flux of water L is 770kg/( m.h). The gas and liquid

*is countercurrent in the tower at isothermal temperature. The equilibrium equation y=0.9x, and gas phase mass transfer coefficient kGa is proportional to V0.8, but it has nothing to do with L. What is the height of the packed layer needed to keep the same absorptivity when the conditions of operation change as follows:

(1)the operating pressure is 2 times as much as the original.(2)the mass flow rate of water is one time more than the original. 3) the mass flow rate of gas is two times as much as the original

Distillation

1 A binary material system is to be separated in a continuous distillation column. The feed is liquid phase and mole fraction in feed is xF=0.42, Mole fraction in the overhead product is xD

=0.95, Givens: The more volatile component’s recovery percent of the overhead product is η=0.92, Calculate: Mole fraction in the bottom product xB=?

2 Certain binary mixed liquid containing mole fraction of easy volatilization component xF 0.35, feeding at bubbling point, is separated through a sequence rectify column. The mole fraction in the overhead product is xD=0.96, and the mole fraction in the bottom product is xB =0.025. If the mole overflow rates are constant in the column, try to calculate

(a)the flow rate ratio of overhead product to feed(D／F)?(b)If the reflux ratio R=3.2, write the operating lines for rectifying and stripping sections

3 A continuous distillation column is to be designed to separate an ideal binary material system，The feed which contains more volatile component xF 0.5, feed rate 100kmol/h, is saturated vapor, the flow rate of overhead product and the flow rate of bottom product are also 50kmol/h. Suppose the operating line for rectifying section is y=0.833x+0.15, the vapor generated in the reboiler enters the column through the bottom plate, a complete condenser is used on the top of column and reflux temperature is bubbling point. Find:

(1) Mole fraction xD of overhead product and the mole fraction xB of bottom product ? (2) Vapor amount condensed in the complete condenser, in mol/h? (3)The operating line for stripping section.

(4) If the average relative volatility of the column is 3 and the first plate’s Murphree efficiencyfrom the column top is Em,L=0.6(represented by liquid mole fraction), find the constituent of gas phase leaving the second plate from the column top.

4 An ideal binary solution of a volatile component A containing 50% mole percent A is to be separated in a continuous distillation column. The feed is saturated vapor, the feed rate is 1000kmol/h, and the flow rate of overhead product and the flow rate of bottom product are also

2500kmol/h. Given: the operating line for rectifying section is y=0.86x+0.12, indirect vapor is used in the reboiler for heating and the total condenser is used on the top of tower. Assume that the reflux temperature is at its bubble point. Find:

(1) reflux ratio R, the mole fraction of overhead product xD and the mole fraction of bottom product xB?

(2) upward flow rate of vapor in the rectifying section(V mol/h,) and down ward flow rate of liquid in the stripping section.(L’ mol/h,) (3) The operating line for stripping section . (4)if relative volatility ?=2.4, find R／Rmin

5 Separate component A and B of mixed liquid in a continuous distillation column, Given: raw liquid flow rate F is 4000 kg·h-1 , mass fraction of A is 0.3. It is required that the A mass fraction in the column bottom liquid can not be beyond 0.05, recovery ratio of A is 88% on the top. Try to calculate the flow rate of the distillated liquid D and its constituent xD at the top of column.( in the terms of molar flow rate and molar fraction) Given: molecular weight of components A and B are 76 and 154, respectively.

6 There is a continuous rectifying operation column, whose the operation line equation is as follows:

Rectifying section: y=0.723x+0.263 Stripping section: y=1.25x-0.0187

if the feed enters the column at a dew point, find (a)the molar fraction of feed、overhead product and bottom product. (b) reflux ratio R.

7 A column is to be designed to separate a liquid mixture containing 44 mole percent A and 56 mole percent B, the system to be separated can be taken as ideal. The overhead product contains 95.7 mole percent A. Given: liquid average relative volatility ?=2.5, min reflux ratio Rmin =1.63, try to illustrate the thermal condition of the feed and to calculate the value of q.

8 A continuous rectifying column operated at atmospheric pressure is used to separate benzene—methylbenzene (甲苯) mixed liquid. The feed is saturation liquid containing 50 mole percent benzene. Given: the overhead product must contain 90 mole percent benzene and the bottom product contains 10 mole percent benzene. If reflux ratio is 4.52, try to calculate how many ideal plates are need? and locate the feed plate. In this situation the equilibrium data of benzene—methylbenzene are as follows

toC 80.1 85 90 95 100 105 110.6 x 1.000 0.780 0.581 0.411 0.258 0.130 0 y 1.000 0.900 0.777 0.632 0.456 0.262 0

9 There is a rectifying column, given : mole fraction of distillation liquid from tower top xD=0.97, reflux ratio R=2, the gas-liquid equilibrium relationship y=2.4x/(1+1.4x); find: the constituent x1 of the down liquid leaving from the first plate and the constituent y2 of the up gas leaving from the second plate in the rectifying section. Suppose the total condenser is used at the top of column.

10 A continuous fractionating column is used to separate 4000kg/h of a mixture of 30 percent

CS2and 70 percent CCl4. Bottom product contains 5 percent CS2, and the rate of recovery of CS2in the overhead product is 88% by weight. Calculate (a) the moles flow of overhead product per hour. (b) the mole fraction of CS2in the overhead product, respectively.

11. A liquid mixture of benzene and toluene is continuously fed to a plate column. Under the total reflux ratio condition, the compositions of liquid on the close plates are 0.28, 0.41 and 0.57, respectively. Calculate the Murphree plate efficiency of two lower plates. The equilibrium data for benzene—toluene liquid and vapor phases under the operating condition are given as follows:

x 0.26 0.38 0.51

y 0.45 0.60 0.72

Drying

1 A wet solid with 1000 kg/h is dried by air from 40% to 5% moisture content (wet basis) under the convective drying conditions. The air primary humidity H1 is 0.001(kg water/kg dry air), and the humidity of air leaving dryer H2 is 0.039 (kg water /kg dry air), suppose that the material loss in the drying process can be negligible. Find: (1) rate of water vaporization W, in kg water/ h.

(2) rate of dry air G required, in kg bone dry air/h, flow rate of moist air, V, in kg fresh air/h. (3) rate of moist material out of dryer, L2, in kg moist solid/h.

2 The wet solid is to be dried from water content 20% to 5% (wet basis) in a convective dryer at an atmospheric pressure. The feed of wet solid into the dryer is 1000 kg/h at a temperature of 40℃. The dry and wet bulb temperatures of air are respectively 20℃ and 16.5℃ before air enters the preheater. After being preheated, air enters the dryer. The dry and wet bulb temperatures of air leaving dryer are respectively 60℃ and 40℃. If heat loss is negligible and drying is considered as constant enthalpy process:

(1) What is the fresh (wet) air required per unit time (in kg/h)? (2) The air temperature TG1 before entering the dryer? (Given: Vaporization latent heat of water at 0℃ is 2501kJ/kg, specific heat of dry air CpB is 1.005 kJ/kg·K, specific heat of water vapor CpA is 1.88 kJ/kg·K)

3 The wet solid material is to be dried from water content 42% to 4% (wet basis) in an adiabatic dryer. The solid product out of the dryer is 0.126kg/s. After the fresh air at a dry-bulb temperature of 21oC and a relative humidity of 40％ is preheated to 93oC, it is sent to the dryer, and leaves the dryer at relative humidity of 60％. If the drying is under constant enthalpy process. (1) Determining the air humidity H1.

(2) If H0=0.008(kg water/kg dry air), H2=0.03( kg water /kg dry air. Find: (a) Dry air flow rate G required, in kg dry air/s.

(b)How much heat is supplied to air by the preheater (in k J/h)?