Gas Absorption Exercises

CHEMICAL ENGINEERING SERIES: GAS ABSORPTION EXERCISES 1.

To increase the absorption factor, (where , G – gas flow rate, S – solvent flow rate) (A) increase both G and S (B) decrease both G and S (C) increase S and decrease G (D) increase G and decrease S

2.

In a packed absorption tower, if the equilibrium and operating lines are both straight lines, then the ratio, HETP/HTUOG _______ the absorption factor. (A) increase with increase in (B) is one at unity value of (C) both (a) and (b) (D) neither (a) nor (b)

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In case of an absorber, the operating (A) line always lies above the equilibrium curve (B) line always lies below the equilibrium curve (C) line can either be above or below the equilibrium curve (D) velocity is more than the loading velocity Absorption with chemical reaction is involved in the removal of (A) carbon dioxide from gaseous stream using alkaline solution (B) benzol from coke oven gas using solar oil/wash oil (C) ammonia from coke oven gas using refrigerated water (D) tar from coke oven gas in primary gas coolers using chilled water Use of raschig rings in place of crushed stones as packing in packed beds (other things being the same) (A) increases pressure drop, increases surface area (B) decreases pressure drop, increases surface area (C) increases pressure drop, decreases surface area (D) decreases pressure drop, decreases surface area

ENGR. RONNIE V. FLORES

6.

At the same gas flow rate, the pressure drop in a packed tower being irrigated with liquid ________ that in dry packed tower. (A) is greater than (B) is lower than (C) is same as (D) cannot be predicted as dats is insufficient

7.

Absorption factor is defined as (A) ratio of the equilibrium curve and the slope of the operating line (B) slope of the operating line and the slope of equilibrium curve (C) difference of the slope of the operating and the slope of the equilibrium curve (D) product of the slopes of the equilibrium curve and operating line

8.

Which of the following quantities need not be fixed before starting the design of a co-current absorber? (A) Heat gain or loss (B) Flow rate of entering liquid (C) Flow rate of gas (D) None of the above

9.

The reciprocal of absorption factor is (A) Selectivity index (B) Relative volatility (C) Stripping factor (D) Murphree efficiency

10.

In an absorption column, the flooding velocity for random packing is ____________ that for stacked/regular packing. (A) greater than (B) smaller than (C) equal to (D) either (a) or (b); depends on tyoe of packing

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CHEMICAL ENGINEERING SERIES: GAS ABSORPTION EXERCISES 11.

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Carbon disulfide is to be absorbed from a dilute gas mixture CS2-N2 into a pure nonvolatile oil at amospheric pressure in a counter-current absorber. The mole fraction of CS2 in inlet gas is 0.05 and the flow rate of gas stream is 1,500 kmol/h. The equilibrium relation is given by: 𝑦 = 0.5𝑥, where 𝑥 is the mole fraction of CS2 in liquid stream. It is desired to reduce the mole fraction of CS2 in the gas stream to 0.005. (A) Calculate the minimum value of L/G, where L is the liquid flow rate in kmol/h, while G is the gas flow rate. (B) Derive the equation for the operating line, if L/G is equal to 1.5 times the minimum value. ANSWER A counter-current plate absorber is to be installed for scrubbing of an air mixture containing 5% ammonia by volume. The scrubber is fed with water containing 0.002 mole NH3 per mole of water. The scrubbing water flows at a rate of 1.0 mole water per mole air. It is necessary to absorb 85% of the ammonia present in the gas by operating the absorber at 200 °C. 𝑚𝑜𝑙 𝑁𝐻3 0.80 𝑚𝑜𝑙 𝑎𝑖𝑟 𝐾= 𝑚𝑜𝑙 𝑁𝐻3 𝑚𝑜𝑙 𝐻2 𝑂 Calculate the concentration of NH3 in the outgoing liquid and estimate the number of stages necessary for this operation. ANSWER Air concentrated with solute P is brought in contact with water. At steady state, the bulk concentrations of P in air and water are 0.3 and 0.02 respectively. The equilibrium equation relating the interface compositions is 𝑦𝑝𝑖 = 0.25𝑥𝑝𝑖 . Assume that the mass transfer coefficients FG and FL are identical, what is the gas phase mole fraction of P at the interface 𝑦𝑝𝑖 ? ANSWER The equilibrium data for a certain absorption may be represented by the equation 𝑌 = 5.2𝑋 2 , where, 𝑋 = kmol of dissolved absorbable material per kmol of pure absorbing liquid and 𝑌 = kmol of gaseous

ENGR. RONNIE V. FLORES

absorbable material per kmol of inert gas. The unit is operated as continuous countercurrent absorption. The entering gas contains 5 moles of absorbable material per 20 mols of inert gas. The absorbing liquid enters the column as a pure material. Determine the minimum ratio of liquid to gas if the gases leaving the tower contain 1 mol absorbable material per 50 mols of inert gas. ANSWER 15.

A dry gas mixture of 1,000 ft3/min containing 60% dry air and 40% SO3 is blown to the bottom of an absorption tower and at the top sprayed by 50 lb/min of 60% H2SO4. The acid liquor coming out at the bottom of the tower is an 84.8% H2SO4. The exit gases leaving the top of the tower has a pressure of 29 in Hg with the water vapor pressure 7 in Hg. What percent of the SO3 of the entering gas is converted to H2SO4? ANSWER

16.

A 5% butane and 95% air mixture from a solvent extraction plant is absorbed in a bubble-plate tower, with 8 ideal plates, at 760 mm Hg pressure and 15.6 °C, to recover 95% of the butane in the butane-air mixture. Butane at 15.6 °C has vapor pressure of 28 psia. Liquid butane has a density of 0.581 kg/L at 15.6 °C. The absorbing non-volatile oil used has a molecular weight of 250 and a specific gravity of 0.90. If Raoult’s and Dalton’s laws apply, calculate the liters of fresh absorbing oil needed per liter of butane recovered from the butane-air mixture. ANSWER

17.

Benzene is removed from air by absorbing it in a non-volatile wash oil at 100 kPa in a counter-current gas absorber. Gas flow rate is 100 mol/min, which includes 2 mol/min of benzene. The flow rate of wash oil is 50 mol/min. Vapor pressure of benzene at the column conditions is 50 kPa. Benzene forms an ideal solution with the wash oil and the column is operating at steady state. Gas phase can be assumed to follow ideal gas law. Neglect the change in molar flow rates of liquid and gas phases inside the column.

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CHEMICAL ENGINEERING SERIES: GAS ABSORPTION EXERCISES Determine the absorption factor for this process. ANSWER 18.

A stripper with one equilibrium stage is stripping 1-chloro-napthalene from water into air. The liquid feed contains 2 x 10-6 mole fraction of 1-chloro-napthalene and the total liquid flow rate is 100 kmol/h. The inlet gas, which is pure air, has total flow rate of 10 kmol/h. The column operates at 25 °C and the Henry’s law constant for 1-chloronaphthalene is 10.96 atm/mol fraction. If an outlet liquid mole fraction of 0.4 x 10-6 is desired, what is the pressure (in atm) of the stripper and what is the mole fraction of 1chloro-napthalene of the outlet air? ANSWER

19.

In a packed tower operated isothermally at a pressure of 3 atm, the over-all mass transfer coefficients have the following values: Gas Phase: KGa – 0.396 kmol/h·m 3·Δy; Liquid Phase: KLa – 13.9 kmol/h·m 3·Δx. Where ∆𝑥 and ∆𝑦 refer to the driving forces in mole fraction. The equilibrium composition of the gaseous and liquid phase are characterized by the equation of Henry’s Law: 𝑝∗ = 𝐻𝑥, where 𝑝∗ - equilibrium partial pressure of transferring component in mm Hg; 𝐻 – Henry’s law constant, mm Hg/mol fraction; 𝑥 – mole fraction of transferring component in liquid phase. What is the numerical value of the Henry’s law constant? ANSWER

20.

A packed tower with an inside volume of 300 cu ft is to be used for an ammonia absorption under such conditions tha KGa = 2.8 lbmol/h·ft3·atm and Henry’s law is applicable to the ammonia solution. The pressuredifference driving force at the top of the column (𝑝 − 𝑝∗ ) is 0.009 atm, and the pressure-difference driving force at the bottom of the column is 0.090 atm. If 550 lb of ammonia, in addition to the inert gases, enters the tower per hour, what percent of the entering ammonia will be absorbed? ANSWER

ENGR. RONNIE V. FLORES

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