Chapter 4, Problem 4.36P

Gas absorption or gas scrubbing is a commonly used method for removing environmentally undesirable species from waste gases in chemical manufacturing and combustion processes. The waste gas is contacted with a liquid solvent in which the potential pollutants are soluble and the other species in the waste gas are relatively insoluble. Most of the pollutants go into solution and emerge with the liquid effluent from the scrubber, and the cleaned gas is discharged to the atmosphere. The liquid effluent may be sent to a waste lagoon or subjected to further treatment to recover the solvent and/or to convert the pollutant to a species that can be released safely to the environment.

A waste gas containing SO₂ (a precursor of acid rain) and several other species (collectively designated as A) is fed to a scrubbing tower where it contacts a solvent (B) that absorbs SO₂. The solvent feed rate to the tower is 1000 L/min. The specific gravity of the solvent is 1.30. Absorption of A and evaporation of B in the scrubber may be neglected.

Stream 3

The gas in the scrubber rises through a series of trays (metal plates perforated with many small holes), and the solvent flows over the trays and through downcomers to the trays below. Gas bubbles emerge from the holes in each tray and rise through the covering liquid, and SO₂ transfers from the bubbles and into solution.

The volumetric flow rate of the feed gas is determined w ith an orifice meter, w ith a differential mercury manometer being used to measure the pressure drop across the orifice. Calibration data for the meter are tabulated here:

/j(mm)	V(m3/min)
1(X)	142
200	204
300	247
400	290

The molar density of the feed gas may be determined from the formula

zmol\ _ 12.2P(atm)

' \fiter/ f(K)

where P and T are the absolute pressure and temperature of the gas. An electrochemical detector is used to measure the SO₂ concentration in the inlet and outlet gas streams: SO₂ in the sampled gas is absorbed in a solution across w hich a fixed voltage is applied, and the mole fraction of SO₂ in the gas is determined from

the resulting current. The calibration curve for the analyzer is a straight line on a semilog plot of y (mol SCWmol total) versus R (analyzer reading) that passes through the following points:

y (log scale)	R (rectangular scale)
0.00166	20
0.1107	90

The following data are taken:

T = 75°F P = 150 psig

/i(orifice meter) = 210 mm /?(SO₂ analyzer) = 82.4 /?(SO₂ analyzer) = 11.6 (outlet gas)

1. Draw' and completely label a process flowchart. Include in the labeling the molar flow' rates and SO₂ mole fractions of the gas streams and the mass flow' rates and SO₂ mass fractions of the liquid streams. Show' that the scrubber has zero degrees of freedom.
2. Determine (i) the orifice meter calibration formula by plotting V versus h on logarithmic axes and (ii) the SO₂ analyzer calibration formula.

Calculate (i) the mass fraction of SO₂ in the liquid effluent stream and (ii) the rate at w hich SO₂ is removed from the feed gas (kg SO₂/min).

The scrubber column trays commonly have diameters on the order of 1-5 meters and perforation holes on the order of 4-12 mm in diameter, leading to the formation of many tiny bubbles in the liquid on each tray. Speculate on the advantages of making the bubbles as small as possible.