CH 101 Lecture Notes - Lecture 19: Pressure Measurement, Barometer

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graysardine594

24 Mar 2016

School

University of Alabama

Department

Chemistry

Course

CH 101

Professor

John Mc Duffie

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Document Summary

Force of the air on the surface of the mercury counter balances the force of gravity on the column of mercury. The pressure of a gas trapped in a container can be measured with an instrument called a manometer. Manometers are u-shaped tubes, partially filled with a liquid, connected to the gas sample on one side and open to the air on the other. A competition is established between the pressures of the atmosphere and the gas. The difference in the liquid levels is a measure of the difference in pressure between the gas and the atmosphere. Pressure of a gas is inversely proportional to its volume. As p increases, v decreases by the same factor. Pressure is caused by the molecules striking the sides of the container. When you decrease the volume of the container with the same number of molecules in the container, more molecules will hit the wall at the same instant.

Related Questions

A gas consisting of NO2 in air is bubbled through a scrubbing tower into which water is fed at a rate of 1 m3 /h. The water absorbs most of the NO2 but none of the air. The volumetric flow rate of the feed gas, V, is determined with an orifice meter, with a differential mercury manometer being used to measure the pressure drop across the orifice. Calibration data for this meter are tabulated below:

h 50 100 200 300 400 500

V 95 142 204 247 290 302

where h is the differential height of the manometer. An electrochemical detector is used to measure the NO2 concentration in the inlet and outlet gas stream. The calibration curve for the analyzer is a straight line on a semilog plot of y (mol NO2/total mol) versus R, (analyzer reading), which passes through the points (20, 0.00166) and (90, 0.1107). The following data are taken: Feed Gas: T = 75Â°F Outlet Gas: R = 11.6 P = 150 psig h = 210 mm R = 82.4 a) Determine the orifice meter calibration formula by plotting V versus h on log-log scale and fitting a trendline through the data. Clearly indicate the units on the constants in the resulting formula. Turn in the calibration curve. b) Determine the NO2 calibration curve formula from the two points given above. Again, clearly indicate the units on the constants in the resulting formula. c) Calculate the mole fraction of NO2 in the liquid effluent stream (neglect the small quantity of water that leaves with the exit gas). d) How much NO2 is removed each hour from the feed gas?

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 highly 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 discharged 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. 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 down comers to the trays below. Gas bubbles emerge from the holes in each tray and rise through the covering liquid, and SO2 diffuses out of the bubbles and into solution. The volumetric flow rate of the feed gas is determined with an orifice meter with a differential mercury manometer being used to measure the pressure drop across the orifice Calibration data for this meter are tabulated here: The molar density of the feed gas may be determined from the formula 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 which 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 semi log plot of y(mol SO2/mol total) versus R (analyzer reading), which passes through the following points: The following data are taken:

Refer to charts and flow diagram from this url:
http://www.solutioninn.com/engineering/chemical-engineering/chemical-processes/gas-absorption-or-gas-scrubbing-is-a-commonly-used-method

(b) Determine (I) the orifice meter calibration formula by plotting V versus h on logarithmic axes and (ii) the SO2 analyzer calibration formula.
(c) Calculate (I) the mass fraction of SO2 in the liquid effluent stream and (ii) the rate at which SO2 is removed from the feed gas (kg SO2/min).
(d) The scrubber column trays commonly have diameters on the order of 1ï¿½5 meters and perforation holes on the order of 4ï¿½12 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 aspossible.

1. SO2 is removed from an air stream by absorption into water at 1atm and 20oC. The air flow rate is 10.0 lbmol/h and the SO2concentration is 3.0%. The exit concentration of SO2 is 0.5%. Thewater flow rate is 1.2 times the minimum (L = 1.2 x Lmin). Thecolumn is 1.0 ft diameter and packed with 25 mm ceramic Raschigrings. kL is 1.3 ft/h and the driving force is concentration(lbmol/ft3). kp is 0.195 lbmol/h ft2 atm and the driving force ispartial pressure (convert to torr). The equilibrium constant at 1atm and 20oC is K = y/x = 20.
a. Make equilibrium graphs of x vs. y and c (lbmol SO2/ft3) vs. p(torr).
b. Use the xy plot to find the specified liquid flow rate.
c. Use a mole balance to find the liquid effluent concentration (xand c).
d. Draw the operating lines on both graphs.
e. Determine the number of theoretical stages using both graphs.(Should be about the same number.)
f. Use the Kremser equation to determine the number of theoreticalstages with K = slope of xy equilibrium line. If A = 1 then youwill have division by 0. So try K = 19 and 21.
g. Determine the NTU and HTU using both overall mass transfercoefficients based on y and p. (Derive these equations with thehelp of class notes.)
h. Determine the height of the packed column.
2. Perform a similar analysis of a stripping column. In this case,100 lbmol/h of water with a concentration of 0.0045 lbmol SO2/ft3is stripped with air to a concentration of 0.0005 lbmol SO2/ft3.The air flow rate is 1.5 times the minimum.
3. Specify as many variations of problems 2 and 3 as you can.

CH 101 Lecture Notes - Lecture 19: Pressure Measurement, Barometer

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