Height per transfer units

Since the terms (1 — xr)xm and (1 — xr) are usually nearly unity, the left-hand portion of Eq. (8.5) is essentially the concentration change dxR experienced per unit of concentration difference xr — xrx) causing the change, and represents a measure of the difficulty of the extraction. I his in turn is designated as Nt the number of transfer units, which when multiplied by the experimentally determined factor HTUy the height per transfer unit, gives the height of the tower. Thus,... 

Benzene is to be absorbed from coal gas by means of a wash-oil. The inlet gas contains 3 per cent by volume of benzene, and the exit gas should not contain more than 0.02 per cent benzene by volume. The suggested oil circulation rate is 480 kg oil/100 m3 of inlet gas measured at 273 K and 101.3 kN/m2. The wash oil enters the tower solute-free. If the overall height of a transfer unit based on the gas phase is 1.4 m, determine the minimum height of the tower which is required to carry out the absorption. The equilibrium data are ... 

In the design of an absorption tower, the most important single factor is the value of the transfer coefficient or the height of the transfer unit. Whilst the total flowrates of the gas and liquid streams are fixed by the process, it is necessary to determine the most suitable flow per unit area through the column. The gas flow is limited by the fact that the flooding rate must not be exceeded and there will be a serious drop in performance if the liquid rate is very low. It is convenient to examine the effects of flowrates of the gas and liquid on the transfer coefficients, and also to investigate the influence of variables such as temperature, pressure, and diffusivity. 

In the general case where the equilibrium hne is curved, the mass transfer rate for gas absorption per differential packed height dZ and unit cross-sectional area of the column is given as... 

The items to note are A separation column 12 inches in diameter can wash at least 125 pounds of ice per hour, the column can be operated to give satisfactory product when only 0 to 5% of the ice is used for wash water, and the height of a transfer unit is about 0.55 foot and this does not appear to be a function of ice, brine, or wash water rates. More complete experimental results on this column are available ( —4). 

Molar flow rate of gas per unit area Gravitational acceleration Specific enthalpy of vapor phase Height of gas film transfer unit Height of liquid film transfer unit Height of overall gas-phase transfer unit Height of overall liquid-phase transfer unit Henry s constant Specific enthalpy of liquid phase Apron clearance... 

Feed to extraction cascade, kg or lb base mixture per hour Height of a transfer unit, m or ft... 

Example 22.5. Gas from a reactor has 3.0 percent ethylene oxide (EO) and 10 percent COj, with the rest mostly nitrogen, and 98 percent of the EO is to be recovered by scrubbing with water. The absorber will operate at 20 atm, using water with 0.04 mole percent EO at 30°C, and the gas enters at SO C, saturated with water. How many transfer units are needed if 1.4 mol HjO are used per mole of dry gas Estimate the diameter of the column and the packed height if l -in. Pall rings are used and the total gas feed rate is 10,000 mol/h. 

C = the gas concentration in parts per million V = the air sample volume in cubic centimeters K = a constant for a given type of indicator tube and test gas H = a mass transfer proportionality factor having the dimensions of centimeters, and known as the height of a mass transfer unit. 

D mols of distillate withdrawn as overhead products per unit of time F mols of mixture fed to column per unit of time H enthalpy, or heat content of vapor h enthalpy, or heat content of liquid H.E.T.P. = height equivalent to a theoretical plate H.T.U. height of a transfer unit... 

The value of is calculated as a sum of die enei for compens don of the pressure drop per a height of mass transfer unit and the energy for pumping of the liquid phase to the same height Because the efficiencies of the pump and of the ventilator are similar, and because the purpose is comparison of different packings, tibe ventilator and pump efficiencies are not taken into account... 

The inU in this tower is expected to be around 1.2 m the gas enters with 630 ppm. The absorbing Uquid enters pure. The gas flux per liquid flux GjL is 0.025. (a) How many transfer units are needed (b) What height of tower is required ... 

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