Transport Processes Flashcards
A 50 mm diameter pipe of circular cross section and with walls 3 mm thick is covered with two concentric layers of lagging, the inner layer having a thickness of 25 mm and a thermal conductivity of 0.08 W/m-K, and the outer layer a thickness of 40 mm and a thermal conductivity of 0.04 W/m-K. What is the rate of heat loss per meter length of pipe if the temperature inside the pipe is 550 K and the outside surface temperature is 330 K?
62.7 W/m
The temperature of oil leaving a co-current flow cooler is to be reduced from 370 to 350 K by lengthening the cooler. The oil and water flowrates, the inlet temperatures and the other dimensions of the cooler will remain constant. The water enters at 285 K and oil at 420 K. The water leaves the original cooler at 310 K. If the original length is 1 m, what must be the new length?
1.86 m
It is desired to warm an oil of specific heat 2.0 kJ/kg K from 300 to 325 K by passing it through a tubular heat exchanger containing metal tubes of inner diameter 10 mm. Along the outside of the tubes flows water, inlet temperature 372 K, and outlet temperature 361 K. The overall heat transfer coefficient from water to oil, based on the inside area of the tubes, may be assumed constant at 230 W/m2 K, and 0.075 kg/s of oil is to be passed through each tube. The oil is to make two passes through the heater and the water makes one pass
along the outside of the tubes. Calculate the length of the tubes required.
5 m
Air at 206.8 kPa and an average of 477.6 K is being heated as it flows through a tube of 25.4 mm inside diameter at a velocity of 7.62 m/s. The heating medium is 488.7 K steam condensing on the outside of the tube. Since the heat transfer coefficient of condensing steam is several thousand W/m2-K and the resistance of the metal wall is very small, it will be assumed that the surface wall temperature of the metal in contact with air is 488.7 K. Calculate the heat flux for an L/D.
. 700 W/m2
In a 1–1 shell and tube heat exchanger, steam is condensing on the shell side at TS °C, and the cold fluid is being heated on the tube side from t1 °C to t2 °C. The following equation relates t2 to the other variables.
Where ‘U’ is the overall heat transfer coefficient, ‘A’ is the heat transfer area, ‘W’ is the mass flow rate and ‘Cp’ is the heat capacity. The tube side coefficient is controlling and the tube side fluid is in turbulent flow. TS = 130°C, t1 = 30°C, t2 = 80°C. If the mass flow rate of the cold fluid is double while keeping all the other conditions it, find the new value of t2 at steady state.
75 deg C
A mixture of benzene vapor and flue gas contains 12.7 mole % benzene and is to be scrubbed continuously in a packed tower operated at atmospheric pressure at 430C. The tower is to be designed to treat 36000 ft3 per hour of entering gas and the exit gas is to contain 1.5 mole % benzene. Pure solvent will be supplied at the rate of 28 lbmol/hr. The solution of benzene in oil may be assumed to follow Raoult’s law and the vapor pressure of pure solvent at 430C is 0.263 atm. It is agreed that the maximum allowable superficial velocity of the gas is 1 ft/sec. Calculate the height of the tower required, assuming that the height of a transfer unit is 2 ft
7.0 ft
A packed tower with an inside volume of 300 ft3 is to be used for an absorption under such conditions that KGa = 2/8 lbmol/hr-atm-ft3 and Henry’s law is applicable to the ammonia solution. The pressure difference 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 0.09 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?
91%
An open cylindrical tank of 6ft diameter is filled to within 2ft of the top with pure CH3OH. The tank is tapered to 4ft diameter. The air within the tank is stationary, but circulation of air immediately above the tank is adequate to assure a negligible concentration of CH3OH at this point. The tank and air space are at 77˚F and 1atm. The diffusivity of CH3OH in air at 77˚F and 1atm is 0.62ft2 /hr. Calculate the rate loss of CH3OH from the tank at steady state
2.7x10-4 lbmol/hr
Ethanol vapor is being absorbed from a mixture of alcohol vapor and water vapor by means of a non-volatile solvent in which alcohol is soluble but water is not. The temperature is 97˚C and P=760mmHg. The alcohol vapor can be considered to be diffusing through a film of alcohol-water vapor 0.1mm thick. The mole % of the alcohol in
the vapor at the outside of the film is 80% and that on the inside, next to the solvent is 10%. The volumetric diffusivity of alcohol-water vapor mixtures at 25˚C and 1atm is 0.15cm2 /s. Calculate the rate of diffusion of ethanol vapor in kg/hr if A=10m2. Assume Fuller et al validity.
17960
An ethanol-water vapor mixture is being rectified by contact with an ethanol-water liquid solution. Alcohol is being transferred from gas to liquid and water from liquid to vapor. The molal flowrates of C2H5OH and water are equal but in opposite directions. The temperature is 95˚C and 1atm. Both components are diffusing through a gas film 0.1mm thick. The molal concentration of alcohol at the outside of the film is 0.8 and that on the inside is 0.1. Calculate the rate of diffusion of alcohol in lb/hr through a film area of 100ft2. The volumetric diffusivity of C2H5OH and H2O mixture is 0.15cm2
/s at 25˚C and 1atm. Assume Chapman-Enskog validity
16160
A thick ethanol-water solution in the form of a stagnant film 2mm thick at 293K is in contact at one surface with an organic solvent in which ethanol is soluble and water is insoluble. At point 1 the concentration of ethanol is 16.8wt% and the solution density is 972.8kg/m3.At point 2 the concentration of ethanol is 6.8wt% and the density is 988.1kg/m3. Thediffusivity of ethanol is 0.74x10-9m2/s. Calculate the steady state flux, NA
9x10-7kmol/m2-s
