Heat and Mass Transfer Flashcards
A single effect evaporator concentrates 1 MT of 10 wt% sucrose solution to 50%. The feed enters the evaporator at 20 deg C and has a specific heat of 1.0. The evaporator is maintained at a vacuum of 600 mmHg. The heat is provided by saturated steam at 8.8 kg/cm2 gage. Assuming that no sensible heat is recovered in the evaporator, calculate the weight of heating steam, in kg, needed for concentrating the sucrose solution.
1020 kg
A solution with a negligible boiling point rise is being evaporated in a triple effect evaporator using saturated steam at 121.1°C. The pressure in the vapor of the last effect is 25.6 kPa abs. The heat transfer coefficients are U1 = 2840, U2 = 1988, and U3 = 1420 W/m2 ·K and the areas are equal. Estimate the boiling point in each of the evaporators.
T1 = 381.7 K
T2 = 363.8 K
T3 = 338.7 K
Consider a four-effect standard vertical tube, each effect has 140 m2 of heating surface, is to be used to concentrate, from 4% to 35% percent solids, the total boiling point elevation is 10 deg C. Forward feed is to be used. Saturated steam is available at 120 deg C, and the vacuum in the last effect corresponds to a boiling temperature of 40 degC. the overall coefficients, in W/m2-K are 2950 in I, 2670 in II, 2070 in III and 1360 in IV, all specific heats may be taken as 4.2 J/g0C and radiation is negligible. Calculate the boiling point of solution in effect number 1.
107.7 deg C
A solution of Na2SO4 in water is saturated at 50 deg C. When a saturated solution of Na2SO4 is cooled, crystals of Na2SO4 x 10H2O separate
from the solution. If 1000 kg of this solution is cooled to 10 deg C, the percentage yield obtained is
Temperature 50, 10
Solubility of Na2SO4 (g per 100g water) 46.7, 9
91%
A saturated solution containing 1500 kg of potassium chloride at 360 K is cooled in an open tank to 290 K. If the specific gravity of the solution is 1.2, the solubility of potassium chloride per 100 parts water is 53.55 at 360 K and 34.5 at 290 K.
What is the capacity of the tank?
Calculate the mass of the crystals obtained, neglecting loss of water by evaporation
3.58 m3 is the capacity of the tank
534 kg mass of crystals
Glauber’s salt, Na2SO4x10H2O, is to be produced in a Swenson– Walker crystallizer by cooling to 290 K a solution of anhydrous Na2SO4 which saturates between 300 K and 290 K. Cooling water enters and leaves the unit at 280 K and 290 K respectively and evaporation is negligible. The solubilities of anhydrous Na2SO4 in water are 40 and 14 kg/100 kg water at 300 K and 290 K respectively, the mean heat capacity of the liquor is 3.8 kJ/kgK and the heat of crystallization is 230 kJ/kg. For the crystallizer, the available heat transfer area is 3 m2 /m length, the overall coefficient of heat transfer is 0.15 kW/m2 -K.
A. Calculate the amount of feed in kg/s
B. The total heat to be removed is
C. How many sections of crystallizer, each 3 m long, will be required to
process 0.25 kg/s of the product?
A. 0.488
B. 76 kW
C. 6
What is the yield of sodium acetate crystals (CH3COONa x 3H2O) obtainable from a vacuum crystallizer operating at 1.33 kN/m2 when it is supplied with 0.56 kg/s of a 40 percent aqueous solution of the salt at 353 K? The boiling point elevation of the solution is 11.5 K.
Data:
Heat of crystallization, q = 144 kJ/kg trihydrate
Heat capacity of the solution, Cp = 3.5 kJ/kg deg K
Latent heat of water at 1.33 kN/m2, λ = 2.46 MJ/kg
Boiling point of water at 1.33 kN/m2 = 290.7K
Solubility of sodium acetate product = 0.539 kg/kg water.
0.18 kg/s
A stream of air at 1000C and 5260 mmHg contains 10% water by volume.
A. Calculate the dew point
B. Calculate the percentage of the vapor condenses and the final composition of the gas phase if the air is cooled to 800C at constant pressure
C. Calculate the percent condensation if, instead of being cooled, the air is compressed isothermally to 8500 mmHg
A. 90 deg F
B. 35%
C. 12%
In a vessel at 101.325 kN/m2 and 300K, the percentage relative humidity of the water vapor in the air is 25. If partial pressure of water vapor when air is saturated with vapor at 300K is 3.6 kN/m2
,
Calculate:
A. The partial pressure of the water vapor in the vessels
B. The humidity of the ai
C.
D.
In a vessel at 101.325 kN/m2 and 300K, the percentage relative humidity of the water vapor in the air is 25. If partial pressure of water vapor when air is saturated with vapor at 300K is 3.6 kN/m2
,
Calculate:
A. The partial pressure of the water vapor in the vessels
B. The humidity of the air
C. The humid volume
D. The percentage humidity
A. 0.9
B. 0.006
C. 0.857 m3/kg
D. 25%
In a process in which it is used as a solvent, benzene is evaporated into dry nitrogen. At 297 K and 101.3 kN/m2, the resulting mixture has a percentage relative humidity of 60. It is required to recover 80 percent of the benzene present by cooling to 283 K and compressing to a suitable pressure. What should this pressure be? The vapor pressure of benzene is 12.2 kN/m2 at 297 K and 6.0 kN/m2 at 283 K.
392 kPa
Air in an amount 1000 cfm at 150 deg F, 20% RH is passed over a refrigerated coil and thereby brought to 60 deg F, 90% RH with the condensed moisture withdrawn at 55 deg F. The air is then reheated by means of an electric heating coil to 150 deg F.
A.Compute the moisture removed in lb/min.
B. Compute the heat removed by the refrigerated coil, expressed as tons of refrigerated coil, expressed as tons of refrigeration.
C. Calculate the wattage of the heating coil required.
A. 1.44
B. 15
C. 25 kW
