CHE 441

Answers to problem set#3

1. For the acetone process shown in Figure 1a (Turton et al, Analysis, Synthesis, and Design of Chemical Processes), check the design specifications for the heat exchanger E-401 (A = 70.3 m2) against the heuristics in Table 3.7 (Lecture Notes). E-401 is a feed vaporizer with the following data for zoned analysis

Zone / Q (MJ/hr) / U(W/m2oK) / T(oC) at the end of zone
I / 590 / 560 / 105
II / 2,280 / 1,140 / 105
III / 680 / 60 / 234

The temperature of stream 2 entering the heat exchanger is 32oC and the temperature of high-pressure steam (hps) is constant at 254oC.

AI = 1.6 m2, AII = 20.1 m2, AIII = 49.2 m2, Total area = 70.9 m2

2. For the acetone process shown in Figure 1a, check the design specifications for the reactor furnace H-401 (duty = 2730 MJ/h, radiant area = 10.1 m2, convective area = 30.4 m2) against the heuristics in Table 3.7. This furnace is used to supply energy to the molten salt, stream 4, which in turn provides heat to the endothermic reactor R-401. The flow rate of the molten salt is 35,100 kg/h and its heat capacity is 1645 J/kgoC.

Radiative area = 10.1 m2, convective area = 30.3 m2

3. For the acetone process shown in Figure 1a, check the design specifications for the phase separator V-402 (D = 0.75 m, L = 2.25 m) against the heuristics in Table 3.2. The vapor flow from V-402, stream 5, is 340 kg/h at 20oC, 1.8 bar, molecular weight 8.56. The liquid flow from V-402 is 2330 kg/h. Liquid density is 814 kg/m3.

D = 0.74 m, L = 2.22 m

4. Estimate the size of the motor necessary to pump a lean oil to the top of an absorption tower operating at a pressure of 445 kPa. The oil is to be pumped from an open tank with a liquid level 3 m above the floor through 46 m of pipe with an inside diameter of 0.078 m. There are five 90o elbows in the line, and the top of the tower is 9.1 m above the floor level. A flow of 2.7 kg/s of lean oil is required. The viscosity of the oil is 15 cP, and its density is 857 kg/m3.Assume that the efficiency of the pumping system including the motor is 40 percent.

3.2 kW

5. What is the pressure loss when 2.14 kg/s of pure benzene at 40oC flows through a 21-m length of straight pipe with an inside diameter of 0.0409 m? The pipeline contains six 90o elbows, one tee used as an elbow (equivalent resistance equal to 60 pipe diameters), one globe valve, and one gate valve. The density of the benzene is 849 kg/m3, and its viscosity at 40oC is 5×10-4 Pa∙s.

38.3 kPa

6. A condenser is to be supplied with 0.0095 m3/s of cooling water at 25oC. What diameter of schedule 40 pipe should be recommended for this application using the economic pipe diameter?

4” schedule 40 pipe

7. Liquid benzene at 38oC with a vapor pressure of 26.4 kN/m2 and a density of 860 kg/m3 is to be pumped at a rate of 0.0025 m3/s from a storage tank to a discharge location 3 m above the liquid level in the tank. The pump with a mechanical efficiency of 60 percent is 1 m above the liquid level. The storage tank is at atmospheric pressure. The pressure at the end of the discharge line is 445 kPa absolute. The inside diameter of the pipe used to transfer the benzene is 0.0409 m. The frictional pressure drops in the suction and the discharge line has been evaluated as 3.45 and 37.9 kN/m2, respectively. Determine the head developed by the pump and the total power requirement. If the pump manufacturer specifies a required net positive head of 3.0 m, will such a pump be applicable for this service?

NPSH = 7.47 m, pump is suitable for this service

8. A centrifugal pump delivers 0.0063 m3/s of water at a temperature of 15oC when the impeller speed is 1800 r/min and the pressure drop across the pump is 138 kPa. If the speed of the impeller is reduced to 1200 r/min, estimate the water delivery rate and the head that is developed by the pump if pump operation is ideal.

0.0042 m3, 6.27 m