A stream of liquid A in an industrial plant, flowing at a rate of 2 kg/s, needs to be heated from 5 "C to 90 °C. Another stream of 5 kg/s of liquid B, at a temperature of 60 "C, is available for...

Thermodynamics


A stream of liquid A in an industrial plant, flowing at a rate of 2 kg/s, needs to be heated from 5 "C to 90 °C. Another stream of 5 kg/s of liquid B, at a temperature of 60 "C, is available for possible heat exchange, but your su- pervisor doubts that this can be of much use, given that its temperature is lower than the target temperature of stream A. Assuming that unlimited heat exchange is possible with the environment (which is at 20 °C), can heating of stream A be accomplished with no external work input? If your answer is yes, show a schematic of your proposed process, and compute the flow rate and exit temperature of stream B and the rate of heat ex change (if any) with the environment. If your answer is no, provide a full quantitative justification for this claim. The heat capacities at constant pressure of the two liquids are 3 k]/(kg K) for A and 2 k)/(kg K) for B, in- dependent of temperature. 3.8 3.12 A device submitted to the Pa- 0.03 kg/s tent Office is shown schemati- sat. steam, P 2 baar liq. + vap. P 2 bar cally in the figure to the right. Its inventor claims that it can Device 0.3 kg/s liq. water, 20 °C 10 kW of electrical generate power continuously, using only 0.03 kg/s of low-pressure (2 bar) saturated steam, which exits as a mixture of liquid and gas also at 2 bar. A stream of cooling water (0.3 kg/s, liquid at 1 bar) is used in the device; the patent application lists its entrance and exit temperatures as 20 °C and 40 °C, respectively. The de- vice does not have any other heat or material exchanges with the environ- - 40°C ment. You are asked to give your fully justified opinion as to whether steady-state operation of this device is (or is not) thermodynamically per- missible. If it is not possible to obtain the electrical power stated, what is the maximum power than can be generated?