Saturated steam at gauge pressure of 2.0 bar is to be used to heat a stream of e

Question

Saturated steam at gauge pressure of 2.0 bar is to be used to heat a stream of ethane. The ethane enters a heat exchanger at 16C and 1.5 bar gauge at a rate of 795 m^3/min and is heated at constant pressure to 93C. The steam condenses and leaves the exchanger as a liquid at 27C. The specific enthalpy of ethane at the given pressure is 941 kJ/kg at 16 C and 1073 kJ/kg at 93 C.

(a)How much energy (kW) must be transferred to the ethane to heat it from 16C to 93C?

(b) Assuming that all the energy transferred from the steam goes to heat the ethane, at what rate in m^3/s must steam be supplied to the exchanger? If the assumption is incorrect, would the calculated value be too high or too low?

(c) Should the heat exchanger be set up for concurrent or countercurrent flow? Explain.

Explanation / Answer

Solution

(a) Heat transferred by stream = heat transfer to ethane

ms (h1-h2) = me (h2-h1)

At 16°C (16 + 273= 289K), 1.5 bar

PV = mRT          (Gas constant of ethane, R = 0.276 kJ/kg.K)

P = RT         

= P/RT       (since P = Pgauge + Patm = 1.5 + 1.0= 2.51 bar)

= (2.51 x 105) / (0.276 x 103 x 289)

= 3.146 kg/m3

Mass flow rate = x volume flow rate

= (3.146 x 795)/60 = 41.68 kg/sec.

Heat transferred by stream = heat transfer to ethane from 16°C to 93°C

                                            = 41.68 (1073 - 941)

                                            = 5501.76 kW

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