Assignment 2 Solutions 2009

Fortheenergybalancethequantit yof interestis

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Unformatted text preview: and explicit ly. b. Prove that at steady­state Q=l.D, where l is the molar heat of vaporizat ion Solution: The control vo lume is the flash tank. We make the fo llowing assumpt ions: · · · · Negligible vapor holdup in the unit Constant stage temperature and pressure No heat loss to surroundings Negligible heat transfer resistance for transfer of Q. For the energy balance, the quantit y of interest is: Total Energy = U + K + P Here, U, K, P represent the internal, kinetic and potential energies of the system, respectively. Assuming thermal equilibrium between the vapor and the liquid streams, we can also neglect the energy balance on the vapor phase. Since the liquid in the tank can be considered stationary Þ dK dP dE dU = = 0 and = d t d t d t d t For liquid systems, one can assume that dU d H @ d t d t H denotes the total enthalpy o f the liquid in the tank (vapor ho ldup neglected). Furthermore, H = Hc p , B , av (T - T f ) re Where: cp,B,av : average mo lar heat capacit y of the liquid in the tank Tref : reference temperature where the specific enthalpy of the liquid is assumed to be zero. The average mo lar heat capacit ies o f the liquid streams can be expressed as (two components A and C): c p , F , av = x c p , A + ( - x F ) c p , C 1 F c p , B , av = x c p , A + ( - x B ) c p , C 1 B c p , D , av = x c p , A + ( - x D ) c p , C 1 D Total...
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