Energy Engineering - Chemical Processes and Technologies

Full exam

1 Chemical Processes and Technologies (Ing. Giorgia De Guido) Past exam paper July 6 th, 2018: Results In the following, the results are given for M 4 = M 6 = 0, so that P flash = 4.08 [atm] and the % Recovery of n-butane in liquid phase = 74 %. 1. Determine which is the state of the feed stream (liquid, vapor or mixed). By solving the Rachford-Rice equation in the unknown vaporization ratio for the feed stream, α = -1.2e11. Therefore, the feed stream is a subcooled liquid. 2. Determine the temperature range in which vapor-liquid equilibrium conditions exist for the considered mixture, assuming that the flash drum is operated at the pressure P flash reported in the data section. The bubble-point equation and the dew-point equation can be solved to determine the corresponding temperatures, at the given pressure P flash . As a result, the temperature range in which VLE conditions exist for the considered mixture is 315.1665 - 333.3495 [K]. 3. Determine the molar flow rates and the compositions (mole fractions) of the outlet streams, the temperature of the flash drum, and the duty required to achieve a recovery of n-butane in the liquid phase as reported in the data section, assuming that the flash drum is operated at the pressure P flash reported in the data section. This is the case of a flash problem at given pressure, P flash , and recovery of one component in the liquid phase. The following equation can be solved in the unknown temperature of the flash drum: ( ) ( ) 1 1 ()0 NC jj r j jr r r zK fT Vy KK K Fz = − == −+ ∑ after calculating the recovery of n-butane (the r-th component) in the vapour phase: 0.26 r r Vy Fz = . As a result: T flash = 321.2360 [K]. 2 The problem is now reduced to the case in which the temperature of the flash drum and the recovery of a component in the liquid phase are known (case 2 of the analysed cases). The corresponding resolution procedure can be followed to obtain the composition of the two outlet phases: Component xi yi C3 0.059266 0.231451 i-C4 0.132538 0.206350 n-C4 (= r ) 0.290793 0.329711 i-C5 0.423220 0.198254 n-C5 0.094182 0.034233 Moreover, V = 23.6571 [kmol/h] and L = 76.3429 [kmol/h]. As for the duty, the following enthalpy balance is solved (assuming the duty to be positive if supplied to the flash drum): (@ )(@ ) (@ )L VL feedflashflash FHT QVHT LHT⋅ + = ⋅ +⋅  .= The calculations can be carried out considering the state of liquid at the temperature of the feed as reference state=(therefore: (@ )Li feedhT =0=[kJ/kmol]). With the given data:= 1 ,, 1 (@ ) (@ ) (@ ) ( ) (@ ) NC VV flashi i flash i NC LL i i feed P i flash feed ev i flash i H T yh T yh T c T T H T = = = ⋅  = ⋅+ ⋅ − +∆  ∑ ∑      = 1 , 1 (@ ) (@ ) (@ ) ( ) NC LL flashi i flash i NC LL i i feed P i flash feed i H T xh T xh T c T T = = = ⋅  = ⋅ +⋅ −  ∑ ∑     = As a result:=Q = 30.3697 [kW]. 3 4. Determine the molar flow rates and the compositions (mole fractions) of the outlet streams, the temperature of the flash drum, and the duty required to achieve the same recovery of n-butane in the liquid phase as at the previous point, assuming that the flash drum is operated at 1 3 flash feedPP = =.= Following the same resolution procedure as for point 3, but for P flash = 8 [atm], the following results are obtained: • T flash = 349.4001 [K]. • Composition of the two outlet phases: Component xi yi C3 0.063281 0.218310 i-C4 0.134446 0.200116 n-C4 (= r) 0.290900 0.329321 i-C5 0.418458 0.213865 n-C5 0.092915 0.038388 • Molar flow rates of the two outlet phases: V = 23.6851 [kmol/h] and L = 76.3149 [kmol/h]. • Q = 133.3477 [kW].