Question
Propylene glycol is produced by the hydrolysis of propylene oxide according to the following reaction In the presence of excess water, the reaction has been found to be first order in propylene oxide. r = kcpo and the rate constant is [6] k = k_0 e^-Pa/RT k_0 = 4.71 times 10^9 s^-1 P_a -18.0 kral/mol Methanol is added as a solvent, and the reaction is performed in a 1000 L CSTR operating 60 degree C. The feed conditions and physical properties are as follows [17]: Assume the mixture is ideal so that in which V_j = M_j/P"_j are the pure component specific molar volumes. Neglect any change in the pure component specific densities with temperature in the temperature range 25-60 degree C. (a) Compute the steady-state concentrations of all components, Q and V_R for the following two situations. 1. A float in the top of the tank is used to adjust Q to maintain reactor volume constant at 1000 L. 2. The reactor is initially charged with pure solvent, and a differential pressure measurement is used to adjust Q to maintain constant reactor mass. Which operation do you recommend, constant volume or constant mass? Look at the conversion of propylene oxide and the total reactor production rate of propylene glycol for the two cases. What are you wasting in constant mass operation? (b) Resolve the constant reactor volume operation under the assumption that all component densities are equal to the density of water. How much error in the conversion and production rate do you commit under this assumption?
Explanation / Answer
Here,two streams are kept to the reactor, the mixture of water-methanol-propylene oxide undergoes a slight contraction in volume due to the mixing effect,Suppose the temperature of the two feeding streams is 14°C before mixing, but increases to 24°C due to the heat of mixing, so this will be the temperature to be taken for the feeding that enters the system.
Here, propylene oxide has a low boiling point (34.3°C at 1 atm). This requires that the mixture used does not exceed an operating temperature otherwise much of propylene oxide vapor would be lost.
Therefore constant volume can be beneficial rather than constant mass as the mass of mixture can be increase along with mixing of reagents.
B.) According to constant component densities as of water no remarkable effect will produce towards reaction rate as water is used here for conversion of propelene glycol.
