CHG 4116 Falling Film Evaporation Chemical Engineering Laboratory

The unit operation studied is a falling film evaporator that concentrates an aqueous sugar solution

under a vacuum pressure condition to account for the heat sensitive product. The evaporator is a

heat exchanger with vertical tubes where a falling film flows downwards by gravity. The goal of

the experiment is to select the optimal parameters that allow high heat transfer coefficients, small

temperature difference and a concentrated liquid product with the highest sugar concentration

possible. Throughout the experiments, samples of the solution have been collected at equal

intervals of time then were weighted in order to calculate their densities which depends on

concentration. A stand curve can then be obtained by using the densities and the sugar

concentrations collected to obtain the final sugar concentration. Another stand experiment was

carried in order to determine the required time of operation for each test which was found to be 30

min. After reaching steady state, multiple temperature profiles can be graphed for each run of

experiment which will be used for heat transfer calculations. The parameters that were aimed to

be changed throughout the experiments affect the heat transfer process and are the flow rate of

recirculation, the vacuum pressure, the initial sugar concentration, the steam pressure and coldwater

flow rate. The flowrate of recirculation influences the nature of the falling film flow and its

viscosity, the vacuum pressure and the steam pressure control the boiling point of the solution

which affects the temperature difference and therefore the heat transfer process. The cold-water

flow rate controls the steam flowrate supplied to the heat exchanger and therefore affects heat

transfer coefficients. A temperature profile was obtained after each test to help determine the heat

transfer coefficients.

To conclude, the optimal vacuum pressure was theoretically found to be -20 psig but

experimentally the results show that the best vacuum pressure was -10 psig. The optimal steam

pressure being 5 psi and the recirculation flowrate being 8 USGPM allow for a final sugar

concentration of 12 wt%. The higher the overall coefficient of heat transfer is, the higher the final

sugar concentration will be since the solute will evaporate due to a better heat exchange process.

The overall heat transfer coefficient and the Reynolds number also influence the final

concentration of sugar and were found to be respectively 1332 763.1 W/𝑚2 𝐾 and 22012.4856