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
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