WFE-SRP Model Applied to Experimental Data

The WFE-SRP computer program was used to analyze the experimen- tal data. As the program has the option of using DIPPR equations to calculate the physical properties and group contribution methods (GCM) to predict the physical properties, a comparison was carried out for the three experimental systems.

WFE-SRP has two models for the prediction of the process side heat transfer coefficient for a wiped film evaporator [11, 14] and two for the falling film evaporator [3, 73], thus giving four different combinations for the heat enhancement factor (βh), and predicting four different exiting concentrations of water. The following sections present the results when the computer program is applied to the experimental data for each system.

6.3.1 Water-Sucrose

The WFE-SRP program was used with Equations 5.6-5.10 for the pre- diction of physical properties, and the modified UNIQUAC equations from Peres and Macedo [75] for the prediction of the activity coefficients (Equa- tions 5.11-5.17), as well as the group contribution methods for physical prop- erties and activity coefficient. Figure 6.17 shows the prediction of the concen- tration of water when using Bott and Romero [11] for the HTC of the wiped film evaporator and Ahmed and Kaparthi [3] for the falling film evaporator, while Figure 6.18 presents the relative error using the same equations. The average error for the combination of the equations was 0.21%, and for the

GCM was 1.36%.

The experiments using water-sucrose were run at different inlet concen- trations of sucrose, varying from 35 to 55 wt percent. They were not run at higher concentrations due to the solubility limit of sucrose in water at 20 ◦C. Outlet concentrations varied from 40 to 66 wt percent of sucrose.

From Figure 6.17 it can be seen that the computer program predicts the exiting concentration of water with good accuracy when using Equations 5.6- 5.10 for physical properties and 5.11-5.17 for the activity coefficient. The GCM option is less accurate. This is mainly due to the estimation of viscosity and liquid enthalpies that are very different from the actual values. When the evaporation rate is low (i.e., concentration of water > 0.95) the prediction is more accurate than when the evaporation rate is high.

Using the combination of equations with the Bott and Sheikh [14] cor- relation for the wiped film evaporator, gives a similar result as shown in Fig- ure 6.19. The average error was 0.10% and 1.38%. Figure 6.20 depicts the relative error for this combination.

From these plots, it can be seen than when using the Equations 5.6- 5.10 for the prediction of physical properties, and the modified UNIQUAC equations from Peres and Macedo [75] for the prediction of the activity co- efficients (Equations 5.11-5.17) in the WFE-SRP program, the prediction of exiting composition of water is very accurate. This confirms that the proposed model predicts the behavior of the water-sucrose system.

Figure 6.17: Predicted versus experimental exiting concentration of water using WFE-SRP for the water-sucrose system when using Equations 5.6-5.17 and GCM for physical properties and Bott and Romero-Ahmed and Kaparthi for HTC.

6.3.2 Water-Glycerol

Figure 6.18: Relative error of the experimental exiting concentration of wa- ter using WFE-SRP for the water-sucrose system when using Equations 5.6- 5.17 and GCM for physical properties and Bott and Romero-Ahmed and Ka- parthi for HTC.

Figure 6.21 shows the prediction of the concentration of water when using Bott and Romero [11] for the HTC of the wiped film evaporator and Ahmed

Figure 6.19: Predicted versus experimental exiting concentration of water using WFE-SRP for the water-sucrose system when using Equations 5.6-5.17 and GCM for physical properties and Bott and Sheikh-Ahmed and Kaparthi for HTC.

and Kaparthi [3] for the falling film evaporator, while Figure 6.22 presents the relative error using the same equations. The average error for the combination

Figure 6.20: Relative error of the experimental exiting concentration of wa- ter using WFE-SRP for the water-sucrose system when using Equations 5.6- 5.17 and GCM for physical properties and Bott and Sheikh-Ahmed and Ka- parthi for HTC.

of the equations was 10.92%, and for the GCM was 6.23%.

glycerol, varying from 38 to 75 wt percent. The outlet concentrations varied from 48 to 90 wt percent of glycerol.

From Figure 6.21 it can be seen that the computer program predicts the exiting concentration of water with excellent accuracy for all the range of exiting water composition. The GCM method works even better than the DIPPR prediction for this system.

Using the combination of equations with the Bott and Sheikh [14] corre- lation for wiped film evaporators, gives a similar result as shown in Figure 6.23. The average error was 9.59% and 2.74%. Figure 6.24 shows the relative error for this combination.

Figure 6.21: Predicted versus experimental exiting concentration of water using WFE-SRP for the water-glycerol system when using DIPPR and GCM for physical properties and Bott and Romero-Ahmed and Kaparthi for HTC.

6.3.3 Water-Ethylene Glycol

The WFE-SRP computer program was also used with the DIPPR equa- tions, as well as the group contribution methods for the prediction of physical

Figure 6.22: Relative error of the experimental exiting concentration of water using WFE-SRP for the water-glycerol system when using DIPPR and GCM for physical properties and Bott and Romero-Ahmed and Kaparthi for HTC.

properties for the six experimental data points. Figure 6.25 shows the predic- tion of the concentration of water when using Bott and Romero [11] for the

Figure 6.23: Predicted versus experimental exiting concentration of water using WFE-SRP for the water-glycerol system when using DIPPR and GCM for physical properties and Bott and Sheikh-Ahmed and Kaparthi for HTC.

HTC of the wiped film evaporator and Ahmed and Kaparthi [3] for the falling film evaporator, while Figure 6.26 presents the relative error using the same

Figure 6.24: Relative error of the experimental exiting concentration of water using WFE-SRP for the water-glycerol system when using DIPPR and GCM for physical properties and Bott and Sheikh-Ahmed and Kaparthi for HTC.

equations. The average error for the combination of the equations was 38.12%, and for the GCM was 4.91%.

These experiments were run at only one inlet concentration of ethylene glycol, around 75 wt percent. The outlet concentrations varied from 83 to 95 wt percent of glycerol.

From Figure 6.25, it can be seen that the computer program predicts the exiting concentration of water with good accuracy for the range of exiting water composition. The GCM method works even better than the DIPPR prediction for this system.

Using the combination of equations with the Bott and Sheikh [14] cor- relation for a wiped film evaporator, gives a similar result as shown in Fig- ure 6.27. The average error was 35.90% and 13.63%. Figure 6.28 shows the relative error for this combination.

Figure 6.25: Predicted versus experimental exiting concentration of water using WFE-SRP for the water-ethylene glycol system when using DIPPR and GCM for physical properties and Bott and Romero-Ahmed and Kaparthi for HTC.

Figure 6.26: Relative error of the experimental exiting concentration of water using WFE-SRP for the water-ethylene glycol system when using DIPPR and GCM for physical properties and Bott and Romero-Ahmed and Kaparthi for HTC.

Figure 6.27: Predicted versus experimental exiting concentration of water using WFE-SRP for the water-ethylene glycerol system when using DIPPR and GCM for physical properties and Bott and Sheikh-Ahmed and Kaparthi for HTC.

Figure 6.28: Relative error of the experimental exiting concentration of water using WFE-SRP for the water-ethylene glycol system when using DIPPR and GCM for physical properties and Bott and Sheikh-Ahmed and Kaparthi for HTC.