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Example 9 – Evaporation

 

Problem

 

Liquid material of 5000 kg/hr flow rate is initially at a mass concentration of 0.13 kg/kg and a temperature of 3 oC. It needs to be concentrated to a mass concentration of 0.57. Material density is 720 kg/m3 at room temperature. Concentration process needs to be performed at around atmospheric pressure. Specific heat of the material without moisture is 1.26 kJ/kg. oC. The boiling point rise of the material can be described by the following duhring lines expressed in a Table 1

 

Mass concentration

(kg/kg)

Start Boiling Point (oC)

End Boiling Point (oC)

Solvent

Solution

Solvent

Solution

0.0

50

50

200

200

0.2

50

52

200

203

0.4

50

55

200

207

0.6

50

59

200

212

 

Table 1. Duhring Lines of Example 9

 

Solution:

 

This can be achieved by a two-effect falling film evaporation process. The initial liquid material is first preheated using part of the secondary vapor from the second effect evaporation to about 85 oC. Then part of the thermally compressed secondary vapor from the first effect is used to further heat the material to nearly the bubble point of the material. It then goes to the first falling film evaporator operating at a pressure of 106 kPa. Water vapor of 265 kPa is used as the heating medium for this evaporator. Vapor and liquid material mixture coming out of the first evaporator goes to a liquid-vapor separator to separate the concentrated liquid with the vapor. Secondary vapor coming out of the separator is compressed with a fresh vapor at 350 kPa using a steam jet ejector. A very small potion of the compressed vapor is used to preheat the feeding material. The majority is used as the heating medium of the second effect evaporation. The second evaporator is operating at 100 kPa. Liquid vapor mixture coming out of the second evaporator goes to another liquid-vapor separator to separate the concentrated liquid material with the vapor. As is mentioned above part of the secondary vapor coming out of the separator is use to preheat the feeding material.

 

A falling film evaporator can be taken as a heat exchange with phase changes. Therefore, a heat exchanger can be used to model an evaporator. The to-be-concentrated material is at the cold side of the heat exchanger. The heating steam is at the hot side of the exchanger.

 

Solution Steps:

 

1.      Start ProcesSimO.

2.      Go to Materials | Drying Materials to bring up the “Drying Materials” dialog. Click “Add” button to add a new drying material. In the “Name” field change the name to “Generic Evaporation Material”. Input 1.26 in the “Specific Heat of Absolute Dry Material” field.

3.      On the same “Drying Materials” dialog click “Duhring Lines” button to specify the duhring lines of Generic Evaporation Material according to Table 1. After all the necessary values are input you should have the following “Durhing Lines” dialog as follows,

 

 

Figure 9a

 

4.      Click “OK” button to close the “During Lines” dialog. Then close the “New Drying Materials” dialog and next close “Drying Materials” dialog.

5.      Go to File | Close to close the current flowsheet.

6.      Go to Materials | New Flowsheet Settings to bring up the New Flowsheet Settings dialog. Select “Generic Evaporation Material” in the Select Drying Material list box. Then click “Set” button to set this material as the drying material of the new system to be created. Close this dialog.

7.      Create a new flowsheet by clicking the first icon  in the toolbar or clicking File | New.

8.      Construct a flowsheet as in Figure 9b.

 

Figure 9b

 

9.      The known and calculated variable values are displayed in Figure 9c.

 

 

Figure 9c

 

Please note that not all the known values of the variables are presented in the statement of the Problem. Please load this example to the software and see all the known values of the variables for all the unit operations.