Reboiler

The reboiler heating elements were designed (by the vendor) to supply a maximum of 7.5 kW at 240 V of three phase power. This was tested by recording the rate at which the reboiler sump temperature increased from ambient with a given reboiler voltage. Two reboiler tests were conducted: 120 V and 180 V. In both cases, the initial reboiler sump volume was 9.5 L. The raw data and instantaneous heating rates are shown in Table 12.5 and the heating rates are plotted in Figure 12.3.

Table 12.5 - Transient Heating Data for the Reboiler

120 V

180V

Time

Temperature

Instantaneous

Time

Temperature

Instantaneous

(mins)

(°C)

Duty (kW)

(mins)

(°C)

Duty (kW)

0

22.2

-

0

28.2

-

1

22.3

0.03

1

28.7

0.15

2.5

23.5

0.24

1.5

29.9

0.59

3

31.4

4.69

2

35.7

3.56

3.5

37.9

3.86

2.5

49.8

8.38

4

44.6

3.98

2.75

55.5

6.77

4.5

52.1

4.46

3

60.7

6.18

5

58.7

3.92

3.25

65.0

5.11

5.75

67.0

3.29

3.5

68.1

3.68

6

68.6

1.90

3.75

71.0

3.45

6.5

72.0

2.02

4

73.0

2.38

7

74.7

1.60

fiizure 12.3 - Heating Rate at 120 V and 180 V

The reboiler elements take some time to reach operating temperature (approximately 220°C) and the heating rate is unrepresentative of the normal process during this period. Immediately after this initial period, there will still be negligible vaporisation of liquid and the supplied duty will essentially produce sensible heat transfer only. Thereafter, the (unmeasured) latent heat effects will predominate and only part of the energy input can be measured via sensible heat changes so that the heating rate is, again, unrepresentative of the normal process. The actual reboiler duties can be calculated from the maximum gradient of each curve in Figure 12.3 according to the first principles formula given by equation (12.1).

n dT

With these considerations, the reboiler elements were estimated to produce 4.2 kW of heating power at 120 V and 7.8 kW at 180 V. The three-phase power output of a mainly resistive element should be calculable from the voltage (V), resistance (R) and the phase angle (<{)) via equation (12.2). This relationship suggests that the power will be proportional to the square of the voltage for constant resistance and phase angle. However, this is inconsistent with the reboiler data suggesting a dependence between the operating temperature and the resistance or the phase angle.

Insufficient data could be collected at 240 V since boiling began soon after the elements reached operating temperature but the duty was estimated to be 12.1 kW by extrapolation according to the empirical relationship indicated by equation (12.3). This estimated maximum duty is significantly higher than the design duty and suggests that a voltage of 150-180 V will be adequate for most conditions.

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  • elliot
    What is the reboiler heat duty with a 1500Watt heating element?
    2 years ago

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