Performance of the thin film evaporator Lab Report

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Performance of the thin film evaporator Lab Report

Executive Summary

The changes in steam pressure and feed flow rates affected the performance of the thin film evaporator. Every experiment involved calculating the heat transfer coefficients. The operating conditions of the evaporator was predicted during the experiment with more focus on maximizing production rate as well as steam efficiency.

The data collected was between the lower limit and upper limit steam pressure. Empirical formulas from Geankoplis were used to calculate theoretical heat transfer coefficients. On the other hand, the experimental heat transfer coefficients were calculated by using the data obtained from the experiment. In  validating the results, the values obtained from experimental and theoretical equations were compared and the trend observed. This was to help in predicting the required operating conditions of the evaporator for a specific concentration goal.  The predictions were then validated by conducting the experiment.

The results indicates that there was higher concentration of sugar  in product at higher team pressure inputs and lower feed flow rates.  It is also important to note the  mass and sugar balance did not close well for lower rates and at higher steam pressure sugar balances did adhere to the initial trend. The theoretical inner heat transfer coefficient was established to be independent of the steam pressure. However, it increased  with feed flow rate. On the other hand, the outer heat transfer coefficient decreased with steam pressure  but remained constant at various feed flow rates. Experimental heat transfer coefficient did not show any trend. The mass of product obtained  to the mass of the mass of condensate used were used to define the steam efficiency. The highest value of steam efficiency  was recorded under lower steam pressure inputs and higher feed flow rates.Performance of the thin film evaporator Lab Report

The highest concentration achieved of-performance of the thin film evaporator Lab Report

Starting with a 4.67% sugar solution, the highest degree of concentration achieved was 24.167%, at the operating conditions of a steam pressure input of 40psig and a feed flow rate of between 119.625 . The highest product flow of 510.006  was obtained at a steam pressure input of 30psig and a feed flow rate of 562.995 ; however, this corresponded to a very dilute product with a concentration of 4.833% sugar. Higher steam pressure inputs and lower feed flow rates both yielded more concentrated solutions. In contrast, steam efficiency was maximized at lower steam pressures and higher feed flow rates. For the given product concentration of 12% sugar with the highest production rate, the operating conditions were predicted to be 161.35  of feed flow rate with a steam pressure input of 37.00 psig. An experiment was conducted at the predicted operating conditions and the product concentration and flow rate obtained were within 10% of the prediction. On the other hand, operating conditions of 214.46  with a steam pressure input of 30.00 psig was predicted for producing 8% sugar solution in the product with the highest steam efficiency. The results obtained by running an experiment did not agree well with the predictions; hence, a second prediction was made with a lower steam pressure input of 27.00psig to obtain the desired product. The experiment validated the prediction as the error was reduced.

A change in the refractometer index reading was observed for the same solution at different temperatures; thus, we recommend carrying out an experiment to test the effect of temperature on the refractometer index. In addition, the wall temperature inside the evaporator was assumed to be constant when in fact it changed as the feed was heated. An investigation on how a function can be developed for this temperature relating it to time is recommended.

Results and Discussion-performance of the thin film evaporator Lab Report

Reproducing the data and results of this lab is possible by understanding the sources of uncertainty. Most of the uncertainties in the data came from timing, temperature sensors, and rotameter and refractometer calibrations. Measuring the refractometer index of more solutions with known concentrations and over a wider range would give a better calibration and lead to more accurate results. The same applies to the rotameter calibration, where measuring flow rates over a wider range of readings and taking as many points possible between the lower and upper limits of the readings set would further decrease the uncertainties in the data. Flow rates of product were calculated by measuring the mass of products obtained in one minute, where time was measured using a stopwatch. A better procedure for calculating the flow rates would decrease the uncertainties in the data drastically. Table 1 (A:21) shows the contribution of some variables to the uncertainties in the data. Time and rotameter had the highest percentages of contribution to uncertainty.

All experiments in this lab were run with two trials that agreed with each other. Nevertheless, running more trials always increases the accuracy and precision of the values obtained. An example of how data were collected for an experiment in this lab is shown in Table 2 (A:9). Performance of the thin film evaporator Lab Report

 

Table 1 (A:21):  Uncertainty Values and Contributions to Uncertainty by Variable-performance of the thin film evaporator Lab Report

Table 16: Uncertainty Analysis Values

Variable Sample Value Absolute Uncertainty Relative Uncertainty (%) Input Variable Contribution of Input Variable (%)
Condensate Mass Flow Rate 239.6 g/min 0.154 g/min 0.064 Condensate Mass 0.39
Time 99.61
Vapor Distillate Mass Flow Rate 102.2 g/min 0.066 g/min 0.065 Distillate Mass 2.11
Time 97.89
Product Mass Flow Rate 29.61 g/min .021 g/min 0.072 Product Mass 20.42
Time 79.58
Feed Mass Flow Rate 147.8 g/min .908 g/min 0.066 Product Mass 1.02
Time 98.98
Heat Transfer Rate 4.589 kW 4.247 W .093 Distillate 48.716
Product .123
Feed .076
Hdistillate 16.09
Hproduct 1.351
Hfeed 33.65
Inside Heat Transfer Coefficient 4.889 kWm2K .686 kWm2K 14.03 Heat Transfer Rate .004
Area .508
ΔTInside 99.49
Outside Heat Transfer Coefficient 1.803 kWm2K 0.095 kWm2K 5.258 Heat Transfer Rate .031
Area 3.617
ΔTOutside 96.352
Overall Heat Transfer Coefficient 1.079 kWm2K 0.048 kWm2K 4.417 hi 49.17
ho 50.77
Wall Thickness 0.047
Wall Thermal Conductivity .005
Total Mass Balance .892 7.573*10-4 .085 Mass of Product 3.64
Mass of Distillate 35.25
Mass of Feed 61.11
Sugar Mass Balance 1.023 0.037 3.612 Product .04
Feed .034
xProduct 1.92
xFeed 49.96

 

Calculated Variable Calculated Value Absolute Uncertainty Relative Uncertainty Input Variable(s) Contribution to Uncertainty
Feed Flow 3.6425 gm/s .1632 gm/s 4.482% STD (Calibration) 16.6%
Rotameter 83.4%
Concentrate Flow 1.862 gm/s .000948 gm/s .051% Time 99.952%
Mass .048%
Distillate Flow 1.420 gm/s .000721 gm/s .051% Time 99.92%
Mass .080%
Overall Mass Balance (out/in) .88 .0404 4.4% Feed Flow 99.99%
Conc Flow .0041%
Dist Flow .0024%
Feed Sugar Fraction .047 .003865 9.093% STD (Calibration) 66.8%
RI 33.2%
Conc Sugar Fraction .07458 .003865 5.18% STD (Calibration) 66.8%
RI 33.2%
Dist Sugar Fraction .00337 .003865 114.6% STD (Calibration) 66.8%
 RI 33.2%
Sugar Mass Balance (out/in) .88 .11 11.9% Feed Flow 14.10%
Conc Flow .0017%
Dist Flow .000002%

Table 2 (A:9): Raw Data Collected at P= 40psig For Different Feed Flow Rates

RPM Feed Flow in

(gm/min)

 

RR Product

(gm)

Distillate

 (gm/min)

Condensate

(gm)

Time (sec) RI Sugar Concentration

(%)

 

1250 119.625 40 19.649 101.736 189.879 55.88 1.3633 24.167
1500 224.422 66 116.061 101.693 251.940 53.62 1.3452 9.083
1750 313.096 88 211.875 92.112 307.188 53.42 1.3422 6.583
2000 393.708 108 302.768 80.544 226.319 53.33 1.3409 5.5
2250 462.229 125 378.081 70.418 200.053 52.93 1.3406 5.25
2500 554.933 148 467.927 59.474 235.084 52.48 1.3398 4.583

 Performance of the thin film evaporator Lab Report

Feed Flow in

(gm/s)

 

Product Mass Empty

(gm)

Mass After

(gm)

Time

 

(min)

Flow Rate

(gm/min)

RI Sugar Concentration

(%)

 

2.35 Concentrate 33.12

32.89

73.78

73.36

1

1

40.57 1.3567

1.3578

15.96
Distillate 33.58

33.76

146.2

146.44

1

1

112.65 1.3331

1.3331

0.47
Condensate 52.53

52.83

666.65

631.14

3

3

198.74

 

3.17 Concentrate 33.34

33.13

126.51

126.17

1

1

137.90 1.3467

1.3468

6.95
Distillate 33.64

34.01

142.63

143.59

1

1

108.28 1.3332

1.3332

0.47
Condensate 52.40

52.10

634.91

689.33

3

3

221.40
4.05 Concentrate 33.00

33.02

171.65

170.17

1

1

93.11 1.3434

1.3430

9.22
Distillate 33.42

33.94

142.05

141.86

1

1

109.29 1.3331

1.3331

0.53
Condensate 51.87

52.83

729.35

703.74

3

3

203.73
5.75 Concentrate 32.83

33.27

283.19

286.47

1

1

251.78 1.3399

1.3398

4.80
Distillate 33.89

34.24

124.78

124.52

1

1

90.59 1.3331

1.3331

0.47
Condensate 52.01

52.49

669.06

718.84

3

3

213.90
7.46 Concentrate 32.75

33.23

405.82

401.29

1

1

370.60 1.3384

1.3387

3.96
Distillate 34.62

34.54

112.49

114.18

1

1

78.76 1.3330

1.3332

0.53
Condensate 51.89

52.10

732.44

757.08

3

3

230.92
7.46 Concentrate            
   
   
  Distillate            
  Condensate            

 

 

Closing the overall and solute mass balances is crucial to obtain more accurate results of calculated variables. Figure 1 (A:7) illustrates the overall mass balances of different rotameter readings at varying steam pressure inputs. Lower rotameter readings, which correlate with lower flow rates, have ratios of total mass out to total mass in that are well above the ideal ratio of 1. The rotameter was calibrated for readings between 40 and 148; therefore, using the equation obtained from the calibration curve line for readings below 40 could give inaccurate results. The correlation between the rotameter readings and feed flow rates is shown in Table 3 (A:17).

 

Table 3 (A:17): Rotameter Reading Correlation with Feed Flow RatesPerformance of the thin film evaporator Lab Report

RR Feed Flow in
40 119.625
66 224.422
88 313.096
108 393.708
125 462.229
148 554.933

 

Figure 1 (A:7): Plot of Mass Out /Mass In vs. Steam Pressure.Performance of the thin film evaporator Lab Report

 

Sugar balances of different rotameter readings at varying steam pressure inputs are shown in Figure2 (A:8). Lower rotameter readings had ratios of mass of sugar out to mass of sugar in that are above 1 due to the limitations of the rotameter calibration. Higher steam pressure inputs did not follow the initial trend of the sugar balance with respect to steam pressure due to experimental errors. A limitation for the use of the refractometer requires the solution being measured to be within a certain temperature range. For lower steam pressure inputs, the solutions were cooled off before their concentrations were measured; however, this procedure was not carried out for the higher steam pressure inputs, and the solutions were measured at temperatures well above the upper limit temperature of the refractometer.

Figure 2 (A:8): Plot of Sugar Out / Sugar In vs. Steam Pressure

Performance of the thin film evaporator Lab Report

Inner, outer, and overall heat transfer coefficients of the thin film evaporator were first predicted and then measured using multiple equations. Table 4 (A:18) shows the average heat transfer coefficients of all experiments in this lab. The predicted inner heat transfer coefficient equation, obtained from Geankoplis[1], covered a region of rotational speeds that was beneath the operating rotational speed in this lab of 645rev/min. In addition, the axial velocity variable in the equation is not fully understood; so, to calculate this variable, some assumptions were made regarding the geometry apparatus of the evaporator. The predicted equations of both the inner and outer heat transfer coefficients are merely empirical equations that cannot be relied on; hence, the huge difference between the predicted and experimental values of those variables was expected. Typical values for the overall heat transfer coefficients of agitated thin film evaporators were found to be around 1 . Both the predicted and experimental values are close to this value.

  Outer HTC

(

Inner HTC

(

Overall HTC

(

Predicted Value 6.368 0.796 0.632
Experimental Value 2.087 4.180 1.128

 

 

 

 

Table 4 (A:18): Average Values of Heat Transfer Coefficients

Performance of the thin film evaporator Lab Report

Minimizing the use of energy is vital both economically and environmentally; thus, a plot relating the energy efficiency to varying steam pressure inputs and feed flow rates was created. Figure 4 (A:10) below represents the ratio of mass of concentrate obtained to mass of steam used for most experiments conducted in this lab. At lower flow rates, less time is needed to evaporate the small quantities of water entering the evaporator, whereas higher steam pressure inputs evaporates water faster. While larger values of this ratio indicate more efficient operating conditions, they do not reflect the degree of concentration achieved in the product. Thus, operating at conditions that yields the highest steam efficiency is not always ideal. For example, operating at 10psig for a rotameter reading of 125 yields the highest steam efficiency of this data, but also corresponds to obtaining a very dilute product with concentration of 4.41% sugar.

 

Figure 4 (A:10): Plot of Steam Efficiency vs. Steam Pressure.

 

A prediction of the performance of the evaporator along with its validation is crucial to assessing the accuracy of the results of this lab. In effect, the feed flow rate and steam pressure input that gives the highest steam efficiency for producing a concentrate of 70g/kg were predicted and then validated by running an experiment. The predicted values along with their validations are shown in Table 5 (A:20). The predictions were based on trends observed in the data, where Figure 5 (A:18) was used to predict the operating conditions required to obtain the desired concentration in the product with the highest steam efficiency allowed. A percentage error between the predicted and experimental values of less than 10% was achieved, and so the objective was met.

 

 

Performance of the thin film evaporator Lab Report

Table 5 (A:20): Evaporator Performance Prediction and Validation II:

Variables Prediction I Validation I Prediction II Validation II
Feed Concentration (gm/kg) 4.75 4.75 4.75 4.75
Feed Flow (gm/min) 176 163.96 200.238 182.5
Steam Pressure (psig) 35 35 35 35
Product Concentration (gm/kg) 11.785 12 11.875 8.0
Concentrate Flow (gm/s) 88.57 92.22 87.254 101.292
Distillate (gm/s) 75.12 54.99 105.61 98.946
% error Concentration 1.05% 32.6%
% error Product flow 26.890% 13.8%
% error Distillate  flow -6.698% 6.31%

 

 

Concentration and Steam Efficiency vs Rotameter Readings

Figure 5 (A:18): Plot of Concentration vs. Steam Efficiency. Squares represent concentration and circles represent steam efficiency.

 

In addition, the operating conditions that gives the highest production rate of a 120gm/kg solution was predicted and then validated by running an experiment at the predicted operating conditions. The predicted values and their validations are presented in Table 6 (A:19), whereas Figure 6 (A:11) was analyzed to predict the operating conditions that yields the desired concentration of the product with the highest production rate.

 

Table 6 (A:19): Evaporator Performance Prediction and Validation I:

Variables Prediction Validation
Feed Concentration (gm/kg) 40 41.22
Steam pressure(gm/min) 40 40
Feed Flow (gm/min) 107.53 107.52
Distillate Flow (gm/min) 94.2 12.6
Concentrate Flow (gm/min)    
Product Concentration (gm/kg) 123.66 124.62
Concentrate Flow (gm/s) 1.04 0.95
% error Concentration 0.77
% error flow -8.83

 

 

 

 

 

 

 

 

Figure 6 (A:11): Plot of Concentration vs. Rotameter Reading. Each line represents a different Steam Pressure.

 

 

[1] Geankoplis, C. J. (2003). 4th edition. Transport Processes and Separation Process Principles:

(Includes Unit Operations). Upper Saddle River, NJ 07458: Prentice Hall Professional

Technical Reference.