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DEPARTMENT OF CHEMICAL ENGINEERING FACULTY OF ENGINEERING, ARCHITECTURE AND SCIENCE Course Number CHE 315 Section No. 02 Course Title Unit Operation Laboratory I Semester/Year Fall 2011 Instructor Dr. Ginette Turcotte Teaching Assistant Samira Ghafoori Lab Report for Experiment NO. 4 Report Title Double Effect Evaporator Efficiency Total Pages Experiment Date October 7 2011 Submission Date October 14 2011 Due Date October 14 2011 Group No. M Students’ Name Student ID Signature Avinash Krishnan 03168 Talal Ali 70764 Harison Perinpanayagam 72818 Leader: Tawsif Zaman 68324 TABLE OF CONTENTS Contents Page Number List of Figures 2 List of Tables 2 Introduction 3 Theoretical Background 4 Experimental Procedure 6 Results and Discussion 9 Error Analysis 12 Conclusion and Recommendation 13 Sample Calculations 14 References 17 Appendices 18 List of Figures: Figure 1 – Schematic diagram of Single effect Evaporator 8 Figure 2 – Schematic diagram of Double effect Evaporator 8 List of Tables: Table 1 – Heat Transfer Results for First Effect 9 Table 2 – Heat Transfer Results for Second Effect 10 INTRODUCTION A fruit juice processing plant wants to install multiple-effect evaporators to produce concentrated orange juice. Before they start using this process, they want to study the performance of a pilot-scale double-effect evaporator using pure water. The purpose of this experiment is to estimate the evaporation capacity and energy efficiency of a double-effect evaporator. In this experiment, the efficiency between the single effect and double effect evaporation was compared. The first half of the experiment was done for single effect evaporation in which the steam was the feed and the produced steam condensate and distillate were collected and measured for efficiency calculations. The second half of the experiment was done for double effect evaporation. In this part of the experiment, the steam from the first effect was used as the heating medium in the second effect. Then, the initial produced steam condensate and the distillates of the first and second effect were collected and measured for efficiency calculations. In most industries, the reason for using multiple effect evaporation is to improve the steam economy. It is expected that the efficiency and capacity should be greater in the double effect evaporation than that of the single effect evaporation. Experimentally, it was determined that the single effect evaporator had an efficiency of 69.09% and a capacity of 11.4 L/hr while the double effect evaporator had an efficiency of 156.44% and a capacity of 14.95 L/hr. As expected, the double effect evaporator had a higher efficiency than the single effect evaporator. THEORETICAL BACKGROUND The process of evaporation is widely used in food and juice industries. This process can be done in a single stage or multiple stages. In a single effect evaporator, a heating medium contains the enthalpy to vaporize a boiling liquid. The vapour removed from a single effect evaporator is usually discarded and thus, there is inefficiency in the process due to the lost energy in the discarded vapour. Multiple effect evaporators are used to increase energy efficiency by using the vapour from one evaporator from one evaporator effect as the heating medium for the next effect. So, the heat in the original heating medium is re-used in the other effects and thus, the evaporation achieved per heating medium in the first effect is substantially increased. Each effect acts as a single effect evaporator in a multiple effect evaporator, but at the expense of a reduced temperature corresponding to the pressure drop from one effect to another. In the experiment, the heating medium fed to the first effect is low-pressure steam and partially condensed in the case of double effect. The second effect uses vapour from the first effect as a heating medium. For the double effect evaporator, the following assumptions will be considered: Negligible heat losses Energy to pre-heat the feed liquor to the first effect is negligible, No leakage or entertainment, The flow of non-condensable is negligible, Superheating and Subcooling effects are negligible. Using the assumptions the difference between the enthalpy of the steam and that of the condensate is simply the latent heat of condensation for the steam, or: qs = ms ?s (1) Where: qS = rate of heat transferred through heating surface from steam ms = rate of steam flow, ?s = latent heat of vaporization, And the enthalpy gained in the liquid side is: q = m?v + mf (H-Hf) (2) Where: q = rate of heat transferred from heating surface to liquid. ?v= latent heat of evaporation of liquid at boiling point Hf = enthalpy of the feed liquid H = enthalpy of liquid at boiling point mf = feed rate m = liquid evaporated Combining (1) and (2) yields: ms ?s= m?v + mf (H-Hf) (3) Since efficiency can be expressed as the number of kilograms of liquid evaporated per kilogram of steam fed, equation (3) can be re-arranged to give: e = m/ms = [?s – ((mf/ms)(H-Hf))]/ ?v (4) EXPERIMENTAL PROCEDURE This experiment was set up to study the differences between the single effect evaporator and the double effect evaporator and thus, divided into two sections. In the first half of the experiment, the single effect evaporator was used under vacuum conditions. Following the lab manual, all the valves were set up correctly. The second effect was used in isolation from the first effect as a single stage evaporator. The liquid feed was flowing directly into the second effect. After closing several valves, the vacuum pump was started. The vacuum pressure was adjusted and kept in the range of -0.4 to -0.6 bars. It was made sure that the steam pressure did not exceed 1.8 bars. The condenser was monitored until the first droplet was observed. Then, the distillate and the steam condensate were collected for 20 minutes. During these 20 minutes, the second effect temperature, inlet and outlet temperatures, T1 and T2, steam pressure, vacuum pressure and flow rate were recorded every 2 minutes. It was made sure the cooling water flow rate was such that the temperature difference between the inlet and outlet temperatures was between 5-10 °C. Also, if the distillate in the second effect vessel was halfway full, certain valves were closed and opened and the distillate was collected in a bucket. After 20 minutes, the vacuum pump was closed along with the main steam valves. Also, the cooling water valve was turned off. Then, the overall distillate and the steam condensate were collected in buckets and measured using graduated cylinders. In the second half of the experiment, two identical evaporators were used to operate as a double effect evaporator. In this experiment, a parallel feed operation was applied. The feed liquid was split in a manner such that one portion passed to the first effect while the remainder went to the second effect. Once again, all the valves were set up correctly by following the lab manual. After closing several valves, the vacuum pump was started. Similar to the first half of the experiment, the rest of the procedure were similar. There were two only notable differences. The first difference was that the temperature readings for the first effect were recorded every 2 minutes during the 20 minute timed-run. The second difference was that the first effect distillate was collected after 20 minutes. After 20 minutes, the main steam valves and the vacuum pump were closed along with the cooling water valve. Then, the distillates and steam condensate were collected in buckets and measured using graduated cylinders. SCHEMATIC DIAGRAMS FOR EXPERIMENT Single Effect Evaporator Steam In Water In Feed Tank Distillate Figure 1: Schematic diagram of Single effect Evaporator Feed Tank 1st Effect Evaporator Steam In Water In 2nd Effect Evaporator Condensate Out 1st Effect Distillate 2nd Effect Distillate Figure 2: Schematic diagram of Double effect Evaporator RESULTS AND DISCUSSION For the single effect evaporator, the rate of heat removed from the condenser was found be constant because the inlet and outlet temperatures were constant. The maximum heat removed from the condenser was 155.83 kJ at 20 minutes. For different times, the heat removed is displayed in Table 2. Similarly, for the double effect evaporator, the rate of heat removed from the condenser varies be constant because the inlet and outlet temperatures were not constant. The maximum heat removed from the condenser was 81.81 kJ at 14 minutes. For different times, the heat removed is displayed in Table 2. Table 1: Heat Transfer Results for Single Effect Time (min) Rate of Heat Removed from the Condenser, q (kJ/s) Total Heat Transfer from Condenser, Q (kJ) 0 0 0 2 7.79152 15.58304 4 7.79152 31.16608 6 7.79152 46.74912 8 7.79152 62.33216 10 7.79152 77.9152 12 7.79152 93.49824 14 7.79152 109.0813 16 7.79152 124.6643 18 7.79152 140.2474 20 7.79152 155.8304 Table 2: Heat Transfer Results for Double Effect Time (min) Rate of Heat Removed from the Condenser, q (kJ/s) Total Heat Transfer from Condenser, Q (kJ) 0 0 0 2 5.84364 11.68728 4 7.79152 31.16608 6 5.84364 35.06184 8 0 0 10 3.89576 38.9576 12 5.84364 70.12368 14 5.84364 81.81096 16 1.94788 31.16608 18 1.94788 35.06184 20 0 0 Throughout the experiment, the steam pressure was kept constant with a varying vacuum level. The purpose of this was to lower the boiling point of water. Increasing the pressure causes more water to be vaporized, increasing the concentration. In turn, this will lead to a greater production rate of both distillate and concentrate. Similarly, by lowering the boiling point of water, the production time is lowered because less time is required to heat the water to the new lower boiling temperature. This will increase the capacity of the experiment. Furthermore, because of the lowered boiling point, less steam will be required to heat the water, which will yield a higher efficiency. Also, in accordance with the ideal gas law and Gay-Lussac’s Law and because the heat is a function of inlet and outlet temperatures, the total heat supplied to the evaporator is increases with lower vacuum pressure. The heat supplied to the evaporator for the single effect was 9741.6 kJ and for the double effect was 11 740.94 kJ. Type of Evaporator Capacity (L/hr) Efficiency (%) Single Effect 11.4 69.09 Two Single Effect 22.8 69.09 Double Effect 1st Effect 7.45 55.84 2nd Effect 7.5 100.6 Total Effect 14.95 156.44 From the experimental data, the single effect evaporator has a capacity of 11.4 L/hr with 69.09% efficiency, which is shown in Table 3. For the double effect evaporator, the one that was considered as two single effects evaporator, the capacity of the systems was calculated to be 22.8 L/hr with efficiency of 69.09%. The overall capacity of the double effect evaporator was found to be 14.95 L/hr with 156.44% efficiency. When the assumption that was made in the double effect evaporator to be two single effects is compared to the regular double effect evaporator, the two gives a difference of 45.61% difference in efficiency. The double-effect evaporator proved to be a more efficient system than the single-effect evaporator. The energy in vapour generated in the first effect is reused as the heat medium for the second effect, rather than being discarded. This way, energy is being conserved for the second effect. In the single-effect evaporator, there is loss in efficiency due to the energy lost in the vapour that is being removed. The resulting efficiency for the double-effect evaporator was 156.44%, whereas the efficiency for the single-effect was 69.09%. The result of the second-effect showed that there is an increase in efficiency when heat from the original heating medium is reused in the subsequent effects. The total heat from the condenser from the 2nd effect evaporator is 11 740.94 kJ. Comparing this value to the total heat from the condenser found in the 1st effect experiment, 9741.6 kJ, an error of the experiment was found to be 17.03%. ERROR ANALYSIS The single effect evaporator had an efficiency of 69.09% and the double effect evaporator had an efficiency of 156.44%, with an experimental error of 17.03%. Even though the double effect evaporator had a higher efficiency than the single effect evaporator as expected, the double effect evaporator had an efficiency of over 100%, which is impossible. This impossible efficiency and percentage error is due to numerous reasons. The major sources of error stem from some of the assumptions that were used to simplify calculations. Firstly, the assumption that there was no heat loss is a source of error because it was observed that there were many hotspots on the apparatus, which in turn reduces the efficiency of the evaporators. Along with the assumption of no heat loss, the assumption that no energy is required to pre-heat the feed liquor to the first effect, the assumption that there were no superheating effects, and possibly the assumption that no leakage or entertainment occurred have also added to the error. In addition to these errors, when operating the double effect evaporator, the timer start is inaccurate because the timer was not started when the first bubble appeared in the first effect, but rather sometime after the first bubble appeared in the first effect. This caused an inaccuracy in each of the readings at their corresponding time, and is a highly likely source of the double effect evaporator’s inflated efficiency. Lastly, while operating the double effect evaporator, the second evaporator was overflowing, so the steam pressure had to be turned off during the last few minutes of the experiment. This altered the capacity and efficiency of the double effect evaporator because the second evaporator takes the steam produced in the first evaporator. CONCLUSION To sum up, the single effect evaporator had an efficiency of 69.09% and a capacity of 11.4 L/hr, while the double effect evaporator had an efficiency of 156.44% and a capacity of 14.95 L/hr. As expected, the double effect evaporator was much more efficient that the single effect evaporator. Along with the double effect evaporator’s impossibly high efficiency, a percentage error of 17.03% was calculated. This experiment would benefit from numerous recommendations. One such recommendation would be to create an adiabatic environment for the apparatus, such that there is negligible heat loss, so that assumption 1 is valid. This could be done be insulating the apparatus. Another recommendation would be to install digital readers on the apparatus, to achieve more accurate results. Lastly, to lower energy consumption, a compressor can be installed to increase the steam pressure to condense the steam at a higher temperature, so that its latent heat is returned to the evaporator to be used by the second effect. SAMPLE CALCULATIONS Equations: q=ms?s (1) q=m?v+mfH-Hf (2) ms?s=m?v+mfH-Hf (3) e=mms=?s-[msmfH-Hf ?v (4) Single Effect Experiment: Heat from steam: q=m?v+mf(H-Hf) m=3.8 L20 min=0.19Lmin=3.17×10-3kg/s ?v @ 860C=2292.74 kJ/kg mf=0.2 L/min=3.33×10-3kg/s Hliq @ 86oC =360.224 kJ/kg Hfeed @ 25oC =104.83 kJ/kg q=3.17×10-3kg/s2292.74 kJ/kg+3.33×10-3kg/s(360.224 kJ/kg-104.83 kJ/kg) q=8.118 kJ/s Q=q×t Q=8.118kJs×(20 mins?60 s1 min) Q=9741.6 kJ capacity= distillatetime=3.80 L20 min=11.4 L/hr efficiency %= distillatecondensate*100%= 3.8 L5.5 L*100%=69.09% Double Effect Experiment: Heat from steam: q=m?v+mf(H-Hf) m=m1+ m2=2.485 L20 min+2.52 L20 min=6.95Lmin=3.90 ×10-3 kg/s ?v @ 840C=2297.86 kJ/kg mf=0.2 L/min=3.33×10-3kg/s Hliq @ 84oC =351.816 kJ/kg Hfeed @ 25oC =104.83 kJ/kg q=3.90 ×10-3 kg/s2297.86 kJ/kg+3.33×10-3kg/s(351.816 kJ/kg-104.83 kJ/kg) q=9.784 kJ/s Q=q×t Q=9.784kJs×(20 mins?60 s1 min) Q=11740.94 kJ %error=|9741.6- 11740.94|11740.94×100%=17.03% First effect for Capacity and efficiency: capacity= distillatetime=2.485 L20 min=7.45 L/hr efficiency %= distillatecondensate*100%= 2.485 L4.45 L*100%=55.84% Second effect for Capacity and efficiency: capacity= distillatetime=2.5 L20 min=7.5 L/hr efficiency %= distillatecondensate*100%= 2.5 L2.485 L*100%=100.60% Total Efficiency = 1st effect efficiency + 2nd effect efficiency = 55.84% + 100.60 % = 156.44% Two single effects evaporator: Capacity=2? distillatetime=2?3.8 L20 min=22.8 L/hr efficiency %= 2?distillate2?condensate*100%= 2?3.8 L2?5.5 L*100%=69.09 % % difference= 100.6-69.0969.09*100%=45.61% REFERENCES Chemical Process Equipment Selection and Design, Couper J. Fair J. Penny W. Walas S. Second Edition, Elservier Inc. APPENDIX Single Effect Evaporator under Vacuum Condition: Temperature and Pressure Readings after steady state conditions Time (min) 2nd effect (oC) Inlet Temperature (oC) Outlet Temperature (oC) Vacuum Pressure (bar) Steam Pressure (bar) 2 86 12 20 -0.425 1.1 4 86 12 20 -0.425 1.1 6 86 12 20 -0.425 1.1 8 86 12 20 -0.425 1.1 10 86 12 20 -0.425 1.1 12 86 12 20 -0.425 1.1 14 86 12 20 -0.425 1.1 16 86 12 20 -0.425 1.1 18 86 12 20 -0.425 1.1 20 86 12 20 -0.425 1.1 Steam condensate collected in bucket: 5.5 L Distillate collected from vessel: 3800 mL Double Effect Evaporator under Vacuum Condition Temperature and Pressure Readings after steady state conditions Time (min) 1st effect (oC) 2nd effect (oC) Cooling Water Inlet Temperature (oC) Cooling Water Outlet Temperature Flow Rate (L/m) Vacuum Pressure (bar) Steam Pressure (bar) 2 72 84 14 20 14 -0.45 0.8 4 74 84 14 22 14 -0.45 0.8 6 84 86 14 20 14 -0.45 0.8 8 92 84 14 14 16 -0.45 0.8 10 98 86 12 16 18 -0.45 0.8 12 98 88 12 18 18 -0.45 0.8 14 98 90 12 18 20 -0.4 0.8 16 98 88 12 14 14 -0.4 0.8 18 98 88 12 14 12 0 0 20 98 88 12 12 10 0 0 Vacuum Pressure: -0.45 bar Pressure of Steam: 0.8 bar Distillate for 1st effect = Condensate for 2nd effect: 2.485 L Condensate collected from 1st effect: 4.45 L Distillate collected from 2nd effect: 2.5 L
