Lecture 8. Design of cooling systems for effluent temperature reduction. L08-1 Energy System Design Update

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1 Lecture 8 Design of cooling systems for effluent temperature reduction L08-1 Energy System Design Update

2 Wastewater Treatment Utility System Wastewater Heat Exchanger Network Separation System Reactor Treatment system - Volume - Concentration - ph - Temperature Environmnent L08-2 Energy System Design Update

3 Effluent Cooling Systems Water-using system Cooling system Freshwater OPERATION 1 OPERATION 2 OPERATION 3 Discharge T env Interaction L08-3 Energy System Design Update

4 Chemical Pollution Distributed Treatment System Example 1 : Effluent Name Effluent 1 Effluent 2 Effluent 3 Flowrate (t/h) Treatment Process Data Environmental Discharge Limit Concentration (ppm) RR= 95% C env = 20 ppm L08-4 Energy System Design Update

5 Graphical Representation of Effluent Stream Data 250 C (ppm) 250 C (ppm) C env = Mass (kg/h) Individual Effluent Stream Data 40 C env = Mass (kg/h) 11.4 Effluent Composite Curve & Treatment Line L08-5 Energy System Design Update

6 Design of Distributed Effluent Treatment Above Pinch 1 2 Treating At Pinch 3 Treatment Process Below Pinch 4 5 Bypassing (Wang and Smith, Chem Eng Sci, 49, , 1994) L08-6 Energy System Design Update

7 Distributed Effluent Treatment 250 C (ppm) E1 40 t/h Treatment 75 t/h C env = 20 E3 E2 25 t/h 10 t/h 15 t/h Mass (kg/h) Minimum treatment flowrate can be targeted for distributed effluent treatment L08-7 Energy System Design Update

8 Comments on the Distributed Treatment Approach Provides a simple way for targeting the minimum flowrate to be treated Gives design guidelines to achieve the targets in practice Provides insights which can be explored and extended to a cooling system for effluent temperature reduction L08-8 Energy System Design Update

9 Distributed Cooling System T 1 Cooling Tower T 1 Cooling Tower T 2 T 2 T env Discharge T env T 3 T 3 Discharge Reduce effluent flowrate to cooling tower L08-9 Energy System Design Update

10 Targeting of Distributed Cooling System T ( C) T ( C) T 1 T 1 Effluent Temperature Composite Curve T 2 T 3 T env = 30 T 2 T 3 T env = 30 Cooling Line Disposable Heat Disposable Heat L08-10 Energy System Design Update

11 Effect of Evaporation Loss E 1 E E F in (T in -T out ) E 2 T in F in CT T out F out F in > F out T dis > T env E 3 T s F s T dis The evaporation loss forces an increase in the CT supply flowrate(f in ) L08-11 Energy System Design Update

12 CT Supply Line Target with Evaporation Loss E 1 E 2 E 3 F 1 T 1 F 21 + f F 22 - f T 2 F 3 T 3 Point A B F in + f T in - T in E NEW F s - f CT T s - T s f : increament of CT supply flowrate F out + f T out A F dis T dis =T env (F out + f)t out + (F S - f)(t S - T S ) = F dis T env Point B (F S - f)(t S - T S ) = (F 22 - f)t 2 + F 3 T 3 Combining the expressions gives F 3 T 3 + F 22 T 2 +F out T out - F dis T env f = T 2 - T out (Assume E NEW E) L08-12 Energy System Design Update

13 Example 2 : Effluent Name Flowrate (t/h) Effluent Effluent Effluent Environmental Discharge Limit Temperature ( o C) T env = 30 o C CT characteristic = 25 o C CW exit temperature L08-13 Energy System Design Update

14 T Effluent temperature composite curve 54.2 o C E t/h E 21.7 t/h 25 o C CT supply line 490 t/h E 2 E 3 CT 510 t/h t/h T dis 30.1 o C Q T dis > T env ( 30 o C) Calculate f : f = F 3 T 3 + F 22 T 2 +F out T out - F dis T env T 2 - T out = 5.4 t/h L08-14 Energy System Design Update

15 Final Design of Distributed Cooling System E 21.7 t/h E t/h t/h t/h CT E t/h t/h E t/h t/h T dis = 30 o C But, Is the cooling cost inversely proportional to the CT flowrate? L08-15 Energy System Design Update

16 Distributed Cooling System Targeting Wet Bulb Temp. limitation T Effluent temperature composite curve Feasible region for distributed cooling system Min. Flowrate Max. Flowrate (Centralised Cooling System) Q L08-16 Energy System Design Update

17 Effect of Distributed Cooling on CT Performance T CT flowrate decrease Range T Range Apporach Apporach R 2 R 1 A 2 A 1 Wet Bulb Temp. Q Flowrate L08-17 Energy System Design Update

18 Effect of Distributed Cooling System on CT Capital Cost Enthalpy Eq. Curve Cost CT capital cost Driving force L/G Operating Line Optimum Flowrate Effect Approach Twbt Range Water Temp Apporach Effect Range Effect Flowrate Approach is more important factor than flowrate and range to achieve high driving force for cooling L08-18 Energy System Design Update

19 Effect of Distributed Cooling Systems on CT Operating Cost Operating Cost Optimum Water pumping cost Air Fan Cost Flowrate Distributed cooling systems increase an air consumption for cooling L08-19 Energy System Design Update

20 Overall cost of Distributed Cooling System Cost Optimum Overall cost Operating cost Capital cost Flowrate To target for distributed cooling systems is an optimisation problem L08-20 Energy System Design Update

21 Optimisation of Distributed Cooling Systems Objective function CT capital cost [ ] : CC = R A WBT F Air fan power cost (FC) : Evaporation Loss - Required air flowrate = Air Humidity difference cfm of air requires one horsepower CW pumping cost (PC) : - Required power = flowrate head density efficiency Overall cost Minimise f = CC + FC + PC Details given in Kim, Savulescu and Smith, Chem. Eng. Sci. Vol. 56 (5), , 2001) L08-21 Energy System Design Update 82

22 Identification of Constraints T T i Pinch Composite curve Pinch Feasibility U T in L T in R T out U T out L A WBT Q i Pinch Q Heat Load L08-22 Energy System Design Update

23 Formulation of Constraints Heat load of cooling system : Q = F C P ( T T ) in out Range : R = T in T out Approach : A = T out T wbt Feasibility constraints : F L F F U T L in T in T U in T L out T out T U out Pinch feasibility constraints : Air humidity : Evaporation loss : Y = f air in E loss air ( T ) in T out = c R F Q + R Q Y air out pinch i = f T pinch i Tin + T 2 out L08-23 Energy System Design Update

24 Example 3 : Effluent Name Flowrate (t/h) Effluent Effluent Effluent Environmental Discharge Limit Temperature ( o C) T env = 30 o C Let s find optimal design of distributed cooling system - Payback year : 3 yr - Interest rate : 15 % - Operating time : 8600 hr/yr - Electricity cost : 0.05 /kwh L08-24 Energy System Design Update

25 Optimal Conditions for Distributed Cooling System Cost 34.2 k Optimum 534 t/h Flowrate Case Flowrate (t/h) Tin ( o C) Tout ( o C) Cost (k /yr) Centralised Distributed (Optimum) L08-25 Energy System Design Update

26 Optimal Design of Distributed Cooling Systems E 1 E 2 E t/h 40 o C 300 t/h 35 o C 450 t/h 32 o C Cost comparision Case 16 t/h 534 t/h 37.3 o C Overall cost E CT Capital cost 7.4 t/h Air fan cost 28.1 o C t/h f = 2 t/h Discharge t/h (k /yr) CW pumping cost 30 o C Centralised Distributed (Optimum) % L08-26 Energy System Design Update

27 Summary A systematic method for distributed cooling system CT thermal performance Distributed cooling system Cooling cost T 1 T 2 T 3 CT T env Discharge Minimum cooling cost L08-27 Energy System Design Update

28 Working Session 8 Design of cooling systems for effluent temperature reduction WS08-1 Energy System Design Update

29 Objectives To reduce the aqueous effluent discharge temperature to 30 C Water Effluent Steam Data - Base Case Effluent Flowrate (t/h) Effluent Effluent Effluent Environmental Discharge Limit Temperature ( o C) T env = 30 o C WS08-2 Energy System Design Update

30 Traditional Option End-of-Pipe Centralised Cooling System E1 E2 E3 200 t/h 40 o C 300 t/h 35 o C 300 t/h 33 o C 800 t/h 35.5 o C CT 30 o C WS08-3 Energy System Design Update

31 Task 1: 1. Open the file WS08.xls 2. Input the cooling water conditions for centralised cooling systems (callout 1). 3. Fill the answer 1 Answer 1: Centralised cooling system k /yr Capital cost of cooling tower Operating cost for air supply Operating cost for water pumping WS08-4 Energy System Design Update

32 Task 2: Draw effluent composite curve T T 1 T T 1 T 2 T 2 T 3 T 3 Environmental limit (30 o C) Q Q * Assume that the heat capacity of cooling water is 4.2 kj/kg o C. WS08-5 Energy System Design Update

33 Answer 2: Effluent composite curve T T 1 T 2 T 3 Environmental limit (30 o C) Q WS08-6 Energy System Design Update

34 Task 3: Indentify the optimisation constraints for targeting of distributed cooling systems. T Composite curve T in U T 2 Pinch T in L T out U T out L T 1 Pinch WBT = 20 o C Q 1 Pinch 1st kink point Q 2 Pinch 2nd kink point Q Heat Load WS08-7 Energy System Design Update

35 Answer 3: 1. CW conditions 2. Pinch feasibility Temp. of CW inlet stream to CT [ o C] Temp. of CW exit stream from CT [ o C] CW flowrate of CT [t/h] Lower bound Upper bound Temperature [ o C] Heat load [kw] 1st kink point 2nd kink point End point WS08-8 Energy System Design Update

36 Task 4: 1. Return to the file WS08.xls 2. Input the constraints for the optimisation of distributed cooling systems (callout 2). 3. From the Tools menu, select Solver and run the Solver 4. Check the answer and fill the table Answer 4 : Distributed cooling systems Capital cost of cooling tower Operating cost for air supply Operating cost for water pumping k /yr WS08-9 Energy System Design Update

37 Task 5: 1. Check the temperature and flowrate conditions of cooling supply line (callout 3). 2. Check the temperature conditions of cooling supply line at kink points (callout 4). 3. Draw the optimal cooling supply line against the effluent composite curve 4. Design the distributed cooling systems. WS08-10 Energy System Design Update

38 Answer Identification of optimal cooling supply line T T 1 T 2 E1 200 t/h 40 o C E2 300 t/h 35 o C? 300 t/h E3 33 o C * No evaporation effect is assumed T 3 Environmental limit (30 o C) 2. Design for distributed cooling systems CT Q WS08-11 Energy System Design Update

39 Working Session 8 Solution SOL08-1 Energy System Design Update

40 Answer 1: Cooling cost for centralised cooling systems E1 E2 E3 200 t/h 40 o C 300 t/h 35 o C 300 t/h 33 o C 800 t/h 35.5 o C CT 30 o C Centralised cooling system Capital cost of cooling tower Operating cost for air supply Operating cost for water pumping Overall cost k /yr SOL08-2 Energy System Design Update

41 Answer 2 : Effluent composite curve T ( o C) 40 T ( o C) Environmental limit (30 o C) 2333 kw 1750 kw 1050 kw Q (kw) Q (kw) * Assume that the heat capacity of cooling water is 4.2 kj/kg o C. SOL08-3 Energy System Design Update

42 Answer 3 : The optimisation constraints T ( o C) 39.4 o C 35 o C 35.5 o C 30 o C 33 o C WBT = 20 o C Q (kw) SOL08-4 Energy System Design Update

43 Answer 3: 1. CW conditions Lower bound Temp. of CW inlet stream to CT [ o C] 35.5 Upper bound Pinch feasibility Temp. of CW exit stream from CT [ o C] CW flowrate of CT [t/h] Temperature [ o C] Heat load [kw] 1st kink point 2nd kink point End point SOL08-5 Energy System Design Update

44 Answer 4: Cooling cost for distributed cooling systems Distributed cooling systems Capital cost of cooling tower Operating cost for air supply k /yr Operating cost for water pumping Overall cost Reduction : 13.5 % SOL08-6 Energy System Design Update

45 Answer Identification of optimal cooling supply line T ( o C) Flowrate: t/h Q (kw) SOL08-7 Energy System Design Update

46 Answer Design for distributed cooling systems E1 200 t/h 40 o C t/h t/h o C CT E2 35 o C 3.3 t/h o C E3 300 t/h 33 o C t/h o C 30 o C * No evaporation effect is assumed SOL08-8 Energy System Design Update