CEDB201 REBOILER DESIGN_2022.pptx

notes on reboiler heat exchanger

1. Horizontal Reboiler Design (HRD) 1

2. Types of Reboiler Distillation column have condenser and reboiler. Three principal types of reboiler are used: Forced circulation Thermosyphon Kettle 2

3. Horizontal Kettle reboiler (HKR) The boiling takes place on tubes immersed in a pool of liquid, there is no circulation of liquid through the exchanger. 3

4. Reboilers Reboiler are used with distillation columns to vaporize a fraction of the bottom product. Reboilers are heat exchangers used to provide heat to the bottom of distillation column. It boils liquid and generates vapours which returned to the column for distillate separation. Heat supplied to column by reboiler at the bottom of the column is removed by the condenser at the top of the column 4

5. Type of Reboilers The choice of the best type of reboiler for a given duty will depend on the following factors. The nature of the process fluid, particular its viscosity and propensity to fouling. The operating pressure: vacuum pressure. The equipment layout, particularly the headroom available Forced-circulation: reboiler is suitable for handling viscous and heavily fouling process fluids. It is suitable for low vacuum operations and for low flow rates of vaporisation. The major disadvantage of this type is that a pump is required which increase the pumping cost. There is also the danger of leakage of hot fluid at the pump seal. 5

6. Type of Reboilers Thermosyphon: Reboilers are most economical type for most applications, but are not suitable for high viscosity fluids or high vacuum operations. A disadvantage of this type is higher installation cost because the column base must be elevated to provide the hydrostatic head required for the thermosiphon effect. Operating pressure less than 0.3 bar. Horizontal reboilers require less space than vertical, but have more complex pipework. Horizontal are more easily maintained than vertical because tube bundle can be easily withdrawn. 6

7. Type of Reboilers Kettle Reboilers: They have lower heat transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials and have a high residence time. More expensive than thermosiphon types as larger shells are required. They are often used as vaporisers. Suitable for vacuum operation. High flow rates of vaporisation, up to 80% of the feed. 7

8. Boiling Heat Transfer The mechanism of heat transfer from a submerged surface to a pool of liquid depends on the temperature difference between the heated surface and the liquid. At low temperature difference, when the liquid is below its boiling point, heat is transferred by natural convention. As the surface temperature increases incipient boiling occurs, vapour bubbles forming and breaking loose from surface. 8

9. Boiling Heat Transfer Then, the agitation caused by the rising bubbles, and other effects caused by bubble generation at the surface result in a large increase in the rate of heat transfer, this is called nucleate boiling. As the temperature is raised further the rate of heat transfer increases until the heat flux reaches a critical value. 9

10. Boiling Heat Transfer At this point, the rate of vapour generation is such that dry patches occur spontaneously over the surface and the rate of heat transfer falls off rapidly. At higher temperature differences, the vapour rate is such that the whole surface is blanketed with vapour and the mechanism of heat transfer is by conduction through the vapour film. Conduction is augmented at high temperature differences by radiation. 10

11. Estimation of Boiling Heat Transfer Coefficients 11 In the reboiler design is pool boiling and convective boiling. Pool boiling is the same as nucleate boiling, such as in a kettle type reboiler. Convective boiling occurs where the vaporising fluid is flowing over the heated surface and heat transfer takes place both by forced convection and nucleate boiling. Boiling is a complex phenomenon, and boiling heat transfer coefficients are difficult to predict with any certainty.

12. Estimation of Boiling Heat Transfer Coefficients 12 Pool boiling: In the nucleate boiling region the heat transfer coefficient is dependent on the nature and condition of the heat transfer surface, and it is not possible to present a universal correlation that will accurate predictions for all systems. It is boiling in the absence of bulk flow; fluid body is stationery. Any possible fluid motion will be due to natural convection currents.

13. Estimation of boiling heat transfer coefficients Other type of boiling: Free/Natural Convective boiling: it is the boiling due to natural boiling, which means fluid motion is governed by natural convection currents. Bubbles do not form on the heating surface until the liquid is heated a few degrees above the saturation temperature (@ 2-6 for ). The liquid is slightly superheated (metastable state). Natural convection ends at an excess temperature of 5   13

14. Estimation of boiling heat transfer coefficients Other type of boiling: Nucleate boiling : is the boiling takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the heat flux is below the critical heat flux. Bubbles form at an increasing rate at an number of nucleation sites as the boiling curve move toward point C. A-B > Isolated bubbles. B-C > Numerous continuous columns of vapour in the liquid.   14

15. Estimation of boiling heat transfer coefficients Other type of boiling: Transition Boiling/Unstable Film Boiling Regime: heat transfer due agglomeration of bubbles and forming layer at solid/liquid interface. When is increased past point C, the heat flux decreases. This is because a large fraction of the heater surface is covered by a vapor film, which acts as an insulation. In the transition boiling regime, both nucleate and film boiling partially occur.   15

16. Estimation of boiling heat transfer coefficients Other type of boiling: Film boiling: It is the process of heat transfer from heated surface to the liquid through radiation (heat transfer is by conduction and radiation across the vapour blanket). The presence of vapour film between the heater surface & the liquid is responsible for the low heat transfer rates in the film boiling region. The heat transfer rate increases with increasing excess temperature due to radiation to the liquid. 2 mechanisms of ht tr. adversely affect each other, causing the total ht. tr. to be less than their sum. 16

17. Estimation of boiling heat transfer coefficients The correlation given by Foster and Zuber, 1955 can be used to estimate pool boiling heat transfer coefficients, in the absence of experimental data. is nucleate or pool boiling heat transfer coefficient, p w is saturation pressure corresponding to the wall temperature, T w ; p s is saturation pressure corresponding to the saturation temperature, T s ; is the surface tension, is the latent heat of vaporization; T s is saturation temperature of boiling liquid.   17

18. Critical heat flux It is important to check the design, and its operating. The heat flux must be below the critical heat flux. Several correlations are available for predicting the critical flux. Zuber et al. 1961 found to give satisfactory predictions for use in reboiler and vaporiser design. Mostinski also gives a reduced pressure for predicting the maximum critical heat flux.   18

19. Reboiler design Design of force circulation reboiler will not be discuss to avoid confusion Design of thermosiphon reboiler will not be discuss to avoid confusion The focus is on the horizontal kettle reboiler design 19

20. Thermal Design of the Kettle reboiler Kettle reboilers and other submerged bundle equipment are essentially pool devices and their design is based on data for nucleate boiling. In a tube bundle the vapour rising from the lower rows of tubes passes over the upper rows. This has two opposing effects: there will be a tendency for the rising vapour to blanket the upper tubes, it the tube spacing is close, which will reduce the heat transfer rate. Palen and small (1964) give a detailed procedure for kettle reboiler design in which the heat transfer coefficient calculated using equations for boiling on a single tube is reduced by an empirically derived tube factor, to account for the effect of vapour blanketing. 20

21. Thermal Design of the Kettle reboiler The maximum heat flux for stable nucleate boiling will be less for a tube bundle than for a single tube. Palen and Small, (1964) suggest modifying the Zuber equation for a single tubes with a tube density factor. The modified equation Zuber equation: is maximum critical heat flux for tube bundle is 0.44 for square pitch and 0.41 for equilateral triangular pitch arrangement N t is the number of tubes in the bundle   21

22. Thermal Design of the Kettle reboiler The tube arrangement, triangular or square pitch will not have a significant effect on the heat transfer coefficient. A tube pitch of between of 1.5 to 2.0 times the tube outside diameter should be used to avoid vapour blanketing. Long thin bundles will be more efficient than short fat bundles. The shell should be sized to give adequate space for the disengagement of the vapour and liquid. The shell diameter required will depend on the heat flux. (refer Coulson and Richardson, Vol 6). 22

23. Thermal Design of the Kettle reboiler The freeboard between the liquid level and shell should not be greater than 0.25 m, to avoid excessive entrainment, the maximum vapour velocity at the liquid surface should be less than that given by the expression. Visit Example 12.12 in text book.   23

24. Thermal Design of the Kettle reboiler Reboiler Duty, Qr V = L + D V = RD + D V = D(R + 1)   24

25. Thermal Design of the Kettle reboiler Energy Balance Condenser Q C = VH V – (L + D) H D = (R + 1)DH V – LH D – DH D Overall energy balance on the column FH F + Q R = DH D + BH B + Q C FH F + Q R = DH D + BH B + (R + 1)DH V – LH D – DH D Q R = BH B + (R + 1)DH V – LH D – FH F Q R = BH B + (R + 1)DH V – RDH D – FH F Calculation of enthalpy C PF = C pmix = ; C pi = a + bT + cT 2 ….   25

26. Thermal Design of the Kettle reboiler ΔT m : Mean Temperature Difference Q = UA Δ T m   26

27. Thermal Design of the Kettle reboiler Δ T 1 = T – T ci ΔT 2 = T – T co ΔT m = F Δ T lm F = correction factor ( if you have more than one tube or shell pass, flow in the exchanger is not always counter current) Heat transfer area Q = UA Δ T m Q and Δ T lm is known and U must be an assume value Select standard layout   27

28. Thermal Design of the Kettle reboiler Tube Diameter: Small tube diameter range 5/8in – 1in: more compact and easier to clean. Larger tube diameter range 1in – 2in: easier to clean, suitable for heavy fouling fluid. BWG: Tube thickness is define by BWG Gauge selected (i) withstand internal pressure, (ii) adequate corrosion allowance Tube pitch, (P T ): P T range 1.5 – 2.0 times tube diameter ( refer C&R vol 6). Slightly larger pitch than normal heat exchanger recommended to avoid vapour blanketing. 28

29. Thermal Design of the Condenser Tubes spaced closely together tend to create a vapour blanketing effect and the consequence of lower heat flux than for wider spaced tube pitches. It is recommended that tube spacing or tube pitch be greater than normal heat exchanger design to ensure than free boiling bubbling movement to avoid the problem of vapour blanketing. Tube length (L) 29

30. Thermal Design of the Kettle reboiler Effective tube length (mean tube length) This needs to be estimated using a chart. Check it later in the example Tube passes (n): U-tubes: 2 passes. Tube material: check C&R Shell side: Support plates and tie rods. Baffles not required to enhance heat transfer but for tube support purposes. (C&R) Recommended spacing ranges from 1m for 16 mm tubes to 2 m fro 25 mm tubes. Specify tube material. 30

31. Thermal Design of the Kettle reboiler Overall heat transfer coefficient, (U): Q = UA Δ T m For initial estimate of U use value from C&R Vol 6, from table 12.1 and for fouling factor (dirt factors) Table 12.2 Ignoring tube wall resistance: thin walled tubes; = 1 Use recommended for h i , h i = h steam = 8000 W/m 20 C and h di = 4000 – 10000 W/m 20 C   31

32. Thermal Design of the Kettle reboiler Shell side heart transfer coefficient, h o : Correlation given by Forster and Zuber is nucleate, pool, boiling heat transfer coefficient, p w is saturation pressure corresponding to the wall temperature, T w ; p s is saturation pressure corresponding to the wall temperature, T w ; is the surface tension, is the latent heat of vaporization; T s is saturation temperature of boiling liquid.   32

33. Thermal Design of the Kettle reboiler Assume h o , calculate T w . Then calculate h nb or h o from Forster and Zuber correlation. Compare ( h o )ass and ( h o ) calc : iterates until both values converge when   33

34. Thermal Design of the Kettle reboiler Verifying assumptions: Start design calculations by (i) assume U; (ii) assume standard tube layout. Calculate overall heat transfer coefficient, U o,calc Calculate areas: : selected heat exchanger will perform satisfactorily : select new unit with more tubes, longer tubes, larger tubes or some combination. Safety factor (SF): SF =   34

35. Thermal Design of the Kettle reboiler LAYOUT U-tube bundle diameter = value of shell ID. Shell diameter = 2 x bundle diameter. Liquid level = maximum 60% of shell diameter 35

36. Thermal Design of the Kettle reboiler Freeboard = shell diameter – liquid level Distance from tube bundle to shell = minimum 40% of shell diameter. Freeboard between liquid and shell should be at least 0.25m. In order to properly handle the boiling – bubbling in a kettle unit, there must be sufficient disengaging space. Height if liquid level above tube bundle The liquid boiling surface should not be greater than 2 in. above the top horizontal tube. The low liquid level allows easier vaporizing of liquid and easier for vapour to disengage. 36

37. Thermal Design of the Kettle reboiler Vapour velocity at surface: Recommend vapour velocity at surface should be (0.6 – 1.0 ft/s) Vapour velocity should be low enough to prevent: bubbles from blanketing the tube through the bundle and thereby preventing liquid contact with the tube. Excessive entrainment, if velocity is too high, entrainment increase 37

38. Thermal Design of the Kettle reboiler Heat Flux (Q/A): Maximum or critical heat flux evaluated using modified Zuber equation is: Nt : for tubes Nt will be twice the actual number of U-tubes Palen and Small recommend Note: if actual heat flux is very much less than 70% of critical flux maximum, it Indicates that: boiling is stable nucleate boiling the formation of bubbles is rapid resulting in strong local velocity within the liquid film and thereby increasing heat transfer.   38

39. Thermal Design of the Kettle reboiler Pressure drop Tube side pressure drop m = 0.25 for laminar flow, Re < 2100 m = 0.14 for turbulent flow, Re > 2100 Np = 2 passes for U-tube L = mean tube length (effective tube length) Shell side pressure drop Usually very low, evaluated as for unbaffled shell (use Kern method on C&R vol 6)   39

40. Thermal Design of the Kettle reboiler Discussion: Some point to note: Will your design work or are further calculation required Justify choices/selection made State assumptions made and justify your assumptions Are any parameters out of range or specific Are there any concerns that you have regarding the design What is “good/bad” about your design, any outstanding feature 40

41. Thermal Design of the Kettle reboiler Suggested sub-headings for discussion 1. TEMA Type: State choice. Justify choice, i.e. say why you selected a particular class and type. Include diagrams in Appendix 2. heat duty and Heat flux: State duty and heat flux What is maximum allowable heat flux. State what it means if your actual flux is less than the maximum allowable flux 41

42. Thermal Design of the Kettle reboiler 3. Exchanger area: State: A available , A required , safety factor (SF) percentage What is recommended safety factor, are you within the range. What does it mean to have a SF = x% 4. Heat Transfer Coefficient and Dirt Factors: Tube side, shell side, overall heat transfer coefficient Is it an estimate from Table. If so state Table no. and source Is it a recommended range. State ref What correlation was used? What does the correlation take into account i.e. say why correlation is suitable for your case 42

43. Thermal Design of the Kettle reboiler 5. Tube sizing and configuration: Standard layout: Tube diameter, BWG, Pitch: size arrangement and length. State values selected. Is it a recommended range, if so give reference. Say why you selected the value for example chose 2-in tube diameter. A large tube diameter was selected as fluid is heavily fouling. Larger tubes are easier to clean. However this means a larger shell is required consequently increasing the cost of the exchanger. Length – note reboiler will be horizontally positioned. Is length suitable in terms of space available and removable of bundle for cleaning and repairs. 43

44. Thermal Design of the Kettle reboiler 6. Shell sizing and configuration: Shell diameter, liquid level, freeboard, height of level above tube bundle, vapour velocity at surface. State your value. It is within recommend range. State what it means if it is within specified/recommended range. 7. Pressure drop: Tube side: state value, is it reasonable within recommended range. Shell side: State was not evaluated as, it is negligible – state reference 44

45. Thermal Design of the Kettle reboiler Example: 12.12 Design a vaporiser to vaporise 5000 kg/h n-butane at 5.84 bar. The minimum temperature of the feed (winter conditions) will be . Steam is available at 1.70 bar (10 psig ). Heat loads: Total heat load of butane = sensible heat + Δ H vap (from ) Add 5% for heat losses Max heat load = 0.05(648.9) + 648.9 = 681 kW Both sides isothermal. So use mean temperature difference ΔT m = 115.2 – 56.1 = 59.1 C   45

46. Thermal Design of the Kettle reboiler Note: If feed is sub-cooled, mean temperature difference should still be based on boiling point of liquid, as the feed will rapidly mixture with the boiling pool of liquid. When calculating heat duty, the quantity of heat required to bring the feed to its boiling point must be included. Assume, U = 1000 W/m 20 C From Table,: Tube layout 1-in, on 1(1/4) in, 16 ft U tubes, assume effective length 15.2 ft, 13 BWG   46

47. Thermal Design of the Kettle reboiler Rough estimate to get effective tube length From figure 10-27, R2, effective length is 15.2 ft From Table 10-9, R2, closest is 38 tubes and bundle diameter is 13(1/4) in. Tube side heat transfer coefficient: For steam, C&R recommends 8000 W/m 2 and dirt factor for steam of 5000 W/m 2   47

48. Thermal Design of the Kettle reboiler Shell-side coefficient: : assume h o = 4000 w/m 20 C   48

49. Thermal Design of the Kettle reboiler Foster and Zuber correlation For example let say, = 4500 W/m 20 C Compare with assumed value of 4000 W/m 20 C = 12.5% Further iteration is required Assume: = 4500 W/m 20 C   49

50. Thermal Design of the Kettle reboiler : assume h o = 4500 w/m 20 C let say For example let say, = 5000 W/m 20 C Compare with assumed value of 4500 W/m 20 C = 11.1% ; Further iteration is required   50

51. Thermal Design of the Kettle reboiler Assume: = 4800 W/m 20 C : assume h o = 4800 w/m 20 C let say For example let say, = 4855 W/m 20 C Compare with assumed value of 4800 W/m 20 C = 1.1% therefore stop iteration   51

52. Thermal Design of the Kettle reboiler Overall Heat Transfer Coefficient: Ignoring tube wall resistance: thin walled tubes; = 1 Use recommended for h i , h i = h steam = 8000 W/m 20 C and h di = 4000 – 10000 W/m 20 C Therefore, h i = 8000 W/m 20 C, h di = 5000 W/m 20 C, h o = 4855 W/m 20 C and h do = 10000 W/m 20 C (assume value)   52

53. Thermal Design of the Kettle reboiler = 1584 W/m 20 C Safety factor (SF): SF = = 63% Exchange or reboiler is too big, therefore further iteration is required.   53

54. Thermal Design of the Kettle reboiler 2 nd Iteration Assume U o = 1584 W/m 20 C Select tube layout: 1-in, on 1(1/4) in, 16 ft U tubes, assume effective length 15,4 ft , 13 BWG Rough estimate of length N = 19 tubes L form figure = 15.4 ft   54

55. Thermal Design of the Kettle reboiler From Table 10-9, R2, closest is 22 tubes and bundle diameter is 12 in. Note: h i = 8000 W/m 20 C, h di = 5000 W/m 20 C, h o = 4855 W/m 20 C and h do = 10000 W/m 20 C (assume value) are not changing and not affecting layout = 1584 W/m 20 C   55

56. Thermal Design of the Kettle reboiler Safety factor (SF): SF = = 12% Final layout: stop iteration Check Maximum critical flux: Modified Zuber equation   56

57. Thermal Design of the Kettle reboiler Palen and Small suggest a safety factor of 0.7 or 70% be applied to the maximum heat flux q cb = 0.7(283224) = 196000 W/m 2 = 196 kW/m 2 Actual heat flux = Note: actual heat flux is less than the critical flux maximum Layout: shell diameter, liquid level, freeboard From Table 10-9, bundle diameter = 12 in = 305 mm. Take (ID) shell diameter as twice bundle diameter. The closest standard diameter is 25 in = 635 mm. Liquid level should be 60% of shell diameter   57

58. Thermal Design of the Kettle reboiler Take liquid level as 400 mm from the base Freeboard = 635 – 400 = 235 mm Freeboard should be at least 0.25 m. Maybe reduce liquid level to 370 or chose bigger shell diameter 58

59. Thermal Design of the Kettle reboiler Vapour velocity at surface:   59

60. Thermal Design of the Kettle reboiler For example: width at liquid level = 0.613 m Surface area of liquid = 0.613 x 2.35 = 1.44 m2 Vapour velocity at surface:   60

61. Thermal Design of the Kettle reboiler Maximum allowable vapour velocity: Therefore actual velocity is well below the maximum allowable velocity. A small diameter could be considered. Tube-side pressure drop: ; m depends on Re, and N p = 2 passes It should be less than 70 kPa Shell side pressure drop: It should be very small or negligible: refer to Kern for recommendation   61

62. Aim and Objectives Evaluate the photocatalytic activity of a locally produced TiO 2 pigment on the degradation of bacteria in river water for potable water purposes and hence optimize the process. The specific objectives are as follows: Evaluate the photocatalytic activity of the mTiO 2 on degradation of organics in river water to produce clean water. Optimize process parameters of temperature, starting pH of the solution and mTiO 2 loading on the degradation of bacteria in river water. Investigate the percentage removal of mTiO 2 and bacteria in treated water using a Polyester Woven Microfiltration Membrane (PEWMM). 62