Rules of Thumb for Design & Development

1. Rules of Thumb forDesign & Development Prof. Dr. Shahid Naveed

2. What are Rules of Thumb ?? Rules of thumb are numerical values and suggestions that are reasonable to assume based on experience. Rules of thumb are application of fundamentals and practical experience. Prof. Dr. Shahid Naveed

3. Rules of thumb: Help us to judge the reasonableness of answers. Allow us to assess the applicability of assumptions. Lead to better understanding of complex systems. Allow rapid order-of-magnitude estimates. Prof. Dr. Shahid Naveed

4. Rules of thumb allow the decisions of: 1. Batch versus continuous process. Set goals. Preliminary scouting of reactor configuration and conditions. 4. Explore mass recycle. Explore separations. 6. Explore energy integration. Prof. Dr. Shahid Naveed

5. Rules of Thumb for Physical & Thermal Conditions Vapor pressure doubles for every 20 oC. The latent heat of vaporization of steam is five times that of most organics. If two liquids are immiscible, the infinite dilution activity coefficient is > 8. 10% salt in water doubles the activity coefficient of a dissolved organic. Freezing temperature may be suppressed 1 oC for every1.5 mol% impurity present. For distillation, the condenser cooling water usage is 15 L/kg of steam to the reboiler. Prof. Dr. Shahid Naveed

6. Physical Property Heuristics Prof. Dr. Shahid Naveed

7. Rules of Thumb for Process Improvement Prof. Dr. Shahid Naveed

8. Continue… Change control: change set points, tighten control variations of key variables. Better inventory control and reduction of fugitive emissions. Identify realistic needs for process units. Optimize the reactor/separation system. Manage the recycle of heat and mass networks; use pinch technology. Substitute reagents, catalysts, solvents, additives. If waste byproducts are formed reversibly, recycle. Prof. Dr. Shahid Naveed

9. Trouble Shooting Statistics First time startup, 75% mechanical electrical failures such as leaks, broken agitators, plugged lines, frozen lines, air leaks in seals. 20% faulty design or poor fabrication, such as unexpected corrosion, overloaded motors, excessive pressure drop, flooded towers 5% faulty or inadequate initial data For ongoing processes 80% fluid dynamical for ambient temperature operations 70% materials failure for high temperature operations Prof. Dr. Shahid Naveed

10. Continue… Averaged Statistics for Frequency of failures: (based on type of equipment) 17% heat exchangers 16% rotating equipment (pumps, compressors, mixers) 14% vessels 12% towers 10% piping 8% tanks 8% reactors 7% furnaces Prof. Dr. Shahid Naveed

11. Continue… Averaged Human Error Statistics: No action taken when some kind of action is desired - - - - - - - 90% Corrective action taken in the opposite direction - - - - - - 5% Corrective action taken on the wrong Variable - - - - - - 5% The most likely operator error is due to in correctly reading / interpreting technical instructions. Prof. Dr. Shahid Naveed

12. Chemical Engineering Equipments… * 1. Piping * 2. Tanks / Vessels / Separators * 3. Heat Exchangers * 4. Distillation Column 5. Reactors 6. Absorbers * 7. Pumps, Compressors 8. Cooling Towers Prof. Dr. Shahid Naveed

13. Continue…. 9 . Boilers 10. Crystallizers 11. Cyclone Separators 12. Filtration Units 13. Vacuum Systems 14. Pneumatic Systems 15. Furnaces and many other unit operations. Only the heuristics for Units with * shall be presented here. Prof. Dr. Shahid Naveed

14. PIPING Prof. Dr. Shahid Naveed

15. Heuristics for Piping A handy relationship for turbulent flow in commercial steel pipes is Where = Frictional Pressure loss, psi/100 equivalent ft of pipe W = Flow rate, lb/hr = Viscosity, cp Valid for N Re 2100-106 = Density, lb/ft3 d = Internal pipe diameter, in. Prof. Dr. Shahid Naveed

16. Line velocities (u) and pressure drop (M): • For liquid pump discharge; u = (5 + 0/3) ft/sec and M = 2.0 psi/100ft • For liquid pump suction; u = (1.3 + 0/6) ft/sec and M = 0.4 psi/100 ft • For steam or gas flow: • u = 200 ft/sec and M = 0.5 psi/100ft • Gas/steam line velocities = 61 m/s (200ft/sec) and pressure drop = 0.1 bar/100m • (0.5 psi/100ft). • Screwed fittings are used only on sizes 3.8 cm (1.5 in) or less, flanges or welding used otherwise. • Flanges and fittings are rated for 10,20,40,103,175 bar (150, 300, 600, 1500 or 2500 psig). Heuristics of Piping Prof. Dr. Shahid Naveed

17. Sizing Steam Piping in New Plants Maximum Allowable Flow and Pressure Drop Prof. Dr. Shahid Naveed

18. Sizing Cooling Water Piping in New Plants Maximum Allowable Flow, Velocity and Pressure Drop Prof. Dr. Shahid Naveed

19. Sizing Piping for Miscellaneous Fluids Prof. Dr. Shahid Naveed

20. Typical Design Vapor Velocities (ft / sec) Prof. Dr. Shahid Naveed

21. Typical Design Velocities for Process System Applications Prof. Dr. Shahid Naveed

22. Net positive suction head (NPSH) of a pump must be in excess of 1.2-6.1 m of liquid (4-20 ft). • Centrifugal pumps volumetric flowrate: • Single stage for 0.057-18.9 m3/min (15-5000 gpm), • 152 m (500 ft) maximum head; multistage for 0.076-41.6 m3/min (20-11,000 gpm), • 1675 m (5500 ft) maximum head. Efficiency 45% at 0.378 m3/min (100 gpm). • Efficiency 70% at 1.89 m3/min (500 gpm), • Efficiency 80% at 37.8 m3/min (10,000 gpm). • 3. Axial pumps for 0.076-378m3/min (20-100,000 gpm), 12 m (40 ft) head, 65-85% efficiency. • Rotary pumps for 0.00378-18.9 m3/min (1-5000 gpm), 15,200 m (50,000 ft head), • 50-80% efficiency. • 5. Reciprocating pumps for 0.0378-37.8 m3 (10-10,000 gpm), 300 km (1,000,000 ft) head max. Efficiency 70% at 7.46 kW (10 hp), 85% at 37.3 kW (50 hp) and 90% at 373 kW (500 hp). Heuristics for Pumps Prof. Dr. Shahid Naveed

23. 1. Outlet temperature for reversible adiabatic process T 2= T1(P2/P1)a 2. Exit temperatures should not exceed 167-204°C (350-400°F); for diatomic gases (CpICv = 1.4) this corresponds to a compression ratio of about 4.   3. Compression ratio should be about the same in each stage of a multistage unit, ratio = (Pn/P1) 1/n, with n stages. 4. Efficiencies of reciprocating compressors: a) 65% at compression ratios of 1.5, b) 75% at 2.0 c) 80-85% at 3-6.   5. Efficiencies of large centrifugal compressors, 2.83-47.2 m3/s (6,000-100,000 acfm) at suction, are 76-78%. Heuristics for Compressors Prof. Dr. Shahid Naveed

24. Heuristics for Thermal Insulation • Up to 345°C (650°F) 85% magnesia is used. • Up to 870-1040°C (1600-1900°F) a mixture of asbestos and diatomaceous earth is used. • Ceramic refractories are used at higher temperature. • Cryogenic equipment -130°C (-200°F) employs insulations with fine pores of trapped air. • Optimal thickness varies with temperature: 1.27 cm (0.5 in) at 95°C (200°F), 2.54 cm (1.0 in) at 200°C (400°F), 3.2 cm (1.25 in) at 315°C (600°F). • Under windy conditions 12.1 km/h (7.5 miles/hr), 10-20% greater thickness of insulation is justified. Prof. Dr. Shahid Naveed

25. Heat Exchangers Prof. Dr. Shahid Naveed

26. Type Designation U-Tube; U-Bundle Only one tube sheet required. Tubes bent in U-shape. Bundle is removable. Significant Feature Applications Best Suited High temperature differentials which might require provision for expansion in fixed tube units. Clean service or easily cleaned conditions on both tube side and shell side. Horizontal or vertical. Bends must be carefully made or mechanical damage and danger of rupture can result. Tube side velocities can cause erosion of inside of bends. Fluid should be free of suspended particles. Limitations Fixed Tube Sheet Both tube sheets fixed to shell Condensers; liquid-liquid; gas-gas; gas-liquid; cooling and heating, horizontal or vertical, re-boiling Temperature difference at extremes of about 200°F. Due to differential expansion Floating Head or Tube Sheet (Removable and no removable bundles) One tube sheet “floats” in shell or with shell, tube bundle may or may not be removable from shell, but back cover can be removed to expose tube ends. High temperature differentials, above about 200°F. extremes; dirty fluids requiring cleaning of inside as well as outside of shell, horizontal or vertical. Internal gaskets offer danger of leaking. Corrosiveness of fluids on shell side floating parts. Usually confined to horizontal units Heuristics for Heat Exchangers Selection Prof. Dr. Shahid Naveed

27. Kettle Tube bundle removable as U-type or floating head. Shell enlarged to allow boiling and vapor disengaging Boiling fluid on shell side, as refrigerant, or process fluid being vaporized. Chilling or cooling of tube side fluid in refrigerant evaporation on shell side. For horizontal installation. Physically large for other applications. Double Pipe Each tube has own shell forming annular space for shell side fluid. Usually use externally finned tube. Relatively small transfer area service, or in banks for larger applications. Especially suited for high pressures in tube above 400 psig. Services suitable for finned tube. Piping-up a large number often requires cost and space. Pipe Coil Pipe coil for submersion in coil-box of water or sprayed with water is simplest type of exchanger. Condensing, or relatively low heat loads on sensible transfer. Transfer coefficient is low, requires relatively large space if heat load is high. Continue… Prof. Dr. Shahid Naveed

28. Open Tube Sections (Water cooled) Tubes require no shell, only end headers, usually long, water sprays over surface, sheds scales on outside tubes by expansion and contraction. Can also be used in water box. Condensing, relatively low heat loads on sensible transfer. Transfer coefficient is low, takes up less space than pipe coil. Open Tube Sections Plain or finned tubes (Air Cooled) No shell required, only end heaters similar to water units. Condensing, high level heat transfer. Transfer coefficient is low, if natural convection circulation, but is improved with forced air flow across tubes. Continue… Prof. Dr. Shahid Naveed

29. Standard tubes are 1.9 cm (3/4 in) OD, on a 2.54 cm (1 in) triangle spacing, 4.9 m (16 ft) long. • A shell 30 cm (1 ft) diameter accommodates 9.3 m2 (100f2); • A shell 60 cm (2 ft) diameter accommodates 37.2 m2 (400f2); • A shell 90 cm (3 ft) diameter accommodates 102 m2 (1,100f2). • 3. Tube side is for corrosive, fouling, scaling, and high-pressure fluids. • 4. Shell side is for viscous and condensing fluids. • 5. Pressure drops are 0.1 bar (1.5 psi) for boiling and 0.2-0.62 bar (3-9 psi) for other services. • 6. Minimum temperature approach are 10°C (20°F) for fluids and 5°C (10°F) for refrigerants. • 7. Cooling water inlet is 30°C (90°F), maximum outlet 45°C (115°F). Heuristics for Heat Exchangers Prof. Dr. Shahid Naveed

30. 8. Heat transfer coefficients for estimating purposes, W /m2°C (Btu/hr ft2 0F): • Water to liquid, 850 (150). • Condensers, 850 (150). • Liquid to liquid, 280 (50). • Liquid to gas, 60 (10). • Gas to gas 30 (5). • Reboiler 1140 (200). • Maximum flux in reboiler 31.5 kW /m2 (10,000 Btu/hr f2). When phase changes occur, use a zoned analysis with appropriate coefficient for each zone. • Double-pipe exchanger is competitive at duties requiring 9.3-18.6 m2 (100-200ft2). • Compact (plate and fin) exchangers have 1150 m2/ m3 (350 ft2 / ft3), and about 4 times the heat transfer per cut of shell-and-tube units. • Plate and frame exchangers are suited to high sanitation services, and are 25-50% cheaper in stainless steel construction than shell-and-tube units Continue… Prof. Dr. Shahid Naveed

31. 13. Air coolers: • Tubes are 0.75-1.0 in. OD. • Total finned surface 15-20 m2 /m2 (ft2 /ft2 bare surface), • U = 450-570 W /m2°C (80-100 Btu/hr ft2 (bare surface) 0F). Minimum approach temperature = 22°C (40°F). Fan input power = 1.4-3.6 kW /(MJ/h) [2-5 hp / (1000 Btu/hr)]. • Fired heaters: • Radiant rate, 37.6 kW /m2 (12,000 Btu/hr ft2). • Convection rate, 12.5 kW /m2 (4,000 Btu/hr ft2 ). • Cold oil tube velocity = 1.8 m/s (6 ft/sec). • Flue gas temperature 140-19SoC (2SD-3S00F) above feed inlet. • Stack gas temperature 345-S10°C (650-950°F). Continue… Prof. Dr. Shahid Naveed

32. Calculation of Tube side Pressure Drop in Shell and Tube Exchangers Prof. Dr. Shahid Naveed

33. Calculation of Tube side Pressure Drop in Air-Cooled Exchangers Prof. Dr. Shahid Naveed

34. For the additional drop for flow through the free area above, below or around the segmental baffles use Where W = Flow in lb/sec NB = Number of baffles in series per shell pass SB = Cross-sectional area for flow around segmental baffles, ft2 Pressure Drop for Baffles Prof. Dr. Shahid Naveed

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37. Approximate Overall Heat Transfer Coefficient, U *

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40. Approximate Overall Heat Transfer Coefficient, U *

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43. Heuristics for Towers

44. Distillation & Absorption • Distillation is usually the most economical method for separating liquids. • For ideal mixtures, relative volatility is the ratio of vapor pressures • Tower operating pressure is most often determined by the temperature of the condensing media, 38-50°C if cooling water is used; or by the maximum allowable re-boiler temperature to avoid chemical ecomposition/ degradation. • Economical reflux ratio is in the range of 1.2-1.5 times the minimum reflux ratio, Rmin. • The economical optimum number of theoretical trays is near twice the minimum value Nmin.

45. Continue… • The minimum number of trays is found with the Fenske-Underwood equation. • Reflux pump are made at least 10% oversize. • The optimum value of the Kremser absorption factor A = (L/mV) is in the range of 1.25 to 2.0. • Reflux drums usually are horizontal, with a liquid hold up of 5 min half full. • For towers about 0.9 m add 1.2 m at the top for vapor disengage-ment and 1.8 m at bottom for liquid level and re-boiler return. • Limit the tower height to about 53 m because of wind load and foundation considerations. An additional criterion is that L/D be less than 30 (20 < L/D < 30 often will require special design)

46. Tray Towers • For reasons of accessibility, tray spacings are made 0.5-0.6 m (20-24 in). • Peak efficiency of trays is at value of the vapor factor in the range of 1.2-1.5 m/s • Pressure drop per tray is of the order of 0.1 psi or 3 in of water. • Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60-90%; for gas absorption and stripping 10-20%. • Sieve trays have holes 0.6-0.7 cm dia., area being 10% of the active cross section. • Valve trays have holes 3.8 cm dia. each provided with a lift-able cap 130-150 caps/m2 (12-14 caps/ft2) of active cross section. • Valve trays are usually cheaper than sieve trays. • Bubble cap trays are used only when a liquid level must be maintained at low turndown ratio • Weir heights are 5 cm (2 in)/ weir lengths are about 75% of tray diameter, liquid rate a maximum of 1.2 m3/min m of weir (8 gpm/in of weir); multi-pass arrangements are used at higher liquid rates.

47. Packed Towers • Replacing trays with packing allows greater throughput and separation in existing tower shells. • For gas rates of 14.2 m3/min (500ft3/min), use 2.5 cm (1 in.) packing; for 56.6 m3/min (2,000 ft3/min) or more use 5cm (2 in) packing. • Ratio of tower diameter/packing diameter should be >15/1. • Because of deformability, plastic packing is limited to 3-4 m (10-15 ft) and metal to 6.0-7.6 m (20-25 ft) unsupported depth. • Liquid distributors are required every 5-10 tower diameters with pall rings and at least every 6.5 m (20 ft) for other types of dumped packing. • Number of liquid distributors should be >32-55/m2 (3-5/ff) in towers greater that 0.9 m (3 ft) diameter and more numerous in smaller columns. • Packed tower should operate near 70% of flooding (evaluated from Sherwood and Lobo correlation)

48. Tanks / Vessels / Separators Prof. Dr. Shahid Naveed

49. Process Units in Common Usages Prof. Dr. Shahid Naveed

50. Continue… Prof. Dr. Shahid Naveed