Heuristics‐Temperature, Pressure and Phase Change

After determining the separation and recycle system, the next step in process design is the consideration of all temperature, pressure and phase change operations

// Pressure

To increase the pressure, the most important consideration to choose the proper equipment is the phase state (gas, liquid or solid) of stream.

• Valves and pumps
• Valves, compressors, fans and blowers

There are different types of pumps and compressors that can be used depending on the process conditions such as flow rate, viscosity, etc.

1. Pumps
2. Compressors, Fans and blowers

// Heuristics for pumps from Table 11.9 Turton et al.

• Power for pumping liquids: kW = (1.67)[Flow(m³/min)][ΔP(bar)]/ε [hp = Flow(gpm) ΔP(psi) /1714/ε] ε = Fractional Efficiency = ε_sh(see Table 9.5)
• Net positive suction head (NPSH) of a pump must be in excess of a certain number, depending upon the kind of pumps and the conditions, if damage is to be avoided. NPSH = (pressure at the eye of the impeller ‐ vapor pressure)/ (ρg). Common range is 1.2‐6.1 m of liquid (4‐20 ft).
• Specific speed NS = (rpm)(gpm)0.5/(head in feet)0.75. Pump may be damaged if certain limits on NS are exceeded, and the efficiency is best in some ranges.
• Centrifugal pumps: 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), 70% at 1.89 m³/min (500 gpm), 80% at 37.8 m³/min (10,000 gpm).
• Axial pumps for 0.076‐ 378 m³/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.
• 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 compressors, fans, blowers and vacuum pumps from Table 11.10 Turton et al.

• Fans are used to raise the pressure about 3% {12 in (30 cm) water!, blowers raise to less than 2.75 barg (40 psig) and compressors to higher pressures, although the blower range is
  commonly included in the compressor range.
• Theoretical reversible adiabatic power= mz₁RT₁[({P₂/ P₁}^a‐ 1)]/ a
  where T₁ is inlet temperature, R =Gas Constant, z₁ =compressibility, m =molar flow rate, a= (k‐ 1)/ k and k= C_p/C_v Values of R: = 8.314 J/ mol K = 1.987 Btu/lbmol R = 0.7302 atm ft³/lbmol R
• Outlet temperature for reversible adiabatic process T₂ = T₁ (P₂/P₁)a
• Exit temperatures should not exceed 167‐204°C (350‐400°F); for diatomic gases (C_p/C_v = 1.4). This corresponds to a compression ratio of about 4.
• Compression ratio should be about the same in each stage of a multistage unit, ratio= (P_n/P₁)^1/n, with n stages.
• Efficiencies of reciprocating compressors: 65% at compression ratios of 1.5, 75% at 2.0, and 80‐85% at 3‐6.
• Efficiencies of large centrifugal compressors, 2.83‐47.2 m³/s (6000 100,000 acfm) at suction, are 76‐78%.
• three‐stage ejector needs 100 kg steam/ kg air to maintain a pressure of 1 Torr.
• In‐leakage of air to evacuated equipment depends on the absolute pressure, Torr, and the volume of the equipment, V in m³ (ft³) according to W= kV213 kg/ h (lb/h) with k = 0.98 (0.2) when P > 90 Torr, k = 0.39 (0.08) between 3 and 20 Torr, and k = 0.12 (0.025) at less than 1 Torr.

// Additional heuristics for compressors, fans, blowers and vacuum pumps from Seider et al.:

• Use a compressor or a stage compressor system to obtain pressures greater than 260 kPa or 30 psig.
• The number of stages, N, for multistage compression systems can be found by assuming a maximum compression ratio of 4 for each stage.

• Outlet pressure/Inlet pressure (Number of stages)
  • < 4 (1)
  • 4 to 16 (2)
  • 16 to 64 (3)
  • 64 to 256 (4)

• When compressing a gas, the entering stream must not contain liquid and the exiting stream must be above its dew point so that avoid damaging the
  compressor.
  • To remove any entrained liquid droplets from the entering gas, a vertical knock‐out drum equipped with a demister pad is usually placed just before the compressor.
  • To prevent condensation in the compressor, especially when the entering gas is near its dew point, a heat exchanger should also be added at the compressor inlet to ensure that the exiting gas is well above the dew point.

// Heating/Cooling

• Heating and cooling is achieved by heat exchangers, refrigeration system, etc.
• In a heat exchanger, the rate of heat transferred is determined by:

• Heat is transferred to or from process streams using either:
  • a. Other process streams
  • b. Heat transfer media (eg. cooling water, steam and products of combustion). In this case the exchanger is referred to as utility exchanger.

Heat transfer media

• Called coolant when heat is transferred to them from process streams.
  • Most common coolant: cooling water
  • Inlet water temperature is typically at 90 °F (32 °C)
  • Exit water temperature is usually at 120 °F (49 °C)

• Called heat source when heat transferred from them to process streams.
  • Most common heat source: Steam

// Heat transfer media from Seider et al.

• When process entering the exchanger is at temperatures higher than 250 °F (121 ° C), it is common to transfer at least some of the heat with the treated boiler feed water.
• When process streams must be cooled to below 100 °F (38 ° C) in utility exchangers, refrigerants are used.
• Even though steam is a great heat source to temperatures as high as about 700 °F (372 ° C), but steam pressures become very high. Thus it is common to use other media for temperatures above 450 °F (232 ° C).

Types of heat exchangers

• Different flow patterns:

Most efficient: countercurrent
For countercurrent flow, temperature difference between streams at either end of exchanger referred to as temperature approach.
Optimum minimum approach depends on temperature levels of fluid streams:

• < 10 °F (‐12 °C) for temperatures below ambient.
• 10 °F (‐12 °C) at ambient temperatures.
• 20 °F (‐7 °C) for temperatures above ambient and below 300 °F (145 °C)
• 50 °F (10 °C) for higher temperatures.

// Heuristics for heat exchangers, Table 11.11 Turton et al.

• For conservative estimate set F = 0.9 for shell‐and‐tube exchangers with no phase changes, q = UAFΔT_lm. When
  ΔT at exchanger ends differ greatly then check F, reconfigure if F is less than 0.85.
• 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) dia., accommodates 9.3 m2 (100 ft²);
  60 cm (2 ft) dia., accommodates 37.2 m² (400 ft²),
  90 cm (3 ft) dia., accommodates 102 m² (1100 ft² ).
• Tube side is for corrosive, fouling, scaling, and high pressure fluids.
• Shell side is for viscous and condensing fluids.
• Pressure drops are 0.1 bar (1.5 psi) for boiling and 0.2‐0.62 bar (3‐9 psi) for other services.
• Minimum temperature approach is 10°C (20°F) for fluids and soc (10°F) for refrigerants.
• Cooling water inlet is 30°C (90°F), maximum outlet 45°C (115°F).
• Heat transfer coefficients for estimating purposes, W/m²°C (Btu/h ft² °F): 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/h ft²). 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 (100‐200 ft²).
• 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.
• 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/h ft2 (bare surface) °F). Minimum approach temperature = 22°C (40°F). Fan input power = 1.4‐3.6 kW/MJ/h) [2‐5 hp/(1000 Btu/h)]
• Fired heaters: radiant rate, 37.6 kW/m² (12,000 Btu/h ft2); convection rate, 12.5 kW/m² (4000 Btu/h ft²); cold oil tube velocity= 1.8 m/s (6 ft/s); approximately equal transfer in the two sections; thermal efficiency 70‐90% based on lower heating value; flue gas temperature 140‐195°C (250‐350°F) above feed inlet; stack gastemperature 345‐510°C (650‐950°F).
• Estimated heat exchanger pressure drops:
  • 1.5 psi for boiling and condensing
  • 3 psi for a gas
  • 5 psi for a low‐viscosity liquid
  • 7‐9 psi for a high‐viscosity liquid
  • 20 psi for a process fluid passing through a furnace
• If possible, heat and cool a stream of solid particles by direct contact with a hot gas or cold gas, respectively. Units such as rotary kiln and a fluidized bed can be used for heating.

// Refrigeration Cycle

Common refrigeration system is the compression refrigeration which is used to cool down a process stream by transferring the heat to a refrigerant in an evaporator.

1. Cooled refrigerant (mixture of liquid and vapour) is passes through an
   evaporator, where it vaporizes, and thus cools down the process fluid.
2. Refrigerant (vapour phase) is then compressed and leaves the compressor as a superheated, compressed vapour.
3. Superheated refrigerant (vapour phase) is then sent to a condenser, where it is cooled and condensed at high pressure.
4. This high pressure refrigerant (liquid phase) is then passed through an expander, where it is partially vaporized and cooled.

Refrigeration cycle efficiency
Expressed by the reversible coefficient of performance, COP_REV and the power requirement for the refrigeration cycle is:

// Heuristics for refrigeration and utilities, Table 11.18 Turton et al.

• A ton of refrigeration is the removal of 12,700 kJ/h (12,000 Btu/h) of heat.
• At various temperature levels: ‐ 18 to ‐ W°C (0 to 50°F), chilled brine and glycol solutions; ‐ 45 to ‐40°C (‐50 to ‐ 40°F), ammonia, freon, butane; ‐ 100 to ‐ 45°C (‐150 to ‐50°F) ethane or propane.
• Compression refrigeration with 38°C (100°F) condenser requires kW / tonne (hp/ ton) at various temperature levels; 0.93 (1.24) at ‐ 7°C (20°F); 1.31 (1.75) at ‐ 18°C (0°F); 2.3 (3.1) at ‐ 40°C ( ‐ 40°F); 3.9 (5.2) at ‐62°C ( ‐80°F).
• Below ‐ 62°C ( ‐ 80 °F), cascades of two or three refrigerants are used.
• In single‐stage compression, the compression ratio is limited to 4.
• In multi‐stage compression, economy is improved with interstage flashing and recycling, so‐called economizer
  operation.
• Absorption refrigeration: ammonia to ‐ 34°C ( ‐30°F), lithium bromide to 7°C (45°F) iseconomical when waste
  steam is available at 0.9 barg (12 psig).
• Steam: 1‐2 barg (15‐30 psig), 121‐135°C (250‐275°F); 10 barg (150 psig), 186°C (366°F); 27.6 barg (400 psi g), 231°C (448°F); 41.3 barg (600 psig), 252°C (488°F) or with 55‐85°C (100‐150°F) superheat.
• Cooling water: For design of cooling tower use supply at 27‐32°C (80‐90°F) from cooling tower, return at 45 ‐ 52°C (115‐125°F); return seawater at 43°C (110°F); return tempered water or steam condensate above 52°C (125°F).
• Cooling air supply at 29‐35°C (85‐95°F); temperature approach to process, 22°C (40°F).
• Compressed air 3.1 (45), 10.3 (150), 20.6 (300), or 30.9 barg (450 psi) levels.
• Instrument air at 3.1 barg (45 psig), ‐ 18°C (0°F) dew point.
• Fuels: gas of 37,200 kJ / m³ (1000 Btu/ SCF) at 0.35‐0.69 barg (5‐W psi g), or up to 1.73 barg (25 psig) for some types of burners; liquid at 39.8 GJ/m³ (6 million Btu/bbl).
• Heat transfer fluids: petroleum oils below 315°C (600°F), Dowtherms below 400°C (750°F), fused salts below 600°C (1100°F), direct fire or electricity above 450°F.
• Electricity: 0.75‐74.7 kW. (1‐100 hp), 220‐550 V; 149‐1864 kW (200‐2500 hp), 2300‐4000 V.