Topics Compression • Chapter 9
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Compression Chapter 9 Based on presentation by Prof. Art Kidnay John Jechura – jjechura@mines.edu Updated: February 9, 2016 Topics • Introduction • Fundamentals Thermodynamics of compression Multistaging Compressor Efficiencies • Drivers • Compressor Types Positive displacement compressors Dynamic Compressors • Capacity and Power Calculations Capacity Power Requirements • Comparison of Reciprocating and Centrifugal Compressors John Jechura – jjechura@mines.edu Updated: February 9, 2016 2 Plant Block Schematic John Jechura – jjechura@mines.edu Updated: February 9, 2016 3 Review of Thermodynamic Principals • 1st Law of Thermodynamics – Energy is conserved (Change in system’s energy) = (Rate of heat added) – (Rate of work performed) System’s energy contributions • Kinetic energy – related to velocity of system • Potential energy – related to positon in a “field” (e.g., gravity) • Internal energy – related to system’s temperature o Internal energy, U, convenient for systems at constant volume o Define Enthalpy, H = U +PV, to describe systems at constant pressure • 2nd Law of Thermodynamics In a cyclic process entropy will either stay the same (reversible process) or will increase • Relationship between work & heat All work can be converted to heat, but… Not all heat can be converted to work John Jechura – jjechura@mines.edu Updated: February 9, 2016 5 Common Paths for Heat and Work • Isothermal – constant temperature, ΔT = 0 • Isobaric – constant pressure, ΔP = 0 • Isochoric – constant volume, ΔV = 0 • Isenthalpic – no heat or heat, constant enthalpy, ΔH = 0 • Adiabatic – no heat transferred, Q = 0 • Isentropic (ideal reversible) – no increase in entropy, ΔS = 0 John Jechura – jjechura@mines.edu Updated: February 9, 2016 6 1st Law for steady state flow • Equation 1.19a (ΔH | ΔU for flowing systems) 'H 'u 2 g 'z Q W 2gc gc • For adiabatic, steady-state, ideal (reversible) flow (using WS as positive value) Ws 'H 'u 2 g 'z 2gc gc P2 ³ VdP P1 'u 2 g 'z 2gc gc P2 Ws | ³ VdP P1 John Jechura – jjechura@mines.edu Updated: February 9, 2016 7 Thermodynamics of Compression • Work depends on path – commonly assume adiabatic or polytropic compression • Calculations done with: PH diagram for ΔH Evaluate integral using equation of state • Simplest EOS is the ideal gas law P2 Ws ³ VdP 'H P1 John Jechura – jjechura@mines.edu Updated: February 9, 2016 8 PH Diagrams Ref: GPSA Data Book, 13th ed. John Jechura – jjechura@mines.edu Updated: February 9, 2016 9 TS Diagram John Jechura – jjechura@mines.edu Updated: February 9, 2016 10 John Jechura – jjechura@mines.edu Updated: February 9, 2016 11 John Jechura – jjechura@mines.edu Updated: February 9, 2016 12 John Jechura – jjechura@mines.edu Updated: February 9, 2016 13 Thermodynamics of Compression • Assume ideal gas: PV = RT • Choices of path for calculating work: Isothermal (ΔT = 0) P2 P2 dP P P1 ³ VdP RT ³ Ws P1 §P · RT ln ¨ 2 ¸ © P1 ¹ • Minimum work required but unrealistic Isentropic (ΔS = 0) • Maximum ideal work but more realistic Polytropic - empirical • Considers some heat loss and gas nonideality John Jechura – jjechura@mines.edu Updated: February 9, 2016 Thermodynamics of Compression Work equations • Isentropic (PVJ = constant) where J = CP/CV Ws J1 / J º RT1 J ª§ P2 · «¨ ¸ 1» M J 1 «© P1 ¹ »¼ ¬ • Polytropic (PVN= constant) where N is empirical constant > J Ws N1 / N º RT1 N ª§ P2 · «¨ ¸ 1» M N 1 «© P1 ¹ »¼ ¬ John Jechura – jjechura@mines.edu Updated: February 9, 2016 15 Thermodynamics of Compression • Calculation of J for gas mixture J ¦x C ¦x C i p ,i i V ,i ¦x C ¦x C R i i p ,i p ,i • Use the ideal gas heat capacities, not the real gas heat capacities • Heat capacities are functions of temperature. Use the average value over the temperature range John Jechura – jjechura@mines.edu Updated: February 9, 2016 16 Example compression calculation Example • Have sales gas (assume pure methane) • Initial conditions: 40oF & 100 psig • Desired outlet pressure: 400 psig Compute work of compression on mass basis • Using PH diagram • Assuming ideal gas and adiabatic compression John Jechura – jjechura@mines.edu Updated: February 9, 2016 17 Example Calculation – PH Diagram H2 = 462 Btu/lb H1 = 370 Btu/lb WS = (H2 – H1) = 92 Btu/lb John Jechura – jjechura@mines.edu Updated: February 9, 2016 18 Example Calculation – Ideal Gas Compression • For methane: J = 1.3 M = 16 T1 = 40°F= 499.67oR P1 = 100 psig = 114.7 psia P2 = 1000 psig = 1014.7 psia R = 1.986 Btu/lb.mol oR Ws J1 / J º JRT1 ª§ P2 · «¨ ¸ 1» M J 1 «© P1 ¹ »¼ ¬ John Jechura – jjechura@mines.edu Updated: February 9, 2016 1.3 1.986 499.67 ª§ 414.7 ·1.31/1.3 1º « » 16 1.3 1 «¬¨© 114.7 ¸¹ »¼ 93 Btu/lb 19 Discharge temperature • For ideal gas compression T2 §P · T1 ¨ 2 ¸ © P1 ¹ J1 / J • For the example problem: T2 414.7 · 499.67 §¨ ¸ © 114.7 ¹ 1.31 /1.3 672.19°R 212.5°F John Jechura – jjechura@mines.edu Updated: February 9, 2016 20 Thermodynamics of Compression • If customer wants 1000 psig… Then pressure ratio of (1015/115) = 8.8 Discharge temperature for this ratio is ~360oF • For reciprocating compressors the GPSA Engineering Data Book recommends Maximum discharge temperature of 250 to 275oF for high pressure systems AND Pressure ratios of 3:1 to 5:1 • To obtain overall high pressure ratio must use multistage compression with interstage cooling John Jechura – jjechura@mines.edu Updated: February 9, 2016 21 Multistaging To minimize work need good interstage cooling and equal pressure ratios in stages. • The number of stages is calculated using 1/ m § P2 · ¨ ¸ © T1 ¹ RP m ln P2 / P1 ln R P • To go from 100 to 1000 psig with a single-stage pressure ratio of 3 takes 2 (1.98) stages & the stage exit temp ~183oF (starting @ 100oF) § 1014.7 · ln ¨ ¸ © 114.7 ¹ m ln 3 T2 ª§ P ·1/ m º T1 «¨ 2 ¸ » «¬© P1 ¹ »¼ ln 8.8 1.98 ln 3 J1 / J ª§ 1014.7 ·1/2 º 499.67 «¨ ¸ » «¬© 114.7 ¹ »¼ 1.31 /1.3 John Jechura – jjechura@mines.edu Updated: February 9, 2016 642.59°R 182.9°F 22 Compressor Efficiencies • Number of definitions for compressor efficiencies • Two major definitions Adiabatic efficiency (also known as the isentropic efficiency): KIS 'H 'S 0 'H actual W'S 0 Wactual Polytropic efficiency: KP J 1 / J N 1 / N • Polytropic efficiency not necessarily 100% at reversible conditions John Jechura – jjechura@mines.edu Updated: February 9, 2016 24 Why use polytropic efficiency? • More closely follows the actual pressure-temperature path during the compression • In general KP > KIS • Can use this to estimate the actual discharge temperature T2 § 1 ·§ J1 · ¨ ¸¨ ¸ J ¹ § P ·© KP ¹© T1 ¨ 2 ¸ © P1 ¹ John Jechura – jjechura@mines.edu Updated: February 9, 2016 25 Compressor efficiency example • Continuation of example given in the beginning of this section… • Actual work required is: Wactual W'S 0 KIS • Assume KIS = 80% then: Wactual 93 Btu/lb 116 Btu/lb 0.80 John Jechura – jjechura@mines.edu Updated: February 9, 2016 26 Compressor efficiency – discharge temperature • GPSA Engineering Data Book suggests the isentropic temperature change should be divided by the isentropic efficiency to get the actual discharge temperature T2,'S 0 §P · T1 ¨ 2 ¸ © P1 ¹ T2,act T1 J1 / J § P2 · ¨P ¸ T1 © 1 ¹ T2,'S 0 T1 J1 / J 1 KIS T2,act John Jechura – jjechura@mines.edu Updated: February 9, 2016 ª§ P · J1/ J º 1» T1 «¨ 2 ¸ «¬© P1 ¹ »¼ ª § P · J1/ J º « ¨ 2¸ 1» P « » T1 «1 © 1 ¹ » KIS « » «¬ »¼ 27 Compressor Drivers • Internal combustion engines Industry mainstay from beginning Emissions constraints Availability is 90 to 95% • Electric motors Good in remote areas Availability is > 99.9% • Gas turbines Availability is > 99% Lower emissions than IC engine • Steam turbines Uncommon in gas plants on compressors Used in combined cycle and Claus units John Jechura – jjechura@mines.edu Updated: February 9, 2016 29 Compressor Types Two basic types: • Positive displacement – compress by changing volume Reciprocating, Rotary screw, Diaphragm, Rotary vane • Dynamic – compress by converting kinetic energy into pressure Centrifugal, Axial John Jechura – jjechura@mines.edu Updated: February 9, 2016 Compressor Pressure and Volume Ranges John Jechura – jjechura@mines.edu Updated: February 9, 2016 31 Reciprocating Compressors • Workhorse of industry since 1920’s • Capable of high volumes and discharge pressures • High efficiency – up to 85% • Performance independent of gas MW • Good for intermittent service John Jechura – jjechura@mines.edu Updated: February 9, 2016 Reciprocating Compressors Drawbacks • Availability ~90 to 95% vs 99+% for others, spare compressor needed in critical service • Pulsed flow • Pressure ratio limited, typically 3:1 to 4:1 • Emissions control can be problem (IC drivers) • Relatively large footprint • Throughput adjusted by variable speed drive, valve unloading or recycle unless electrically driven John Jechura – jjechura@mines.edu Updated: February 9, 2016 Reciprocating Compressors - Principle of Operation Double Acting – Crosshead Single Acting - Trunk Piston Typical applications: Typical Applications: All process services, with any gas and up to the highest pressures and power Small size standard compressors for air and non dangerous gases Courtesy of Nuovo Pignone Spa, Italy John Jechura – jjechura@mines.edu Updated: February 9, 2016 35 Reciprocating Compressors - Compression Cycle 5 4 Pressure 1 p0 3 2 Volume Suction 1 2 3 4 5 Discharge Courtesy of Nuovo Pignone Spa, Italy John Jechura – jjechura@mines.edu Updated: February 9, 2016 36 Reciprocating Compressors - Main Components Pulsation Bottles Crankcase Cast Iron Crankshaft Counterweight Forged Steel for balancing Crosshead Cast Steel Ballast for balancing of inertia forces Slide Body Distance Pieces Pneumatic Valve Unloaders for capacity control Forged Cylinder Main Oil Pump Connecting Rod (die forged steel) Rod Packing Oil Wiper Packing Piston Rod Piston Cast Cylinder Cylinder Valve John Jechura – jjechura@mines.edu Updated: February 9, 2016 Rotary Screw Compressor Left rotor turns clockwise, right rotor counterclockwise Gas becomes trapped in the central cavity “ The Process Technology Handbook” Charles E. Thomas, UHAI Publishing, Berne, NY, 1997. John Jechura – jjechura@mines.edu Updated: February 9, 2016 42 Rotary Screw Compressors Oil-free Oil-injected • First used in steel mills because handles • Higher throughput and discharge pressures “dirty” gases • Max pressure ratio of 8:1 if liquid injected with gas • High availability (> 99%) Leads to low maintenance cost • Volumetric efficiency of ~100% • Has two exit ports Axial, like oil-free Radial, which permits 70 to 90% turndown without significant efficiency decrease • Pressure ratios to 23:1 • Small footprint (~ ¼ of recip) • Tight tolerances can limit quick restarts • Relatively quiet and vibration-free • Requires oil system which filters and cools • Relatively low efficiency 70 – 85% adiabatic efficiencies • Relatively low throughput and discharge pressure John Jechura – jjechura@mines.edu Updated: February 9, 2016 Rotary Screw Compressor Courtesy of Ariel Corp John Jechura – jjechura@mines.edu Updated: February 9, 2016 oil to 140oF • Oil removal from gas • Oil compatibility is critical Widely used in propane refrigeration systems, low pressure systems, e.g., vapor recovery, instrument air Oil-Injected Rotary Screw Compressor Courtesy of Ariel Corp John Jechura – jjechura@mines.edu Updated: February 9, 2016 Two-stage screw compressor Courtesy of MYCOM / Mayekawa Mfg John Jechura – jjechura@mines.edu Updated: February 9, 2016 Dynamic Compressors • Two types Centrifugal • High volumes, high discharge pressures Axial • Very high volumes, low discharge pressures • Use together in gas processing Centrifugal for compressing natural gas Axial for compressing air for gas turbine driving centrifugal compressor John Jechura – jjechura@mines.edu Updated: February 9, 2016 48 Centrifugal compressors • Single stage (diffuser) • Multi-stage Bett,K.E., et al Thermodynamics for Chemical Engineers Page 226 John Jechura – jjechura@mines.edu Updated: February 9, 2016 49 Two-stage compressor John Jechura – jjechura@mines.edu Updated: February 9, 2016 Centrifugal Compressor Courtesy of Nuovo Pignone Spa, Italy John Jechura – jjechura@mines.edu Updated: February 9, 2016 Centrifugal Compressors – Issues Surge • Changes in the suction or outlet pressures can cause backflow; this can become cyclic as the compressor tries to adjust. The resulting pressure oscillations are called SURGE Stonewall • When gas flow reaches sonic velocity flow cannot be increased. John Jechura – jjechura@mines.edu Updated: February 9, 2016 52 Centrifugal Reciprocating Inlet Volume Flow Rate John Jechura – jjechura@mines.edu Updated: February 9, 2016 Stonewall Line Axial e Lin Surg Pressure Head e Surge Gas Turbine – Centrifugal Compressor Axial Compressor Centrifugal Compressor Exhaust Gas Air Fuel Gas Low Pressure Gas High Pressure Gas Combustion Turbine John Jechura – jjechura@mines.edu Updated: February 9, 2016 Industrial Gas Turbines Ref: GPSA Data Book, 13th ed. John Jechura – jjechura@mines.edu Updated: February 9, 2016 59 What is “heat rate”? • Heat rate is the amount of fuel gas needed (expressed heating value) to produce a given amount of power Normally LHV, but you need to make sure of the basis • Essentially the reciprocal of the thermal efficiency Thermal efficiency 2544 Btu LHV Heat rate, hp hr Example: Dresser-Rand VECTRA 30G heat rate is 6816 Btu/hp·hr Thermal efficiency 2544 6816 0.3732 • Includes effects of adiabatic & mechanical efficiencies John Jechura – jjechura@mines.edu Updated: February 9, 2016 Gas Turbine Courtesy of Nuovo Pignone Spa, Italy John Jechura – jjechura@mines.edu Updated: February 9, 2016 60 Gas Turbine Engine Gas Turbine Engine From: F.W.Schmuidt, R.E. Henderson, and C.H. Wolgemuth, “Introduction to Thermal Sciences, second edition” Wiley, 1993 Fuel P2 P1 Compressor Combustion chamber shaft P3 shaft Turbine P4 Atmospheric air Load Combustion products Assumptions To apply basic thermodynamics to the process above, it is necessary to make a number of assumptions, some rather extreme. 1) All gases are ideal, and compression processes are reversible and adiabatic (isentropic) 2) the combustion process is constant pressure, resulting only in a change of temperature 3) negligible potential and kinetic energy changes in overall process 4) Values of Cp are constant John Jechura – jjechura@mines.edu Updated: February 9, 2016 Gas Turbine Engine P2 = P3 Temperature T 3 Patm = P1 = P4 2 4 1 Entropy S wS = -∆h = -CP∆T (9.1 and 1.18) Note the equations apply to both the compressor and the turbine,since thermodynamically the turbine is a compressor running backwards Neglecting the differences in mass flow rates between the compressor and the turbine, the net work is: wnet = wt – wc = CP(T3 – T4) – (T2 -T1) Since (T3 – T4) > (T2 – T1) (see T – S diagram) Since wnet is positive work flows to the load John Jechura – jjechura@mines.edu Updated: February 9, 2016 Air & Hot Gas Paths Gas Turbine has 3 main sections: • A compressor that takes in clean outside air and then compresses it through a series of rotating and stationary compressor blades FRESH AIR EXHAUST COMPRESSION MECHANICAL ENERGY John Jechura – jjechura@mines.edu Updated: February 9, 2016 64 Air & Hot Gas Paths Gas Turbine has 3 main sections: • A combustion section where fuel is added to the pressurized air and ignited. The hot pressurized combustion gas expands and moves at high velocity into the turbine section. FRESH AIR EXHAUST COMBUSTION COMPRESSION MECHANICAL ENERGY John Jechura – jjechura@mines.edu Updated: February 9, 2016 65 Air & Hot Gas Paths Gas Turbine has 3 main sections: • A turbine that converts the energy from the hot/high velocity gas flowing from the combustion chamber into useful rotational power through expansion over a series of turbine rotor blades FRESH AIR EXHAUST COMBUSTION EXPANSION COMPRESSION TURBINE MECHANICAL ENERGY John Jechura – jjechura@mines.edu Updated: February 9, 2016 66 GT - Principle of Operation Theoretical Cycle Fuel Gas Combustion Chamber ~650 - 950°F 1 Air 2 3 ~1800 2300°F Axial Compressor Exhaust Gas (~950°F) Temperature °F 3 Real Cycle 4 4 2 Centrifugal Compressor H.P./L.P. Turbine Simple Cycle Gas Turbine 1 A B Entropy Ideal Cycle Efficiency Kid= 1- (T4-T1)/(T3-T2) = 1 – (P1/P2)(J-1)/ J John Jechura – jjechura@mines.edu Updated: February 9, 2016 Modeling Gas Turbine with Aspen Plus • Basics to tune model Combine heat rate & power output to determine the fuel required Determine the air rate from the exhaust rate Adjust adiabatic efficiencies to match the exhaust temperature Adjust the mechanical efficiencies to match the power output John Jechura – jjechura@mines.edu Updated: February 9, 2016 68 Centrifugal Compressors vs. Reciprocating Compressors Centrifugal Reciprocating Constant head, variable volume Constant volume, variable pressure Ideal for variable flow Ideal for constant flow - MW affects capacity + MW makes no difference ++Availability > 99% - Availability 90 to 95% + Smaller footprint - Larger footprint - ηIS = 70 – 75% + ηIS = 75 – 92% CO & NOx emissions low Catalytic converters needed - Surge control required ++ No surge problems ++Lower CAPEX and maint. ++Fast startup & shutdown (maint cost ~1/4 of recip) John Jechura – jjechura@mines.edu Updated: February 9, 2016 70
