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Cen tr trifugal ifugal co m pr pres esss o r operations The w et gas com pr press essor or is used used as an ex amp le in this article article review review ing co m pr press essor or performance, operating conditions and basic control philosophy – an aid to und er ersta standin ndin g the inter interactions actions influen cing co m pr press essor or perfor perform m ance an d con tr trol ol Ton y Barletta Barletta an d Scott Scott W Go lden Proce Pr ocess ss Con sult ing Services Services In c
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he FCC wet gas compressor’s major function is reactor pressure control. The machine must compress gas from the main column overhead receiver to gas plant operating pressure press ure wh il ilee m aintainin g stable regenregenerator-reactor differential pressure (Figure 1). Typically, reactor-regenerator differential diff erential pressure pressure m ust be controlled within a relatively narrow +2.0psi to –2.0psi (+0.14 (+0.14 to –0.14 bar) ran ge to p ermit stable catalyst circulation. The wet gas compressor and its control system play a vital role in maintaining steady reactor operating pressure. To be sure, optimum FCC operation requires balancing regenerator an d reactor pressures pressures to wet gas and air blower constraints. Nonetheless, reactor pressure is presumed constant throughout this articl articlee to simplify discussions. Reactor operating pressure is regulated by the main column overhead receiver pressure and system pressure drop from the reactor to the overhead receiver. The wet gas machine needs to have sufficient capacity to compress receiver recei ver wet gas to th e gas plant o perating pressure. Reactor effluent composition, overhead receiver pressure and temperature, and gasoline endpoint all
Figure 2 Fixed speed compressor and inter-condenser system influence the amount of wet gas and its molecular weight. Variability in main column overhead receiver pressure or unstable system pressure drop produce reactor pressure swings. These can cause catalyst circulation problems and other operability operabili ty concerns. Reactor operating pressure is set by main column overhead receiver receiver pressure pressure and system pressure drop. System pressure drop depends on equipment design and operation, while compressor and control system performan ce set receiver receiver
Figure 1 Regenerator-reactor differential pressure
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pressure. Wet gas compressors operate at fixed or variable speed. speed. Fixed speed com pressors throttle compressor suction while variable variable speed m achines u se steam turbines or variable speed motors to control receiver pressure. If necessary, compressor surge control systems recycle gas to ensure inlet gas flow rate is maintained above the minimum flow (surge point or line). Even when receiver pressure is stable, rapid system pressure drop changes from tray flooding and dumping will cause rapid chan ges in in reactor pressure. pressure. Most motor driven compressors operate at fixed speed using suction throttle valves to vary pressure drop from the main column overhead receiver to the com pressor inlet (Figure (Figure 2). Th Th e pressure controller man ipulates the thrott le valve valve position and pressure drop to maintain constant receiver pressure. Normal system pressure drop variation is slow and predictable. Therefore, receiver pressure can be adjusted to maintain constant reactor pressure. As long as the throttle valve is not fully open, then the compressor has excesss capacity. exces capacity. On ce th e th rottle valve is fully open and spillback valve is close cl osed, d, the m achine can n o longer comcompress wet wet gas flow flow to th e gas plant operating pressure. Generally, reactor temperature or feed rate is reduced to
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permit the compressor throttle valve to regain pressure control so that flaring can be avoided. Variable speed compressors use steam t urbines or variable speed motors to control receiver pressure. Speed is adjusted to change the operating point on the compressor map to meet the system flowhead requirements for stable reactor pressure control. As Figure 3 Compressor schematic with inter-condenser system pressure drop increases, receiver pressure is reduced. Thereinter-condenser systems. A single flowfore, machine speed must be increased polytropic head and flow-polytropic to com press the h igher gas fl flow ow rate and efficiency curve represent overall to m eet higher head requiremen requiremen ts. performance. They have lower efficienOnce the turbine governor is widecy and the gas temperature leaving is open or the variable speed motor is generally near 300°F rather than 200°F operating at maximum speed or amps, with an inter-cooled design. These feed rate or reactor reactor temp erature mu st be machines must compress all wet gas reduced to lower wet gas rate to the from inlet conditions to the gas plant compressor capacity. operating pressure, resulti resulting ng in higher Fixed or variable speed motors and power consumption. turbines must have sufficient power to Stable op era erating ting range comp res resss th e m ass flow flow rate of gas while meeting the differential head between Each wet gas compressor section must the overhead receiver and the gas plant. be operated with in its stable flow flow range. Otherwise, reactor temperature or feed At fixed speed, the compressor curve rate must be reduced to decrease the begins at the surge point and ends at amount of receiver wet gas flow to the stonewall, or choke flow. Surge point is driver limit. an unstable operating point where flow is at m inimu m . At At surge, the com press pressor or Com pres pressor sor Design suffers from flow reversals that cause Wet gas machines use six to eight vibration and damage. At the other end impellers (stages) to compress gas from of the curve is the choke (or stonewall) the main column overhead receiver to point. At the cho ke point, th e inlet flow flow the gas plant operating pressure. Most is very high and the head developed have inter-stage condensing systems very low. Flow through the machine after the first three or four stages (lowapproaches sonic condition, or Mach stage) stag e) that cool the comp res ressed sed gas, gas, con1.0. Polytropic efficiency also drops dense a small portion and separate the rapidly near ston ewall ewall.. gas and liquid phases (Figure 3). InterFor variable speed compressors, there stage receiver gas is then compressed in is a region between the surge and the last three or four stages (high-stage). stonewall lines where there is stable Inter-stage condensers reduce gas temmachine performance (Figure 4). The perature and raise compressor efficiency by 5–7%, but they also consume pressure drop. Separate flow-polytropic head and flowpolytropic efficiency curves are needed to evaluate overall compressor system performance. These curves have inlet gas flow rate on the X-axis and polytropic head developed and polytrop ic efficiency efficiency on th e Y-axis Y-axis.. Consequen tly tly,, overall com press pressor or performance and power consumption depend on each compressor section’s performance curves and th e effects effects of th e intercondenser system. Evaluating overall performance of th ese com com pressors is more complex than a machine without inter-cooling, but fundam entall entally y the same. Figure 4 Variable speed flow-head map Som e compressors do n ot have
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compressor flow-polytropic head can be varied anywhere within this region. Because there is no throttling, all power goes into compression, which minimises power consumption. Stable com press pressor or performance is defined between these two flow-head limitations. Some machine designs can vary flow by 25% or more between surge and stonewall points, while others have on ly 6–8% 6–8% flow variation variation b etween th ese limits. Compressors with with small (narrow) stable flow regions need to have robust surge control systems. The head-flow curve basic slope is relatively flat near the surge point and becomes steeper as inlet flow is increased. The impeller blade angle determines the shape of the curve an d t he comp res ressor sor effici efficiency. ency.
Basic compressor control Reactor yield and condenser operating temperature and pressure change throughout the day. Therefore, the gas rate from the main column receiver is variable. Consequently, the compressor control system system m ust be capable of main taining constan t receiver receiver pressure. Thu Thu s, fixed speed compressors have suction throttle valves and variable speed machines change speed to compensate for gas rate changes. Because the compressor press or inlet gas flow flow rate is no t con stant and may be below the surge point or line, the compressor is typically designed desig ned with a surge cont cont rol system. system. Surge control ensures that inlet flow rate is maintained above minimum (surge point or surge line) at all times. A flow fl ow m eter in the com press pressor or suction or discharge and inlet temperature and pressure press ure are used to calculate th e actual flow fl ow rate (ICFM) (ICFM) into th e low- and h ighstage of comp ress ression. ion. As suction flow (ICFM) (ICFM) decreases toward th e surge line line (or point ), the spillback control valve opens to recycle gas from discharge to suction to raise inlet flow rate. Spillback flow is kept at minimum to reduce power consumption. Compressors with inter-condensers need two independent spillback systems from discharge to the suction of each section. Spillback streams should be routed in front of upstream exchan gers so the heat of compression is removed (Figure 2).
Without in ter ter--con denser The simplest wet gas compressor to evaluate is a fixed speed machine with no inter-condensers. It has a single flow-head
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Figure 5 Flow-head curve for a six-stage compressor curve (and single flow-efficiency curve) with surge point rather than a line. Although molecular weight does affect aff ect developed h ead, typical molecular weight variations in a gas oil cracker do not materially change compressor flowhead performan ce curve. Whereas, resi resid d crackers processing varying amounts and quality of residues may have as much as eight number variations in molecular weight, thus the flow-head performan ce curve is is affected. affected. The The m anufacturer sho sho uld provide curves at maximum and m inimum molecul molecular ar weig weight. ht. Figure 5 is the p erf erforman orman ce curve for a six-stage FCC wet gas compressor that is also discussed in this article’s case study. The compressor develops a fixed polytropic head for a given inlet flow rate and th e curve can can be used to predict compressor performance at different process conditions. The suction throttle valve plays an important role for a fixed speed motor driven compressor. Throttle valve pressure drop controls overhead receiver pressure (Figure 6) so that reactor pressure is stable. Throttle valve position and pressure drop compensate for changes in receiver gas flow rate or receiver pressure set point changes. Because compressor discharge pressure is held constant by the gas plant pressure controller, suction pressure will vary and fol follow low the fl flow-head ow-head curve. When gas rate leaving the overhead receiver recei ver is higher th an fl flow ow at th e surge point, the spillback is closed. Hence, compressor suction pressure will ride up and down the flow-head flow-head curve as long as th e th rottle valve is is generating pressure pressure drop an d n ot fully open. As As compressor inlet flow rate approaches the surge point, the spillback valve opens recycling gas to ensure sufficient inlet flow into th e m achine. When the spillback spillback is is open , spillback spillback flow flow rate determin es the operating point on the curve. Flow rate must always be maintained above the surge point with suction pressure deter-
Figure 6 Receiver 6 Receiver pressure control: compressor suction throttling
mined by the polytropic head generated at the m inimum fl flow ow control point. point. Since the amount of gas leaving the overhead receiver depends on reactor effluent composition and overhead receiver recei ver condition s, the comp ress ressor or suction pressure will vary. As As previously d iscussed, the compressor has unused capacity as long as the suction throttle valvee is not full valv fully y open . When establishing operating conditions to stay within an existing machine’s capacity, or if considering a revamp, determining the compressor suction pressure pressure needed to m eet the proposed operation is critical. Compressor suction pressure is calculated from the comp ress ressor or p erf erforman orman ce curve. Because Because the flow and head terms are affected by suction pressure, estimating this pressure is an iterative process. Centrifugal compressors generate a fixed polytropic head (and not a fixed discharge pressure) at a given inlet flow rate – with suction press pressure, ure, gas molecular weight weight an d gas temp erature all influinfluencing both inlet flow rate and polytropic head. The polytropic head equation is shown in Equation 1 below. where
MW Molecular Molecul ar weig weight ht Zavg Average com pressibil pressibility ity T1 Suction temp erature, °R °R n Com press pression ion coeff coeffici icient ent P1 Suction pressure, psia P2 Discharge pressure, psia Understand ing each variabl variable’ e’ss imp act on in let flow flow rate and polytropic head is important. Molecular weight and suction pressure have a significant influence on performance, while compressor discharge pressure (P2 ) is fi fixed xed an d tem perature effects are small. Gas molecular weight (MW) is primarily controlled by
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reactor effluent effluent comp ositi osition on . As molecular weight decreases the inlet flow rate increases. Because the compressor discharge pressure is fixed, compressor suction pressure must be high enough to generate gener ate the head corresponding corresponding to the inlet flow flow rate into th e comp ress ressor. or. Again, for a fixed speed compressor, as long as the com press pressor or th rottle valve valve is not fully open, it h as unu sed capacity. capacity. Thus, molecular weight changes simply cause throttle valve position and pressure drop to adjust, to m aintain receiver pressure. But once the throttle valve is fully open, the machine is operating at maximum capacity. Inlet gas temperature has little influence on compressor capacity because it is based on absolute temperature. Hence, a 20°F rise in temperature changes the head term by only 3% and the flow term a similar amount. Compressor suction pressure has a large in in fluence on inlet gas flow flow rate. For a fixed mass flow rate, raising suction pressure decreases inlet volume by the absolute pressure ratio. At constant receiver pressure, compressor inlet pressure is determined from the flow-head curve for a fixed speed compressor. Using the compressor curve shown on Figure 5, the suction pressure will be th at needed to satisfy satisfy the inlet flow flow and head term simultaneously. Throttle valve pressure drop will vary to maintain inlet flow flow rate between between 10 400 and 11 100icf 100icfm m , while m eeting the gas plant discharge pressure. As lon g as th e receiver gas flow flow rate is above the surge point, then th e spill spillback back valve will be closed. However, as gas flow approaches surge, the spillback valve open s to main tain flow in a stable region of the curve. Suction pressure is a dependent variable as long as the throttle valve has pressure drop. Once the control valve is wide open, the gas rate m ust be reduced or the suction p res ressure sure increased to reduce the inlet volume into the compressor.
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Variable speed compressors have an operating region between the surge and stonewall lines as previously illustrated in Figure 3. Because variable speed comp resso ressors rs do not incorpor incorporate ate a th rottle valve, valve, main colum n overhead receivreceiver pressure controller varies speed to maintain receiver pressure. As long as the receiver gas flow rate is above the surge line flow rate, there will be no spillback. However, if receiver gas flow is below the surge line, then the spillback willl open to m eet the m inimu m flow. In wil the previous Figure 3, maximum compressor capacity occurs when the compressor is operated at maximum speed. As long as the compressor driver had sufficient energy, then minimum overhead receiver operating pressure will be based on the flow-head developed at 110% of the rated speed for the curve shown in Figure Figure 3.
Capacity: driver pow er Compressor power consumption is a function of the mass flow, polytropic head, polytropic efficiency, and gear losses. Compressor shaft horsepower (SHP) is shown in Equation 2 below: Compressor SHP = [(mH ρ)/ ( n ρ 33000)] 1.02 where Hρ Polytropic Poly tropic h ead m M a ss flo w r a t e o f ga s nρ Polytropic efficiency 1 .0 .0 2 2 % ge ar ar lo lo ss sse s SH P Sh a ft ft h o r se se p ow ow er er. Unlike variable speed compressors, fixed speed compressors have suction throttle valves to control overhead receiver pressure. Satisfying the flowhead curve requires pressure drop across the control valve and throttling always wastes energy. Because variable speed compressors change speed to match the process flow-head requirements, they consume less energy. However, the capital cost of steam turbines or variable speed motors is higher than a fixed speed mot or or..
With inter inter--con denser Figure 3 is a schematic of a six-stage compressor with three-stages (low-stage) in front and three-stages (high-stage) behind the inter-condenser. Gas must still be compressed from the overhead receiver recei ver pressure pressure to th e gas plant o perating pressure. However, th e low-stage disdischarges intermediate pressure gas (65–90psig (65– 90psig)) to an inter-condenser wh ere it is cooled from approximately 200°F to 100–130° 100–13 0°F F, depen ding on wheth er air or cooling water is used. The condenser outlet stream is sent sent to a separator drum where th e gas and liquids (oil (oil and water) are separated . Inter-stage drum gas is fed to the high-stage section of the compressor. Each section has its own flow-
polytropic head and flow-polytropic efficiency effi ciency curves. Because the inter-condenser system condenses a portion of the low-stage gas,, m ass rate and gas m olecul gas olecular ar weight (typically four to five numbers lower than main column overhead receiver gas) is lower into the high-stage. This reduces com com press pressor or power consum ption and allows for more efficient compressor design. Because the low-stage and high-stage sections have separate operating curves, each requires an independent surge control system. One part of the compressor may be operating with spillback spil lback to avoid surge, while while th e oth er may have t he spil spillback lback closed. closed. Fixed speed compressors with an inter-cooler inter-c ooler have a suction th rottle valve valve in th e low-stage low-stage to cont rol main colum n overhead receiver pressure while highstage discharge pressure is controlled by the gas plant sponge absorber or amine contactor pressure controller. Typically, there is no throttle valve in the suction line to the high-stage. Consequently, discharge pressure from the low-stage and suction pressure to the high-stage are depend ent vari variables ables.. Inlet flow flow rate into into the low- and h ig ighhstages must always be above the surge point. Separate spillback systems from the low-stage discharge to inlet of the main column condensers and from the high-stage discharge to the inlet of the inter-condenser maintain flow above surge for for both com pressor sections. sections. Thus, suction and discharge pressure from the first three-stages and suction and discharge pressure from from th e last three-stages three-stages are dependent on each other, inter-condenser system pressure drop, and the amou nt of material conden sed. While principles of compression are the same, overall performance is more difficult diffi cult to evalu ate. Fortun ately, process flow models, such as Simsci’s proprietary ProII or Provision, allow the lowand high-stage flow-polytropic head curves and flow-polytropic efficiency curves, and the inter-condenser system, to be rigorously modelled. Thus, interstage operating pressures can be determined through an iterative process without excessive calculations.
Prior to the revamp, reactor pressure was 20psig with with a com press pressor or inlet p res res-sure of approximately 4-5psig. The 40% higher feed rate and higher gas from undercutting, together, would have increased inlet flow rate by more than 50% if the 4psig suction pressure was maintained. Thus, a new parallel compressor or one new larger compressor would have been required. Because this solution was h igh cost, alternatives were were considered. When evaluating compressor performance, the complete system from the reactor through the compressor outlet needs to be evaluated as a single system. system. Undercuttin g gasoline gasoline in creas creases es wet gas flow rate if no other changes are made because it decreases the amount of liquid product from the main column overhead receiver. Gasoline “sponges” C 3 + hydrocarbons leaving the condenser. Therefore, Therefore, und ercutting reduces sponging at fixed receiver temperature and press pressure. ure. For For examp le, un dercutting 10% of the gasoline raises wet gas flow rate by app roximately 5–7%. As As the percent undercutting increases, so does the amount of wet gas from the overhead receiver. Increasing mass flow rate through the compressor without large changes in the inlet flow rate requires higher suction pressure. System pressure drop from the reactor to the compressor inlet must be reduced. In this case, the reactor p ress ressure ure operated at approximately 20psig and the system pressure drop was 16 Psi, resulting in a 4psig compressor suction suction press pressure. ure. High pressure-drop components included reactor line coke formation, main column overhead system and column internals pressure drop. Table 1 shows each major component and its measured pressure drop. Substantial reduction in system pressure drop was required to lower compressor inlet flow rate to within within 10 400 to 11 200 inlet inlet cubic feet per minute stable operating range. Increasing compressor suction pressure raises raises conden sation in th e overhead receiver, decreases inlet flow rate and increases compressor capacity. In this instance, main column overhead receiver pressure and temperature needed to
Case study
Reactor system pressure drop
Minimising compressor modifications An FCC unit was revamped to increase unit capacity by 40%. In addition, gas plant limitations required undercutting gasoline to produce heavy naphtha from the main fractionator to reduce liquid loading through the gas plant. The existing compressor was a sevenstage machine with no inter-condenser and a 4400 h orsepower motor. A suction thrott le valve valve controlled main frac fractionationator overhead receiver pressure.
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Components React or cyclones React or vap our lin e React or l ine coke M ain colum n Con d enser Pi p ing Fl ow cont ro rol/ me met er erin g Tot al
Table 1
∆P, psi
2 1 4 3 3 2 1 16
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be 12psig and 100°F to m ainCompressor drivers must tain in let flow flow rate within staalso be able to supply the ble operating range. Coking power requirement throughin the reactor line, main colout the stable operating umn pressure drop, piping, range of the compressor. flow fl ow metering, and con denser Power consumption depends pressure drop all had to be on mass flow rate through reduced. Otherwise, a new the machine, polytropic polytropic head parallel compressor would developed and compressor have been needed. polytropic efficiency. FlowOverhead condenser polytropic efficiency curves exchanger surface area, coolare supplied by the manufacing water (CW) flow rate, and turer and allow power conCW tem perature all influence sumption throughout the wet gas flow rate. Each 1°F stable flow range to be easily reduction in receiver tempercalculated. Figure 9 shows a ature lowers wet gas rate by comparison between the approximately 1%. Thus, power requirements of a decreasing temperature by Figure 7 Receiver seven- and six-stage compres7 Receiver operating pressure: before and after revamp 10°F lowers the wet gas rate sor. By eliminating one stage by about 10%. Although temperature higher m ass flow flow rate. Hence, the exis existtof compression, machine horsepower has little effec effectt on gas volum volum e, main coling 4400hp motor would have to be requirements were reduced below the umn overhead receiver temperature has replaced. Replacing a motor or steam existing 4400hp motor. a large impact on condensation. Costturbine can be very costly because it Centrifugal machines are used in effective changes that reduce receiver often requires major utilities system many refinery units to compress gases. temperature should always be considmo difi difications. cations. Althou Althou gh the compress compressor or The principles of compression are the ered to maximise existing compressor OEM or compressor expert should do same irrespective of the process units. capacity or to minim is isee m odifi odifications. cations. comprehensive compressor analyses, the Only the gases compressed and the proIn this instance, raising receiver presrevamp engineer can perform prelimicess variables are different. Understandsure from from 5 to 12psig and lowering lowering tem nary review by making simplifying ing centrifugal compressor flow-head perature to 100°F decreas decreased ed th e amo un t assumptions such as equal head rise per an d flow-effici flow-efficiency ency cu rves is an essen tial of wet gas produced so th at th e gas inlet inlet stage of compression to determine when evaluating process changes or flow rate was within the existing comwhether removing a stage of compreswhen rev revamping. amping. pressor capacity. Raising compressor sion or trimming the impellers is needed. inlet pressure (and lowering temperIn th is instan instan ce, the seven-stage seven-stage comature) decreased inlet flow rate, but it pressor developed about 9500ft of head Tony Barletta is a chemical engineer with Process Consulting Services in Houston, also raised compressor discharge presrise per stage of compression. Therefore, sure. Since centrifugal compressors elim eli m inating a stage would reduce develTexas, USA. His primary responsibilities are develop fixed polytropic head , discharge oped head by about 9500ft. Because conceptual process design (CPD) and pro- cess design for refinery revamps. He holds a pressure (P2 in Equation 1) from the comp ress ressor or discharge pressure pressure n eeded to existing seven stages of compression be approximately 210psig to meet gas BS degree in chemical engineering from the Lehigh Le high Univers Universitity. y. would have been 350psig, which plant operating pressure and to stay E-mail -mail:: t barlet barletta ta@ @revamps revamps.com .com exceeded the m aximum all allowable owable workworkbelow the 250psig pressure relief valve ing pressure (MAWP) (MAWP) of the m ajor (PSV) settings, the required polytropic Scott W Golden is Golden is a chemical engineer with Process Consulting Services. His previous equipment in the gas plant. Hence, head at 12psig suction and 210psig disexperience includes refinery process eng- compressor head had to be reduced. charge pressure was approximately Compressor power requirements also 46 000ft of head. Thu s, de-stagi de-staging ng was a ineering and distillation troubleshooting and design. He has written more than 80 techni- have to be considered. Both the flowpractical solution. Figure 8 shows the head and flow-polytropic efficiency comparison between the flow-head cal papers on revamping, trouble-shooting, curve is needed to evaluate power concurves for the original seven-stage and distillation. He holds a BS degree in chemical engineering from the University of sumption. Power requirement would machine and a de-staged six-stage comhave been been m ore than 5100hp due to the pressor press or generated by the OE OEM. M. Maine. Ma ine. E-ma -mail: il: sgold sgolden@
[email protected]
Figure 8 Flow-head 8 Flow-head curve for seven- and six-stage compressor
Figure 9 F 9 Flow-power requireme requirement nts s for 7- and 6-stage compresso compressor r
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