E thylene plant capacities in re-cent decades have increasedwell beyond 1.5 MMTPY (mil-lion metric tons per year) and

are now around 2.0 MMTPY. TheCracked Gas Compressor (CGC) isone of the most critical pieces of ro-tating equipment present in modernethylene plants.

The purpose of the CGC is to com-press gases from the cracker for sepa-ration in downstream units within theprocess plant. The compressor handlesprocess gas, which is a complex mix-ture of cracked gases containing sub-stantial quantities of high molecularweight hydrocarbons, such as C4s, C5sand C6s.

Therefore, any reduced capacity orunscheduled downtime of the CGC neg-atively impacts overall production andplant economics. Fouling is one of thetypical causes for reduced capacity or per-formance deterioration in CGC operation.

Typically, a CGC train consists of twoor three bodies of multistage compressorsdriven by steam turbines (STs). Foulingthat occurs in a single multistage crackedgas compressor has a crippling effect onthe overall performance of the train.

Fouling is caused mainly by three dif-ferent mechanisms: free radical polymer-ization, condensation and thermal degra-dation to coke. Polymerization occurs whentwo or more unsaturated monomers withreactive double bonds (or consisting of thesame elements in the same proportions byweight but differing in molecular weight)react to form another compound havinghigher molecular weight and differentphysical properties.

Compounds, such as ethylene (C2H4),Propylene (C3H6), Butene (C4H8) withinthe gas stream may react with heavier mo-lecular weight (i.e., C6, C7, C8 hydrocar-bon compounds) resulting in polymer for-mations. These polymer formations andfouling rates tend to increase exponentiallywith temperature.

As such, the polymer chain grows, andthe molecular weight of the polymer in-creases until it becomes insoluble andclings to the metal surface. With time, thesepolymer deposits reduce to a coke-like sub-stance on internal parts of the compressor(Figure 1).

Fouling and performance Surface roughness has a major impact oncompressor performance. Impeller and dif-fuser performance depends on the presenceof a smooth surface finish. Fouling aggra-vates the degree of surface roughness andaffects the component efficiencies at indi-vidual and collective levels. Additionally,this fouling reduces the fixed volume ofthe internal components, ultimately de-creasing the gas-pass area within the im-peller (Figure 2).In a two-stage impeller compressor theperformance of the first-stage impeller hasa significant effect on the performance ofthe second-stage impeller. Decreased flowarea due to fouling, lowers the efficiencyand discharge pressure of the first impeller(Figure 3).

Therefore, the predicted suction pres-sure of the second impeller is no longer thesame as the actual suction pressure. As aresult, the pressure, temperature and flowto the second stage also change, therebylowering second-stage efficiency.

This also leads to high temperatureswithin the compressor internals. To meetthe predicted discharge pressure, then, thecompressor has to work harder. As a result,the speed and power of the compressor in-creases. This rise in power results in higherthan expected operating expenses

As mentioned earlier, surface roughnesscontributes to compressor performance.The effects of surface roughness within thegas passage due to fouling can be estimatedusing ICAAMC Reynolds Number correc-tion formulas.

If surface roughness is worse than theimpeller design condition, losses can beexpected at the impeller surface resultingin a lower pressure coefficient. Therefore,a shift in the operating point from the de-sign point brings about a change of pre-dicted polytropic head, flow coefficient andimpeller efficiency (Figure 4).

OIL&GAS

2 Turbomachinery International • Month 2018 www.turbomachinerymag.com

COMPRESSOR FOULING CRACKED GAS COMPRESSOR FOULING AND ANTI- FOULING TECHNOLOGIESBY PALLAVI BADDAM

Figure 1. Cracked gas compressor fouling: CGC LP casing after operation (left), and HP casing (right).As a result of fouling, the accumulation of polymers on the internal surfaces adversely affects overallperformance. Changes in feedstock or cracking severity can change or shift the dominant foulingmechanism.

Anti-fouling technologies A certain amount of fouling is, inevitable;however, it can be controlled. Several anti-fouling mechanisms have been used by op-erators. As the fouling mechanism changes,the effectiveness of the mitigation methodmay also shift. It is usual for process licen-sors and end users to dictate the type ofanti-fouling mechanism needed.

Anti-fouling technologies can broadlybe divided between conventional and un-conventional techniques. One example ofan unconventional approach involves theuse of chemical treatments or anti-foulantswithin the process gas.

Its main function is to prevent fouling byinhibiting chemical reactions. These formu-lations contain an inhibitor and antioxidant.The inhibitor reacts with monomers beforethey can form insoluble polymers. The anti-oxidant reduces oxidative polymerization.Researchers are constantly coming up withanti-foulant formulations that can preventpolymerization at its initial phase.

Conventional anti-fouling technologiesused by ethylene producers include:

CGC COMPRESSOR COATINGS Compressor internals are coated to avoidcorrosion and foulant deposition on the sur-faces. Use of coatings has a minimal effi-ciency decrement. They are generally ap-plied to diaphragms, inlet guide vane (IGV)and rotor assemblies (Figure 5). Processlicensors, purchasers and OEMs mutuallyagree upon the compressor componentsthat need coating based on the type of serv-ice and the process gas used.

Mitsubishi Heavy Industries Compres-sor Cooperation (MCO) uses SermaLoncoatings if requested, typically a three-layercoating. The foundation is a tightly adherentlayer of sacrificial aluminum–filled ceramic.

This galvanic coating prevents corro-sion of structural hardware. The interme-diate layer in a SermaLon is an organiccoating containing metalo-compounds,which prevent corrosion by modifying thechemistry of environmental corrodants.The outermost layer is an organic materialcontaining PTFE. It acts as a barrier againstcorrodants in the environment and limitsfouling. This incurs a nominal drop ofcompressor efficiency.

CGC COMPRESSOR WATER INJECTIONEthylene producers typically add water tothe process gas compressor to lower thegas discharge temperature. Water vaporizesin the compressor stage, absorbing someheat of compression and lowering stagedischarge temperatures.

As fouling increases at high dischargetemperatures, water injection is used in ap-plications for more precise temperature con-

www.turbomachinerymag.com Turbomachinery International • Month 2018 3

Flow path area is reducedas if impeller flow coefficientbecame smaller

Foul

Build-up of polymerand hydrocarbon deposits

withfouling

withoutfouling

EfficiencyEfficiency curvewithout fouling

Efficiency curvewith fouling

Flow coefficient

Pressure coefficientcurve without fouling

Pressure coefficientcurve with fouling

Pressurecoefficient

Fig. 2. Volume reduction due to narrow passage within the impeller as a result of fouling. This isequivalent to using an impeller with a smaller flow coefficient. As a result, the efficiency of theimpeller decreases.

Fig. 3. Stage 1 Aerodynamic performance with and without fouling. As can be seen, fouling causesa reduction in the gas passage area which impedes the impeller. As the impeller design wasoptimized for a particular flow coefficient, fouling means a drop in impeller efficiency.

Figure 4. A rough surfaceinside a compressor dueto fouling

bLAckwhIte

4 Turbomachinery International • Month 2018

trol. It can either be continuous or intermittent. Typically, the waterquantity is around 1% of the total process flow. When wash nozzlesare requested, the purchaser or the process licensor should providedischarge temperature limits to calculate the water flow rate.

Experience with this method had demonstrated significant de-crease in temperature (~10°C) due to water injection (Figure 6).

CGC COMPRESSOR WASH OIL INJECTIONTo prevent efficiency lossesdue to fouling during long-term operation, wash oil isinjected at regular intervalsin CGCs (Figure 7). Wash oilinjection nozzles are usuallyinstalled on the suction pip-ing as well as the return bendon each stage.

Wash oil injection en-sures that polymer depositsdo not adhere to internal surfaces. Oil quality is important andshould be free of impurities. Some of the best wash oils havearomatic contents greater than 60 % and boiling points higherthan 300°C. This ensures that the oil remains liquid, allowing itto dissolve and scour polymer from the metal surfaces and min-imize deposition.

OEMs have the responsibility to ensure that the droplet size ismaintained to avoid erosion due to water or oil injection. The lo-cation of injection nozzles should be optimized to improve washefficiency. CFD analyses should be used to determine the optimumoil injection location

The effectiveness of water and oil injection cannot be esti-mated by an OEM alone since the operating history and usagepattern is unknown. To resolve a fouling problem, collaborationis required.

Figure 5. SermaLon coating of internal components

Figure 6. Water injection nozzles are located on the casing in order tobe able to inject the water spray into the compressor stages.

Figure 7. Compressor internals can bekept free of polymer deposits by usingoil injection

Pallavi Baddam is the Proposal Manager for

Mitsubishi Heavy Industries Compressor Inter-

national (MCO). He has a Master’s Degree in

Mechanical Engineering. MCO is currently

working with an operator to evaluate the opti-

mum wash area and oil quantity, as well as

how to mitigate erosion potential. The results

will be presented in a follow-up article. For more information, visit

www.mhicompressor.com/en