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Chapter 1

Ethylene Production

NOTE: The following chapter is thefirst of many to be released as partof a Chemical sourcebook. Thesechapters will be released to eDocsas they are completed and whenfully developed, compiled into onesourcebook.

Ethylene is one of the most importantpetrochemical intermediates and is a feedstock formany various products. End products made withethylene include food packaging, film, toys, foodcontainers, bottles, pipes, antifreeze, carpets,insulation, housewares, etc. Chemicals that aremade from ethylene in order to produce these endproducts are polyethylene, ethylene dichloride,ethylene oxide, ethylbenzene, and vinyl acetate,just to name a few.

Global ethylene capacity utilization has remainedabove 90% since 2004 until 2008's economicmeltdown. In 2007, 2 million tonnes per year (tpy)of ethylene capacity was added, according to theOil & Gas Journal. As of January 1, 2009, globalcapacity was 126.7 million tpy. Capacity has beenadded in recent years due to expansions and

debottlenecking at existing plants, as well asgreenfield plants being built in the Middle East andAsia. Due to the change in market conditions andthe economy, there is an over‐supply of ethylenecapacity. Many plants have been taken offline inthis time period, are operating at reduced rates, orare undergoing turnarounds. As the ethylenemarket rebounds, capacity will increase. In fact,based on new capacities announced and plantsthat are under construction, global ethylenecapacity is expected to be at 162 million tpy by2012, ahead of the demand growth.

There are five major licensors of ethylene plants:KBR; Technip; Linde; Shaw, Stone & Webster;and Lummus. While ethylene production differsslightly by licensor, the overall process is fairlysimilar (see Figure 1‐1). There are also somedifferences in the process coming from the type offeedstock being used. Some of these differenceswill be highlighted. This chapter will cover thegeneral steps in ethylene production and willdiscuss the critical valve applications within anethylene plant, what valve challenges thoseapplications present, and the recommendedEmerson solutions.

Figure 1‐1. General ethylene process (naptha fed cracker)

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Figure 1‐2. General ethylene furnace schematic

I. FurnaceThe two primary feedstocks for ethyleneproduction are naphtha and natural gas (ethane,propane, butane, etc.). The first step in theproduction of ethylene is to take the feedstock andcrack it into ethylene and other various products ina furnace. This process is called pyrolysis.Pyrolysis is the thermal cracking of petroleumhydrocarbons with steam, also called steamcracking. The main types of commercial furnacesare the ABB Lummus Global furnace, Millisecondfurnace (KBR), Shaw� furnace (ultraselectivecracking furnace), Technip furnace, and the LindePYROCRACK� furnace. See Figure 1‐2 for ageneral schematic of an ethylene furnace.

The feed hydrocarbon stream is pre‐heated by aheat exchanger, mixed with steam, and thenfurther heated to its incipient cracking temperature(932�F to 1256�F or 500�C to 680�C dependingupon the feedstock). At this point, it enters areactor (typically, a fired tubular reactor) where it isheated to cracking temperatures (1382�F to1607�F or 750�C to 875�C). During this reaction,hydrocarbons in the feed are cracked into smallermolecules, producing ethylene and co‐products.

The cracking reaction is highly endothermic,therefore, high energy rates are needed. Thecracking coils are designed to optimize thetemperature and pressure profiles in order tomaximize the yield of desired or value products.Short residence times in the furnace are alsoimportant as they increase the yields of primary

products such as ethylene and propylene. Longresidence times will favor the secondary reactions.

Table 1‐1. Furnace Reactions

Primary Reactions SecondaryReactions

Feedstock/steam

Ethylene C4 products

Propylene C5 products

Acetylene C6 products

Hydrogen Aromatics

Methane C7 products

Etc. Heavierproducts

Maximum ethylene production requires a highlysaturated feedstock, high coil outlet temperature,low hydrocarbon partial pressure, short residencetime in the radiant coil, and rapid quenching of thecracked gas. Valves in the furnace section play acritical role in maximizing ethylene production andthroughput.

There are three critical control valve applications inthe furnace area: dilution steam ratio control, feedgas control, and fuel gas control. Each will bediscussed in further detail in the subsequent text.

Dilution Steam Ratio ControlThe quantity of steam used (steam ratio) varieswith feedstock, cracking severity, and design ofthe cracking coil. Steam dilution lowers thehydrocarbon partial pressure, thereby enhancingthe olefin yield. Because of this, it is important toobtain the appropriate ratio and maintain proper

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control of that ratio. Steam helps to reduce cokingdeposits by reacting with coke to form carbondioxide (CO2), carbon monoxide (CO), andhydrogen (H2) and also reduces the catalytic effectof the reactor coil's metal walls, which tend topromote coke formation. An improper ratioreduces efficiency of the cracker and can result inthe need for more decoking cycles, thus resultingin less furnace uptime. It is necessary thatdecoking be performed on a regular basis. This istypically done by burning out the coke with amixture of steam and air. Time intervals fordecoking will depend upon several factorsincluding, but not limited to the type of furnace,how the process is operated, feedstock type, andthe types of coils utilized.

Precise control of the steam dilution valve isnecessary to maintain the proper steam ratio,which can greatly affect the efficiency of thefurnace. Due to the process conditions seen by thedilution steam control valve, it requires the use ofgraphite packing. Graphite packing often leads tohigher friction than one would see with the use ofPTFE packing. This added friction contributes tohigh deadband and high variability, thus the loopmay become unstable. With high deadband andvariability, it's difficult to have precise control withinthe valve, which leads to issues controlling theloop. Due to the location of the valve (near thefurnace), high ambient temperatures are possible,thus making the location a variable to considerwhen selecting the actuator and relatedaccessories.

The Fisher� control valve solution for the dilutionsteam ratio control valve is typically a Fishereasy‐e� sliding‐stem valve or a Fisher GXsliding‐stem valve. Because of the frictionconcerns mentioned previously, graphite ULFpacking is recommended. This packing meets theprocess temperature requirements and has muchlower friction than standard graphite packing. Aspring‐and‐diaphragm actuator should also beused as they are proven to provide precise controlin the field as well as in Fisher valve testingfacilities.

The use of a Fisher FIELDVUE� digital valvecontroller with Performance Diagnostics (PD) canbe utilized to monitor control valve assemblyperformance and allow for predictive maintenance.When the performance is degrading, the next timethe furnace is brought down for maintenance(typically decoking), valve maintenance can bescheduled ahead of time to bring the valveassembly back to an optimal performance level.

Feed Gas Control

The feed into an ethylene furnace can be ethane,propane, butane, gas oil, or naphtha. Variation inthe type of feedstock used is related to availability.Plants in the Middle East tend to use natural gasfeedstock because it is plentiful in the region andis a low cost feedstock. Asia has a largeavailability of naphtha and, therefore, is inclined touse it more frequently as a feedstock. Plants canalso be designed to handle different types offeedstocks, allowing them more flexibility tochange based upon availability and cost.

The feed gas control valve controls the flow offeedstock used in the ethylene plant. Tight controlof the valve is critical in the steam dilution valve sothat the proper reaction ratio can be maintainedwithin the furnace. Control of the reaction ratio offeedstock to steam will affect the reactionefficiency and percentage of conversion toethylene. Due to the process conditions, the use ofgraphite packing is required. Graphite packingoften leads to higher friction than with the use ofPTFE packing. This added friction contributes tohigh deadband and high variability, thus the loopmay become unstable in automatic. With the highdeadband and variability, it's difficult to haveprecise control within the valve, which then leadsto issues controlling the loop. Due to the locationof the valve (near the furnace), it can see highambient temperatures. This should be aconsideration when selecting the actuator andrelated accessories. While most of these issuesmimic those of the steam dilution valve, there is anadditional variable to consider: the use of lowemission packing to reduce the emissions of thefeedstock for environmental and safety concerns.

The Fisher control valve solution for the feed gascontrol valve is typically an easy‐e valve or GXvalve. Because of the friction concerns mentionedand the desire to reduce emissions, use of FisherENVIRO‐SEAL� graphite ULF packing isrecommended. A spring‐and‐diaphragm actuatorshould also be used as they are proven to provideprecise control in the field and in Fisher valvetesting facilities. A FIELDVUE digital valvecontroller with PD can be utilized to monitor controlvalve assembly performance and allow forpredictive maintenance. When the performance isdegrading, the next time the furnace is broughtdown for maintenance (typically decoking), valvemaintenance can be scheduled ahead of time tobring the valve assembly back to an optimalperformance level.

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Fuel Gas ControlFuel gas regulates the temperature of the furnaceby controlling the fuel to the burners. Specialtemperature profiles are applied along the crackingcoil to avoid long residence times at lowtemperatures. This is because low temperaturesfavor the oligomerization reactions involved in theformation of secondary products. Oligomerizationis a chemical process that converts monomers to afinite degree of polymerization (i.e. products thatare not desirable in ethylene production). Becausespecial temperature profiles are applied, thetemperature control of the cracker is critical. Thegoal is to maintain the optimum temperature inorder to favor the desired primary reactions andproduce the most ethylene possible.

Due to the nature of the fuel, many plants utilizeemissions control packing to limit the emissions ofthe fuel gas. This is for environmental concerns aswell as general safety concerns. As with the othervalves in the furnace area, due to location, the fuelgas valve may also see high ambienttemperatures. Depending upon on the ambienttemperatures for each particular application,special care may need to be taken in selecting theactuator and accessories.

The Fisher control solution for the fuel gas controlvalve is typically an easy‐e valve or GX valve. Dueto emissions concern, use of ENVIRO‐SEALgraphite ULF packing is recommended. Aspring‐and‐diaphragm actuator should also beused as they are proven to provide precise controlin the field and in Fisher testing facilities. AFIELDVUE digital valve controller with PD can beutilized to monitor control valve assemblyperformance and allow for predictive maintenance.When the performance is degrading, the next timethe furnace is brought down for maintenance(typically decoking), valve maintenance can bescheduled ahead of time to bring the valveassembly back to an optimal performance level.

II. Quench TowerCracked gases leave the furnace at 1382�F to1607�F (750�C to 875�C). The gases must becooled immediately in order to preserve thecurrent composition of the gas and preventundesirable side reactions from taking place.These side reactions are generally the secondaryreactions listed in Table 1. The quench tower can

Figure 1‐3. Quench tower

Used with permission of Qenos Pty Ltd

use either quench oil or quench water. Generally,only quench water is used on natural gas‐basedsystems whereas naphtha plants use quench oiland may use a quench water tower as well.

For situations in which a quench oil tower is beingused for a naphtha fed plant, the quench oil is anextremely erosive fluid. It is usually dirty withentrained carbon particles. In order to have along‐lasting solution, the erosive nature of the fluidmust be taken into account when selecting anappropriate valve.

The Fisher V500 eccentric plug rotary controlvalve is a well‐suited solution for this applicationas it was specifically designed to control erosive,coking, and other hard to handle fluids. It shouldbe operated in the reverse flow position for erosiveservice as this will help move the downstreamturbulence away from the shutoff surface. For thequench oil application, a hardened trim should beapplied, either Alloy 6 or ceramic. Ceramic trim isthe typical solution. Sealed metal bearings areavailable to help prevent particle buildup and valveshaft seizure. The seat ring is reversible and willhelp improve the lifetime of the construction.

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Figure 1‐4. V500 eccentric plugrotary control valve

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III. Cracked Gas CompressorAfter the cracked gas has been cooled in thequench tower, the next step in the process iscracked gas compression. A turbine drivencentrifugal compressor is utilized to perform thiscompression and there are typically four to fivestages, with intermediate cooling. The number ofstages necessary depends primarily upon thecracked gas composition and the temperaturelevel of the cooling medium. All of the throughputof the ethylene plant will pass through a crackedgas compressor, so performance and reliability ofthis unit are especially important. The compressoris also an extremely expensive piece ofequipment, resulting in a large percentage of theoverall capital of the plant.

An antisurge control system is designed to protectthis asset. The system is designed to provide afaster response than adjusting the turbine speed tocontrol the onset of surge. The controller looks atmultiple variables to prevent the onset of surge. Itrequires fast, accurate response in order toprevent surge conditions. The characteristics of asurge condition are fast flow reversal (measured inmilliseconds), excessive compressor vibration,increase in flowing media temperature, noise, andit may cause the compressor to “trip”.Consequences of surge situations are substantialand may include shortened compressor life, loss ofefficiency, reduced compressor output, andmechanical damage to seals, bearings, impellers,etc.

Antisurge control valves present many variouschallenges. The key challenge is ensuring valvereliability. There is an extended period between

Figure 1‐5. Antisurge control system

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maintenance cycles and it is important to ensure areliable control valve assembly solution. Theantisurge valve is the main piece of equipment thatprotects the compressor from damage caused bya surge. When these valves are called upon tomove, they are required to stroke very quickly,typically in the open direction only. For example,valves with travels up to 20 inches (50.8 cm) havebeen required to stroke in as little as 0.75seconds. This can necessitate oversized actuatorconnections and the use of volume booster(s) andquick exhaust valve(s). The improper selection ofthese accessories will result in poor valveperformance and tuning difficulties. During a surgeevent, the pressure drop and flow rateexperienced by the valve can be high, causingexcessive levels of noise. This must be consideredin valve selection, although noise controlthroughout the entire range of valve travel may notbe required. This valve may also be required tothrottle intermittently from 0 to 100% open. Thesecases require the valve to have fast, accuratecontrol for incremental step sizes. Any delays cancause a surge to occur. The antisurge valve mustbe able to pass the highest possible outputcapacity of the compressor. Application of a

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Figure 1‐6. WhisperFlo Trim cage

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multiplying factor to the compressor capacity figureis common and may lead to selection of anoversized valve. Valves with too much capacityoften have controllability issues and can causeunstable operation.

Emerson has developed an optimized antisurgepackage to meet the challenging demands of thisapplication. Some highlights of this package will bediscussed, but for more information, please seethe brochure titled “Fisher Optimized AntisurgeControl Valves”. These valves use a spoked plugversus the traditional balanced plug. With thetraditional plug, when the valve is asked to movevery quickly, there is not enough area in thebalance holes to keep the plug in a balanced state;therefore, creating a differential pressure situationbetween the top and the bottom of the plug. Thisdifferential pressure case can lead to pluginstability. The spoked plug has large balanceareas so that this does not occur. For surge eventswhere noise is a concern, a Fisher valve with aWhisper Trim� III or WhisperFlo� trim isrecommended.

Emerson has the engineering capability tocharacterize these trims in order to meet thespecific application needs and tailor a solutiontowards them. For situations when the valveassembly is called upon to move quickly,mechanical air cushions have been added to theactuator cylinder to provide controlled decelerationto help protect actuator and valve components.

The Fisher optimized antisurge package alsoincludes a FIELDVUE digital valve controller withthe Optimized Digital Valve (ODV) tier. There arefeatures within this tier and ValveLink� softwareto meet the needs of the application. For example,factory expertise is not required to tune the Fisheroptimized antisurge valve. A technician can simplyuse ValveLink software's performance tuner or thestabilize/optimize feature with real time graphics.Configuration and tuning can also be performed

Figure 1‐7. Rich amine letdown system

remotely by operators as process requirementschange. This feature gives plant operators andtechnicians the capability to tune this assembly inthe field. The lead‐lag filter in the ODV tier can beused to improve the response to small amplitudesteps by overdriving the set point. Asymmetricadjustments allow the response to be setindependently in the open and closing directions.Integrated, real time graphics allow adjustments tobe done remotely as well. Also, diagnostics can becollected, viewed, and analyzed using ValveLinksoftware to look at items such as packing friction,air path leakage, actuator spring rate, and benchset. Partial stroke tests can also be performed tocheck the health of the valve and ensure theantisurge valve is going to move when it isrequested to.

IV. Acid Gas RemovalThe acid gas removal system is typically locatedbetween the 3rd and 4th or between the 4th and5th stages of the compressor. In all processconfigurations, acid gas removal must be locatedupstream of the drying unit in order to avoidformation of ice and hydrates in the followingfractionation steps. Acid gases are typicallyscrubbed on a once‐through basis or incombination with a regenerative chemical.Regenerative pre‐scrubbing, before a final sodiumhydroxide treatment, is applied for high sulfurfeedstocks. This will reduce the sodium hydroxideconsumption. Regenerative scrubbing can employalkanolamines. The use of alkanolamines shouldthen require the use of a rich amine letdown valveas seen in Figure 1‐7.

After any free liquids are removed from the gas atan inlet scrubber, the gas passes to the absorber

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section. Here, it rises counter‐currently in closecontact with the descending amine solution.Purified gas leaves from the top of the absorber.Lean amine enters the tower at the top where itflows across trays and downward against the flowof the gas. At the bottom of the absorption tower,the acid gas rich amine leaves through the richamine letdown valve that is actuated by aliquid‐level controller. The rich amine then goes toa flash tank, operating at a reduced pressure,where large portions of the physically absorbedgases are offgassed. From there, the rich aminegoes through various processes to be regeneratedand the cycle starts over again.

The rich amine letdown valve is a demandingapplication because the process has entrained gasin solution. As the fluid passes through the letdownvalve, it takes a pressure drop due to the pressuredifferential between the tower and the flash tank.As this pressure drop takes place in the valve, alarge amount of outgassing occurs. Outgassing iswhen the entrained gas comes out of solution. Asa result of outgassing, the valve has a two phaseflow. One phase is the liquid amine and the otheris CO2 and/or hydrogen sulfide (H2S) that comesout of solution. This two‐phase flow may produceexcessive vibration and may be very erosive dueto high velocity impingement of the liquid phase onthe valve trim.

Outgassing is very similar in effect to flashing andrequires special consideration in the proper choiceof valve, trim style, and materials. Generally, theoverall approach is dependent on the severity ofthe pressure drop experienced. Although somesizing methods predict cavitation, small orificeanti‐cavitation trim should not be used on thisservice for two reasons: first, the vapor cushionsany cavitation bubble implosions and thencavitation damage should not be experienced andsecond, the accelerated gas breakout that, in turn,accelerates the liquid, would rapidly erode themultiple passage trim structure because ofincompressible fluid impingement.

For pressure drops of 300 to 600 psi (20.7 to 41.4bar), use of slotted (Whisper Trim I) or drilled hole(Whisper Trim III) trim styles installed in the flowup direction are recommended. The slotted ordrilled hole cages “break up” the flow, minimizingthe potential energy available to be dissipatedduring the outgassing process. Many relativelysmall sources of energy do not possess thedamage capabilities of fewer large sources. Byflowing the process fluid up, these small sourcesof energy are kept away from other critical trimparts. Standard hardened cage, plug, and seat

Figure 1‐8. Whisper Trim I cage

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Figure 1‐9. DST

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parts are recommended. For pressure drops over600 psi (41.4 bar), use of a slotted Whisper Trim Icage made of solid Alloy 6 is recommended. Ahardened valve plug and seat ring should also beused.

Other options for this condition include the use of aFisher NotchFlo� Dirty Service Trim (DST) valveor DST‐G trim designed for outgassing. For all richamine letdown applications, NACE materials arelikely specified.

V. DryingThe cracked gas is saturated with water beforecompression and after each intercooler stage.Moisture must be removed before fractionation toprevent the formation of hydrates and ice.Temperatures of -148�F (-100�C) would form ice

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Figure 1‐10. DST‐G

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compounds that could block pipes and/or damageequipment. Typically this is accomplished bychilling and by adsorption on molecular sieves.The drying process is similar to that of a two bedpressure swing adsorption (PSA) skid. Olderplants also use absorption by a glycol scrubbingsystem or adsorption on alumina. Drying isarranged before the first fractionation step,typically after the last compression stage. Multipleadsorption beds make continuous water removalpossible. One or more adsorption beds are inoperation while at least one unit is beingregenerated. The inability to dry because ofmolecular sieve issues will shut down the plant.

Generally, line‐sized butterfly valves orquarter‐turn ball valves are used in molecularsieve switching valve applications. Over‐sizing canoccur and high cycle demands will cause wear onthe valves. As a result, galled bearings and sealwear will also occur. In the case of high outputtorque, bed lifting of the adsorption beads canoccur if the valve is controlling poorly or opens tooquickly and “jumps” out of the seat. This candamage the adsorption beads and cause them torub or abrade together and create dust. Thisreduces the drying effectiveness of the adsorptionbeads. The dust or fines from bead wear can getstuck in the bearing area and cause damage. Atthe very worst, it can cause seizing of the valve.The adsorption bead dust or fines can also causeseal wear.

Emerson has experience and success inmolecular sieve applications for the ethanolindustry. Fisher A81 valves with a 316 SSTchrome plated disc and UHMWPE seal technology

Figure 1‐11. Molecular sieve drying system

Figure 1‐12. ENVIRO‐SEAL PTFE packing system

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have been used with good results. The UHMWPEseal allows for tight shutoff. Tight shutoff allows forimproved bed drying. PTFE lined PEEK bearingsshould also be used as these have been shown inmolecular sieve applications to last longer thanwire mesh bearings. ENVIRO‐SEAL packing mayalso be considered to avoid leakage of the crackedgas.

Use of a FIELDVUE digital valve controller canincrease the response to set point at the beginningof the adsorption and regeneration cycle without

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overshoot. It precisely controls the rate of openingto eliminate adsorption bead bed disturbance.

VI. Distillation ColumnsThe fractionation section receives the compressedcracked gas at a pressure of 464 to 551 psi (32‐38bar) for further fractionation into different productsand fractions at specified qualities. This is donethrough a series of distillation columns andhydrogenation reactors. Cryogenic separation isthe predominant method for cracked gasseparation. Although gas separation processes viaadsorption, absorption, or membrane technologyhave made progress in the recent past, they havenot found major applications within the ethyleneindustry. Today, three processing routes havegained commercial importance, with the maincharacteristics being the first separation step andthe position of the acetylene hydrogenation. Theseroutes are demethanizer first with tail‐endhydrogenation, deethanizer first with front‐endhydrogenation, and depropanizer first withfront‐end hydrogenation. The following is a listingof the various distillation columns and theirfunctions:

� Demethanizer: Demethanization of thecracked gas separates methane as an overheadcomponent from C2 and heavier bottomcomponents. Concurrently, hydrogen is removedfrom the cracked gas stream and may be obtainedas a product by purification before or afterdemethanization. Methane is typically used as aplant fuel or sold. C2 and heavier components aresent to the recovery system.

� Deethanizer: Deethanization of cracked gasseparates acetylene, ethylene, and ethane asoverhead components from C3+ bottomcomponents.

� Depropanizer: Depropanization separatespropane and lighter fractions as overheadcomponents from C4+ fractions as bottomcomponents.

� C2 splitter or ethylene fractionation: Ethylenefractionation separates ethylene as a high‐purityoverhead product from ethane, which is combinedwith propane and recycled for cracking.

� C3 splitter or propylene fractionation:Propylene fractionation separates propylene as achemical grade overhead product or more

frequently as polymer grade propylene frompropane. Propane is recycled for cracking.

� Primary fractionator: With liquid pyrolysisfeedstocks (naphtha fed plants), the primaryfractionation column is the first step in the crackedgas processing route. Cracked gas enters thecolumn and it is contacted with circulating oil and,at the top of the column, with a heavy pyrolysisgasoline fraction obtained from the subsequentwater quench tower. Cracked gas leaves the top ofthe primary fractionator free of oil but stillcontaining all the dilution steam. Hot oil, whichfunctions as a heat carrier, is collected at thebottom of the column. After cooling, it isrecirculated as reflux to the middle section of theprimary fractionator and to the quench nozzlesdownstream of the transfer line heat exchangers.

Distillation columns occur in all types of chemicalplants. The objective is to separate a feed streaminto light‐component and heavy‐componentproduct streams. It relies on the relative volatilitybetween the components that make up the feedstream. The high volatility (lighter) componentsboil at a lower temperature than the low volatility(heavier) components. Therefore, when heat isadded to the column through a bottom reboiler, thelighter materials are vaporized and rise to the topof the column. The overhead vapors are cooleduntil they condense and become a liquid again.

The efficiency of distillation depends on theamount of contact between the vapor rising andthe liquid falling down the column. Therefore,some of the overhead liquid product is sent back(refluxed) to the top of the column. Increasing thereflux will improve the purity of the overheadproduct. However, it also requires more heat fromthe reboiler to re‐vaporize the lighter componentsin the reflux stream. Some distillation columns canoperate with a side reboiler as well, such as thedemethanizer. The operation of a distillationcolumn is a balancing act between product purityand energy usage. If the amount of vapor andliquid traveling through the column becomes toogreat, the column can “flood.” Too much reflux, toomuch reboil heat resulting in too much vapor, orboth can causing flooding. When flooding occurs,the efficiency of the distillation column isdramatically reduced with corresponding drops inproduct purities.

Figure 1‐13 shows the general schematic of adistillation column. The valves associated with thisare the feed, reflux, bottom product, overheadproduct, pressure control, and reboil valves.

Feed valves are usually used as flow or levelcontrol loops. An upstream unit or process often

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Figure 1‐13. Schematic of a basic distillation column

controls the feed valve. Unstable feed flow willmake the distillation column difficult to control. Aproblem valve will often cause the feed flow tooscillate. As a result, the column will alternatebetween too little and too much reboil heat.Depending upon the size and number of trays inthe column, the effect of a swing in the feed willtake anywhere from several minutes to more thanan hour to reach the ends of the column.Sometimes, the reboil and reflux control willamplify the swings. As a result, meeting productpurity targets becomes more difficult. Operationspersonnel will normally respond by over‐purifyingthe products, wasting energy to compensate forthe problematic feed control valve.

The reflux valve is typically either a flow or columntemperature control loop. It is used to adjust thepurity of the overhead product. The higher thereflux rate, the more pure the overhead productwill become. However, raising the reflux raterequires more reboil heat and will eventually floodthe tower. A poorly operating reflux valve has thesame effects as a bad feed valve. Product puritieswill oscillate and the column will be difficult tocontrol. This ultimately affects the efficiency of thecolumn.

The bottom product valve is used to control thelevel in the bottom of the column. It can cause thelevel to change quickly and dramatically. Issueswith this valve reduce efficiency or at worst, cause

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flooding in the column. The overhead productvalve is used to control the level in the overheadreceiver. It can also cause the level to changequickly and dramatically in the column. Problemswith this application can reduce efficiency.

Pressure control valves provide the back pressureto the column. This is very important for controllingthe stability of the tower. Stable pressure isrequired to ensure that temperature changesreflect composition changes and not pressurechanges. With too small of a valve, the pressureresponse will be very sluggish. Too large of avalve and small valve movement will cause a largepressure swing. Oscillating column pressure anddifficult control result with either over‐ orunder‐sizing the valve.

The reboil valve controls the amount of heat putinto the column by the reboiler. In many cases,steam is used as a heat source. The service isvery clean and fugitive emissions are not aconcern. Steam valves are usually very reliable;however, a problematic valve will make the columndifficult to control precisely. This is especially trueif the column feed is subject to frequent changes.Not all reboilers use steam though. Highertemperature process streams can also be used toprovide heat for lower temperature processes suchas using cracked gas streams.

Whichever distillation column application is beingdiscussed, the control valve solution needs toprovide accurate and reliable control. The valvescan affect column efficiency, stability, columnenergy usage, flooding, etc. An easy‐e valve orGX valve is recommended and are typically CF8Mconstructions. The demethanizer and deethanizermay require the use of cryogenic valveconstructions. A FIELDVUE digital valve controllershould be utilized to achieve tight control.Diagnostics are key in these applications forpreventative maintenance since distillationcolumns have long periods between shutdowns,and loss of a distillation column will shut down theentire ethylene plant.

VII. Propylene and EthyleneRefrigerationRefrigeration in ethylene plants is important andcostly. Refrigeration optimization is vital in plantdesign. Typically, two different refrigerationsystems are employed. The propylene and

ethylene refrigeration compression trains are theother two compressors in addition to the crackedgas. These compressors also have antisurgesystems and, therefore, antisurge valves. Theantisurge valve challenges and solutions are thesame as highlighted in Section III: Cracked GasCompressor, though it is important to note thatvalve sizes may differ for each compressor.

VIII. Hydrogen PurificationHydrogen is produced in the cryogenic section ofthe plant. The purity is typically 80‐95% volume.However, it contains approximately 1000 parts permillion (ppm) of CO. It needs to be purifiedbecause CO is a poison for hydrogenationprocesses. One of the hydrogenation processes isthe acetylene conversion to ethylene. Typically theethylene specification requires less than 1 ppm ofacetylene. The other hydrogenation process isMAPD (methylacetylene and propadiene)conversion to propylene and propane. This is donefor economic reasons and to remove thesecomponents from the propylene product. The mostcommon process for hydrogen purification is PSA.

The control valves used in PSA applications are ina very high cycle service, seeing 100,000 to250,000 cycles per year. The valves and actuatorsare expected to stroke up to once every threeminutes. The stroking speeds are required to befast and controlled. Uncontrolled opening cancause pressure/flow spikes. Depending upon thetype and size of a PSA skid, the amount and typeof control valves will vary. PSA skids can useglobe and/or rotary valves. Control valve shutoff isa major concern because it affects PSA unitefficiency. Bi‐directional flow conditions will alsoexist. Also, if valve leakage causes contaminationfrom one PSA bed to another, industrial gas puritycan be compromised.

A GX valve is the recommended globe valvesolution for PSA service. It can handle strokingspeeds up to 1.5 ‐ 2 seconds. Class VI shutoff isnecessary and bi‐directional shutoff is standard inthis construction. Certified emission controlpacking is standard to reduce potential hydrogenleakage. The recommended high performancebutterfly valve construction requires the use of apressure assisted UHMWPE seal. PTFE linedPEEK bearings that can withstand long cycle liferequirements are used. Live loaded PTFEENVIRO‐SEAL packing is available to reducehydrogen leakage.

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Figure 1‐14. PSA basic flow scheme

These constructions have undergone extensivetesting within the flow lab in Marshalltown, Iowa,United States to ensure that they meet thedemands of PSA service. PSA testing is used toverify the life cycle of Fisher digital valves. PSAtesting was designed and set up with input frommajor PSA licensors to represent the PSA processas accurately as possible. Both the GX andbutterfly valve solutions have undergone rigoroustesting to prove they can meet the high cycledemands of this service.

IX. Power/Steam TurbineAs with most process plants, there is a power sideto the ethylene plant. The valve applications in thisarea are very similar to those seen in traditionalpower plants. The conditions may not be assevere but the applications and recommendedsolutions are very similar. Heat from the crackedgas is recovered through a Heat Recovery SteamGenerator (HRSG) to generate steam. Steam isused to run turbines and for other processes withinthe plant, i.e. steam to the pyrolysis furnace. TheHRSG system has a feedwater, condensate,desuperheating, and blowdown system. Thecritical valve applications are boiler feedwaterrecirculation, boiler feedwater start‐up andregulator, continuous blowdown valves, andcondensate recirculation.

The boiler feedwater system begins at thedeaerator and ends at the inlet to the economizer.The main components are the deaerator, theboiler feed pump, and the high pressure feedwater

heaters. The main purpose of the boiler feedwatersystem is to condition the feedwater for entry intothe boiler. The deaerator removes unwantedoxygen from the feedwater, which in turn preventscorrosion in the entire piping system. The boilerfeed pump raises the pressure and the highpressure feedwater heaters raise the temperatureof the feedwater. The critical valves within theboiler feedwater system are the boiler feedwaterrecirculation, feedwater startup, and feedwaterregulator valves.

In order to protect the feed pump, there must be arecirculation system. The boiler feed pumprecirculation valve takes feedwater from the boilerfeed pump and recirculates it to the deaerator. It isthere to protect the pump from cavitation andexcess temperature rise. There are three basicmethods of providing feed pump recirculation. Twoolder methods are continuous recirculation andon/off recirculation. The current method ismodulating recirculation. This provides minimumrecirculation flow to protect the pump and optimizeefficiency. It requires a high technologyrecirculation valve.

The recirculation valve typically experiencescavitation and if not properly taken into accountwith valve selection, cavitation damage will result.Because of the cavitation, tight shutoff is required.Any fluids leaking past the valve will cavitate andcause damage to the seat. A leaking recirculationvalve can cause decreased unit capacity, repeatedmaintenance, and repeated trim replacement.Plugging can occur if feedwater is not clean. Acommon issue with all feedwater applications iscorrosion due to materials chosen. Amine orhydrazine treated feedwater is corrosive to Alloy 6.

1-13

Figure 1‐15. Boiler feedwater recirculation valve

Figure 1‐16. Cavitrol IV trim

W5662

If the feedwater is treated, use of this materialshould be avoided.

It is important to select a valve that can combatdamaging cavitation. Typical recommendations forthis application include an easy‐e sliding‐stemcontrol valve or a Design EH or HP globe valvewith Cavitrol� III trim. For extremely high pressuredrops, Cavitrol IV trim can be used.

In the case of unclean feedwater with particulates,a Fisher NotchFlo DST valve is an optimal choice

Figure 1‐17. NotchFlo DST valve

W8433

because it has the ability to minimize cavitationand allows for the passage of particles to preventclogging. In the case of treated feedwater, 440C isthe recommended material.

The feedwater startup and feedwater regulatorapplications will be discussed separately.However, it is not uncommon to see these twoapplications combined into one valve. Thefeedwater startup valve is used to initially fill theboiler. Depending upon the design, this can bethrough the main feedwater pumps or thecondensate pumps. The valve transitionsoperation to the feedwater regulator valve, orvariable speed drive, once drum pressure hasbeen built up.

1-14

During drum fill operation, the boiler is underminimal pressure. This causes the entire pressuredrop to be taken across the feedwater startupvalve. Because of this, the formation of cavitationbecomes a concern. Sizing of the startup valvemust be done in combination with the feedwaterregulator valve. This is to ensure that thefeedwater regulator valve does not experience anyservice conditions that lead to damagingcavitation. The most common split is that 80%capacity in the startup valve is equal to 20%capacity in the regulator valve. Once the transitionto the regulator valve has begun, the startup valvecloses. Improper use is one of the main issuessurrounding two valve feedwater systems. Forexample, the startup valve is not being used at alland the regulator valve is being used to performboth functions. This can be a major problem if theboiler feedwater regulator was not sized orselected to perform both functions. There can alsobe an issue if switching between the startup andthe regulator valve is happening too quickly.

Because of the cavitation concerns and taking thefull pressure drop, the startup valve should utilizesome form of anti‐cavitation trim. Typically, inprocess plants, since the pressures are not ashigh as power plants, Cavitrol III trim is selected.440C trim is recommended for the case of treatedfeedwater. For cases where one valve isperforming the startup and regulator duties,characterized Cavitrol trim can be designed tohandle the cavitating conditions at startup andthen standard equal percentage or linearcharacteristic for steady‐state conditions tomaximize capacity. Another common issue in boththe startup and regulator valves is to see themoperated below the minimum operating point. Thiscan cause “gear‐toothing” damage on the plug.Damage can be limited by utilizing a lower metalpiston ring or matched plug/cage combination.This limits the amount of clearance flow betweenthe plug and cage thus minimizing erosion effects.Another solution is to use the protected inside seattechnology with Cavitrol III trim. This technology isdesigned so that the shutoff surface is notexposed to potential erosion. Protecting theshutoff surface will extend the sealing life of thetrim. Using the low travel cutoff feature of theFIELDVUE digital valve controller is ideal. Theinstrument can be setup so that the valves do notthrottle below a minimum point.

The continuous blowdown application is constantlyremoving concentrated water from the drums andremoves a significant level of suspended solids.Typically, this application is flashing. Flashing is asystem phenomenon and, therefore, cannot be

Figure 1‐18. Gear‐toothing

W9693

prevented. The best way to handle it is to minimizethe amount of damage being caused by flashing.Use of an angle valve with a downstream liner isrecommended to minimize the amount of damagecaused by flashing. It is much more economical toreplace a liner than to replace an entire valve.

The condensate recirculation valve is similar to thefeed pump recirculation valve in that it alsoprotects the pump from cavitation. Inlet pressureand temperature differ from the feedwater system.The dissimilarities from the feedwater systeminclude the inlet pressure and temperature. Inletsizing often indicates that flashing is occurring.The end user needs to ensure that there is not asparger or diffuser downstream emitting backpressure on the valve. This will cause cavitationrather than flashing. Cavitation can also causenoise and vibration. Tight shutoff is needed on thisapplication because it prevents loss of condenservacuum, loss of condensate pressure and flow tothe deaerator, and saves money in terms ofwasted pump horsepower. Valve selection istypically an EWT valve with Cavitrol III trim. Flow isusually 25‐35% of the condensate pump's fullcapacity. Class V shutoff is highly recommendedto minimize leakage past the seat because it cancause damage.

X. Flare SystemIn an ethylene plant, vent to flare systems are onseveral of the unit operations such as the quenchtower, distillation columns, steam systems, etc.Vent valves are used to depressurize the unit forsafe shutdown and, possibly, for startup as well.Due to the high pressure drop and high mass flow,they are severe service applications. They are alsocritical reliability applications as part of the safetyshutdown systems. These valves are closedexcept in flare scenarios. Plant personnel need toensure that the valves move when the processrequires flaring.

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The first challenge for vent valves is to haveoptimum sizing and selection of the valve with asilencer/diffuser. It is a balancing act when sizingthis system as a result of optimizing the noiseattenuation by adjusting how much pressure dropeach component is taking. Because they directlyaffect each other, they should be considered anengineered solution and sized together as asystem. Tight shutoff is a major concern as anyleakage causes a loss in plant efficiency. Class Vseat load is recommended as it will minimizeleakage past the vent valve while in the closedposition. This is usually a high noise applicationdue to the pressure drop and high mass flow.Operation may be intermittent and for a shortduration so high noise may be tolerable.Awareness and concern for structurally intolerablenoise levels are necessary. Noise requirementsare likely to be driven by plant and regulatorynoise requirements.

Globe or angle valves with a Whisper Trim III orWhisperFlo trim for noise attenuation, are thetypical recommendation for this application. Thereare some exceptions to this. Vent valves on thequench tower may be able to use butterfly valveswith diffusers due to the low pressures in thisparticular application. Also, cryogenic valves maybe needed on some of the distillation column vent

valves. Because this is a valve that normally sitsclosed but needs to move when called upon, theFIELDVUE DVC6000 SIS (Safety InstrumentedSystems) is an optimized solution. As plants arepaying more attention to their safety loops andperforming safety evaluations, these are beingtagged as SIS applications because they are keyto ensuring that the process can flare in the eventthey are needed (shutdown, startup, andemergency event). Partial stroke testing can beperformed using the DVC6000 SIS. This can bedone without interrupting normal operation andrequires the valve to move from 1 to 30% of itstotal travel.

XI. ConclusionEthylene plants use hundreds of control valvesthroughout the entire production process andunderstanding the various applications is essentialin order to apply an engineered solution towardsthem. With the selection of an appropriate Fishercontrol valve solution, plant performance willimprove as a result of enhanced reliability,variability, safety, etc.

www.Fisher.com

Chapter 2

Polysilicon Production

I. Overview:

Polysilicon, or polycrystalline silicon, is a keycomponent used in the production of manyelectronic devices used today includingcomputers, cell phones, game consoles, PDAs,mp3 players, car and home electronics, digitalcameras, medical devices, etc. Polysilicon isformed to make ingots, wafers, fabricated wafers,and integrated circuits. It is then manufactured intonecessary parts. Polysilicon is also a keycomponent in the production of solar cells for solarpower. The solar energy market is growing asproducers come closer to reaching the level of gridparity. Grid parity is the cost at which photovoltaicsare competitive against other forms of electricitygeneration. Once grid parity is reached, demandfor photovoltaic products will increase significantly.

There is not just one type or grade of silicon orpolysilicon. A supplier may classify the polysiliconproduced into fifty or more different grades withdifferent specifications unique to the user and theapplication. However, this chapter will only refer tothree main types:

� Metallurgical grade silicon (MG‐Si) is 95‐99%pure and it is produced from quartz crystals.

� Solar grade polysilicon is approximately 6Nto 7N pure.

� Electronic or semiconductor grade polysiliconis the purest form at grade 9N to 11N.

In referring to purity levels of polysilicon, it iscommon to use the number of nines past thedecimal point, i.e. 6N pure is actually 99.999999%.

Figure 2‐1. Silicon WafersW9770

Polysilicon has historically been a cyclical market.There are times when capacity addition cannotmeet rising demand. At other times, oversupply isa concern. Market fluctuations do not greatly affectestablished industry and larger downstreamcustomers as they generally work on long termcontracts, typically five to eight years. Majorproducers of polysilicon include REC Silicon,Hemlock Semiconductor, Wacker‐Chemie, andTokuyama. The market was in a period of extremedemand in 2006‐2007; however, analysts arecurrently predicting an oversupply beginningsometime in 2010. Research by Photovoltaic‐Techin March of 2009 shows there has been an over50% increase in polysilicon capacity from majorproducers through 2012, compared to just ninemonths earlier. A conservative model of the futurepolysilicon market, taking an adjustment for gridparity, was presented by Hemlock SemiconductorVP Gary Homan in Photovoltaics International. Itshows a growth of polysilicon in the 3.3x rangefrom 2008 to 2014, or an increase from 64,000 to209,000 metric tons (MT) in thousands. The rapidrate of market growth means that periods ofoversupply are short‐lived (typically one to three

2-2

Figure 2‐2. Solar PanelsW9771

years); however, various complex factors influencethis trend.

Government incentives have a major effect onmarket needs as interest in using solar power asan energy source continues to grow. Theeconomic viability of solar power has beencritically dependent upon a state subsidy of someform. This mainly occurred in Japan and Germanybut was used in Spain in 2008 as well. It alsodepends upon the grid prices of conventionalelectricity in the region. The American Recoveryand Investment Act includes over US$6 billion inloan guarantees for renewable energy projects,solar in particular. Industry representatives have

estimated the bill will create a total of 119,000 jobsbetween 2010 and 2011. Analysts also anticipategrid parity to exist in a number of large marketsbetween 2012 and 2015 at which pointgovernment incentives are no longer necessary tosupport additional capacity installation.

Two major production processes are used to makepolysilicon:

� Siemens process

� Fluidized bed reactor process.

This chapter will discuss each of them separately.Critical valve applications and specialconsiderations for valves used for polysiliconproduction will also be discussed.

II. Siemens Process:The Siemens process, or a modified Siemensprocess, accounts for more than 90% of the globalinstall base for polysilicon production. This processcan be used to produce solar or semiconductorgrade polysilicon. The general process is nolonger protected but several technology suppliersdo license their own specific variants to theprocess. There are three main steps to thisprocess:

1. Preparation and purification of trichlorosilane(or occasionally silane)

2. Chemical vapor deposition (CVD)

3. Recovery

2-3

Figure 2‐3. Generic Polysilicon Production Process

The general chemical reactions for the main steps of the Siemens process are shown below:

TCS � �production

�Si� 3HCl � HSiCl3 �H2

�HSiCl3 �HCl � SiCl4 �H2

Polysilicon� �production�(CVD)

�HSiCl3 �H2 � Si� 3HCl

�HSiCl3 �HCl � SiCl4 �H2

STC� �conversion

�SiCl4 �H2 � HSiCl3 �HCl

Nomenclature :Trichlorosilane(TCS) :

SiliconTetrachloride(STC) :

Silicon :

��

��HSiCl3��SiCl4��Si

Figure 2‐4. Chemistry of the Polysilicon Process

Preparation and Purification ofTrichlorosilaneIn step one; silicon is converted to trichlorosilane(SiHCl3), silicon tetrachloride (SiCl4),

dichlorosilane (SiH2Cl2), or silane (SiH4). Thoughchlorosilanes are more commonly used thansilane, there is a major advantage to using silaneas its deposition temperature is about two‐thirdsthat of trichlorosilane. However, silane is not

2-4

Figure 2‐5. Flow Chart of the Preparation and Refining of Trichlorosilane

widely used because it is more difficult to workwith and ignites spontaneously in air.Trichlorosilane is used as the feed material inapproximately 75% of worldwide polysiliconproduction by the Siemens process. Chlorosilanescan be produced at the polysilicon manufacturingsite or at a nearby plant and transported to thepolysilicon site while maintaining the purity level ofthe fluid; however, larger polysilicon plants aretypically producing their feed materials onsite or atan adjacent plant.

Dry hydrogen chloride (HCl) gas is passed througha heated MG‐Si, particle sizes smaller than 40 μm,in a fluidized bed reactor. This is done at a HClratio of 1:3. The reaction takes place at around600°F (315°C) and 500 psig (34.5 barg). Theresulting chemicals of this reaction are a blend ofchlorosilanes, hydrogen, HCl, and unreactedsilicon. Two chlorosilane species predominate:trichlorosilane (TCS) at 90% and tetrachlorosilane,also referred to as silicon tetrachloride (STC), at10%.

The TCS produced includes some contaminants ofboron, aluminum, and phosphorous. Thesecontaminants are removed in a multistagefractionation process. To ensure properseparation, TCS is cooled and the liquid is flashedthrough a multistage fractionation tower. In the firststage, hydrogen is separated and recycled back tothe process. The first stage column bottom productis flashed through the second stage column wherethe TCS and STC are separated. The TCS isfurther purified through fractionation and otherpurification methods to get ultrapure TCS for useas the reactor feed. Typically, there are eight toten purification towers installed in series for eachplant. Sometimes complexing agents are used toremove phosphorous and boron chlorides from theprocess. Impurities are removed as metalchlorides in solution of TCS/STC. It is “impossible”to measure impurities at such low levels, so anover‐design of the columns can occur and puritylevels cannot be measured until the final product.TCS is typically stored in a tank and then fed to

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the chemical vapor deposition reactors asnecessary.

Chemical Vapor DepositionUsing chemical vapor deposition to producepolysilicon was first developed by Siemens andHalske in 1952 and patented in 1956. It is a batchprocess that takes place in a reactor. This reactormay be called a Siemens dome reactor, reductionreactor, or a chemical vapor deposition reactor.The basic reactor design is a bell jar with threestarter polysilicon rods, also called filaments.These filaments are approximately 7‐8 mm sizeand are placed into an inverted U shape. Somecompanies have their own proprietary reactordesign. Others purchase reactor technology fromcompanies such as GT Solar. Reactors can bebuilt to have more capacity, but this requiresextensive engineering and design to ensure thatthere is uniform performance across the entirereactor surface. Some points to consider onchoosing between multiple reactors or one largerreactor include:

� Turnaround time due to the batch nature ofthe process

� Growth rates are nonlinear across the lengthof a run (reach a point of diminishing return in theprocess)

� Equipment failure consequences onoperability

� Purity levels

Filaments are electrically heated to 1832‐2012�F(1,000‐1,100�C) and voltage and current areadjusted throughout the batch. This process ishighly energy intensive. The pressure of thereactor is approximately 85‐270 psig (5.9‐18.6barg). Pure TCS is blended with a carrier gas,typically hydrogen, to form the gas phasefeedstock to the polysilicon reactors. Moleconcentrations of TCS in the carrier gas aretypically 5‐20% with exceptions to 50%. During thestart of the batch, hydrogen is recycled through thereactor. Once the desired temperature is reached,TCS is mixed with hydrogen at a varying ratio andfed to the reactor. A gas distribution system,essentially a series of nozzles, is designed toproduce uniform polysilicon deposition across thediameter and throughout the length of the rod.About 22 pounds (10 kg) of trichlorosilane isrequired to produce 2.2 pounds (1 kg) of

polysilicon. The exhaust gases must be recycledfor the process to be cost effective.

The reactor is kept cold during the vapordeposition process. It is double walled and uses awater jacket for cooling. The filaments are the onlyitems heated so that the reaction only takes placeon the filaments themselves. Newly formedpolysilicon therefore deposits directly on thefilaments. The amount of time filaments are in thechemical vapor deposition process is proprietary tothe polysilicon manufacturer, but one batch lastsapproximately three to seven days with atwenty‐four hour turnaround time. The growth rateof the filament is approximately one mm/h. Thefinal filament diameter is approximately 200 mmwith a filament length of greater than two mm andeight to sixteen filaments per reactor. Theeconomic effects of an unplanned shutdown of areactor are market dependent but can range fromUS$60,000 to US$100,000 per run of the reactor.

RecoveryThe chemical vapor deposition reactor has lowconversion efficiency, about 10‐20%. Thus, STC isrecovered and recycled. STC separation andconversion is a complex process involvingdistillation columns, cryogenics, and pressureswing adsorption. STC converters are chemicalvapor deposition reactors with platinum filaments.This recovery process enables plants to recyclethe silicon tetrachloride and eliminates byproductsthat are difficult to handle. The hydrogen that isproduced in the chemical vapor depositionreaction is purified through a pressure swingadsorption process, then compressed andreturned to the process.

Supporting the whole plant are the offsite andutilities. This includes HCl synthesis and storage,either cryogenic or aqueous storage; exhaust gasrecovery system; and utilities. There is a causticscrubber for vent gas treatment that needs tohandle 0‐100% turndown. Also present is anemergency gas discharge tank of caustic with amini scrubber on top. All drains in the plant mustbe sealed for safety reasons. All chlorides must beprecipitated out before discharge of wastewater.On the utility side, the cooling water is the mostcritical process for plant safety. It is used to coolreactors and quench exhaust gases. These typesof polysilicon plants are large consumers ofelectricity. Every reactor has its own electricalcontrol equipment, transformers, and rectifiers.

The final processing step is packaging, which is amanual process. The finished polysilicon filaments

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Figure 2‐5. Chemical Vapor Deposition of Granular Silicon

are taken to a clean room where they are brokenup into cut rod, chunk, chip, or fines to meet sizerequirements per customer specifications.

III. Fluidized Bed Reactor:The fluidized bed reactor process is a closed loop,continuous reaction process that is becomingmore common in polysilicon production. It istypically used to produce solar grade polysilicon.Polysilicon is produced in granular pellet form.Main benefits of this process over the Siemensprocess are that the fluidized bed reactor plantsare typically smaller for the equivalent throughput,use much less energy, and polysilicon beads canbe harvested continuously without having toshutdown the reactor. Less energy is requiredbecause the heat transfer is much more efficient,as the gases are heated going into the reactor.According to REC Silicon, this process will reduceenergy consumption in the silane to polysiliconstep by 80‐90% from the Siemens process. Thegeneral process is trichlorosilane or silaneproduction, polysilicon deposition in the fluidizedbed reactor, and recovery/utilities.

Fine metallurgical grade silicon powder, granulesof about 100 μm diameter, is heated to 1340°F(727°C). This silicon powder is let in through anopening at the top of the reactor and fluidized bythe upflowing silane or TCS gas. The silanedecomposes into silicon and hydrogen on the hotsilicon particles. The silicon particles grow andwhen large enough, approximately one mmdiameter, they fall to the bottom of the reactorwhere they are removed. There is some unwanteddeposition of silicon on the hot walls of the reactor.A newer version of this process avoids this issuedue to choice in reactor wall material.

IV. Critical Valve Applicationsand Special Considerations:While most valves used for polysilicon productioncould be considered standard valve configurations(Vee‐ball, easy‐e, and GX valves), severalapplications are critical to the process and specialconsiderations must be taken into account whenselecting these valves, such as cleaning andprocess fluid containment. It can be said, though,that around the reactors, no one valve is morecritical than another. However, there are somecritical applications in the cooling water, vent gas,and recovery sections that have the potential to

2-7

shutdown the entire process and expose the plantto significant safety risks.

Critical Control Valve ApplicationsIn the TCS purification section, there are severalpurification steps involving distillation columns.Distillation columns are found in various chemicalprocesses and have the same basic setup. Theobjective is to separate a feed stream intolight‐component and heavy‐component productstreams. It relies on the relative volatility betweenthe components that make up the feed stream.High volatility (lighter) components boil at a lowertemperature than low volatility (heavier)components. Therefore, when heat is added to thecolumn through a bottom reboiler, lighter materialsare vaporized and rise to the top of the column.Overhead vapors are cooled until they condenseand become a liquid again.

Efficiency of distillation depends upon the amountof contact between the vapor rising and the liquidfalling down the column. Therefore, some of theoverhead liquid product is sent back, or refluxed,to the top of the column. Increasing reflux willimprove the purity of the overhead product;however, it also requires more heat from thereboiler to re‐vaporize the lighter components inthe reflux stream. Operation of a distillation columnis a balancing act between product purity andenergy usage. If the amount of vapor and liquidtraveling through the column becomes too great,the column can “flood”. Too much reflux, too muchreboil heat resulting in too much vapor, or both cancausing flooding. When flooding occurs, efficiencyof the distillation column is dramatically reducedwith corresponding drops in product purities.

Figure 2‐7 shows the general schematic of adistillation column. The valves associated with thisare the feed, reflux, bottom product, overheadproduct, pressure control, and reboil valves.

Feed valves are typically used as flow or levelcontrol loops. An upstream unit or process oftencontrols the feed valve. Unstable feed flow willmake the distillation column difficult to control. Aproblem valve will often cause feed flow tooscillate. As a result, the column will alternatebetween too little and too much reboil heat.Depending upon the size and number of trays inthe column, the effect of a swing in feed will takeanywhere from several minutes to more than anhour to reach the ends of the column. Sometimes,reboil and reflux control will amplify the swings. Asa result, meeting product purity targets becomes

more difficult. Operations personnel will normallyrespond by over‐purifying the products, wastingenergy to compensate for the problematic feedcontrol valve.

The reflux valve is typically either a flow or columntemperature control loop. It is used to adjust thepurity of the overhead product. The higher thereflux rate, the more pure the overhead productwill become. However, raising the reflux raterequires more reboil heat and will eventually floodthe tower. A poorly operating reflux valve has thesame effects as a bad feed valve. Product puritieswill oscillate and the column will be difficult tocontrol. This ultimately affects the efficiency of thecolumn.

The bottom product valve is used to control thelevel in the bottom of the column. Poor control ofthis valve can cause the level to change quicklyand dramatically. Issues with this valve reduceefficiency or at worst, cause flooding in thecolumn. The overhead product valve is used tocontrol the level in the overhead receiver. It canalso cause the level to change quickly anddramatically in the column. Problems with thisapplication can reduce efficiency.

Pressure control valves provide the back pressureto the column. This is critical to controlling thestability of the tower. Stable pressure is required toensure that temperature changes reflectcomposition changes and not pressure changes.With too small of a valve, the pressure responsewill be sluggish. Too large of a valve and smallvalve movement will cause a large pressure swing.Oscillating column pressure and difficult controlresult with either over‐ or under‐sizing the valve.

The reboil valve controls the amount of heat putinto the column by the reboiler. In many cases,steam is used as a heat source. The service isvery clean and fugitive emissions are not aconcern. Steam valves are usually very reliable;however, a problematic valve will make the columndifficult to control precisely. This is especially trueif the column feed is subject to frequent changes.Not all reboilers use steam though. Some useother high temperature process streams.

Regardless of distillation column application, thecontrol valve solution needs to provide accurateand reliable control. Valves can affect columnefficiency, stability, column energy usage, flooding,etc. An easy‐e valve or GX valve is recommendedand are typically CF8M constructions. AFIELDVUE digital valve controller should beutilized to achieve tight control. Diagnostics arekey in these applications for preventative

2-8

Figure 2‐7. Schematic of a Basic Distillation Column

maintenance since distillation columns have longperiods between shutdowns.

For vapor deposition applications, such as thefeed valves to either the Siemens reactor or to thefluidized bed reactor and the vent valves on theSiemens reactor, tight control is necessary. Feedvalves are critical in controlling the vapordeposition rate of producing polysilicon. Due to thecritical nature of these valves, online diagnosticsshould be used to maintain performance and toperform predictive maintenance. In the Siemensreactor, envelope size is an importantconsideration as there is not always ample roomaround the reactor to fit the valves. The GX valveis a good fit as one of the design criteria was to

reduce the overall envelope size. Due to the hightemperature of the reactor, remote mounting of theFIELDVUE digital valve controller may also beused.

For the vapor deposition reactor, cooling water isanother critical application. If for any reason,cooling water is lost to a chemical vapor depositionreactor, even if electricity and gas supplies areisolated immediately, the reactor vessel will get sohot that it is likely to fail and potentially melt. It isimportant to employ predictive maintenance on allcooling water applications so that maintenancecan be scheduled ahead of time to avoid controlvalve failures during operation.

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Figure 2‐9. PSA Basic Flow Scheme

Figure 2‐8. GX with DVC2000W8787

In the recovery section, there are critical valveapplications surrounding the hydrogen purification

and compression. Hydrogen purification is donethrough pressure swing adsorption (PSA). Controlvalves used in PSA applications are in a very highcycle service, seeing 100,000 to 250,000 cyclesper year. Valves and actuators are expected tostroke once every three minutes. Stroking speedsare required to be fast and controlled. Uncontrolledopening can cause pressure/flow spikes.Depending upon the type and size of a PSA skid,the amount and type of control valves will vary.PSA skids can use globe and/or rotary valves.Control valve shutoff is a major concern because itaffects PSA unit efficiency. Bi‐directional flowconditions also exist. If valve leakage causescontamination from one PSA bed to another,industrial gas purity can be compromised.

A GX valve is the recommended globe valvesolution for PSA service. It can handle strokingspeeds up to 1.5 ‐ 2 seconds. Class VI shutoff isnecessary and bi‐directional shutoff is standard inthis construction. Certified emission controlpacking is standard to reduce potential hydrogenleakage. The recommended high performancebutterfly valve construction requires the use of apressure assisted UHMWPE seal. PTFE linedPEEK bearings that can withstand long cycle liferequirements are used. Live loaded PTFEENVIRO‐SEAL packing is available to reducehydrogen leakage.

These constructions have undergone extensivetesting within the flow lab in Marshalltown, Iowa,United States to ensure that they meet thedemands of PSA service. PSA testing is used toverify the life cycle of Fisher digital valves. PSAtesting was designed and set up with input from

2-10

Figure 2‐10. ENVIRO‐SEAL Packing Systems

W8616

major PSA licensors to represent the PSA processas accurately as possible. Both the GX andbutterfly valve solution have undergone rigoroustesting to prove they can meet the high cycledemands of this service.

Figure 2‐11. Antisurge Control System

W8950‐1

Hydrogen also needs to go through a compressorbefore it is recycled back to the process. Typically,a reciprocating compressor is used for polysiliconplants. An antisurge control system is design toprotect the compressor as it is an expensive pieceof equipment and critical to the process. Thesystem is designed to provide a faster responsethan adjusting the turbine speed to control theonset of surge. The controller looks at multiplevariables to prevent the onset of surge. It requiresfast, accurate response in order to prevent surgeconditions. Characteristics of a surge condition arefast flow reversal (measured in milliseconds),excessive compressor vibration, increase inflowing media temperature, noise, and it maycause the compressor to “trip”. Consequences ofsurge situations are substantial and may includeshortened compressor life, loss of efficiency,reduced compressor output, and mechanicaldamage to seals, bearings, impellers, etc.

Antisurge control valves present many variouschallenges. The key challenge is ensuring valvereliability. There is an extended period betweenmaintenance cycles and it is important to ensure areliable control valve assembly solution. Theantisurge valve is the main piece of equipment thatprotects the compressor from damage caused bya surge. When these valves are called upon tomove, they are required to stroke very quickly,typically in the open direction only. This cannecessitate oversized actuator connections andthe use of volume booster(s) and quick exhaustvalve(s). Improper selection of these accessorieswill result in poor valve performance and tuningdifficulties. During a surge event, pressure dropand flow rate experienced by the valve can behigh, causing excessive levels of noise. This mustbe considered in valve selection, although noisecontrol throughout the entire range of valve travelmay not be required. This valve may also berequired to throttle intermittently from 0 to 100%open. These cases require the valve to have fast,accurate control for incremental step sizes. Anydelays can cause a surge to occur. The antisurgevalve must be able to pass the highest possibleoutput capacity of the compressor. Application of amultiplying factor to the compressor capacity figureis common and may lead to selection of anoversized valve. Valves with too much capacityoften have controllability issues and can causeunstable operation.

Emerson has developed an optimized antisurgepackage to meet the challenging demands of thisapplication. Some highlights of this package will bediscussed, but for more information, please seethe brochure titled “Fisher Optimized AntisurgeControl Valves”. These valves use a spoked plug

2-1 1

versus the traditional balanced plug. With thetraditional plug, when the valve is asked to movevery quickly, there is not enough area in thebalance holes to keep the plug in a balanced state;therefore, creating a differential pressure situationbetween the top and the bottom of the plug. Thisdifferential pressure case can lead to pluginstability. The spoked plug has large balanceareas so that this does not occur. For surge eventswhere noise is a concern, a Fisher valve with aWhisper Trim III or WhisperFlo trim isrecommended.

Emerson has the engineering capability tocharacterize these trims in order to meet thespecific application needs and tailor a solutiontowards them. When the valve assembly is calledupon to move quickly, mechanical air cushionshave been added to the actuator cylinder toprovide controlled deceleration to help protectactuator and valve components for longer travelapplications.

The Fisher optimized antisurge package alsoincludes a FIELDVUE digital valve controller withthe optimized digital valve (ODV) tier. Featureswithin this tier and in AMS ValveLink softwaremeet the needs of the application. For example,factory expertise is not required to tune the Fisheroptimized antisurge valve. A technician can simplyuse ValveLink software s performance tuner orthe stabilize/optimize feature with real timegraphics. Configuration and tuning can also beperformed remotely by operators as processrequirements change. This feature gives plantoperators and technicians the capability to tunethis assembly in the field. The lead‐lag filter in theODV tier can be used to improve the response tosmall amplitude steps by overdriving the set point.Asymmetric adjustments allow the response to beset independently in the open and closingdirections. Integrated, real time graphics allowadjustments to be done remotely as well. Also,diagnostics can be collected, viewed, andanalyzed using ValveLink software to look at itemssuch as packing friction, air path leakage, actuatorspring rate, and bench set. Partial stroke tests canalso be performed to check the health of the valveand ensure the antisurge valve is going to movewhen it is requested to.

The conversion of STC in the recovery processuses distillation columns. These applications arethe same as discussed in the TCS purification.See the above text for more details.

The final step in the Siemens process is themanual breaking and packaging of the polysilicon

inside a clean room. This requires an industrialheating, ventilating, and air conditioning (HVAC)system to meet the air cleanliness requirements ofthe clean room. Industrial HVAC means usingindustrial grade control equipment to control theenvironment to support the process, not humancomfort. These facilities should be designed toprovide the necessary protection to minimize therisk of contamination and to ensure thatappropriate air quality (temperature, humidity andparticulates) is maintained, especially in areaswhere product is exposed to the environment. Thisis critical to maintaining the purity of the productthat is such a concern throughout the entireprocess. The HVAC system needs to provide goodperformance, be reliable, and be low maintenance.While the valve applications are generally simple,it is important to have an accurate (<1% device)and reliable valve assembly to ensure overallsystem performance. Generally, the Baumann24000 series works well in these applications.There are a variety of valve sizes, materials, andtrims available to meet each individual application.The small light weight design is ideal forinstallation in tight areas such as around airhandling equipment, which is typically located inceiling crawl spaces above the clean room. Also,adding a FIELDVUE DVC2000 reduces start uptime by allowing for simple local push‐buttoncalibration, while adding remote diagnosticcapability to monitor valve performance in difficultto access installations.

There are many toxic, lethal, and explosive fluidsthat need to be contained throughout thepolysilicon process. Chlorine, HCl, TCS, STC, andhydrogen are flammable, corrosive, toxic, andexplosive. Silane ignites spontaneously with air.HCl or TCS plus air gives hydrochloric acid, sosystems containing these fluids need to be keptcompletely dry. Depending upon the process fluid,government regulations, and customerspecifications, either ENVIRO‐SEAL packing orENVIRO‐SEAL bellows will need to be used.ENVIRO‐SEAL packing limits emissions whileENVIRO‐SEAL bellows eliminates emissions.

Special CleaningContamination is a major factor in the productionof polysilicon because the end product needs tomeet such stringent purity levels. Residual grease,oil, etc. in process equipment after maintenance,testing, or initial construction causes productquality issues and produces off‐specificationproduct. Phosphorous is a big contaminant andcan ruin batches. Some greases that are approvedfor oxygen use are not acceptable in polysilicon

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Figure 2‐12. 24000 with DVC2000

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Figure 2‐13. GX with ENVIRO‐SEAL Bellows

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environments. The cleaning process requiredvaries from one polysilicon producer to the next.Emerson has created and utilizes standarddegreasing levels for Fisher control valves; ClassA for non‐oxygen fluids and Class AAA for oxygenapplications. Typically, polysilicon projects have

required some form of Class A degreasing. Thismay be modified to specify a certain cleaner or anon‐phosphate rinse at the end of the cleaningprocess. Emerson has also worked with polysiliconmanufacturers to tailor a cleaning process to meettheir specific needs.

Safety Instrumented SystemsDue to the nature of the process fluids involvementand the reactions taking place, safety is animportant and critical part of the productionprocess. Safety instrumented systems (SIS) areput in place to ensure protection against accidentssuch as toxic chemical releases, facilityexplosions, or fires. The SIS valve is present totake the process to a safe state when specified(dangerous) conditions are violated. Loops areevaluated by plants to determine what safetyintegrity level (SIL) level is needed for that safetyapplication. Several Fisher products have beencertified and are suitable for use in SISapplications. These products include theFIELDVUE DVC6000 SIS, GX, Vee‐ball, HP,easy‐e, 8580, A81, and the Control‐Disk alongwith the respective actuators for the valves.Typically, safety valves operate in one staticposition and only move upon an emergencysituation. Without mechanical movement for longperiods of time, unreliability inherently increases.To ensure availability on demand, SIS valves mustundergo regular testing. This includes not only fullstroke tests, but also partial stroke tests that canbe used to increase the time between full prooftests. The DVC6000 SIS can be utilized to performpartial stroke testing. It can move the valvebetween 1 and 30% from its original position.Partial stroke tests can be scheduled via theautomatic test scheduler. If an emergency demandoccurs, the response will override the testing.Using smart positioner technology with partialstroke testing provides for predictive maintenancethrough diagnostic information.

V. Conclusion:As technical uses for polysilicon grow along withthe demand for solar power, the polysilicon marketwill continue to expand. Reliability, safety, control,purity levels, and variability are all affected by thecontrol valves used in the process. It is importantto understand the polysilicon process in order toapply the proper control valve solution for variousapplications.

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Emerson Process Management Marshalltown, Iowa 50158 USASorocaba, 18087 BrazilChatham, Kent ME4 4QZ UKDubai, United Arab EmiratesSingapore 128461 Singapore

www.Fisher.com

The contents of this publication are presented for informational purposes only, and while every effort has been made to ensure their accuracy,they are not to be construed as warranties or guarantees, express or implied, regarding the products or services described herein or their useor applicability. All sales are governed by our terms and conditions, which are available upon request. We reserve the right to modify or im­prove the designs or specifications of such products at any time without notice. Neither Emerson, Emerson Process Management, nor any oftheir affiliated entities assumes responsibility for the selection, use or maintenance of any product. Responsibility for proper selection, use,and maintenance of any product remains solely with the purchaser and end user.

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