NATURAL GAS SPECIFIC COURSE
3 CRYOGENIC CRYOGENIC PROCESS (EXPANSION (EXPANSION &TURBOEXPANER) &TURBOEXPANER) 2009
Eng./ ALy Nassr 1. INTRODUC INTRODUCTION TION
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The use of turbo-expanders in gas processing plants began in the early sixties. By 1970, most new gas processing plants for ethane or propane recoery were being designed to incorporate the particular adantages characteristic of an expander producing usable wor!. The trend in the gas processing industry continues toward increased use of the turboexpander. The first turboe turboexpande xpanderr applic application ation for natural gas proce processing ssing was accom accomplished plished usin us ing g "o "oto tofl flow ow te tech chno nolo logy gy in Tex exas as in th thee ea earl rly y 19 19#0 #0s. s. $t dr dram amat atic ical ally ly demonst dem onstrat rated ed how ef effic ficient iently ly the exp expans ansion ion tur turbine bine cou could ld con conden dense se hea heaie ier r components of the gas stream, while at the same time proiding power to recomp co mpre ress ss th thee le lean aner er ga gas. s. Th Thee cu curr rren entt ra range nge of "ot "otof oflo low w Tur urbo boex expa pande nderr%enerators grew from that original application. &orty years later, close to ',000 units are in operation around the world, and "otoflow has proen, many times oer, its ability to engineer machinery that deliers higher power leels, functions at ext extrem remee ope operat rating ing tem temper peratu atures res,, and ach achie iees es gre greate aterr pre pressur ssuree rat ratios ios.. (ur turboexpander experience and technology hae become inaluable resources for eery segment of the natural gas and hydrocarbon industries. The rapid growth of "otoflow turboexpander technology has been a story of continuous improement in expander design, rotor and bearing design, efficiency optimi)ation, and control systems. This growth has been drien by the needs of the industry to increase capacity, reduce costs, and maximi)e reliability. By adopting an internal business structure that parallels the structure of the oil and gas industry, %* *nergy is able to ad addr dres esss th those ose ne need edss dir direc ectly tly,, as an ac acti tie e pa part rtne nerr in th thee se sear arch ch fo forr mo more re effectie solutions. &our of %* *nergy+s (il %as Business nits are closely inoled in the application of Turboexpander-%enerator solutions for natural gas and related processes. The extraction of %/ is generally preceded by treatment of the gas to remoe water, sulphur compounds and other contaminants. ( remoal and nitrogen re2ection may also be carried out depending on composition of the inlet gas. 3ethod for the separation of %/ can generally be diided into cryogenic and non-cryogenic non-cr yogenic systems. systems. *arly attempts attempts at recoery recoery of these li4uids li4uids were made using usin g lea lean n oil abs absorp orption tion 5no 5non-cr n-cryoge yogenic nic66 and mec mechan hanica icall ref refrig rigera eration tion.. The absorption process was later deeloped into refrigerated absorption process and in recentt years, enhanced recen enhanced absorp absorption tion proce processes sses inoling both refrigeration refrigeration and presaturation hae been deeloped. The introduction of oule Thompson ales and turbo expanders in 19#0s made significant contributions to the achieement of cryogenic conditions. The schemes using these technology were initially designed with minimal heat integration and no or little column reflux. These were later deelop de eloped ed into sch scheme emess tha thatt gen genera erated ted colu column mn ref reflux lux and max maximi imi)e )e the hea heatt integration for high %/ recoeries and optimi)e the plant profitability.
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2.NGL RECOVERY 8
eeral technologies hae been ealuated as the main process to separate methane from the %/, such as, lean oil absorption, refrigeration, and turbo expander technologies.
Absorption Plants or Lean Oil Plant
The most efficient lean oil absorption plants recoer only about '0 percent of the ethane, 90 percent of the propane, and 100 percent of the butane and heaier hydrocarbons from the gas. :dditional heat is re4uired to separate the products from the lean oil, and additional cooling is re4uired in order to re-li4uefy. the raw products before fractionation. /ean oil absorption plants usually hae higher operating costs than refrigeration plants or turbo expander plants. Therefore, this old type process was dropped from selection.
Refrigeration Plants
$f the ma2or purpose of a plant is to condition rich gas to meet certain pipeline specifications, the mechanical refrigeration plant may be the proper selection. "efrigeration is used to condition produced gas to meet pipeline hydrocarbon dew point specification, Btu specification, limited li4uid recoery of heaier hydrocarbons such as ;< or a combination of these ob2ecties. The straight refrigeration plant is limited to chilling the gas stream to the range of -='> to -'0>. This limits product recoery to about #0 percent of the propane and much less ethane at typical plant operating pressure. :s the intention of the pro2ect is to recoer heaier components starting from *thane, this technology was s!ipped.
Cryogenic Turbo Expander Plants
ryogenic turbo expander plants is capable to recoer from #0 to 90 percent of the ethane, 90-9? percent of the propane, and 100 percent of the butane and heaier hydrocarbon components from the rich gas. : turbo expander plant is compact and relatiely simple to install and operate. The inlet gas to a turbo expander plant must hae essentially all of the water and ( remoed to preent hydrate dry ice formation to the leel of respectiely 1 ppm and 100 ppm for @( depending on the cryogenic temperature achieed. Turbo expander plants hae less process e4uipment 5towers and external heating6 than lean oil absorption plants, but they hae more mechanical e4uipment 5gas heat exchangers and recompressors6. $f ethane recoery is the ob2ectie, the expander process is the most economical means for recoering a high percentage of ethane and heaier hydrocarbons from a gas stream. : turbo expander plant is the first design considered because it is comparable in cost to a refrigeration plant, but it is more efficient and achiees greater li4uid recoery.
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3. CRYOGENIC GAS PLANTS:
&irst of all , %/ recoery is also called ethane recoery , < recoery or cryogenic gas plants. The word, cryogenics, defined as being Athe science that deals with the production of ery low temperatures and their effects on the properties of matter.A The cryogenic gas plant is built around a process that produces ery low temperatures in order to affect the properties of natural gas, namely to cause certain of the gass components to condense to li4uid. The gas components which condense form a li4uid hydrocarbon mixture !nown as atural %as /i4uids or %/. The %/ mixture contains a number of aluable hydrocarbons that can subse4uently be separated into indiidual products, such as propane, gasoline and petrochemical feedstoc!. The cryogenic process has application as the most economical means for recoering a high percentage of all hydrocarbons heaier than methane. pecifically, the cryogenic process aims to recoer ethane and heaier hydrocarbons, such as propane, butane and gasoline components. Cifferent ariations of this process are capable of remoing more than ?;D of the ethane and essentially all of the heaier hydrocarbons found in produced natural gas. By contrast, other processes may be more appropriate when the goal is to recoer 2ust propane and heaier components. The main adantage of the cryogenic plant, in terms of recoered product, is therefore its ability to recoer ethane or high propane recoeries. Ehen we chill a gas to condense %/, we hope to extract or recoer the maximum amount of aluable %/. @oweer, a substantial amount of methane will also condense to li4uid and become part of the mixture. The methane is undesirable for two reasons8 1. The methane has )ero alue as an %/ component . . 3ethane eleates the apor pressure of the %/ mix and may cause difficulties in li4uids transportation. Therefore, almost all of the methane is remoed from the %/ in the demethani)er tower. There are costs associated with operating a demethani)erF these include the capital cost of the e4uipment and its operating cost, which includes seeral factors. ince methane must be re2ected from the %/, it stands to reason that the cost of demethani)ing will be less if the amount of methane condensed is less. Thus, one criteria in the design and operation of the turboexpander plant is to minimi)e the amount of methane that li4uefies while still maximi)ing the recoery of ethane and heaier hydrocarbons. The 4uantity of methane that can be remoed in the demethani)er depends upon its design. $t will
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hae been designed for some maximum case, where the maximum amount of methane condensed and re2ected corresponds to the maximum recoery of ethane. Turboexpander plants can usually recoer 9?
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4.Theory of Gas Expansion
Ee hae seen how turboexpander plants remoe both sensible and latent heat from a feed gas stream to condense hydrocarbon li4uids. :ny gas will possess both sensible heat and its latent heat of apori)ation. @eat is a form of energy, so a gas contains heat energy. : gas may contain other forms of energy as well, such as pressure energy and elocity. The total energy, of all !inds, possessed by a gas is called its Ainternal energyA. :nother name for this internal energy is enthalpy. $n gas processing, the enthalpy of a gas, or a mixture of gases, is primarily dependent on the pressure and temperature of the gas. 5The elocity 5or !inetic6 energy of a gas changes little as the gas moes through the plant, so it is considered to be negligible and therefore can be ignored in the discussion which follows.6 Cifferent gases will hae different enthalpies at the same temperature and pressure 5because the different gases hae differing latent heats of apori)ation and specific heats6, so the enthalpy of a gas mixture is also, therefore, composition-dependent.
5. VALVE EXPANSION: JT PLANTS
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5.1 The oule!Tho"son effect # $n thermodynamics, the ouleGThomson effect or ouleGHelin effect or HelinG oule effect describes the temperature change of a gas or li4uid when it is forced through a ale or porous plug while !ept insulated so that no heat is exchanged with the enironment. This procedure is called a throttling process or ouleG Thomson process. :t room temperature, all gases except hydrogen, helium and neon cool upon expansion by the ouleGThomson process .
Fig(1): Professor William Thomson
The effect is named for ames Irescott oule and Eilliam Thomson 5fig.16, 1st Baron Helin who discoered it in 1?; following earlier wor! by oule on oule expansion, in which a gas undergoes free expansion in a acuum. $n practice, the ouleGThomson effect is achieed by allowing the gas to expand through a throttling deice 5usually a ale-&ig.6 which must be ery well insulated to preent any heat transfer to or from the gas. o external wor! is extracted from the gas during the expansion.
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5.$ %echanis" of expansion # The adiabatic 5no heat exchanged6 expansion of a gas may be carried out in a number of ways. The change in temperature experienced by the gas during expansion depends not only on the initial and final pressure, but also on the manner in which the expansion is carried out. i. $f the expansion process is reersible, meaning that the gas is in thermodynamic e4uilibrium at all times, it is called an isentropic expansion. $n this scenario, the gas does positie wor! during the expansion, and its temperature decreases. ii. $n a free expansion, on the other hand, the gas does no wor! and absorbs no heat, so the internal energy is consered. *xpanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas may either increase or decrease, depending on the initial temperature and pressure. iii. The method of expansion discussed in this article, in which a gas or li4uid at pressure I1 flows into a region of lower pressure I ia a ale or porous plug under steady state conditions and without change in !inetic energy, is called the ouleGThomson process. Curing this process, enthalpy remains unchanged Temperature change of either sign can occur during the ouleGThomson process. *ach real gas has a ouleGThomson 5Helin6 inersion temperature aboe which expansion at constant enthalpy causes the temperature to rise, and below which such expansion causes cooling. This inersion temperature depends on pressureF for most gases at atmospheric pressure, the inersion temperature is aboe room temperature, so most gases can be cooled from room temperature by isenthalpic expansion. :s a gas expands, the aerage distance between molecules grows. Because of intermolecular attractie forces 5see Jan der Eaals force6, expansion causes an increase in the potential energy of the gas. $f no external wor! is extracted in the process and no heat is transferred, the total energy of the gas remains the same because of the conseration of energy. The increase in potential energy thus implies a decrease in !inetic energy and therefore in temperature.
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FIG.(2):
J.T.VALVE.
oule-Thomson Jales The principal function of a -T ale is to obtain isenthalpic cooling of the gas flowing through the ale. These ales generally are needle-type ales modified for cryogenic operation. They are an important component in most refrigeration systems, particularly in the last stage of the li4uefaction process. oule-Thomson ales also offer an attractie alternatie to turboexpanders for small-scale gas-recoery applications.
5.& .T. Expansion Process # The use of the oule-Thomson 5-T6 effect to recoer li4uids is an attractie alternatie in many applications. The general concept is to chill the gas by expanding the gas across a -T ale. Eith appropriate heat exchange and large pressure differential across the -T ale, cryogenic temperatures can be achieed resulting in high extraction efficiencies. The main difference between the -T design and turboexpanders is that the gas expansion is adiabatic across the ale. Irocesses which use the cooling effect of the expansion of a gas across a ale or cho!e are sometimes called /T 5/ow Temperature eparation6 or /TK 5/ow Temperature *xtraction6 units.5fig.=6
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&ig.5=6 .T. expansion process
5.'. Applications of (T )al)e operation# 1. Ilant startup operations, . @andling gas flows in excess of expander capacity, and =. &or continued operation during those times when the expander is down for maintenance.
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6. The Tr!oexpan"er Pro#ess : turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce wor! that is often used to drie a compressor . Because wor! is extracted from the expanding high pressure gas, the expansion is an isentropic process 5i.e., a constant entropy process6
*.1. +istory The possible use of an expansion machine for isentropically creating low temperatures was suggested by arl Eilhelm iemens 5iemens cycle6, a %erman engineer in 1?;7. :bout three decades later, in 1??;, *rnest olay of Belgium attempted to use a reciprocating expander machine but could not attain any temperatures lower than L9? > because of problems with lubrication of the machine at such temperatures. $n 190, %eorges laude, a &rench engineer, successfully used a reciprocating expansion machine to li4uefy air. @e used a degreased, burnt leather pac!ing as a piston seal without any lubrication. Eith an air pressure of only '0 bar, laude achieed an almost isentropic expansion resulting in a lower temperature than had before been possible. The first turboexpanders seem to hae been designed in about 19=' or 19=; by %uido Mer!owit), an $talian engineer wor!ing for the %erman firm of /inde :%. $n 19=9, the "ussian physicist Iyotr Hapitsa 5fig.'6 perfected the design of centrifugal turboexpanders. @is first practical prototype was made of 3onel metal, had an outside diameter of only ? cm , operated at '0,000 reolutions per minute and expanded 1,000 cubic meters of air per hour. $t used a water pump as a bra!e and had an efficiency of 79 to ?= percent. 3ost turboexpanders in industrial use since then hae been based on Hapitsas design and centrifugal turboexpanders hae ta!en oer almost 100 percent of the industrial gas li4uefaction and low temperature process re4uirements. $n 197?, Iyotr Hapitsa was awarded a obel physics pri)e for his body of wor! in the area of lowtemperature physics
&ig.5'6 . Iyotr Hapitsa 5obel pri)e 197?6
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*.$ Turboexpander Process # 3any hydrocarbon turboexpanders, or expanders for short, are employed in processes for the purpose of extracting heaier hydrocarbons from the gas stream. &ig.5;6 is a simplified schematic for such a process. The high-pressure gas mixture enters the process at condition 516and is cooled in a gas-to-gas heat exchanger. sually some li4uids are condensed at 56 and are separated out in the high-pressure separator. The oerheads from this separator, 5=6 , are then expanded nearly isentropically in the expander and additional li4uids are condensed at 5'6 . These li4uids are remoed in the low pressure separator and the oerheads, 5;6 , are used for precooling the incoming gas in the gas-to-gas exchanger. The warmed gas at 5#6 is sent to the booster compressor drien by the expander where the gas pressure is increased to conditions at 576 . &or a natural gas dew point control process, the gas at 1 is to be conditioned for pipeline transmission. The pressure drop between 5=6 and 5'6 is usually relatiely small because the re4uired temperature drop between 5=6 and 5'6 is low. This is because only limited 4uantities of hydrocarbon li4uids need to be remoed to ma!e the hydrocarbon dew point acceptable. &or a natural gas ethane extraction process, the gas at 1 is being processed to remoe essentially all of the < hydrocarbons from the stream. $n this case, the pressure drop between 5=6 and 5'6 is relatiely large and the resulting low temperature at 5'6 facilitates proper condensation in the demethani)er tower. $n this process, relatiely large 4uantities of li4uid are often extracted.
&ig.5;6 . Typical expander process
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*.&. Expander %ain Co"ponents # A typical expander (fig.6) is containing the following :
&ig.5#6 . Typical cross section of expander
*.&.1 ,ariable Expander -nlet uide ,anes # Jariable inlet guide anes 5$%J65fig 76 are used to regulate mass flow to the expander 5&igure 76. This design proides precise control and high efficiency oer a broad operating range. The inlet guide anes achiee these design benefits by using a uni4ue proprietary mechanism that incorporates a pressure actuated sealing ring. :n internally mounted mechanism translates the linear motion of the pneumatic $%J actuator into the rotational moement of the ad2usting ring.
&ig576 8 $nlet guide Janes
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*.&.$ Expander and Co"pressor /heels # The radial inflow expander, utili)ing ariable inlet guide anes, produces high efficiencies oer a broad operating range. areful design of the wheel blade angles and contour optimi)es aerodynamic performance without compromising the mechanical design integrity of the wheel. : typical expander wheel is shown below. 5fig.?6
&ig5?6 8 expander wheel
*.&.& Expander and Co"pressor 0haft # expanderNcompressor shafts5fig.96 are of rigid design and operate below the first bending critical speed and torsional resonance. (n certain applications wheels are attached to the shaft on a special tapered profile, with cylindrical !eys and !eyways
&ig596 8 expanderNcompressor shaft
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*.'. Expander Auxiliaries# *.'.1 Lubrication 0yste" The expander-compressor has a bearing on each end of the shaft. These bearings must be continuously lubricated with clean lubricating oil5fig.106 of the approed type at the proper temperature. /ubrication failure for a short-period of time may result in a bearing failure which may seriously damage the machine . The lube oil system is shown in the drawing below. Iressures shown are typical for a plant in which the expander outlet pressure is 070 !Ia O=00 psiP. &low is as follows8 (il from the reseroir enters one of the pumps which is able to raise its pressure seeral thousand !Ia Oseeral hundred psiP. : control ale in the pump discharge line releases excess pressure through a spilloer line that returns to the reseroir. (il from the pump enters a temperature control ale, which is positioned by a temperature controller that allows some of the oil to by-pass the cooler to maintain a constant temperature. The oil then flows through one of two filters to remoe solid particles and enters the bearings on each end of the shaft. The oil flows out of the bearings5fig. 116 and drops to the bottom of the housing and flows by graity into the reseroir, and the cycle is repeated.
&ig.5106 8 Typical /ube oil system
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&ig.5116 8 Typical 2ournal bearing
*.'.$ %agnetic earing2fig.1$3 #
A si$ni%#an& &e#hno'o$i#a' a"(an#e o##rre" in )*+* ,hen -T %rs& in#orpora&e" /a$ne&i# !earin$s in&o a hy"ro#ar!on &r!oexpan"er as an a'&erna&i(e for &he #on(en&iona' !earin$. Sin#e &ha& ini&ia' s##ess0 &his &e#hno'o$y has fr&her /a&re" so &ha& &r!oexpan"ers ,i&h /a$ne&i# !earin$s are no, #onsi"ere" s&an"ar" pra#&i#e for #er&ain app'i#a&ions. The app'i#a&ion of /a$ne&i# !earin$ &e#hno'o$y &o hy"ro#ar!on &r!oexpan"ers has i/por&an& a"(an&a$es o(er &ra"i&iona' oi' !earin$ "esi$ns. So/e of &he a"(an&a$es of /a$ne&i# !earin$ &e#hno'o$y are: 1 Eliminates Risk of Process Contamination : Sin#e &he '!e oi' is #o/p'e&e'y re/o(e" fro/ &he sys&e/0 &here is no ris2 of oi' /i$ra&in$ in&o &he pro#ess. 1 3earin$ Losses: The fri#&iona' 'osses of /a$ne&i# !earin$s are sa''y 'o,er &han #on(en&iona' oi' !earin$s. This a""i&iona' po,er is no& 'os& !& re#o(ere" as sef' ener$y for #o/pression or po,er $enera&ion. 1 Eliminates the Risk of Oil Dilution: In oi' !earin$ sys&e/s0 &he /ixin$ of hea(ier sea' $as an" oi' #an res'& in a /easra!'e "i'&ion of &he '!e oi'. If no& proper'y #on&ro''e"0 &his /ay #ase ins#ien& oi' (is#osi&y0 re"#e" 'oa" #apa#i&y0 an" in#rease" (i!ra&ion. This po&en&ia' pro!'e/ is e'i/ina&e" ,i&h /a$ne&i# !earin$s. 1 Lower Utility And Maintenance Costs: 3y e'i/ina&in$ &he '!e oi' sys&e/0 &he e'e#&ri# po,er an" o&her &i'i&y reire/en&s are re"#e". -ain&enan#e reire/en&s on p/ps0 %'&ers0 #oo'ers0 an" o&her /ain&aine" #o/ponen&s are a'so re"#e" or e'i/ina&e". 1 Reduced Weiht And !"ace: E'i/ina&ion of &he '!e oi' sys&e/ re"#es &he o(era'' ,ei$h& an" "i/ensions of &he sppor& s2i". This /a2es /a$ne&i# !earin$s i"ea''y si&e" for oshore app'i#a&ions0 or ,here(er spa#e is 'i/i&e".
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1 En#ironmental $m"act: As &he /a$ne&i# !earin$ sys&e/ "oes no& reire '!e oi'0 p'an& s&ora$e reire/en&s0 #'eanp0 an" "isposa' is e'i/i na&e". 1 !afety: E'i/ina&ion of oi' fro/ &he sys&e/ res'&s in #'eaner /a#hinery "e#2s0 ,i&h re"#e" ris2 of %re an" persona' in7ry.
&ig.516 8 Typical 2ournal bearing
*.'.& 0eal as2fig.1&3 8 8se a si&a!'e $as s&rea/ ,i&h %'&erin$ an" pressre #on&ro' &o /ain&ain proper $as pressre a& &he shaf& sea's. The sea' $as sho'" !e in&ro"#e" !efore &he '!e oi' sys&e/ is s&ar&e" !e#ase &here /i$h& !e a pressre pse& ,hi#h ,o'" p& eno$h oi' in&o &he pro#ess &o #ase a pro!'e/. Ea#h of &he /ain ro&a&in$ #o/ponen&s 9ra"ia' !earin$s0 &hrs& !earin$s0 an" shaf& sea's #an !e "a/a$e" or ero"e" !y i/proper oi' %'&ra&ion0 'a#2 of oi' ;o,0 i/proper $as "ehy"ra&ion0 an" i/proper sea' $as %'&ra&ion.
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&ig.51=6 8 Gas Sea'
Thrust Control
$n any type of centrifugal deice, thrust forces deelop which tend to moe the shaft toward one end or the other. $f it were to moe laterally along its axis, the impellers would touch the casing and 4uic!ly wear out. $n an expander-compressor, thrust bearings on each end of the shaft preent lateral moement. @oweer, the thrust forces against the bearings must be controlled at a moderate leel to preent bearing failure and serious damage to the machine. The thrust force is due to a difference in suction and discharge pressure acting on the front and rear face of an impeller. /oo! at the expander impeller in the following drawing. @igh pressure inlet gas enters at the tip of the impeller, and lea!s around the labyrinth impeller seal to the rear face and exerts a force to the left. /ow pressure outlet gas pressure is imposed on the front or left side of the impeller. $n order to neutrali)e the thrust in the expander impeller, gas which lea!s around the labyrinth seal on the rear face is slightly aboe outlet pressure.5fig.1'6
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&ig.51'6 8 Thrs& 3a'an#in$ sys&e/
C4$ ree6e(up in Turboexpander Processes # 0 can be expected to free)e 5form a solid phase6 if the temperature of the process is less than -?0>&, the triple point of 0. $n the typical turboexpander process, temperatures lower than the triple point are usually necessary to achiee the desired leels of ethane and propane recoery. arbon dioxide has a limited solubility in both the li4uid and apor hydrocarbon phases at typical turboexpander plant process conditions. Thus, the concentration of 0 in both the li4uid and apor phase must be considered along with the temperature-pressure leels in the turboexpander plant demethani)er. Ehite et al. hae presented a generali)ed correlation for predicting the conditions under which ( free)eup can occur. Bergman and Qarborough hae performed a series of ( free)e out experiments on light hydrocarbon systems. Their wor! resulted in correlations similar to the one gien by Ehite. The li4uid free)eup cures from these two correlations are essentially identical except at the high temperature end 5-100>&). The possibility of ( free)eup should be chec!ed at the outlet of the turboexpander and in the top section of the demethani)er column. 0 free)eup can be aoided by8 516
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remoing the ( from the gas before processingF 56 increasing the pressure of the demethani)er 5to increase temperatures in the column6. ertainly ( remoal is the most positie and direct approach to controlling ( free)eup. $n those cases where ( remoal is not economically feasible, either higher than normal demethani)er column pressures will hae to be maintained andNor the process conditions ad2usted to remoe the ( from the ery low temperature part of the process. Irocess condition ad2ustment to remoe ( early in the process 5at temperatures aboe about - 100>&6 will be ery tric!y and is probably not feasible. $f the expander li4uid is fed to the top tray of a demethani)er, the ( will concentrate in the top e4uilibrium stages. This means that the most probable condition for solid ( formation may be seeral trays below the top of the tower rather than at expander outlet conditions.
Turboexpander Efficiency Esti"ation
:s with compression efficiencies, there are two leels of turboexpander efficiency estimation procedures. The first leel of efficiency estimation is a blind estimation procedure. The second leel of efficiency estimation is based on selected turboexpander machine parameters and the results of the first leel of turboexpander efficiency. :ll turboexpander efficiencies and efficiency estimation procedures are based on the adiabatic reersible assumption. This assumption is usually alid. @oweer, if there is significant heat lea! into the turboexpander, unrealistic alues for the efficiency will result. The obsered efficiencies would probably be lower than expected. Turboexpander efficiencies typically range from about #0D to about ?;D. /ower efficiencies hae been obsered in some cases. : best first guess for turboexpander efficiency would be in the range of 70-7;D. :fter preliminary calculations hae been performed, this initial assumption can be ad2usted using a procedure to be outlined later.
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The predicted performanceNli4uid recoery of a turboexpander plant are fairly sensitie to the estimated Rand actual6 turboexpander efficiencies. :s a general rule, we find that on the aerage a 1 D change in the turboexpander efficiency causes a =-'>,& change in the predicted outlet temperature. There are three problems associated with poor estimation of the turboexpander efficiency8
1. :ny errors in the efficiency are directly reflected in the recompression power re4uirements. :dmittedly, these effects may be small, but on ery large plants, they can become more significant. 2. The errors in the predicted li4uid formation directly affect the < li4uid recoery and the heat balance on the demethani)er. *rrors here are probably more significant than the errors in the recompression load. 3. &alse 0 free)eup problems may be predicted if too high efficiency is used. onersely, a potential ( free)eup may not be properly predicted if the efficiency is too low. This issue will be discussed in a later section. The efficiency of a compressor drien by a turboexpander appears to be about ;D lower than the turboexpander efficiency. This obseration is completely empirical but seems to match the performance of seeral operating plants.
7esign para"eters of a turboexpander #
There are two factors affecting performance characteristic of turboexpander 8
1. 3ass percent li4uid in the expander outlet 2. *xpansion ratio across the machine ome feel that about 0 mass percent li4uid in the expander outlet is the maximum li4uid formation that can be tolerated in the expander. is not formed in the area of the wheel, but that li4uid is formed in the outlet no))le of the expander. "esidence times in the area of the wheel are on the order of nanoseconds 510A seconds6. This is too short a time for big 5micron Ssi)e6 drops to form. &urther, the ability of some specially adapted turboexpanders to handle li4uids tends to refute this limitation. $n compression systems, maximum compression ratios of three to four are routinely used. The ma2or reason for this limitation is the discharge temperature of the gasF a secondary reason is the loading on the thrust bearings in the machine. Thrust bearing loading
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Natural gas processing course
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problems hae also been adanced as reason for limiting expansion ratios to three or four in turboexpanders. $f the turboexpander is driing a compressor, the thrust bearing loading problems may be eased somewhat and higher expansion ratios may be permissible. ome plants are operating with expansion ratios greater than four with no apparent difficulty. $n spite of this, we normally would not recommend an expansion ratio greater than about four without careful consideration of the potential problems inoled. : oule Thomson ale is normally installed in parallel with the turboexpander. This installation is made for seeral reasons8 516 startupF 56 continued operation if the turboexpander failsF 5=6 permit operation at flow rates aboe the turboexpander limits. o-called constant entropy no))les hae been used in. the space industry for years. The oule Thomson control system might be replaced with one of these constant entropy no))les to maintain turboexpander performance of the plant at high through puts or if the turboexpander is out of serice. :mong the factors that must be considered in the purchase of a turboexpander plant is the high speed of the turboexpander 5rpm ?0006. Because of this high speed, maintenanceNpart replacement problems can crop up particularly in remote installations. nit noise leels are high, special precautions must be ta!en to protect the operator from this noise.
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Co"parison of the oule(Tho"son and Turboexpander Processes
@ow much, if any, difference in the li4uids recoery and temperature leel should be expected between the turboexpander and oule Thomson processesU This 4uestion cannot be answered specifically for eery system, the results will be contingent on the temperature-pressure-compositions encountered in the specific case. @oweer, one can say that if the operating pressure leels are identical, the turboexpander process will generally produce lower temperatures and more li4uids than the oule Thomson process. &or these conditions, the turboexpander process will re4uire less recompression horsepower to recompress to the residue gas to pipeline pressure. learly the turboexpander process has a far better capacity for recoering li4uidsF essentially 100D of the '< are recoered in the li4uid stream, while only #1D of the '< are recoered in the li4uid for the oule Thomson process. This incremental li4uid recoery is due simply to the much lower temperature achieed in the turboexpander process 5about 7;>& lower6.
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Natural gas processing course
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References 1. ". . 3:CC(K , @. *"B:" , V%: (C$T$($% :C I"(*$%W , ampbell Ietroleum eries 5an. 19?6. . VTurboexpander @andboo!W , 3afi-Trench orporation. =. A%I: *ngineering Cata Boo!A *leenth *dition. '. http8NNen.wi!ipedia.orgNwi!iNoule-ThomsonXeffect. ;. http8NNen.wi!ipedia.orgNwi!iNTurboexpander .
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Natural gas processing course
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Thanks ,
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