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HYDROCARBON LIQUIDS EXTRACTION FROM NATURAL GAS

IN A NEWLY COMBINED PROCESS

ARMAND KOSKAS AND ARI MINKKINEN

INSTITUTO FRANCES DEL PETROLEO

Francia

TREVOR WOOD

CHART HEAT EXCHANGERSUK

Presented at

Venezuelan Gas Processors Association (AVPG)XIV International Gas Convention

May 10 - 12, 2000Caracas, Venezuela

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AVANCES TECNOLOGICOS EN GAS

AVPG, XIV Convención de Gas, Caracas, Mayo 10 al 12, 2000. Página 2

Hydrocarbon Liquids Extraction from Natural Gas

in a newly combined process

Armand Koskas and Ari Minkkinen IFP

Trevor Wood Chart Heat Exchangers

Abstract

Today, the natural gas processor is called upon to make deeper and more selective liquids

recovery with a minimum sales gas shrinkage to improve the return on gas processinginvestments. Fortunately, recovered hydrocarbon liquids, including ethane, will be increasingly in

demand to feed the forecasted growing appetite of the world-wide petrochemical industry. Thekeen interest in high liquids recovery at the lowest cost cannot be overstated when it involves the processing of ethane, propane and heavier hydrocarbon rich wet natural gases.

Gas dehydration and liquids extraction processes which are low in cost, easy to operate and makehigh ethane plus recovery with the lowest cold or cryogenic energy requirements are therefore in

high demand. In addition, these processes need to be environmentally-friendly to meet “ever-greener” world-wide standards. A single technology is often not enough to meet all the desiredobjectives. A synergistic combination of technologies is needed.

This paper presents two well proven technologies, each which could stand alone, but whichcombine to give an excellent synergy in gas processing for deep and selective natural gas liquidsextraction. State-of-the-art dephlegmator technology is integrated with the well established,

environmentally-friendly, methanol based Ifpexol process to give an advanced versiontrademarked Dephlexol .

Case studies to be presented show that this process affords significant cost savings whileimproving liquids extraction performance and respecting all environmental standards.

Introduction

Technologies which comprise today's advanced gas processing and NGL extraction schemes arenumerous and well documented in the hydrocarbons processing literature [1]. While the subject

of this paper is not to make an overview of these technologies, it will suffice to say that many of the most successful schemes use turbo-expanders in clever combinations with heat exchangersand distillation columns to achieve the highest and most selective separation of C3 plus

hydrocarbons from ethane and lighter natural gases. Top of the nineties percent propane recoveryis becoming the standard for NGL extraction plants. Today, even ethane recovery is finding a

renewed importance as feedstock to ethane steam cracking projects due to its advantageouslyhigh conversion to ethylene. The resultant deeper liquids extraction and higher selectivitydemand is becoming ever more difficult to achieve in conventional simple turbo-expander

schemes without resorting to deeper cryogenics and complex multiple distillation steps. These areoften prohibitively expensive and difficult to rationalize in today's highly competitive economic

climate unless the liquid product price premiums are significant.

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Simplicity and low cost have become the principal drivers for the development of advancedliquids extraction technologies. To this end, cost effectiveness should be the principal attribute to

promote , not outright liquids recovery performance at any cost. Engineering design should strivefor simplicity; integrate unit operations, combine services and maximize process synergies of

known and proven technologies. A process flow diagram conceived to look uncluttered and

simple, will make that process easier and less costly to operate.One substantial step to simplicity is the reduction of the equipment piece count. This can be

achieved by physically and mechanically combining equipment functions and services instead of piping them as individual sequential steps. A disciplinary synergy of chemical, mechanical and

civil engineering is needed.The process developed by IFP with Marston is a good example of disciplinaryengineering synergy. It is also a good example of a remarkably cost effective solution for deep

liquids extraction from wet natural gases. The brazed aluminum plate-fin heat exchanger operating as a run-back reflux condenser, referred to as a dephlegmator many decades ago, is

integrated to a modern advanced refrigerated or turbo-expanded cold separator process.Dehydration, pre-requisite for cryogenic level freeze protection, conventionally achieved bycomplicated molecular sieve adsorption is replaced by simple low cost methanol injection and

recovery technique within the technology ( i.e. the process)Today there are countless, dephlegmator type, plate-fin heat exchangers in reflux run-back

operation within the refining, chemical, petrochemical and gas processing industries. Marston,can claim a substantial share of the worldwide reference installations in relevant cryogenic gas processing. Likewise, , by the application of the process, is well established today

as a foolproof simultaneous dehydration and gas liquids extraction technology in manyinstallations around the world. The synergistic combination of these two commercially proventechnologies, as the process, is the subject of this paper, attributes and benefits of

which will be discussed and illustrated in ample detail.

Advanced Cold and Cryogenic Gas Processing

All natural gases are water wet and all contain methane as the principal component. The methanegenerally becomes the main and/or bulk saleable product after dehydration, purification andnatural gas liquids (NGL) removal. When gas prices are low and the natural gas is rich in heavier

hydrocarbons, NGL co-product sales can become as/or more important than gas sales. In suchcases, the NGL extraction process configuration needs more scrutiny in order to fully maximize

production revenue.To extract NGL from natural gas, cold and/or cryogenic processing is today still the most reliableand cost effective way of doing so. The degree of cold and the manner in which it is applied

needs to be optimized taking into account feed gas composition ( i.e. richness in hydrocarbonsheavier than methane ), feed and product battery limit pressures and process layout constraints.

Advanced gas processing aims to maximize recovery and selectivity at the warmest coldtemperature levels and minimize refrigeration duties at coldest cold temperature levels. The keyto achieving this at the lowest investment cost is the dephlegmator run-back refluxing type cold

separator process.

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Dephlegmator heat exchange technology

As defined in Perry's Chemical Engineers Handbook [2], Dephlegmation, or partial

condensation (according to them), refers to a process in which a vapor stream is cooled to a

desired temperature such that a portion of the less volatile components of the stream is selectively r emoved from the vapor by condensation . Although the word dephlegmation is rarely

seen or heard in modern English usage today, the concept is seeing a resurgence in cryogenicseparation designs for gas processing and petrochemical plants. One advanced recovery scheme

proposed by a well known engineering firm for ethylene separation from cracked gases is chiefly based on the use of dephlegmator heat exchanger technology [3]. A number of such units have been installed successfully in grass roots and revamp projects.

The deplegmation concept that is incorporated within the Dephlexol process to be described isillustrated schematically and artistically by the drawing of Figure 1 below.

Rich Gas

Lean Gas Outlet

Rich Gas

Cold Fluid # 1

Outlet

Cold Fluid # 2

Outlet

Figure 1

The Dephlegmator Principle

Chilling and partial condensation of up-flowing rich gas in dedicated channels is achieved byindirect counter-current heat exchange with cold fluids in adjacent channels. The condensing

hydrocarbons draining counter-currently downward directly against the vapor in the samechannels cause a refluxing action as in a rectification zone of a structured packed distillationcolumn. The end result is that mass and heat transfer take place simultaneously and the

composition of the gas leaving the up-flowing channels is much leaner in heavier hydrocarbonsthan the gas entering them.

Marston's experience

Since the early 60s Marston, today a part of the Chart Industries Incorporated, has fabricated andinstalled over 50 dephlegmator type brazed aluminum plate and fin heat exchangers around the

world. Referred by Marston as reflux or run-back condensers, operating pressures have rangedfrom 8 (120) to 44 Bar (640 psi) and W/H/L core sizes from 150/225/400 mm to 600/759/3750mm. Reflux or dephlegmator condensing heat exchangers are used in many other applications

besides gas processing and ethylene recovery [4]. Among the myriad of applications are helium

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and argon extraction from natural gas, hydrogen and CO2 purification and ammonia purge gasseparation.

Manufacturers of brazed aluminum plate-fin heat exchangers since 1950, Marston is firmlyestablished as a world-leader in compact heat exchange technology. In the specific area of

dephlegmators, Marston have developed high efficiency exchangers with sophisticated internals

design which accommodates simultaneous heat and mass transfer effects without flooding. Aspecial corrugated design is used for the counter-current refluxing stream, which delivers the

optimum combination of free-flow area and hydraulic mean diameter. High heat and masstransfer performance is achieved with limited pressure drop and an acceptably safe and

predictable margin from flooding. To become successful, takes considerable know-howaccumulated over years of design experience and operating unit feed-back coupled with the latest3D CAD and most rigorous computer simulation programs.

Moreover, in the early to mid nineties, IFP had undertaken advanced pilot scale research on theuse of plate-fin heat exchangers in dephlegmation service at their Solaize ( near Lyon ) R&D

center. The process under development jointly with NAT ( today PROSERNAT) at the time wascalled Extrapack [5]. The full commercialization was not achieved due to various circumstances, but not related to the economic interest of the process. The documented know-how gained from

this pilot experience was fruitful to establish an extensive technological expertise within IFP for the design and simulation of dephlegmation type processes. Today this expertise is used to good

advantage in the design of the Dephlexol process when using the state-of-the-art dephlegmator technology from Chart Marston.

The state-of-the-art

Figure 2 depicts a single core of a modern state-of-the-art reflux condenser or dephlegmator. A

number of parallel cores are positioned vertically to permit gravity flow liquid run-back and theseare manifolded together to provide the appropriate surface for almost any application and

capacity.

Figure 2

Refrigerant

Refrigerant

Liquid RefluxFeed Gas

Export Gas

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The Ifpexol Technology

The Ifpexol technology using methanol as a single solvent for dehydration, hydrocarbon liquidsextraction and acid gas removal was developed by IFP over many years of laboratory, benchscale and pilot plant research and development effort. The first patents covering the process date

back to 1986 [6] and since this time many additional improvement patents have been awardedaround the world. The first Ifpex-1 unit successfully started up in June 1992 in Canada at Petro-

Canada's East Gilby gas plant [7]. Today the Ifpexol technology is fully commercialized and assuch is a viable answer to many worldwide gas processing objectives from dehydration, dew point control, NGL extraction to acid gas removal. The installation of an Ifpex-1 unit in Alberta,

Canada, treating 140 MMScfd of wet natural gas for AEC West at their Sexsmith gas plant in a propane refrigerated cold process is a typical example of a simple foolproof application of this

technology [8].

Ifpexol Process

The Ifpexol process consists of two integral parts, each of which can also stand-alone; Ifpex-1and Ifpex-2. In the Ifpex-1 process, shown in simplified flow diagram of Figure 3, water

removal (dehydration) is achieved simultaneously by co-condensation with hydrocarbons in acold process. Methanol presence in appropriate concentration assures hydrate and ice freeoperation of the cold process down to temperatures below minus 90°C (minus 130°F). Methanol

is simply recovered from the decanted aqueous phase by stripping with a slip stream of feed gas.Since the beginning of the year 2000, this technique is now industrially proven in 15 operating

installations around the world and is gradually being recognized as the most simple, trouble freeand environmentally friendly dehydration process available [9].

Cold

Process

Methanol

Reject Water

Raw NGLWet RichFeed Gas

Dry Lean Sales Gas

Figure 3

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The second step of gas treatment, if required, is acid gas removal. This is achieved with theIfpex-2 process (not shown). Refrigerated methanol solution removes H2S and other sulfur

compounds such as COS and mercaptans by their high solubility in cold methanol. CO2 with

lower solubility is also removed but to a lesser and controllable extent. This process technology

is not a pertinent subject of the present paper. It is, however, amply covered in a number of

recent published references dealing with acid gas removal [10].

Ifpex-1 for Dehydration and NGL Extraction

As shown in the typical process flow diagram above, the inlet feed gas is split into two streams;one stream flows counter-currently through the Ifpex-1 contactor against the rich methanol

solution, while the other stream by-passes the contactor. The by-pass stream and the Ifpex-1contactor outlet are recombined, methanol make-up is injected and then the entire stream is fed tothe cold process. During its subsequent cooling, the gas stream is chilled to the required

temperature, thereby condensing simultaneously hydrocarbons and a methanol/water mixture.The two liquid phases and the gas phase are separated in a three phase low temperature separator

(LTS); the methanol/water mixture (heavy phase) is recycled back to the contactor, while theresidual gas and hydrocarbon liquid phases leave the Ifpex-1 section.In the contactor, the flow of gas is counter-current to the downward flow of methanol/water

mixture. During its passage upwards through the packed column, the relatively warm gas is ableto strip the methanol from the mixture due to methanol's high volatility. The methanol/water

mixture gets progressively leaner with respect to methanol during its downward passage throughthe packing.As the gas entering the contactor is water saturated, it does not have any additional water

carrying capacity and hence the water makes it way down the contactor and out at the bottom.The water leaving the contactor contains only traces of methanol. The flow split of inlet gas thatis fed to the contactor is optimized to minimize the total cost of the column, including packing.

The water from the Ifpex-1 contactor is of high quality, similar to steam condensate. When theIfpex-1 unit is located downstream of an amine unit, this stream provides a source of high quality

make-up water for the amine system and also recovers any amine carryover.As indicated in Figure 3, a small methanol stream is introduced ahead of the heat exchangers tomaintain methanol concentration in the water boot to compensate methanol exits from the

process with the hydrocarbon vapor and liquid streams leaving the low temperature separator.The methanol is not lost from the production slate, as it becomes a part of the products adding

fuel value nearly equivalent to that of methane. Moreover, the make-up methanol does not needto be completely free of water. Any water coming in with the make-up will be convenientlyremoved ( de-watered) to the Ifpex-1 contactor bottoms.

Methanol Recovery from Hydrocarbon Liquids

To reduce methanol inventory logistics and operating costs with deep liquids recovery schemesas will be described further on, water wash techniques can be used to recover methanol from the

hydrocarbon liquids. Conventional proven techniques, incorporating static mixer settlers and/or coalescors can be used. For very high recovery, multistage separators or packed wash columns

can also be considered as depicted in Figure 4 below.

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The Ifpex-1 contactor bottoms water can conveniently serve as the make-up source of circulatingwash water. The return of dilute wash water with the recovered methanol is received in the same

Ifpex-1 contactor at some intermediate level. This imposes a dual bed contactor for a singlecolumn or a second contactor expressly designed for water wash service. In the latest designs,

the second contactor approach is recommended with the two contact services combined in the

same column, as shown, each receiving a slip stream of water saturated methanol free strippinggas from the feed ( generally a 50/50 split ).

Cold ProcessCold Process

MeOH

IFPEX-1 Contactor

WetGas

Feed

MeOH Water Return

Clean Wash Water

Dry Lean Sales Gas

Stabilizer

Water Wash

Column

MeOH free NGL

Reject Water Figure 4

Most of the 15 operating Ifpex-1 units now incorporate some manner of water wash for methanolrecovery. The overall methanol consumption is substantially reduced for systems operating in

cryogenic applications since vapor losses become negligible at very low temperature and theliquids are easy to wash for high methanol recovery.

Environmental Considerations

Benzene and other aromatics that are present in the inlet gas feed stream are classified ashazardous air pollutants. Conventional glycol dehydration processes will absorb the benzene and

other aromatics in the feed stream along with some heavier hydrocarbons. These are then emittedto the atmosphere during the regeneration process. In many cases an incinerator is required todestroy the off-gases from the glycol regenerator before they are emitted to the atmosphere,

resulting in even greater amounts of emissions.A molecular sieve process requires an external heat source for regeneration of the molecular

sieve beds which also results in the emission of greenhouse gases to the atmosphere.

The Ifpex-1 process, on the other hand, is an environmentally friendly process in that there are noemissions whatsoever. The Ifpex-1 process uses the inherent energy available in the feed gas

stream being dehydrated to accomplish the stripping of the methanol laden water stream returnedfrom the cold process. Furthermore, water is recovered at the bottom of the contactor as a high

quality water stream which can be re-utilized as mentioned. This improves water conservationwhich can be an important factor in certain water scarce areas of the world.

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Dephlexol, the Ideal Process Synergy

The combination of the Ipex-1 process for the dehydration aspect with an advanced cold processscheme incorporating a dephlegmator for the NGL extraction aspect gives an ideal synergy in the

process.

In the most simple form as depicted in Figure 5 the basic Ifpex-1 methanol stripping system islocated upstream of a externally refrigerated cold process where the dephlegmator reflux run-

back condenser is mechanically integrated with the cold separator drum.

Wet RichGas Feed

Water

Dry Lean Sales Gas

External Refrigeration

Dephlegmator

IFPEX-1 Contactor Methanol

NGL

Cold Separator

Figure 5

As previously explained, raw wet feed gas is split into a by-pass and a stripping gas stream. Theslip stream of water saturated, methanol free stripping gas is passed to the bottom of thecontactor. The upwards flowing gas strips essentially all the methanol contained in the decanted

methanol laden water entering the top of the contactor. A methanol free water stream,representing the moisture removed from the feed gas, is withdrawn from the bottom of the

contactor. This water stream can be recovered for re-use.The overhead vapor is combined with the by-pass gas and the mixture containing the recoveredmethanol is passed to the cold process. An additional methanol make-up is injected to the gas

before the chilling. A pre-cooling in a gas/gas heat exchanger ( which can also be a plate-finexchanger) precedes the dephlegmation step, where the deepest chilling is achieved. As the cold

separated vapor containing residual methanol passes upwards through the refluxing channels it is

progressively chilled by the counter-current down-flowing cold streams and refrigerant inadjacent channels. Condensing hydrocarbons and residual water are continually drained

downward against the up-flowing vapor creating a fractionation zone which rectifies thecomposition of the residue gas thus restricting substantially its equilibrium C3 plus content.

Since the uppermost vapor contains the least amount of condensable hydrocarbons, the lowesttemperature chilling duty is greatly reduced thereby economizing significantly on the highest costrefrigeration demand. Moreover the bulk NGL liquid fraction in the cold separator is separated at

a warmer temperature resulting in a substantially lower light ends content and lower methanol

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concentration requirement for freeze protection. These factors afford processing benefits such aslower downstream stabilization duty and reduced methanol circulation and make-up.

Auto-Freeze protection within the dephlegmator

The cold separated hydrocarbon rich vapor from the cold separator which is passed upwards tothe bottom open passages of the integral vertical dephlegmator contains equilibrium vapor

pressure methanol and water. Due to the much higher volatility of methanol relative to water,over ten times more methanol than water is always present in the up-flowing rich gas. This over

abundance of methanol in the rich vapor has been shown in IFP's analysis to be auto-protective tofreezing upon further chilling in a dephlegmator type heat exchanger. As the residual water condenses with decreasing temperatures it absorbs by the high solubility of methanol in water

more than adequate amount of methanol from the vapor phase to give an non-freezingmethanol/water concentration in the disperse aqueous liquid phase. Together with the methanol

dissolved in the co-condensing continuous hydrocarbons phase and the resultant commingling of the two liquid phases results in an non-obstructive counter-current downward flow of the coldliquids, water phase occluded in the hydrocarbons, to a warmer zone against the warming vapor.

In any event, the detail mechanical design of the dephlegmator incorporates emergencyintermittent hard piped methanol injection and a special Marston designed proprietary

distribution system at the top of the rising vapor passages. This injection, while not needed innormal full capacity operation, will give positive assurance of trouble-free operation at start-upand turndown conditions.

High Pressure Gas Processing

When the wet rich gas feed to the gas processing plant is available at such high pressure that NGL extraction by chilling alone is not practical, if not impossible, then pressure letdown by

expansion is the best approach. Such schemes are well established in the gas processing industryand have the advantage of providing the chilling duties required by auto-refrigeration due to the

expansion of high pressure gas. This type of scheme is also well suited for the Dephlexol processas depicted in the simplified process flow diagram of Figure 6.

Wet RichGas Feed

Water

Dry Lean Sales Gas

Stabilized NGL

IFPEX-1 Contactor

Dephlegmator / Cold Separator

Methanol

Figure 6

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High pressure, wet rich gas feed is treated in the same manner as previously described beingdehydrated by chilling in the presence of methanol while the methanol is recovered by the Ifpex-

1 process technique. After pre-chilling in the gas/gas heat exchanger, the first high pressure (HP)three phase cold separator removes the condensed water/ methanol stream to the Ifpex-1

contactor and a first cold hydrocarbon condensate to be sent to the top of the downstream

stabilizer. This high pressure condensate, when flashed across a level control valve to thestabilizer pressure, creates a very cold vapor and liquid feed to the top of the column. The liquid

portion serves as a very beneficial reflux to reduce the loss of heavier hydrocarbons to theoverhead whereas the vapor portion is passed to the overhead line connected to the cold separator

to be rectified in the dephlgemator.

At the HP separator, the high pressure vapor, containing equilibrium concentration of methanol,

water and hydrocarbons yet to be recovered, is passed through a turbo-expander to let down the pressure. The isentropic expansion results in a sharp temperature drop and condensation of

residual water/methanol and hydrocarbons and the release of motive power to be harnessed in there-compression of the final residue gas. The methanol contained in the turbo-expander suction isgenerally enough to prevent freezing at the discharge. A dedicated methanol injection quill to the

turbo-expander suction can be foreseen in case of freezing risk.This three phase discharge from the turbo-expander is passed to the top of the multi-channeled

dephlegmator containing an integral purposely designed vapor/liquid separator/distributor. Bothextremely cold phases pass downward through their respective channels providing the principalauto-refrigeration to the dephlegmation process. The unwanted lighter components in the cold

liquid phase that are re-vaporized provide a substantial amount of the refrigeration duty. Some of the wanted hydrocarbons also re-vaporize but are subsequently recovered by rectification actionin the dephlegmator.

The reheated turbo-expander fluids are collected and exit at the bottom of the dephlegmator to be

passed to the low pressure cold separator mechanically linked beneath the dephlegmator assembly. Also fed to this cold separator is the rich overhead vapor from the stabilization columnwhich can be designed either as a demethanizer (for the case of ethane plus recovery) or as a

deethanizer (for the case of propane plus recovery). The combined rich vapors flow upwardsthrough the refluxing channels of the dephlegmator while being chilled by the counter-current

cold stream in adjacent channels as previously described. Again the equilibrium methanol contentof the rich vapor stream being nearly ten times higher than that of residual water preventsfreezing during the upward passage of the vapor stream. The resultant collected residual

water/methanol heavy liquid phase run-back and feeds are separated from the cold hydrocarbon NGL in a water boot. This stream is returned via the HP separator water boot to the Ifpex-1

contactor for methanol recovery. The separated cold NGL stream is then pumped to the top of thestabilizer column where the dissolved methane and/or ethane is stripped accordingly. Thestabilizer column is essentially a simple stripper with a single feed to the top. Rigorous analysishas shown that there is only very marginal benefit with a separate feed location for the pumpedliquid feed. A single feed is simpler and less costly. The bottoms of the stabilizer containing

dissolved methanol can then be passed to a water wash system as was shown in Figure 4 for complete recovery of methanol.

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Stand-alone dephlegmation process

The very same IFP patented HP turbo-expander cold process scheme shown in Figure 6 can also be considered without the Ifpex-1 dehydration system pertaining to full Dephlexol process

schemes. A molecular sieve or glycol dehydration system could precede the cold process and

provide completely water dry gas to the dephlegmation section. While such a scheme can givethe equivalent NGL recovery performance as Dephlexol it should be noted that it would be more

complicated with a higher equipment count and higher investment cost. According to theauthors, the interest of such a configuration is mostly for revamp schemes which already have a

molecular sieve or glycol dehydration rather than for grass-roots applications. Moreover, theenvironmental attributes of the Ifpex-1 dehydration scheme would not be retained if glycol and/or molecular sieves dehydration was used.

Case Studies of the Dephlexol Process

A number of case studies have been undertaken to evaluate the recovery performance benefits of

various Dephlexol process configurations. The most simple schemes consist of refrigerated cold processes with the addition of the dephlegmated cold separator as was depicted in Figure 5already described.

A Simple Case Study Comparison

The first simple case study concerns a rather low pressure (LP) gas feed where it is desired torecovery as much NGL as possible with a fixed external refrigeration duty for both. A

conventional scheme of gas/gas heat exchange and propane refrigerated chiller leading to a coldseparator drum without dephlegmator as in Figure 3 is compared with the scheme depicted inFigure 5 having a dephlegmator integrated with the cold separator. The process simulations and

cost analysis was conducted on the basis given in Table 1 below: Table 1

Basis for Study

Feed Gas Capacity 2.8 MScmd (100 MM Scfd)

Typical rich gas LHV 1000 Btu/Scf

Gas price 2.5 $ / MM Btu

LPG price 175 $ / ton

Energy consumption Same for both cases

Dephlexol extra investment cost 150 000 $

Both schemes were simulated rigorously and the results of the simulations were evaluated in

respect to differences in the production of sales gas and LPG and the gross revenues generated.The comparative results are tabulated in Table 2 hereafter:

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Table 2Comparative Results

Process Conventional Dephlexol

LPG tons / year 64 800 82 400

Sales gas trillion Btu / year 32.0 31.3Gross production revenue 91.0 million $ 92.0 million $

Increase revenue 1.0 million $ / year

Simple payout for extra cost 2 months

The above results show a very quick payout time for the addition of a dephlegmator to aconventional propane refrigerated cold process. This type of application is very easy to retrofit toan existing gas plant with or without an Ifpex-1 upstream dehydration process.

A High Pressure Lean Gas Case Study

The second case study undertaken with considerably more scrutiny with the help of aninternationally reputed engineering company concerns the comparison of the patented HP turbo-

expander Dephlexol scheme with a leading competitive advanced cryogenic scheme. The basisfor this real case study for an North African gas field is given in the Table 3 below. Note that the

gas is very lean in C3 plus, having less than 2 gal/Mscf (GPM), making high propane recoveryrather challenging.

Table 3Basis of Study

Capacity 9.2 MScmd (325 MMScfd)

Feed Gas Pressure 70 Bar abs.Residue Gas Delivery Pressure 80 Bar abs.

Lean Gas Composition (C3 plus GPM) (1.70 ) see Table 4

Objective Recover over 95% of the C3 in the feed

Table 4

Gas Composition

Component Mol% Component Mol%

Nitrogen 0.62 i Pentane 0.30

CO2 3.64 n Pentane 0.268

Methane 83.19 Hexane 0.33Ethane 7.35 Heptane 0.1897

Propane 2.63 Octane 0.1665

i Butane 0.44 C 9 Plus 0.1208

n Butane 0.735 Total 100.000

The competitive scheme for the sake of this comparative evaluation can be depicted in Figure 7.

This scheme consists of a molecular sieve dehydration upstream of a classical brazed aluminum

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plate fin heat exchanger configuration and a two column ethane refluxed non- reboileddemethanizer absorber coupled with a refrigerated refluxed deethanizer.

1

WetGas Feed

Molecular Sieves

NGL

C3 Refrig

Regeneration

Figure 7

Dry Residue Gas

The Dephlexol scheme which achieves 95.6% propane recovery performance is basically thesame as was depicted in Figure 6 with the addition of a booster compressor to be able to deliver

the residue gas at 80 Bar Abs. and a water wash system to recover the methanol from the NGL asshown in Figure 8 below. The water wash considered is a single stage mixer coalescer designed

for 90% methanol recovery. Remaining methanol is recovered in the downstream "Pasteurized"depropanizer overhead drum ( not shown) while removing the residual soluble and entrainedwater wetness of the water washed NGL by distillation [11].

T u r b o E x p a n d e r S c h eme

HP Wet

Gas Feed

Dry residue gas

Methanol

NGL

Water

Figure 8

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A comparative summary of the main process features and power inputs of each scheme is given

in Table 5 hereafter together with the total equipment piece count.

Table 5

Summary of Requirements

Scheme Competitive Cryogenic Dephlexol

Dehydration Molecular sieves Ifpex-1

Regeneration power MW 1.0 negligible

Chilling train Plate-fin exchangers Dephlegmator

Turbo-expander yes yes

Propane refrigeration MW 6.0 none

Booster compression MW 16.0 24.0

Distillation columns two one

Water wash none yes

Major equipment count 24 18

The competitive cryogenic scheme attains a propane recovery in excess of 95% due to the

replacement of equilibrium lost propane in the overhead vapor of the demethanizer column bysub-cooled ethane reflux. However to provide the ethane wash, an overhead refrigerated

deethanizer column was used with a reboil duty of 11.0 MW at 110°C. A 6.0 MW propanerefrigeration system, including air cooled condensers, is required to make the ethane reflux.The Dephlexol scheme contains no external refrigeration but to achieve the same propane

recovery in excess of 95% a deeper turbo-expansion with subsequently higher booster compression power is required. The trade-off with refrigeration power consumption of the

competitive scheme is almost compensated by its absence in the Dephlexol scheme. The

Dephlexol scheme as shown in Figure 8 requires a water wash system to keep the methanolconsumption reasonably low. But in any event, the methanol consumption cost can be kept below

or equivalent to the molecular sieve replacement consumption costs of the competitive scheme.Moreover the recycle, heating and cooling of regeneration gases adds approximately 1.0 MW of

power consumption to the competitive scheme which is not required for the Ifpex-1 system. Thereboil duty of the stripper column in the Dephlexol scheme is 10.0 MW at 70°C allowing aslightly lower heat consumption but, more importantly, at a significantly lower waste heat level.

A comparative tabulation of the investment costs of the two equivalently performing schemes is presented in Table 6 hereafter:

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Table 6Comparative Installed Cost in million U.S. Dollars (1st Qt.1999 basis)

Scheme Competitive Cryogenic Dephlexol

Dehydration 5.0 2.0

Chilling train 2.3 4.0Motive equipment 12.7 16.6

Propane refrigeration 9.0 none

Separators 3.7 5.4

Distillation 6.0 1.0

Total (TIC) 38.7 29.0

Savings afforded 9.7 million dollars

The preceding tabulation accredits the Dephlexol process scheme with a 25 % installed cost

savings over this one example competitive cryogenic process. A comparison of the operating

costs for this example is summarized in Table 7 below:Table 7

Comparison of Operating Costs in million US dollars/year

Variable Costs Competitive Cryogenic Dephlexol

Energy @ 2.5 $/MMBtu 5.2 5.4

Chemicals / Supplies 0.4 0.4

Sub total 5.6 5.8

Fixed Costs

Maintenance/Labor @ 5%TIC 1.9 1.4

Capital Charges @ 10%TIC 3.8 2.9

Sub total 5.7 4.3Grand Total 11.3 10.1

Savings afforded 1.2

Despite having a slightly higher energy consumption, the overall operating costs, taking into

account the above fixed costs structure, affords the Dephlexol scheme with about 10% savingsover the competitive cryogenic scheme. Other optimized cryogenic schemes can obviously bedesigned and engineered to give closer capex and/or opex results and thus more or less savings.

However, the objective of this paper was not to compare only cost savings with potentialchallengers. Most noteworthy intangible advantages of the Dephlexol process are its

environmental, security and simplicity attributes for which costs are difficult to ascertain:

• No emissions of volatile organic compounds (VOCs)

• Saturation water can be recovered for re-use

• No fired heaters required; less maintenance higher security

• Less overall CO2 emissions

• No intermittent solids handling required

• No dust filter and/or glycol filter replacements

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• Minimized volatile hydrocarbon inventory ( i.e. no propane refrigerants)

• Higher safety for operating personnel

• Lower foot print for space limited applications

• Operationally and environmentally friendly

Conclusion

In the pursuit of higher returns on gas processing plant investments, the importance of

maximizing NGL recovery at the lowest cost and minimizing sales gas shrinkage cannot beoverstated in today's economic climate. Simplicity is the key to lower costs and to this end water

in gas plant feeds should be accommodated; not extracted. Methanol protection is the best way toaccommodate water to deep cryogenic levels and the Ifpex-1 process is the least costly way torecover methanol.

To maximize production revenues, deep NGL extraction needs not only efficiency but selectivity

as well. In this regard, the modern adaptation of the dephlegmator principle is the answer. To

optimize the performance of the dephlegmator experience and know-how coupled with the latestand most rigorous computing resources is a must.

The synergy provided by the Ifpexol technology from IFP with the plate-fin heat exchanger expertise from Chart Heat Exchangers Division is incontestable. This provides the foundation for

the Dephlexol process for deep NGL extraction with many tangible and intangible benefits.

References

[1] GPSA Engineering Data Book 11thedition 1998 Volume II Section 16

Gas Processors Suppliers Association Tulsa Oklahoma USA[2] Perry R.H. and Green D.W. Perry's Chemical Engineer's Handbook 6th edition p. 18-3

[3] Prickett R.D.,Bush K.E. and Cruey G. " " Hydrocarbon Processing March 1998[4] Finn A.J, Chemical Engineering

May 1994 p. 142-147[5] Rojey A., Larue J., Minkkinen A. and Amande J.C.

4th. EC Symposium Berlin 1992

[6] Rojey A., Larue J. et al. U.S. Patent 4 775 395 October 1986[7] Minkkinen A., Larue J. and Patel S. "

" Oil & Gas Journal June 1, 1992 p. 65[8] Holcek R.G., Minkkinen A.

presented at the CGPA quarterly meeting October 1996 .

[9] Johnson A. and Moore B.K." AEC West" Proceedings of the 1997 Laurance Reid Gas Conditioning Conference

Norman Oklahoma USA[10] Minkkinen A., Rojey A., Charron Y. and Lebas E. "

" Les Entretiens IFP May 14, 1998, p 56-80 Gas Cycling A new approach

editions Technip 1998[11] GPSA Engineering Data Book 11th edition 1998 Volume II Section 20

p 20-43 Gas Processors Suppliers Association, Tulsa Oklahoma USA

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