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Chemical Engineering Science 56 (2001) 4979–4991www.elsevier.com/locate/ces

Hydrate plug prevention by anti-agglomeration

Z. Huoa, E. Freer a, M. Lamar a, B. Sannigrahi b;c; 1, D. M. Knaussc, E. D. Sloan Jr.a;∗

aDepartment of Chemical Engineering, Center for Hydrate Research, Colorado School of Mines, 1500 Illinois Street,

Golden, CO 80401-1887, USA bDepartment of Chemistry, Clark Atlanta University, 223 J.P. Brawley Dr., S.W. Atlanta, GA 30314, USA

cDepartment of Chemistry, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401-1887, USA

Received 11 January 2001; received in revised form 6 April 2001; accepted 30 April 2001

Abstract

Dispersing hydrates into a condensate phase by anti-agglomerants is an alternative to kinetic or thermodynamic inhibitors to prevent hydrate plug formation in a gas production pipeline. In this work, both commercially available surfactants and synthesizedanti-agglomerants were tested in high-pressure apparatuses at typical pipeline conditions. Candidates from families of commerciallyavailable surfactants, chosen based on their hydrophilic–lipophilic balance (HLB), were tested in an H2O-hydrocarbon mixtureof 30% water and 70% octane (volume). It was found that, at 3 wt% of the water mass with a synthetic natural gas, somecommercial surfactants (Span 20, Span 40, Span 60, Span 80) could keep hydrate particles suspended in a range of condensate typesand shear numbers at 4

◦

C and 8:27 MPa. However, a synthesized chemical dodecyl-2-(2-caprolactamyl) ethanamide was a moreecient dispersant at 0:75 wt% of the water mass. Both synthesized and commercial chemicals passed 5-day shut-in tests basedupon torque measurements and visual hydrate observations. Flow-loop testing is needed to extend this work to ÿeld applications.? 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Pipeline; Hydrate; Plug; Prevention; Dispersion; Anti-agglomerant

1. Introduction

Hydrates are crystalline inclusion compounds in whichguest molecules (eg. CH4; CO2) stabilize the cagesformed by hydrogen-bonded water molecules at lowtemperature and high pressure. In order for hydrates to

be stable, the guest must occupy a minimum fraction of the cages. The hydrate structure is primarily determined

by the size of the guest molecules. Hydrate is a problemto the oil and gas industry because it can form plugsin pipelines under certain T; P conditions and causeremarkable production loss.

The past decade has seen the gas and oil industry mov-ing towards deep-water exploration and production (cur-rently over 10,000 feet of water depth (Mehta, Walsh,& Lorimer, 2000), where pressures and temperaturesare ideal for hydrate formation. This has brought new

∗ Corresponding author. Tel.: 1-303-273-3723; fax: 1-303-273-3730.

E-mail address: [email protected] (E. D. Sloan).1 Current address: Department of Chemistry, Clark Atlanta Uni-

versity, 223 J.P. Brawley Dr., S.W. Atlanta, GA 30314, USA.

challenges for hydrate prevention in transportation. Ma- jor eorts are being put forth to use traditional thermody-namic inhibitors and new inhibition measures to preventhydrate blockage.

Thermodynamic inhibitors prevent hydrate formation by shifting the equilibrium conditions so that lower temperatures and higher pressures are required to form hy-drates. The eectiveness of these inhibitors is well known,

but large concentrations are needed, which sometimesimpact project economics.

A second prevention method is by kinetic inhibitors,which do not shift the hydrate equilibrium conditions;rather, they decrease the rate at which hydrates form, pre-venting plugs for a period longer than the free water res-idence time in a gas line. Kinetic inhibitors are eectiveat lower concentrations than thermodynamic inhibitors,

but they do not perform well at pipeline= well shut-inconditions or at higher subcooling (i.e. T , the dier-ence between hydrate equilibrium temperature and oper-ating temperature at a given pressure). As hydrocarbonexploration moves to deeper water, inhibitors eective atlarger T are necessary.

0009-2509/01/$- see front matter ? 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 0 0 9 - 2 5 0 9 (0 1 ) 0 0 1 8 8 - 9

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4980 Z. Huo et al. / Chemical Engineering Science 56 (2001) 4979–4991

A third method, called anti-agglomeration, is intendedto be eective at very high T or at shut-in conditions(Behar, Kessel, Sugier, & Thomas, 1991; Sloan, 2000).Anti-agglomerants (AA’s) suspend hydrate crystals incondensate because the ends of AA molecules have qual-ities attractive to both hydrates and oil. This combination

leaves hydrates dispersed as small masses in oil and pre-vents the accumulation of hydrate under proper water = oilratios. This method, while not preventing hydrate forma-tion, does prevent hydrate blockages in pipelines. Somenatural surfactants have shown positive results, but itwas dicult to separate and characterize those surfactants(Fordedal et al., 1996; Jakobsen et al., 1996). The pur-

pose of this work was to investigate synthetic surfactantswhich might act as anti-agglomerants.

2. Apparatuses and experimental procedures

Three dierent types of apparatuses were used to testthe performance of anti-agglomerants for dierent pur-

poses.

2.1. High-pressure apparatuses

Two identical high-pressure apparatuses (HPA) wereused to evaluate the performance of new inhibitors atsimilar conditions to sea-oor pipelines; an apparatusschematic is shown in Fig. 1. Each HPA consists of an Autoclave Engineers’ reactor (rating: 32:75 MPaat 204

◦

C; volume: 300 cc; model: 88-03300-1) con-

nected to an AutoclaveJ

high-pressure reservoir (rating: 40 MPa at 343

◦

C; volume: 500 cc, model:76-3202-1). The stainless-steel reactor was placed in atemperature-controlled water bath, while the reservoir stayed at room temperature. A custom-designed im-

peller mixed the reactor contents. The impeller shaft wasmagnetically coupled to a constant DC motor made byDayton Electrics (2A846A). A refrigeration unit and twoimmersion heaters allowed for temperature control of thewater bath. The pressures and temperatures of the sys-

Fig. 1. High-pressure apparatus.

Table 1Green Canyon gas

Component Mole fraction

Methane 0.872Ethane 0.076Propane 0.031

n-Butane 0.008iso-Butane 0.005n-Pentane 0.002iso-Pentane 0.002 Nitrogen 0.004

tem were monitored by a data acquisition system (DAS)from Intellution Inc. (V4.01) through pressure transduc-ers (Heise, model 623; accuracy: ±0:17 MPa) and ther-mistors (Omega, ON402-pp, accuracy: ±0:1

◦

C). Reactor pressure was also monitored by a Heise pressure gauge(OM86849, accuracy: ±34:5 KPa). In this paper, ratings

and accuracy statements are based on manufacturer’sspeciÿcations.

The reactor was thoroughly cleaned after each testwith methanol and DI water, and then dried with com-

pressed air. The reactor was ÿlled with a mixture of 36 ccde-ionized water and 84 cc octane together with certainamount of chemicals. After equilibrated with bath temper-ature,it was then charged with Green Canyongas (Table1)to 0:7 MPa and vented three times to purge air before be-ing charged to the desired pressure and starting the mo-tor and the data acquisition system. The reactor was alsomaintained at constant pressure with gas charged from

the reservoir via the regulator. The reservoir was usuallycharged to 35 MPa before starting experiments. Systemtemperature and pressure were reported every minute tomonitor hydrate formation. Green Canyon gas was a syn-thetic natural gas similar to the gas produced in the GreenCanyon area in the Gulf of Mexico. It was obtained fromScott Specialty Gases. All chemicals were tested with thisgas in HPA.

In all tests, the reactor liquid content was 30% (vol-ume) water and 70% condensate with a total amount of 120 cc; the bath was maintained at 4

◦

C, and the pressureat 8:27 MPa (typical sea-oor production conditions (Er-ickson & Brown, 1994)). The estimated subcooling was11:5

◦

C at a ÿnal pressure of 8:27 MPa. Chemical concen-trations were always based on water mass. The mixingspeed was 300 rpm. As suggested by industrial partners,the condensates tested were hexane, heptane, octane, de-cane, dodecane and hexadecane, and they were obtainedfrom Fisher Scientiÿc Company with 99+% purity. Con-densates were used as received without further checkingtheir purity.

As shown in Table 2, the gas composition in HPAis no longer the same as Green Canyon gas due to the

presence of octane. All gas composition and subcooling predictions in this paper were based on CSMHYD (Sloan,

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Z. Huo et al. / Chemical Engineering Science 56 (2001) 4979–4991 4981

Table 2Gas composition in HPA



i-Butane 0.0006n-Butane 0.0008 Nitrogen 0.0061n-Pentane 0.0001i-Pentane 0.0001Octane 0.0002

Fig. 2. Sapphire screening apparatus.

1998). The uncertainty for gas composition was 5%, andthe subcooling calculation was within 0:5

◦

C.

2.2. Sapphire screening apparatus

A sapphire screening apparatus was used to test the per-formance of combinations of AA and poly(vinyl capro-lactam) (PVCap) at low mixing compared to HPA. Asshown in Fig. 2, the sapphire screening apparatus (SSA)consisted of 10 sapphire tubes with a maximum operat-ing pressure of 10:34 MPa at room temperature. Each of the tubes had an internal diameter of 1:07 cm, wall thick-ness of 0:1 cm, and length of 15:24 cm. Each tube con-tained a stainless steel ball with a diameter of 0:89 cmwhich rolled as the tube was rocked back and forth.

Table 3Gas composition in SSA tubes



i-Butane 0.0002n-Butane 0.0002 Nitrogen 0.0063n-Pentane 0.0000i-Pentane 0.0000Octane 0.0002

This ball had two functions: it enhanced mixing in thetubes and it indicated hydrate formation upon cessationof rolling. All 10 tubes were mounted in a stainless steelrack that was rocked at a 30

◦

angle on both sides every7 s by a step motor made by Superior Scientiÿc com-

pany (MO93-FF-206). Each tube was operated as an in-dependent isochoric reactor. As shown in the ÿgure, eachsapphire tube was ÿtted with a proximity sensor. The

proximity sensor created an electromagnetic ÿeld in eachtube. When the rolling ball passed the sensor, the elec-tromagnetic ÿeld was broken, creating a recorded elec-tronic impulse. Once hydrate plug formed, the ball was

blocked from traveling. We used “ball stop time” asthe criterion for hydrate plug formation in the sapphirescreening apparatus. Better inhibitors allowed longer ballstop times. The entire assembly was submerged into atemperature-controlled bath (0:5

◦

C). Data from this ap-

paratus were obtained by the same DAS.The solution to be tested (80% octane) was preparedand injected into each tube while out of the bath. Therack was then submerged into the temperature-controlled

bath. In order to obtain high T at low pressure, the tubeswere ÿrst charged with propane (99.92% purity, GeneralAir Company) to 0:24 MPa while the rack was rocked tosaturate propane in the condensate. The tubes were thencharged to 3:45 MPa by Green Canyon gas from the topof each tube while the rack was vertical. The rack wasÿnally set in the horizontal position and rocking began.By adopting this charging procedure, a propane-rich va-

por (Table 3) was obtained and the apparatus was oper-

ated at a subcooling of approximately 13:8◦

C with a ÿnalworking pressure of 2:62 MPa.

2.3. Optical cell

Optical experiments used a high pressure visual cellto observe the performance of dierent chemicals. Asshown in Fig. 3a (top view), this cell was constructedof three brass plates. The middle plate contained a 5 ccTeon sample chamber bound on either side by sapphirewindows. The two outer plates sealed the sample chamber and had a maximum internal pressure rating of 15 MPa

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Fig. 3. High-pressure visual cell. (a) top view; (b) side view.

at 70◦

C. A Neslab RTE-220 cooler circulated coolantthrough cell ports to maintain cell temperature. The cellwas insulated in a Textilite block with an inserted OmegaRTD in the cell to monitor the temperature to within±0:3

◦

C. Gas was supplied to the cell through a small

injection port and the pressure was monitored using aHeise pressure gauge with an accuracy of ±34:5 kPa.A stir bar, magnetically coupled to a Corning PC-320laboratory stirrer, mixed the cell contents at 1200 rpm.Cell contents were recorded using a Sony CCD camerawith an Olympus SZ60 microscope. Fig. 3b is a side view

picture of this cell.The sample chamber was ÿlled with 2 cc of octane

(99+% pure) and 0:5 cc of de-ionized water. Chemicaladditives varied in composition; however, the cited massfraction was always relative to the water phase. After thecell equilibrated to 0:5

◦

C, the sample chamber was ÿrst pressurized to 274 KPa with propane, and ÿnally pres-surized to 9:06 MPa with Green Canyon gas. The ÿnalvapor was composed of approximately 92% methane, 2%ethane, 6% propane and trace amounts of other compo-nents in Green Canyon gas and the subcooling for thisoperation was approximately 21

◦

C.

3. Chemicals

The major purpose of this work was to disperse hy-drates into condensate by chemicals with surfactant

properties. Those chemicals can be divided into two cat-

egories: commercially available and custom-synthesized,made in the Chemistry Department at the ColoradoSchool of Mines under the supervision of Professor D.M.Knauss.

3.1. Commercially available surfactants

Because it was not possible to test all commer-cially available surfactants, one surfactant was chosenfrom each commercially available family based on itshydrophilic–lipophilic balance (HLB). HLB provides anapproximation of the emulsion type made by a surfactant(Schick, 1967; Rosen, 1989). In this study, chemicalsthat had HLB values from 3 to 6 (with a few exceptions)were used to obtain water in oil (W= O) emulsion (Rosen,1989). However, it is not uncommon for chemicals inthis range to have oil in water (O= W) emulsion, andsome chemicals outside this range provide W= O emul-

sion. The HLB values were obtained for each chemicalfamily in McCutcheon’s (1996) handbook. In this work,we focused exclusively on non-ionic surfactants becausethey usually are non-toxic, their performance is not afunction of the hardness of water, and they work well inlow dielectric liquids. Table 4 summarizes the commer-cial surfactant families tested, the chemicals chosen fromeach family with HLB values, and the performance for each chemical based on HPA tests. Chemical formulascan be found in McCutcheon’s (1996) handbook. Com-mercially available surfactants were used as receivedfrom the vendor without further puriÿcation.

3.2. Design of synthesized chemical

The adsorption of surfactants onto the surface of dispersed particles is an essential prerequisite for the surfac-tants to be eective (Ottewill, 1967). Based on computer simulation, Makogon (1997) determined that the anchor-ing of one of the best kinetic inhibitors PVCap on thehydrate surface is due in part to the steric ÿt of the pen-dant lactam ring into the partially completed 51264 cavityat the sII hydrate surface. The bulk of the lactam groupacts as a pseudo-guest within the partial hydrate cavity,while the oxygen on the carbonyl group hydrogen bondsto the hydrate at the top of the partial cavity.

The above kinetic inhibitor concept inspired the ideathat the lactam ring might be a good surfactant head-groupto adsorb onto the hydrate surface. Since very few lac-tam based commercialized surfactants were available, agroup of lactam-ring-based chemicals were synthesized.As shown in Fig. 4, lactam-rings (or other functionalgroups that are known to interact with the hydrate surface)were used as the head-group with dierent long chain hy-drocarbon tails. Dierent connecting groups such as car-

bonyl and nitrogen between head and tail groups wereemployed. These connections also acted as agents that

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Table 4Candidates from surfactant families

Family name HLB range Chemical tested HLB Result

BRIJ 4.9 –20 BRIJ 72 4.9 O= W emulsionIgepal 3–20 Igepal CO210 4.6 Faileda

Tergitol 8–20 Surfonic N-10 3.4 Failed

Surfonic 3.4 –20 Tergitol TMN-3 8.1 FailedPluronic 1–30 Pluronic L31 1–7 Failed Nolpalcol 3.8–20 Nolpalcol 1-TW 4.1 FailedMYRJ 10 –20Span 1–7 All Good b

for HLB 3–6Tween 9 –20 Tween 65 10.5 FailedAbil N= A Abil WE09 N= A FailedSurfadones N= A Surfadone LP-100 N= A Failed

aMotor current change larger than 50 mA. bMotor current change less than 50 mA.

Fig. 4. The design of new chemicals.

adjusted the hydrophilic–lipophilic balance of the chem-icals. More than 50 chemicals based on lactam-rings andmore than 20 chemicals based on other functional groupswere synthesized. Fig. 5 is a summary of the most eec-tive synthesized chemical structures as tested by HPA.

Each chemical was puriÿed by column chromatographyor by recrystallization and then used as it was.

As shown in Fig. 5a, the ÿrst class of synthesizedchemicals that demonstrated anti-agglomeration prop-erties were the alkyl-2-(2-caprolactamyl)ethanoates (or alkyl-2-( N -hexahydro-2H-azepin-2-one-yl)ethanoates).The length of the alkyl chain was varied from 8 to20 carbon atoms to adjust the HLB and the perfor-mance. Modiÿcations to this general structure were alsomade to improve the anti-agglomeration properties.Class 2 (Fig. 5b) compounds, alkyl-2-(2-caprolactamyl)ethanamides (or alkyl-2-( N -hexahydro-2H-azepin-2-one-yl)ethanamides), were also synthesized with dif-ferent length of alkyl chains (from 8 to 16 car-

bon atoms in length). Class 3 (Fig. 5c) was the N , N -dimethylalkylamides. These chemicals were producedwith dierent length alkyl chains. Class 4 (Fig. 5d)chemicals were short chain poly(vinyl caprolactam)swith controlled alkyl end groups. The length of thealkyl end groups and the length of the PVCap chainwere modiÿed to produce a large variety of compoundswith dierent surfactant properties. These compoundswere synthesized by a chain transfer agent controlledfree radical polymerization of vinyl caprolactam usingalkylthiols of dierent chain lengths.

The resulting oligomers were therefore a combina-

tion of monoalkyl-, dialkyl-, and nonalkyl-terminatedchains. Class 5 (Fig. 5e) was synthesized by a sim-ilar chain transfer controlled free radial polymeriza-tion of N; N -diethylacrylamide with dierent amountsof t -dodecanethiol chain transfer agent. The length of the poly( N; N -diethylacrylamide) chain was controlled

by the amount of chain transfer agent. The resultingoligomers were a mixture of monoalkyl-, dialkyl-, andnonalkyl-terminated chains similar to those produced inClass 4.

4. Results

4.1. Motor current as a criterion of hydrate plug formation in HPA

In the HPA, if a certain amount of gas was con-sumed, a plug was expected to form in the cases withno inhibitor or kinetic inhibitors. The objective of us-ing anti-agglomerants was to disperse hydrate particlesinto the condensate phase. Instead of preventing hydrategrowth (as in the kinetic inhibitor tests), hydrates wereallowed to form, but only as small dispersed particles. Inthis case, gas consumption was no longer a valid criterionfor failure because a large amount of gas consumptiondid not necessarily mean that a plug was formed.

Direct observation of hydrate formation was notallowed because the HPA cell was non-visual. Gas con-sumption indicated hydrate formation, but motor currentchange was chosen as the criterion of a surfactant’s suc-cess after a careful study of the system. The DC motor current was a measure of the torque needed to mix thereactor contents. Once a plug formed, the impeller wasrequired to overcome friction between the plug and thereactor wall, resulting in a higher torque, or higher motor current.

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Fig. 5. Most eective synthesized chemical structures.

Three factors in HPA might change the motor current:viscosity change, dispersion formation, and plug forma-

tion. A set of motor current experiments was performedwith a clear tube of the same HPA cell diameter to evalu-ate various motor current changes. Fig. 6a shows two ex-

periments. In the ÿrst experiment, 48 cc water and 72 ccoctane were loaded into the test tube to observe the motor current. At about 220 min, as shown in the upper part of the ÿgure, 1:8 g of Tween 20 was also loaded to obtaina viscous emulsion. The viscosity of the emulsion wasat least 10 times higher than that of water as determined

by an Ostwald viscometer, but only a slight motor cur-rent increase (approximately 20 mA) was observed. Inthe second experiment, as shown in the lower portion of Fig. 6a, 84 cc of octane was ÿrst loaded into the reac-tor, and 36 g of polystyrene powder (d ¡ 0:053 mm) wasmixed in with strong agitation at about 150 min in order to simulate the eect of hydrate dispersion formation onmotor current. After the powder was added, a slight motor current increase (approximately 20 mA) was observed.

Fig. 6b shows the motor current eect of an ice plugformation. When the ice plug formed (conÿrmed byvisual observation), the motor current spiked as highas 100 mA. Further experiments in the stainless steelcell showed similar motor current results. As shown inFig. 7a, a motor current spike as high as 1000 mA wasobserved when hydrates formed (as conÿrmed by gas

consumption); after opening the cell, the depicted solidhydrate plug with ¿ 95% of the cell contents was found

stuck ÿrmly around the impeller. In another case, withuse of a successful AA chemical, as shown in Fig. 7b,no apparent motor current change was observed after hydrate formation; after opening the cell, hydrates werefound dispersed in octane. These results showed that if there was no plug formation, there was no apparent mo-tor current change. On the other hand, if there was largemotor current change, there was plug formation.

Visual observations after opening the cell in many experiments indicated that, if the motor current change wasless than 50 mA, there was no plug. Therefore, 50 mAof motor current change was used as a criterion for screening chemicals. Gas consumption, however, wasalso plotted to indicate hydrate formation. A total “gasconsumption” of approximately 2 moles indicated 100%water to hydrate conversion. The gas consumption shownin Fig. 7 (and the following ÿgures) included both thegas consumed by water and the gas dissolved in conden-sate and was magniÿed by a factor of 3.33.

4.2. SSA and HPA results for synthesized chemicals

Hexadecyl-2-(2-caprolactamyl)ethanoate (also calledUA377, synthesized at the University of Akron by

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Fig. 6. (a) Viscosity change and dispersion formation eects on motor current. (b) Plug formation eects on motor current.

Dr. William Brittain’s group) was the ÿrst successfulchemical synthesized (general structure shown in Fig.5a). This chemical was initially tested in the sapphirescreening apparatus together with PVCap to see the syn-

ergy between these two chemicals. As shown in Fig. 8, asmall amount of this chemical (0.1–0:5 wt%) improvedthe performance of PVCap by a factor of at least 40 atSSA conditions. The arrows at the top of the bars indi-cate that those experiments were arbitrarily stopped after a long running time. Similar chemicals showed almostidentical results.

For anti-agglomeration studies, all 70 chemicals syn-thesized at the Colorado School of Mines were tested at2 wt% in HPAs. Many lactam-ring-based chemicals andsome chemicals with other functional groups passed themotor current criterion.

The chemical dodecyl-2-(2-caprolactamyl)ethanamide(CDDA, with general structure shown in Fig. 5b) per-formed best of all synthesized chemicals. As shown inFig. 7b, 2 wt% of CDDA gave no motor current change

when hydrates formed, as indicated by gas consump-tion. In the same plot, the picture shows that there wasno plug around the impeller, and all hydrates were inthe form of small particles after the cell was opened.The concentration was reduced to test the limits of thischemical. As shown in Fig. 9, at 0:75 wt% of this chem-ical, the motor current uctuation was still small and nohydrate plug was observed upon opening the cell. A fur-ther reduced concentration was tested; it was found thatthis chemical did not work well at 0:5 wt% and 8:27 MPa,allowing a very loosely packed hydrate plug and a motor current change of 60 mA. When the pressure was

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Fig. 7. (a) Agglomeration as a function of motor current change. (b) Agglomeration as a function of motor current change.

increased for 0:75 wt% CDDA, a small plug containing

approximately 20% of the water mass was observed atthe upper limit of 11 MPa (T = 12:5

◦

C). 0:75 wt%of this chemical passed a 5-day shut-in test at 70 vol%octane and 8:27 MPa.

4.3. HPA results for commercial surfactants

Commercial surfactants are readily available and usu-ally inexpensive. Due to the large variety of commer-cially available surfactants, it was not possible to test allsurfactants. Fortunately, it was not necessary to test allsurfactants due to two reasons:

First, the surfactants of each family have very similar

structures and similar chemical properties, so one or twochemicals from each family provided a general idea aboutthe eciency of other chemicals in the same family.

Second, some of the surfactants are synthesized asdetergents or solubilizers, which usually make oil in wa-ter (O= W) emulsions under our test conditions (Rosen,1989). The O= W emulsions fail immediately in the HPA

because water is the continuous phase. For success,water in oil (W= O) emulsion was required to maintainwater = hydrate particles dispersed.

In this work, the chemical chosen from each family was based on its HLB. Some families have no surfactants with

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Fig. 8. Synergy of PVCap and UA377.

Fig. 9. Performance of 0:75 wt% CDDA.

the desired HLB value. In such cases, the chemical thathad an HLB close to 3–6 was selected. Table 4 lists allsurfactant candidates and their performance. Surfadonewas a group of chemicals with a 5-membered lactam ringas head group. Their HLB values were not available, butthey were only oil soluble and they did not give stableemulsions at 2 wt%, so HLB values were probably lessthan 3.

Commercially available surfactants were tested at3 wt%. As shown in Fig. 10, most surfactants did not

pass the motor current criterion at this concentration.

Only 2 motor current lines are shown in this ÿgure be-cause others performed similarly, and since all chemicalshad almost identical gas consumption, only one line for gas consumption is shown. Initially we found Span 80gave no motor current change, after which we tested allSpan chemicals. As shown in Fig. 11, most Span chem-icals passed the motor current criterion at 3 wt%, whileSpan 65 and Span 85, with very low HLB values, hadmotor current change greater than 50 mA. In this ÿgure,only one motor current line is shown for Span 65 andSpan 85 and one motor current line is shown for Span

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Fig. 10. Most surfactants do not prevent agglomeration.

Fig. 11. The performance of Span chemicals.

40 and Span 60 because each two chemicals performedvery similarly. Table 5 summarizes performance of allSpan chemicals.

After Span chemicals were found, comprehensive testswere performed on Span 20 at 3 wt% and 8:27 MPa. Con-clusions from these tests are as follow:

1. Span 20 worked well in dierent condensate types(C6; C7; C8; C10; C12, and a mixture of 50% C6, 25%C7, 10% C8, 5% C10, 5% C12, 5% C16).

2. Span 20 worked well at shear numbers from160 (s−1) to 700 (s−1). The calculation of shear numbers was based on the method of Tatterson(1991). According to Mr. G. Shoup, a BP special-ist in pipeline ow, the shear number in a pipelinemay vary from 150 (s−1) to 500 (s−1) for turbulentow.

3. Span 20 passed a 5-day shut-in test at 3 wt% withoutmotor current spikes larger than 50 mA before andafter the shut-in period.

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Table 5Span chemicals

Commercial name Chemical name Manufacturer HLB Performance

Span 20 Sorbitan monolaurate ICI 8.6 GoodSpan 40 Sorbitan monopalmitate ICI 6.7 GoodSpan 60 Sorbitan monostearate ICI 4.7 Good

Span 80 Sorbitan monooleate ICI 4.3 GoodSpan 65 Sorbitan tristearate ICI 2.1 FailedSpan 85 Sorbitan trioleate ICI 1.8 Failed

Fig. 12. Sequential images of hydrate formation in visual cell. (a) 80 vol% octane, 20 vol% water, T = 21◦C; (b) 80 vol% octane, 20 vol%water, 5 wt% UA 377, T = 21◦C.

4. 2 wt% Span 20 passed the motor current crite-rion with the addition of 5 w% propylene glycol atT = 11:5

◦

C. It was shown by Salager (1977) thatalcohol stabilizes emulsions.

However, 3 wt% Span 20 failed at 11 MPa, and

no Span chemical alone was eective at 2 wt% as ananti-agglomerant.

4.4. Visual observations of hydrate formation

Visual observations of hydrate formation through thesapphire-windowed cell (Fig. 3) were made to formulatethe mechanism of anti-agglomeration, with and withoutchemical additives. Fig. 12a (side view) shows hydrateformation in the absence of anti-agglomerants, which re-sulted in total agglomeration of hydrate particles within21 s of initial hydrate formation. In this sequence the gas–

liquid hydrocarbon interface marked the upper bound-ary of the images. The water phase surrounded the stir

bar, which was ellipsoid in shape, and the octane phasewas bounded by the gas–liquid hydrocarbon interface andthe top of the stir bar. The liquid hydrocarbon phase isclearly distinguishable in Fig. 12a after complete hydrateagglomeration. The hydrate phase is shown as the dark solid mass at the bottom of the cell and the liquid hy-drocarbon phase lies between the upper interface and thehydrate mass.

In Fig. 12b (side view), 5 wt% of UA377 was addedand a marked dierence was observed; however, the onlyexperimental dierence between experiments in Fig. 12aand b was the addition of the anti-agglomerant. WithUA377, hydrate particles formed at the water–octane in-terface and were immediately dispersed into the liquid hy-drocarbon phase. This process continued until the entirewater phase was converted to hydrate without agglom-

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eration. The hydrate slurry could be thoroughly shearedseveral hours after the experiment was initiated, whichveriÿed the proposed mechanism of anti-agglomeration.

5. Discussion

Based on the working mechanism of non-ionic sur-factants (Rosen, 1989) and the proposed mechanism for PVCap as a kinetic inhibitor in hydrate growth (Mako-gon, 1997), a three step mechanism is proposed to ex-

plain the anti-agglomeration eects of Span chemicalsand CDDA and similar chemicals.

1. Span or CDDA stays in the water–oil interfacialarea with or without emulsions because of their hy-drophilic and lipohilic characteristics. All successfulSpan chemicals gave stable water in oil emulsions,while CDDA could keep the system emulsiÿed only

with continuous mixing.2. When water to hydrate conversion occurred in the

emulsiÿed system, the chemicals remain anchored onhydrate particle surfaces. The chemicals may be con-sidered to be both hydrophilic and “hydrate-philic” inthis regard. The reason why most surfactants do notwork may be due to their inability to attach to the hy-drate surface because they are not “hydrate-philic”.

3. The long hydrocarbon tails extend from the particlesurface into the dispersing liquid hydrocarbons. Thusthe interfacial tension between the particles and thedispersing liquid is substantially lowered. A steric bar-

rier is formed due to the preferred tail-solvent inter-actions, and this keeps hydrate particles suspended(Tardos, 1986; Parÿtt & Picton 1968; Rosen, 1989).

Both commercially available surfactants and designedchemicals have advantages and disadvantages. For example, Span chemicals are very cheap compared tosynthesized chemicals, but in order to be eective, their concentrations have to be as high as 3 wt%, and emul-sions made by these chemicals are much harder to break.In contrast, CDDA is eective at very low concentration(0:75 wt%), but the cost of this chemical can be muchhigher (lab-made CDDA was approximately 40 timesmore expensive compare to commercial Span chemi-cals). However it is possible to develop new method tomake CDDA at lower cost, and it is also possible to

perturb this chemical structure to improve eciency.

6. Conclusions

1. DC motor current was established as an in-house cri-terion for hydrate agglomeration in HPA experiments.Motor current changes less than 50 mA distinguishedgood chemicals.

2. UA377 (hexadecyl-2-(2-caprolactamyl)ethanoate)and similar chemicals showed good synergy withPVCap in preventing hydrate plug formation in SSAtests.

3. HPA tests showed that CDDA (dodecyl-2-(2-caprolactamyl)ethanamide) could prevent hydrate

particles from agglomeration at very low concentra-tion (0:75 wt%). CDDA passed a 5-day shut-in test at0:75 wt%.

4. Span chemicals were the best among commer-cial surfactants tested in HPA. At low subcooling(11:5

◦

C); 3 wt% Span chemicals passed the motor current criterion at dierent shear numbers (from 150to 700 s−1) and dierent condensate types (from C6 toC12), but they did not work well at higher subcooling(12:5

◦

C) and lower concentration.5. Further experiments such as ow-loop test are neces-

sary to quantify how Span chemicals or CDDA-likechemicals can be applied into ÿeld productions.

Acknowledgements

The authors wish to thank our consortium membersARCO, BP, Chevron, Department of Energy, Marathon,Mobil, Petrobras, Phillips, Shell, Texaco and Unocal for their ÿnancial support on this work.

The authors wish to thank Dr. J.P. Long andMr. R. Miner for their construction of the sapphirescreening apparatus based on a Conoco Inc. design.

The authors wish to thank Dr. William Brittain’s group

at the University of Akron for synthesizing UA 377.The authors also wish to thank Mr. Ben Bloys of Tex-aco for ordering the Surfadone chemicals, and Mr. GeorgeShoup of BP for providing pipeline shear numbers.

References

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