Increased liquid extraction in the Kollsnes dew point process by using mixed glycols

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Kollsnes dew point process by using mixed glycols

Andreas Tomasgaard

Master of Energy and Environmental Engineering Supervisor: Even Solbraa, EPT

Department of Energy and Process Engineering Submission date: June 2015

Norwegian University of Science and Technology

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P ^REFACE

This thesis concludes five years of study towards a master of science in “Energy and Environmental Engineering” at the Norwegian University of Science and Technology (NTNU) in Trondheim. It is a collaboration between the Department of Energy and Process Technology at NTNU, and Statoil Research and Development Center (R&D) in Trondheim. The thesis counts thirty credits, and stretched over a period of twenty weeks in the second semester of the academic year 2014-2015. It is a continuation of the specialization project carried out during the previous semester. The main academic supervisor is associate professor at NTNU and Statoil, Even Solbraa.

First and foremost I would like to thank Even Solbraa for guiding me throug the work. Help has never been far away, and I have enjoyed the luxury of quick responses to all questions, large or small. In addition, a sincere thank you to laboratory engineer Ole Johan Berg at Statoil R&D, for his help in setting up, conducting and guiding the experimental work. I would also direct a special thank you to the Kollsnes gas plant, represented by senior engineer Tom Georg Eriksen, and process engineer Julien Francis Faber. They were always ready to answer my questions regarding everything from the various Kollsnes processes to scope of the work.

Working with this thesis has been exiting. By continuously developing my knowledge, I have come to see how theory from five years of study is used in real life gas processing applications. I am grateful for this opportunity.

Trondheim June 2015

Andreas Tomasgaard

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V

S ^AMMENDRAG

Væskeutskillelse er et viktig og utfordrende del av naturgassprosessering. Fjerning av tyngre komponenter fra gassen er nødvendig for å møte salgsgass-spesifikasjoner. I tillegg er tyngre gasskomponenter i væskefase, kalt kondensat, et energirikt petroleumsprodukt, med høy kommersiell verdi. Optimalisert kondensatproduksjon fordrer lave temperaturer og høye trykk, forhold som samsvarer med en større tendens til utfrysning og driftsproblemer. Økt væskeproduksjon er ønskelig. Samtidig er det essensielt å opprettholde nødvendig sikkerhet både i dagens og framtidens gassprosesseringsscenarier.

For øyeblikket benyttes an vannløsning av ren MEG og vann som hydrathemmer ved Kollsnes.

Duggpunktseparatoren drives med [78 vekt %, 77 vekt %] MEG inn/ut, ved en temperatur på - 23℃. Denne studien undersøker potensialet for økt væskeproduksjon i gassprosesseringsanlegget, ved å redusere driftstemperaturer i duggpunktprosessen. Hydratdannelse og MEG utfrysning, så vel som begrensninger i prosesseringsutstyr er vurdert. Målet er å utlede nye driftsforhold for maksimal trygg væskeutskillelse.

To simuleringsmodeller er utviklet, og implementert med to ulike tilstandsligninger.

Hydrokarbon-dominerte beregninger er utført i HYSYS (8.6) med SRK som tilstandsligning.

NeqSim, med CPA ligningen er brukt når polare komponenter dominerer, særlig ved beregning av MEG utfrysning og hydratdannelse, så vel som gass-væske likevekter i MEG regenereringsanlegget. Modellene er validert mot felt og design data fra Kollsnes, og brukt til å utlede driftsforhold som maksimerer trygg væskeproduksjon. I tillegg er frysepunkttemperaturer for MEG i kontakt med kondensat undersøkt eksperimentelt. Resultatene samsvarer godt med NeqSim simuleringer, hvilket gir kredibilitet til utledningene av optimale driftsforhold.

Resultatene fra studien viser et betydelig optimaliseringspotensial for væskeproduksjon. Endring av MEG konsentrasjonen til [83 vekt %, 81 vekt %] in/ut, tillater reduksjon av driftstemperaturer til minimumsverdien -26℃ i duggpuntseparatoren. Dette øker kondensatproduksjonen med rundt 20%, og gir en årlig inntektsvekst på ca. 25 MNOK pr. tog. Nye MEG konsentrasjoner reduserer energiforbruket til regenerering. Innsparingene er imidlertid moderate, ca. 0,1 MNOK/år. pr. tog.

Bruk av blandede glykoler (MEG og TEG) som hydrathemmer tillater lavere driftstemperaturer i duggpunktseparatoren enn ren MEG. En blanding av 80 mol % MEG - 20 mole% TEG (62 vekt

% MEG – 38 vekt % TEG) gir trygg reduksjon av driftstemperatur til -30℃. Dette gir 24% økning i kondensatproduksjon sammenlignet med en optimalisert MEG prosess, og 37 MNOK vekst i årlig inntekt. Imidlertid er det ikke driftssikkerhet, men kapasitet i eksportkompressorer som utgjør begrensningen for temperaturreduksjon i duggpunktseparatoren. Drift ved minimum tillatt temperatur, -26℃, oppnås ved bruk av ren MEG-vann som hydrathemmer. Det er følgelig ikke noe direkte potensial for bruk av blandede glykoler i dagens anlegg på Kollsnes. På den annen side er det et betydelig potensial for økt væskeproduksjon ved optimalisering av allerede etablerte prosesser.

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A ^BSTRACT

Liquid extraction is an important and challenging part of natural gas processing. Removing heavy hydrocarbon components from the gas is necessary in obtaining sales gas specifications. In addition, liquefied heavy gas components, commonly termed condensate, is an energy dense petroleum product, with high commercial value. Optimized condensate production requires low temperatures and elevated pressure, which corresponds with an increasing tendency to solid formation and operational problems. Producing more liquid from natural gas is desirable due to high energy content and sales price. At the same time, it is imperative to maintain process safety in both current and future gas processing scenarios.

Currently, an aqueous solution of pure MEG and water is injected as hydrate inhibitor on Kollsnes. The dew point separator operates with [78 wt%, 77 wt%] MEG in/out, at a temperature around -23℃. This work investigates the potential for increased liquid extraction in the gas plant, by reducing operating temperatures in the dew point process. Hydrate formation and MEG solidification conditions, as well as plant capacity limitations, are taken into account. The objective is to derive conditions for maximized safe and feasible liquid extraction, and quantify the optimization gain, both in terms of condensate production and increased income.

Two simulation models of the Kollsnes process have been developed, and implemented with two different equations of state. Hydrocarbon-dominated calculations are performed in HYSYS (8.6), implemented with SRK equation of state. NeqSim, implemented with the CPA EoS, is used when polar components dominate, particularly for predicting MEG freeze out and hydrate formation conditions, as well as VLE calculations in the MEG regeneration system. The models are validated against field and design data from Kollsnes, and used to derive conditions for maximum allowable liquid extraction. In addition, experiments are preformed to investigate MEG solidification temperatures under the influence of condensate. Measured MEG freezing points correspond well with NeqSim simulations, adding credibility to the derived optimized conditions.

The work shows considerable potentials for optimizing the Kollsnes dew point process in terms of increased liquid extraction. Changing the injected MEG-water concentration to [83 wt%, 81 wt%] MEG in/out, allows safe reduction of the dew point separator temperature to process minimum of -26℃. This increases condensate production by roughly 20%, adding up to a net income increase of around 25 MNOK/year pr. dew point train. New MEG concentrations reduce the energy required for regeneration. However, the savings are relatively moderate, adding up to 0,1 MNOK/year for the amount of MEG in one dew point train.

Aqueous solutions of mixed glycols (MEG and TEG) have lower freezing points than pure MEG- water. Injected as hydrate inhibitor, a mixture of 80 mole% MEG – 20 mole% TEG, (62 wt%

MEG – 38 wt% TEG) allows safe reduction of the dew point separator temperature to -30℃. This increases the condensate production by 24% compared to the optimized MEG case, giving a net

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growth in income of 37 MNOK/year from one dew point train. However, the capacity of the export compressors rather than solid formation limits temperature reduction in the dew point separator. Reduction to process minimum is obtained with pure MEG-water as hydrate inhibitor, and as for today, there is no direct utilization for mixed glycols on Kollsnes. There is however a considerable potential for increased liquid extraction by optimizing current operations with MEG.

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A BBREVIATIONS

Approx. Approximate/Approximately

Atm. Atmosphere

barg bar gauge

℃ Degrees centigrade

CH4 Methane

Cond. Condensate

Concentr.range Concentration range

CO2 Carbon Dioxide

CPA Cubic Plus Association

DEG Di- Ethylene Glycol

DP Dew Point

EoS Equation of State

Exp. Experimental

℉ Degrees Farenheit

Frac. Fraction

GLF Gas loading factor

h Hour

H2S Hydrogen Sulfide

HC Hydrocarbon

HHC Heavy Hydrocarbon

JT Joule Thompson

kg Kilogram

kg/h Kilogram pr. hour

kPa Kilo Pascal

LHV Lower Heating Value

Liq. Liquid

ln Natural logarithm

LNG Liquefied Natural Gas

LP Low pressure

LPG Liquefied Petroleum Gas

m Mass

Min. Minimum

min. Minute

MEG Mono- Ethylene Glycol

MNOK Million Norwegian Kroner

mole% Mole percent

MSm³/d Million Standard Cubic meter pr. day

MW Mega Watt

NG Natural Gas

NGL Natural Gas Liquid

Oper. Operational

Oper.temp Operational temperature

Pa Pascal

PFD Process flow diagram

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ppmv. Parts per million (volume)

PT Pressure-Temperature

PVT Pressure-Volume-Temperature

Sat. Saturated/saturation

sec. Second

Sm³ Standard cubic meters

Spec Specification

SLE Solid-liquid equilibrium

SRK Soave Redlich- Kwong (equation of state)

TEG Tri- Ethylene Glycol

Temp. Temperature

ton/h Tons pr. hour

Vap. Vapor

Vap.press. Vapor pressure

VLE Vapor-liquid equilibrium

Vol. Volume

Vol% Volume %

wt% Weight percent

W Watt

kW Kilo watt

kWh Kilo watt hours

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N OMENCLATURE

A_cross Cross sectional area

C_p Heat capacity at constant presure C_v Heat capacity at constant volume

C^drag Drag coefficient

D_drop Droplet diameter

f Fugacity

G Gibbs free energy

g Gravity constant

h Specific enthalpy

m Mass

𝑚̇ Mass flow

M Molar mass

n Number of moles

P Pressure

P_i Partial pressure of component i P_sat Saturation pressure

P_vap Vapor pressure

𝑄 Duty

Q_cond Condenser duty

Q_reb Reboiler duty

R Universal gas constant

R̅ Universal gas constant molar form

s Specific entropy

T Temperature

V Volume

V̇ Volume flow

𝑉̇_𝑔 Gas volume flow

v_t Terminal settling velocity

v Velocity

x Mole fraction in liquid

y Mole fraction in gas

Z Compressibility factor

𝜌 Density

𝜌_𝑙 Liquid density

𝜌_𝑔 Gas density

𝜇 Chemical potential

∆ Delta (difference)

𝛾 Activity

𝜑 Fugacity coefficient

~ Approximately

𝑥 Multiplied by

𝜕

𝜕𝑥

Partial derivative with respect to component x

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I NTRODUCTION

Natural gas contain considerable amounts of energy. However, direct use of the gas as it appears in the subsurface is in most cases not feasible. Prior to being utilized as energy source, natural gas is subject to a wide range of industrial operations, commonly termed as “gas processing”. Gas processing covers all industrial processes a natural gas stream is subject to as it moves from reservoir to end user. The various operations have different areas of applications, but they all aim at maximizing safe harvest of energy from the gas.

It is custom to extract liquids, both water, and in particular heavy hydrocarbons from the gas stream. This is commonly handled by a particular branch of gas processing employing low temperatures in the extraction process. The energy content of hydrocarbons increases for heavier components, corresponding with an increasing boiling point. When a gas stream is cooled, it thus condenses out an energy dense liquid, rich in heavy hydrocarbons. This mechanism is utilized in liquid extraction processes, where gas streams are exposed to considerable cooling, ideally in combination with elevated pressure. Condensed, valuable liquid is separated out in downstream separators.

Low temperatures and elevated pressure gives optimal conditions for both liquid and solid formation. The presence of water in low temperature processes induce the risk of hydrate and ice formation, which potentially can block piping and damage process equipment. To ensure safe operations it is often custom to dehydrate the gas upstream low temperature processes. The goal is to produce a gas dryness corresponding to a water dew point below the operating temperature of the process. This ensures that no liquid water is present, and gives problem free, operations, safe from hydrate formation.

Some situations however does not feature upstream gas dehydration. The Kollsnes plant receives gas from the large Troll gas-condensate field in the northern part of the North Sea. On Kollsnes the gas is subject to cooling and expansion in a sequential cooling and condensation process.

There is no upstream dehydration, and the process simultaneously removes water from the gas, produces liquid hydrocarbons (NGL and condensate) and gives a sales gas with specifications suitable for the European market.

Hydrates and freeze out in the low temperature processes on Kollsnes is avoided by upstream MEG injections. MEG provides hydrate inhibition, but also limits the process in terms of safe temperature reduction and water removal. Theoretically, the freezing point of the injected MEG solution, and the hydrate formation temperature of the gas stream, limits the process in terms of temperature reduction. As the ability to produce natural gas liquids increase for decreasing temperatures, the MEG solution ultimately appears to be the limiting factor for liquid extraction on Kollsnes

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MEG and TEG are two commonly used glycols in gas processing, with a freezing point of -13℃

and -7℃ respectively. It is known that a mixture of glycols, like MEG and TEG, have a lower freezing temperature than the components alone. In processes like Kollsnes, using such a mixture for hydrate inhibition, would change the process limitations imposed by the MEG solution. Mixed glycols is expected to increase the potential for safe temperature reductions, with corresponding extraction of more liquids.

Kollsnes produces large amounts of both sales gas and condensate. Vast gas resources on the Troll field make the plant account for almost 40% of all Norwegian gas deliveries, with an average daily capacity adding up to around 143 million standard cubic meters of sales gas, and 8500 standard cubic meters of condensate [1]. The substantial gas and condensate production gain a large income for the plant operator, as well as ensuring energy safety for a broad range of consumers and end users.

Condensate is in most cases a more valuable petroleum product than sales gas both in terms of energy content and price. It contains a larger share of heavy hydrocarbons, with a higher energy content. In addition, it is in liquid phase at atmospheric conditions, which simplifies storage and transport, and generally gives a more energy dense fluid. Extracting more condensate is thus favorable for the producer, gaining even larger income from sale of a more valuable petroleum product. However, condensate production also has positive impacts on a larger scale. The world is in a continuously growing need for energy, corresponding with increasingly scarce fossil resources. Maximizing feasible energy harvest from natural gas seems to be a good, if not necessary step in meeting a future with expanding energy demands. Investigating increased liquid extraction on Kollsnes is thus in the very heart, not only of gaining a larger income for the plant operator, but in providing more energy to consumers and markets with a growing demand.

S

COPE OF WORK

This work investigates the potential for increased liquid extraction in the Kollsnes gas plant.

Previous operational problems are identified, together with a detailed review of relevant plant processes. Statoil documentation is the basis for developing two simulation models of the plant, implemented with two different equations of state. The models are evaluated and validated against available field and design data from Kollsnes, leading to a choice of which thermodynamics that are suitable for analyzing the respective processes.

The plant is simulated under design conditions, implemented with current MEG concentrations in the separators. Discussion of these results, together with process safety considerations, derive an optimized simulation case for maximum allowable liquid extraction on Kollsnes. Experimental investigations extend the work to real situations. The MEG solidification temperature is approximated under Kollsnes dew point separator conditions. This either add credibility, or disproves simulated safety considerations, and investigates the effect of real condensate on MEG solidification.

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Final results from the optimized simulation case are given towards the end of the thesis, and is subject to economic evaluations. The potential for mixed glycols is treated in a subsequent chapter, reporting both operational limitations and potential economic gain. Final discussions focuses on potential sources of error, flaws and their impact on the results, before a final conclusion summarizes the findings.

The work in this thesis follow a line of logical development. Discussions and preliminary conclusions all aim at deriving operating conditions that are economically and practically feasible, and allow safely increased liquid production on Kollsnes. The optimized simulation case quantifies the full potential for liquid extraction according to this work, and reports the maximum optimization gain from improved operating conditions. Figure 1-1 illustrates the sequential nature of the work.

Figure 1-1– Logical development of the work

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T

HE STRUCTURE OF THE REPORT

This sub chapter briefly explains the content and idea behind each chapter in the report.

Chapter 1 motivates the topic of the report, explains the background, and introduces the general problem of increased liquid extraction. The idea is to briefly introduce the reader to benefits and challenges from increased liquid extraction, explain the development of the work and the report, and motivate further reading.

Chapter 2 introduces general concepts of low temperature natural gas processing, including the theory of condensation, the phase envelope and basic process configurations. The idea is to highlight the theoretical background for several processes on Kollsnes by explaining how basic thermodynamics is utilized in low temperature natural gas processing.

Chapter 3 introduces solid formation in low temperature natural gas processing. The chapter explains hydrate formation and inhibition, as well as solidification of eutectic systems. In addition it introduces specific problems related to MEG in low temperature gas processing, and explains process safety terms used throughout the report. This chapter also introduces mixed glycols for low temperature gas processing applications. The idea is to introduce the reader to concepts of solid formation in low temperature natural gas processing, which is a major part of this work. In addition, the goal is to explain particular terms used in the work, related to MEG systems

Chapter 4 introduces the Kollsnes plant, and explains various plant processes according to Statoil documentation. The idea is to review Kollsnes processes of particular importance for this work, and explain the flow through the plant. Hopefully, by this, the reader gain knowledge of the Kollsnes processes, needed to understand the development of the simulation models, and the discussions in subsequent chapters.

Chapter 5 presents a review of previous troubleshooting related to the Kollsnes gas plant. In particular it presents a conference paper concerning operational problems in the plant at the end of the 1990ies. Understanding the considerations of Chapter 5 is important in understanding the entire problem of increased liquid extraction and safe temperature reduction on Kollsnes. The idea is to introduce the reader to process limitations in the plant, and to explain the conclusions from previous troubleshooting, which has an impact on current plant operations.

Chapter 6 explains how the process simulator Aspen HYSYS calculates selected thermodynamic properties. In addition the chapter introduces the other process simulator used in this work;

NeqSim, and provide preliminary discussions concerning the areas of application for the different simulators. Solid formation and VLE calculations in NeqSim is also validated against available experimental data from literature. The idea is to introduce the reader to HYSYS and NeqSim, and explain basic property calculation methods for the two simulation tools. In addition the validation of NeqSim add credibility to later results.

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Chapter 7 explains the development of the simulation models, and how they are implemented in HYSYS. The idea is to explain how the models are built, which is considered a step in increasing the readers understanding of the work. In addition, a detailed explanation of the model development enable others to perform the same work, and investigate the validity of the conclusions.

Chapter 8 validates the simulations against available field data, and design (PFD) values. In addition the chapter compares HYSYS and NeqSim, and concludes on which thermodynamics that are appropriate for analyzing the respective processes. The idea is to add credibility to the simulation results. The system is tuned according to conditions under which field or design data exists. The simulation outputs are investigated, and potential deviations from the existing data are discussed and as far as possible, explained. In order to suggest new operational conditions on Kollsnes, and predict the potential for increased liquid extraction, it is imperative to know the accuracy in the simulations.

Chapter 9 gives results from simulations of the plant according to design/current conditions. The results are used in discussions of the potential for increased liquid extraction in the Kollsnes dew point process. In addition, the chapter include process safety considerations, and discusses the MEG injection system and MEG concentrations. Chapter 9 concludes with theoretical results in the form of increased liquid production at design conditions, and suggests improved operating conditions that are practically feasible in an optimized simulation case. The idea behind the chapter is to report results from plant simulations at design/current conditions. Optimized operating conditions cannot be suggested without in depth knowledge of current/design plant operations. In addition the chapter discusses process limitations, as well as process safety and the MEG system. Detailed knowledge of this is imperative in suggesting improved operating conditions for maximized liquid extraction.

Chapter 10 accounts for the experimental work related to MEG solidification temperatures in a MEG-water system rich in MEG, under the influence of condensate. The chapter explains the experimental procedure, and measurement uncertainties. In addition it gives simulation results from the reactor system, and discusses differences between the experiments and real operations on Kollsnes. Results from the experiments are reported and discussed, concluding on approximate solidification temperatures for aqueous 80 wt% and 90 wt% MEG under the influence of condensate. The idea of the chapter, and the experimental work is to validate the simulated MEG solidification curve. The potential for MEG solidification plays a key role in process safety, and experimental investigation are included to see if NeqSim gives appropriate freeze out temperatures under Kollsnes conditions. Finally, the chapter also compares results with previous troubleshooting in the dew point process.

Chapter 11 gives results from the optimized simulation case. Chapters 8 through 10 derive practically feasible, new operating conditions that maximize safe liquid extraction on Kollsnes.

Chapter 11 report the results, and quantify potential increases in liquid production, income and MEG regeneration and condensate stabilization energy use. The chapter follow the same template as Chapter 9, but excludes already treated process safety considerations. The idea is to give a final answer to the question of the potential for increased liquid extraction from the Kollsnes dew point

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process. Optimized operational conditions are derived in previous chapters. Chapter 11 shows how implementing them affects the simulations.

Chapter 12 explains the potential for mixed glycols on Kollsnes. This includes new operating conditions, and further increases in liquid extraction and income. The idea is to introduce mixed glycols on Kollsnes, show the theoretical potential for implementing new operational conditions, and quantify their implication.

Chapter 13 provides the final discussions of this work. Generally, results are discussed and explained while they are presented. As preliminary findings are used to derive the optimized simulation case, it was determined not to collect all discussions in a final discussion chapter.

Chapter 13 discusses possible flaws and sources of error in the work on a more superior level.

The focus is on outcomes from potential errors, and the importance of choices such as thermodynamic models, safety margins etc. The idea is to give final considerations to the validity of the findings, explain choices not yet explained, and discuss “what ifs” related to the work.

Chapter 14 concludes on the most important findings from this work.

Chapter 15 suggests further work on basis of findings and considerations from the thesis

The explanations above give the following summary of the report structure. Chapters 2, 3 and 5 are connected to the first block of Figure 1-1, and provide theoretical background, and review of previous work. Chapter 4, 6 and 7 explain the Kollsnes plant, the simulation tools, and the development of the simulation models, connected to the second block in 1-1. Chapter 8 validates the simulations, adding credibility to the results, and enabling a choice of thermodynamics, connected to the third block in 1-1. Chapter 9 gives results from design/current plant simulations, discusses the potential for increased liquid extraction, and gives limitations in terms of process safety and plant capacity. The chapter is connected to the fourth block in 1-1, and derives the optimized simulation case. The fifth block is connected to Chapter 10, presenting the experimental work, which investigates the validity of MEG solidification simulations. Chapter 11 is connected to the sixth block, presenting results from the optimal case derived on basis of intermediate findings. The seventh block and Chapter 12 gives the potential for mixed glycols, succeeded by the eight block, and final discussions in Chapter 13. The two last Chapters 14 and 15 conclude this work, and suggest further work. They are connected to the last block, the ninth, in Figure 1- 1.

Hopefully, by this explanation the reader can follow the line through the report and the work. The goal is to help obtaining a structural overview, and through that increase the understanding of considerations, choices and findings throughout the thesis.

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D

EVIATIONS FROM THE PROJECT DESCRIPTION¹

The topic for this work was always increased liquid extraction from the Kollsnes dew point process. However, this was initially believed to demand the use of mixed glycols rather than MEG for hydrate inhibition. Thus both the title of the work, and one of five main tasks concern the use of mixed glycols on Kollsnes.

As the work went on, it became clear that the real limitation for increased liquid extraction was the capacity of the export compressors rather than process safety. Temperature reduction to process minimum could be maintained with pure MEG as hydrate inhibitor, substantially decreasing the importance of investigating mixed glycols on Kollsnes. In agreement with supervisor Even Solbraa, as well as Kollsnes senior engineer, Tom Georg Eriksen it was thus determined to shift the scope of the work. The main concern is still increased liquid extraction, however, the new approach is to optimize current MEG operations, and quantify increases in liquid production, income and energy use. Mixed glycols are still introduced in this work, and are subject to a limited study towards the end of the report, but this is mainly to highlight the potential for other plants. Currently there is no direct utilization for mixed glycols on Kollsnes.

1 This is written in retrospect, and touches findings presented in later Chapters

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L OW TEMPERATURE NATURAL GAS PROCESSING

Liquid extraction is important in natural gas processing. In short, low temperature gas processing utilizes temperature reduction and variations in dew point temperature to liquefy and extract certain components from the gas, while others remain in gas phase. Liquid extraction is crucial in production of sales gas with specifications suitable for the European market. In addition, the extracted liquid is an energy dense byproduct, enriched with heavy hydrocarbons, called condensate. Liquid water and condensate is removed simultaneously in downstream low temperature separators [2], ensuring problem free gas pipeline transportation, and income from sale of a valuable petroleum liquid. [3].

The following chapter introduces selected elements in low temperature gas processing believed to be of particular relevance for this work. In addition, the chapter provides simplified process configurations, and basic theoretical insight for understanding low temperature gas processing as it appears in this thesis.

C

ONDENSATION IN LOW TEMPERATURE NATURAL GAS PROCESSING Condensation, phase transition from vapor to liquid phase, generally occurs when the temperature of a vapor/gas drops below its saturation temperature [4]. Physically, this is explained by the ratio of molecular kinetic energy in a vapor, against the intermolecular forces in the corresponding liquid phase. Temperature and kinetic energy are equivalents, and reducing the temperature of vapor directly means reducing the average kinetic energy of the vapor molecules. As the temperature drops, the vapor molecules reach a certain energy level where their molecular kinetic energy equals the intermolecular forces in their liquid phase. At this particular energy level, named the dew point temperature, or saturation temperature of the vapor, the molecules are in the limit of existing in vapor phase, and the system is in so-called vapor-liquid equilibrium (VLE).

Further reduction in kinetic energy results in a phase transition where the molecules are drawn together by intermolecular forces, and condenses out as a separate liquid [5].

2.1.1 The effect of pressure

The saturation temperature and pressure depends strongly on each other. High pressures push molecules together, strengthening the intermolecular forces in the liquid phase. The result is a higher energy requirement to remain in gas phase, resulting in a higher dew point temperature.

[5]. The latter is illustrated by Figure 2-1 - the vapor liquid equilibrium line for butane. Butane is liquid for pressure and temperature conditions to the left of the equilibrium line, wears the area to the right of the equilibrium gives butane in gas phase. As predicted, the dew point temperature

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increases with increasing pressure. This effect is readily utilized in gas processing by maintaining an elevated pressure level, either to extract more liquid or to extract liquid by a smaller temperature reduction.

Figure 2-1 - Vapor - liquid equilibrium for butane (NeqSim)

2.1.2 The effect of molecular weight

Figure 2-2 illustrates another important feature with condensation. The chart shows vapor-liquid equilibrium lines for selected hydrocarbon components with varying molecular weight. As seen, the dew point temperature at a given pressure increases for increasing molecular weight. This can be understood from increases in so called dispersion forces, induced by an increasing number of electrons in the heavier substances [5]. As the electrons have the same charge, they repel each other, are dispersed, and redistribute the electron density in one molecule relative to another. This creates what is called instantaneous dipoles that attract each other, strengthening the intermolecular forces among the molecules. Stronger intermolecular forces increase the energy required to stay in gas phase, and thus increase the dew point temperature of the substance [5].

The effect is utilized in gas processing to separate heavier components as liquid, while maintaining lighter components in gas phase. This simultaneously controls the dew point of the gas, and produces the energy dense condensate with high commercial value.

Figure 2-2 - Vapor- liquid equilibrium for selected hydrocarbons with varying molecular weight (NeqSim) 0

5 10 15 20 25 30

-50 0 50 100 150

Pressure [bar]

Temperature [℃]

0 5 10 15 20 25 30

-100 -50 0 50 100 150

Pressure [bar]

Temperature [℃]

Butane Propane Ethane

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2.1.3 Condensation of water

Figure 2-3 illustrates the final condensation feature to be explained by showing vapor – liquid equilibrium lines for butane and water. Even though butane has more than three times the molar mass of water, the dew point temperature at a given pressure is considerably higher for water. At atmospheric conditions of approximately one bar, the dew point temperature of water is 100℃

compared to slightly below 0℃ for butane. The explanation is the strong hydrogen bounding forces among water molecules in liquid phase [5], which demand relatively high molecular kinetic energy for the water molecules to be in gas phase. In gas processing, the effect of the hydrogen bounding is utilized to condense out water simultaneously with much heavier components from the gas stream. This simplifies regulation of the water content in the natural gas, which is important to reach both energy content and transport specs, and to protect downstream process equipment from the presence of liquid water [3].

Figure 2-3 - Vapor - liquid equilibrium for butane and water (NeqSim)

T

HE PHASE ENVELOPE

Vapor – liquid equilibrium diagrams as given above, show the phase behavior of single components under influence of pressure and temperature. The phase diagram is represented by one single equilibrium line, and the system is either vapor, liquid or in equilibrium.

Multicomponent mixtures like natural gas on the other hand, may exist either as a vapor, liquid, or in a two-phase vapor-liquid mixture. The phase behavior of such systems is described by a characteristic curve called the phase envelop [6]. In principle, the phase envelope can be displayed by several variables, it is however custom to use pressure and temperature (PT) diagrams on the same form as those given above.

Figure 2-4 shows a typical phase envelope for sales gas from an onshore processing facility.

Analogous to Figure 2-1, the natural gas is in pure vapor phase to the right of the envelope, and in pure liquid phase to the left. The area enclosed by the phase envelope features a two-phase mixture of vapor and liquid, where the phase of a specific component depends on the particular dew point temperature of that component at the given pressure. The top point of the curve is called the cricondenbar, and is the highest pressure where the gas exists as a two-phase mixture, wears

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-50 0 50 100 150 200 250

Pressure [bar]

Temperature [℃]

Water Butane

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the right extremal point, called the cricondenterm, is the highest possible temperature where the gas is in two phase [6].

Figure 2-4 - Typical phase envelope for sales gas from an onshore processing facility (HYSYS)

The part of the envelope marked in red is the dew point line, at which gas exists as saturated vapor. The mixture is pure vapor to the right of the line, wears crossing the line features condensation of gas components. The dew point line of the envelope can be understood as the vapor equilibrium. When the gas cools at constant pressure, the dew point line intersection will be the point where the first drop of liquid condenses out as a separate phase.

The part of the envelope marked in blue is the bubble point line, at which the gas exists as saturated liquid. To the right of the line, the gas is a two-phase mixture of vapor and liquid, wears crossing the line features a pure liquid natural gas phase. Analogous to the dew point line, the bubble point line can be understood as a liquid equilibrium. When the gas cools at constant pressure, the first liquid condenses when the dew point line is crossed. Moving through the phase envelope, intermolecular forces will dominate over kinetic energy for an increasing amount of gas components. The result is that an increasing amount of liquid condenses out as the temperature drops. At the bubble point line, there is an equilibrium between the kinetic energy of the last vapor component, and the intermolecular forces of its liquid phase. Further cooling crosses the bubble point line, the last vapor bubble condenses, and the natural gas is pure liquid.

For cases of liquid extraction, the phase envelop is used to predict which areas of temperature and pressure that corresponds to condensation of components in vapor phase. Moreover, the envelope is used to predict the necessary liquid extraction and compression to ensure single-phase gas pipeline transportation. The phase envelope also symbolizes dew point control of the gas stream.

Reducing the dew point of the gas is equivalent with producing a leaner phase envelope. A gas dew point specification of say -15℃ at 52 bar may be seen directly from the phase envelope dew point line at the specified pressure. Throughout this work, the phase envelope is reported several times, to symbolize the changing composition and liquid extraction as the gas moves through the processes from reservoir to end user.

0 10 20 30 40 50 60 70 80

-180 -150 -120 -90 -60 -30 0

Pressure [bar]

Temperature [℃]

Dew point line Bubble point line

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B

ASIC PROCESS CONFIGURATIONS

– E

XPANSION AND SEPARATION Temperature reduction is the most important process feature in low temperature gas processing.

In principal, there are several ways of cooling a gas stream to extract liquids, and selection of preferable method depends upon desired products and processed volume of gas, as well as inlet composition and pressure [2]. Refrigeration systems with cooling against a refrigeration medium is one possibility, particularly important when the inlet pressure is low [2]. Cooling by expansion is another, in which a rapid depressurizing of the gas stream induces the temperature drop. The latter can only be applied when the process inlet pressure is sufficient to provide the required pressure drop [2].

This work is mainly concerned with cooling by expansion, for which the principal configuration is given in Figure 2-5 below. Feed gas enters the first separator, separating liquid and gas phase upstream the depressurizing. Pressure is released in the depressurizing unit, resulting in a temperature drop, which condenses out heavy hydrocarbon components and in some cases liquid water. For processes with water condensation, hydrate inhibitors, often MEG is injected upstream the depressurizing, to ensure problem free operations. A separator downstream the depressurizing separates the phases to streams of dry, lean gas for recompression, and a liquid mixture of heavy hydrocarbons, water and hydrate inhibitor. Further processing of the liquid mixture produces stable condensate, water and regenerated hydrate inhibitor. In principal, there are two ways of preforming expansion and cooling processes, depending on the chosen method for depressurizing.

Figure 2-5 – General expansion and separation unit (HYSYS)

2.1.4 Joule- Thompson expansion

Joule- Thompson expansion utilizes the Joule- Thompson effect, which describes how the temperature of a gas changes with pressure. As the gas depressurizes in the Joule- Thompson (JT) valve, the distance between the molecules increase, increasing their average potential energy. The expansion produces zero work, and can be assumed to yield limited heat transfer to the surroundings. Hence, from the principal of conservation of energy, the kinetic energy of the gas molecules decreases, which is equivalent with a decrease in temperature [7]. If the kinetic energy decreases below the intermolecular forces in the liquid phase of one of the gas components, this component condenses out as a separate liquid.

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A JT-valve is essentially a control valve, and is a simple and inexpensive way to reduce gas temperature [2]. The depressurizing process follows a line of constant enthalpy in the PT diagram, as visualized by the dotted line in Figure 2-6.

Figure 2-6 – Phase envelopes for JT – expansion and turbo expansion [6]

2.1.5 Expander processes

In expander processes, the gas is depressurized through a turbo expander. Turbo expanders are essentially centrifugal compressors running backwards, and are able to extract work from the expansion process [2]. The work can be utilized directly for recompression in an expander – compressor unit, or to run electrical generators.

In addition to the ability to produce work, the main difference between a JT-valve and the turbo expander is the exit temperature from the depressurizing process. JT-valves are isenthalpic irreversible processes, meaning that enthalpy is constant. Ideal turbo expanders on the other hand, are thermodynamically reversible, isentropic processes producing work [2, 6]. The depressurizing follow a line of constant entropy in the PT-diagram as visualized by the whole line in Figure 2-6.

The two lines in the diagram illustrate how the temperature pressure gradient is larger for isentropic than isenthalpic processes. The result is a larger decrease in temperature for a given decrease in pressure, and thus, more liquid condenses using turbo expanders than by use of JT- valves. The isentropic turbo expansion process provides the largest possible heat removal from a gas stream for a given pressure reduction, while generating useful work [2]. The simple thermodynamic inequality given below summarizes the principal difference in temperature reduction between isentropic (s=constant) and isenthalpic (h=constant) depressurizing.

(2.1)

Figure 2-7 summarizes the two principal solutions for depressurizing and cooling. It should be stressed that these are simplifications. Real life processes as they are treated later in this work, include elements such as cold gas heat integration, and hydrate inhibitor injection. However,

|𝜕𝑇

𝜕𝑃_{𝑠=𝑐𝑜𝑛𝑠𝑡}| > | 𝜕𝑇

𝜕𝑃_{ℎ=𝑐𝑜𝑛𝑠𝑡}|

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preliminary descriptions in this chapter show how the physical principals of condensation and the phase envelope is utilized for gas processing applications.

Figure 2-7 – Basic expansion and separation by Joule- Thompson (left) and turbo expander (right) (HYSYS)

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S OLID FORMATION IN LOW TEMPERATURE NATURAL GAS PROCESSING

Low temperature gas processing liquefies and extracts heavy hydrocarbon components and water at low temperatures and under elevated pressure. In addition to maximizing liquid production, these conditions also create an optimal environment for solid formation. In the processes treated in this work, solid natural gas hydrates, and crystallized glycol impose severe safety issues, and their formation may induce harm on pipes and downstream process equipment. The following chapter thus introduces the formation of these solids, and aims to provide a background for treating and understanding their formation and prevention. In addition, the chapter introduces important process limitations, and terms being used throughout the work.

N

ATURAL GAS HYDRATES

Gas hydrates are a group of solid crystalline compounds called clatherates [2]. Generally, hydrates form from cage like structures of host material, in which smaller guest molecules are trapped. For the case of natural gas hydrates, the host material is water, clustering together to cavities of hydrogen bounded water molecules. Smaller hydrocarbon molecules are trapped inside the cavities, stabilizing them to hydrate crystals [8, 9]. Natural gas hydrates resemble ice, or wet snow, in appearance, but are less dense [10]. Despite the similarity to ice, interaction between gas and water under elevated pressure may form hydrates at temperatures considerably above the freezing point of water. [10]. In addition to small hydrocarbon molecules, also nitrogen, oxygen, carbon dioxide and hydrogen sulfide may act as guest molecules [2].

Hydrates induce operating problems, as the crystals form a solid, non-flowing compound, which may grow to plugs chocking gas flow in pipelines, flow lines and processing equipment. Hydrate plugs may grow to considerable size, within minutes, making them far more difficult to discover in terms of pressure drop compared to wax and scale [2].

Hydrate formation

Hydrates can form when water and hydrocarbons are present within the hydrate formation area in terms of pressure and temperature. In principal, both liquid water and water vapor may form hydrates. However, hydrate formation from water vapor is a kinetic slow and rare reaction. At least twenty water molecules are required pr. gas molecule, to form the stabilizing cage structures [2]. Moreover, several cages must combine in forming the hydrate lattice. The water vapor content in a natural gas mixture is in most cases very low, corresponding to a very low concentration of

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water molecules. The probability for a sufficient number of water molecules in gas phase to interact and form hydrates is thus extremely low, [2] and the presence of liquid water often lists as a condition for hydrate formation.

Studies of vapor/liquid/solid (VLS) equilibrium constants predict hydrate formation by generating hydrate phase diagrams. Figure 3-1 highlights a typical hydrate phase diagram for a system of methane and water. The red curve is the equilibrium line, at which methane/water and the solid hydrate phase are in equilibrium. As indicated by the figure, hydrates can form at pressure and temperature conditions to the left of the equilibrium line, wears systems operating to the right of the equilibrium will avoid hydrate formation.

Figure 3-1 also illustrates how hydrate formation increases with decreasing temperature and increasing pressure. The principle is analogous to the vapor-liquid equilibrium considerations of the previous chapter. Broadly speaking, the host material only forms cage structures when the kinetic energy of the molecules are sufficiently small to be overcome by their intermolecular forces. Thus, for natural gas hydrates to form, the kinetic energy of the water molecules must be smaller than the attractive intermolecular forces of the hydrogen bound that forms the cages. The trapped guest molecules stabilizes the cages, allowing them to form crystals at temperatures above the freezing temperature of water. The implication for low temperature gas processing is as predicted; low temperatures and elevated pressures that maximize liquid extraction also maximizes the tendency to hydrate formation.

Figure 3-1 – Hydrate phase diagram for two component system of methane and water (NeqSim)

Different natural gas compositions will change the hydrate phase diagram in terms of changing the equilibrium line. Figure 3-2 gives hydrate formation conditions for natural gases with various specific gravities. The diagram illustrates how hydrates form at decreasing pressure and temperature for increasing specific gas gravity. Thus, the more heavy components in the gas mixture, the larger is the hydrate formation area. However, this is only valid as long as the gas molecules fit in the cage structures. Components heavier than C5 will not contribute to hydrate formation, and will rather have a small effect as dilute, preventing hydrates. The effect is however insignificant in most cases, due to low heavy component concentrations [2].

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0 5 10 15 20 25

Pressure[bar]

Temperature[℃]

Hydrates can NOT

form Hydrates

can form

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Figure 3-2 – Hydrate equilibrium diagram for various gas gravities (NeqSim)

Figure 3-3 relates the hydrate equilibrium line to the phase envelope of a gas mixture. Continuous cooling the gas in vapor phase moves the system to the left in the PT diagram, towards intersection with the dew point line. Further temperature reduction condenses heavy hydrocarbon components and water as separate liquid phases. However, the temperature reduction, and thus the liquid extraction, is limited by the potential for hydrate formation in the gas system. Assuming a constant pressure of 60 bar, this particular gas mixture cannot be cooled safely below approximately -5℃

before penetrating the hydrate area. Potential hydrate formation thus limits not only liquid extraction, but also dew point control of the gas stream, and appears as a limiting factor for optimized low temperature gas processing.

Figure 3-3 – Hydrate equilibrium line and the phase envelope (NeqSim & HYSYS)

Hydrate formation is a reversible process, meaning that removal of hydrates is possible by reversing the conditions that caused them to form [11]. Thus, both heating and depressurizing will cause hydrates to dissociate. Depressurizing pipes or process equipment should however be done

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-10 0 10 20 30

Pressure [bar]

Temperature [℃]

Methane

80 mole% methane 20 mole% ethane

80 mole% methane 10 mole% ethane 10 mole% propane

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-80 -60 -40 -20 0 20 40

Pressure [bar]

Temperature [℃]

Phase envelope Hydrate equilibrium line

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with care, on both sides of the hydrate plug in order to prevent large pressure differences accelerating plugs through the system [9]. Heating is possible, but generally, hydrates are a serious operational problem along the entire gas value chain, avoided by preventive measures.

General hydrate prevention

Kidnay et al. lists three general ways of preventing hydrates from plugging a natural gas stream.

[2]: Operation outside the hydrate formation region, dehydration of the gas, and injection of hydrate inhibitors upstream the hydrate forming conditions.

Operation outside the hydrate formation region implies maintaining operations to the right of the hydrate equilibrium curve in all gas processes. This provides certainty that no hydrates form, but will in many cases be contradictory to low temperature gas processing. Feasible levels of liquid extraction and dew point control require low temperatures and elevated pressure. Thus, in many cases, optimal processing conditions penetrate the hydrate formation region, making operations outside an inappropriate solution.

Gas is dehydrated by either absorption or adsorption depending on demanded gas dryness. The method prevents hydrate formation by utilizing that liquid water must be present for hydrates to form effectively. Removing water from the gas stream reduces the water dew point to the extent where no free water condenses out during the desired process, and no hydrate formation occurs.

Gas dehydration is also a step in ensuring correct combustion characteristics of the gas [6], and is generally an important element in gas processing. However, dehydration units are costly, and not always technically or economically feasible. Dehydration is also less common in onshore field operations [2], as in this work, where there is no upstream gas dehydration prior to the low temperature processes. As a result, water removal, condensate production and gas dewpointing occurs as simultaneous processes in low temperature separators. This saves the expenses of installing and running upstream dehydration units, however, it also gives processes operating inside the hydrate formation region, at the presence of liquid water.

Neither gas dehydration, nor operating outside the formation region is preferred in the processes treated in this work. Instead, they apply a very common hydrate preventive measure, named chemical hydrate inhibition.

Chemical hydrate inhibition

Kidnay et al. lists three types of chemical hydrate inhibition, through antiagglomerate, kinetic and thermodynamic inhibitors [2]. Antiagglomerates inhibit hydrate formation by preventing smaller hydrate particles from agglomerating into large size plugs. Kinetic inhibitors interfere with the construction of host material cages to slow down the hydrate crystal formation [2]. Both are used to prevent hydrates, however, this work concentrates on the use of thermodynamic inhibitors, and agglomerate and kinetic inhibitors will not be subject to further discussion.

Increased liquid extraction in the Kollsnes dew point process by using mixed glycols

Kollsnes dew point process by using mixed glycols

Andreas Tomasgaard

P REFACE

S AMMENDRAG

A BSTRACT

Table of Contents

A BBREVIATIONS

N OMENCLATURE

I NTRODUCTION

S

T

D

L OW TEMPERATURE NATURAL GAS PROCESSING

C

2.1.1 The effect of pressure

2.1.2 The effect of molecular weight

2.1.3 Condensation of water

T

B

– E

2.1.4 Joule- Thompson expansion

2.1.5 Expander processes

S OLID FORMATION IN LOW TEMPERATURE NATURAL GAS PROCESSING

N

Hydrate formation

General hydrate prevention

Chemical hydrate inhibition

P ^REFACE

S ^AMMENDRAG

A ^BSTRACT