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Copyright 2008 GASEX 2008. All Rights Reserved.

Processing of High CO2 and H2S Gas with

Controlled Freeze Zone

Technology

Jaime A. Valencia and Beverly K. Mentzer

ExxonMobil Upstream Research Company

Houston, Texas

INTRODUCTION

Global gas reserves and new resource opportunities are becoming increasingly

challenging, with as much as 1/3 of global reserves having significant amounts of CO2

and H2S. Fields with >30% CO2 and >10% H2S are not uncommon, and in the extreme,

methane can be less than 50% of the total gas stream. This makes the economic

viability very challenging when smaller amounts of methane and other light

hydrocarbons in the full well stream must bear the added cost of producing, removing

and disposing of the larger amounts of contaminants, mainly CO2 and H2S. There is a

clear need for the more economical processing of resources with decreasing

hydrocarbon content. Furthermore, the focus on CO2 levels in the atmosphere has made

release of CO2 to the environment less desirable, leaving geosequestration as the most

promising alternative where there is no market for CO2. Similarly, sulfur production

from H2S has saturated many markets and prompted the need for an alternate way to

dispose of H2S. CFZ technology, developed at ExxonMobil Upstream Research

Company, is being advanced to the commercially-ready stage in response to these

needs.

THE CFZ PROCESS

The first choice for many separations in the gas industry is one that takes advantage of

the simplicity provided by differing relative volatilities and phase behavior, such as

flash drums, separators, distillation and fractionation towers. In the presence of

methane, and to yield sales-quality gas, such separations imply very low, cryogenic,

temperatures. Carbon dioxide solidifies at the low temperatures needed for methane

purification.

While the concept of CO2 separation by solidification was first suggested over 50 years

ago, most preferred avoiding solidification conditions and the handling of very cold

solids or slurries. Thus, it is not surprising that conventional technologies for the

removal of CO2 from natural gas are based on principles that do not involve cryogenic

temperatures. Most are solvent-based, capturing CO2 with a chemical, physical or

hybrid solvent, and reversing the process to discharge the captured CO2, and to

regenerate the removal capacity of the solvent.

CFZ is a cryogenic process for the single step separation of CO2 and H2S from

natural gas involving the controlled freezing and remelting of CO2. It enables the

production of sales quality gas at lower costs, and it is capable of advantageously

handling gases with a wide range of CO2 and H2S content.

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Rather than avoiding the solidification of CO2, the CFZ process controls its freezing

and remelting to take place in a specially designed chamber under specifically

controlled conditions. Below this chamber, or CFZ section, methane is stripped or

recovered from the bottoms liquid stream, which contains CO2 and other acid gas

contaminants, via conventional distillation. Above the chamber the CO2 content is

reduced, if necessary, to meet pipeline or LNG feed quality via conventional distillation

in a rectifying section.

Liquid from the upper conventional distillation section that is about to enter

solidification conditions is sprayed into the CFZ chamber, which is very open and

unobstructed. As the liquid droplets fall, they encounter warmer temperatures. Methane

and any lighter components such as nitrogen, if present, vaporize. The residual

concentration of CO2 in the droplets increases, leading to solidification. The solids that

form fall onto a liquid layer at the bottom of this chamber that is maintained above

solidification temperatures. A liquid, beyond solidification conditions, emerges from

the bottom of the CFZ chamber and is directed to the stripper section to recover

valuable methane.

Figure 1. The CFZ Process

Similarly, vapors from the bottom conventional distillation (stripper) section rise

through the CFZ chamber and encounter colder temperatures. CO2 condenses or

frosts onto the falling spray droplets or solid crystals, further contributing to the

solidification process. Their removal from the vapor stream results in a product exiting

the top of the CFZ chamber that is significantly depleted in CO2. The solids formed

in the CFZ section are pure CO2, thus providing greater separation factors and high

efficiency for this section.

The CFZ process normally operates at constant pressure, the selection of which can

be optimized primarily based on field gas pressure, sales gas pressure requirements,

Feed

C1/CO2

CO2

C1

Top Conventional Distillation

Bottom Conventional

Distillation BCD

TCD

CFZ

-120 F

T

30 F

LIQUID

500 PSIA

VAPOR

L + V

S + V

1 C 2 CO x,y

L+V Controlled Freeze

Zone

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tower size and refrigeration requirements. Operating temperatures will be dictated by

the actual feed compositions and prescribed product specifications. To exemplify

conditions in the tower, operations at 500 psia (3,448 kPa) would involve overhead

temperatures around -125F (-87C), bottoms temperatures around +30F (-1C), and CFZ section temperatures in the -70 to -110F (-57 to -79C) range.

Fundamentally, the CFZ technology provides the ability to more economically

process natural gas without imposing limitations on the level of CO2 or H2S

contamination it may have. It separates the CO2, and any other acid gas components

present, in a liquid stream that can be easily pumped for geosequestration or for use in

enhanced oil recovery operations, while yielding a high quality methane product.

INCENTIVES

The major benefits include:

- Capital savings Being a simpler process involving fewer processing steps, the CFZ

process results in less equipment for all applications, with additional savings

derived from less weight and footprint for offshore applications.

Significant reduction or complete elimination of solvents and additives The CFZ process does not involve solvents or the continued use of

additives.

- Lower acid gas injection costs The bottoms stream of the CFZ unit is a high pressure liquid stream that

contains the CO2 and acid gas components. Typical conventional

technologies reject CO2 and acid gas components as a low pressure gas

stream that must undergo costly recompression for underground reinjection.

The costs of boosting the pressure of an already relatively high pressure

liquid CFZ bottoms stream to geosequestration or EOR reservoir

pressures can be significantly less.

- Lower sales gas recompression costs Similarly to the bottoms stream, the overhead sales gas stream is also

produced at relatively high pressure, which can minimize the recompression

requirements for its delivery to sales pipelines.

- Environmental benefits CFZ allows the economic geosequestration of CO2 and provides an

alternative to sulfur production plants when coupled with acid gas injection.

- Provides for product flexibility: gas suitable for pipeline sales or LNG production.

- All additional sulfur-bearing species that might be present are separated and directed along with the H2S and CO2 into the bottoms liquid stream.

- Potential improvement in oil production if CO2 is used for enhanced oil recovery.

In particular, the CFZ technology can deliver these benefits when applied to the

following:

- Natural gas reserves with a high (>10%) acid gas content; the higher the acid gas content, the greater the benefits of the CFZ technology compared to

alternatives processes.

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- Applications where the acid gas disposal will be done via reinjection for geosequestration or EOR purposes.

- Fuel gas treating and LNG pretreatment applications.

THE CLEAR LAKE PILOT PLANT OPERATION

The CFZ technology was invented at Exxon Production Research in 1983 and was

patented in 1985.1 A 0.6 MMSCFD pilot plant was built at Exxons Clear Lake Gas

Plant in Pasadena, Texas (near Houston) in 1985. It was operated in 1986 and 1987.

Figure 2. The Clear Lake CFZ Pilot Plant

The Clear Lake CFZ Pilot Plant was the first facility to successfully demonstrate the

freezing and remelting of CO2 as part of a natural gas separation process.

The pilot plant processed feed gases with CO2 contents ranging from 15 to 65% CO2, at

pressures of 550 to 600 psia (3,792 to 4,137 kPa). While it was designed to achieve

pipeline quality overheads, its overhead gas stream not only met pipeline quality, but

approached LNG feed quality by reducing its CO2 content to a few hundred parts per

million. Methane losses in the bottom stream were targeted for 1%, but better

performance of as low as 0.5 % was achieved.

The Clear Lake Pilot Plant operations were very successful at demonstrating the

concept of controlled freezing and remelting of CO2 and provided valuable operating

and design information.

CFZ TECHNOLOGY DEVELOPMENT

Additional field activities were not pursued due to the oil price collapse in the mid 80s,

which further impaired economics of highly sour resources. However,

commercialization studies and the pursuit of refinements did continue. A total of 7 U.S.

patents (and associated world patents) have been granted to ExxonMobil for CFZ

technology, and others are pending. Also, in parallel and synergistic activities,

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ExxonMobil has designed and operated large scale acid gas injection facilities in

several countries.

SCALE-UP PERSPECTIVE

While pilot plant operations are crucial to demonstrating the technical viability of a

new processing concept, they are not sufficient to deem a technology ready for

commercialization, particularly when the commercial scale involved is orders of

magnitude greater than that of the pilot facility.

Areas of commercial interest to ExxonMobil include facilities that would process

hundreds of millions to billions of standard cubic feet a day of natural gas. To properly

plan for a scale up of this magnitude, ExxonMobil is currently building a commercial

demonstration plant (CDP). Data from this plant will not only allow scale-up all the

way to world scale plant size, but also broaden the operational experience to feeds

containing H2S and to a greater range of operating conditions.

THE COMMMERCIAL DEMONSTRATION PLANT AT LABARGE

In 2007 ExxonMobil began the design of a CFZ commercial demonstration plant at

its Shute Creek gas treating facility (SCTF) in LaBarge, Wyoming, USA.

Figure 3. Shute Creek Gas Treating Facility

SCTF processes over 700 MMSCFD of natural gas which contains 65% CO2 and 5%

H2S. The bulk of the CO2 is sold for enhanced oil recovery. A concentrated H2S acid

gas stream is reinjected. This is one of the largest acid gas injection operations in the

world; it is equivalent to a sulfur production of 1,300 LT/D.

Figure 4. Acid Gas Injection (AGI) unit and injection well

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The LaBarge facility was chosen to host the commercial demonstration plant because it

will allow testing of the CFZ technology not only with CO2 but also H2S. In addition,

its acid gas injection facilities will also allow the CFZ unit to demonstrate the ability

to pump its bottoms for geosequestration.

The operational objectives of the CDP include the demonstration of processing of

higher volumes of gas with a wide range of CO2 and H2S content under a variety of

operating conditions. That data will allow the establishment of scale-up parameters, and

the development of process, equipment and mechanical design information that will

enable the design and operation of up to BSCFD-range CFZ facilities.

Figure 5 CFZ commercial demonstration plant design

The CDP is currently in the engineering, procurement and construction phase and will

be operational in 2009.

SUMMARY

The CFZ technology involves a simpler single-step process for the separation of acid

gases from methane. It imposes no limitation on the amount of CO2 or H2S present in

the feed gas; thus, it provides for more economical processing of very sour gases.

Further, the acid gas components are discharged as a high pressure liquid stream,

providing for a more economical reinjection of this stream, either for geosequestration

or for enhanced oil recovery purposes. Its integration with acid gas injection provides

an alternative to sulfur plants for H2S disposal.

REFERENCES

1. Valencia, J. A. and Denton, R. D., Method and Apparatus for Separating Carbon Dioxide and Other Acid Gases from Methane by the Use of Distillation

and a Controlled Freeze Zone, U.S. Patent 4,583,372, August 6, 1985.