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HYBRID MEMBRANE/ABSORPTION PROCESS FOR ACID GAS REMOVALIN FLNG APPLICATIONS

James Zhou Howard Meyer Dennis Leppin

Gas Technology Institute

ABSTRACT

GTI and PoroGen Corporation are working jointly on developing an advanced membrane contactor technology for FLNG applications. The membrane contactor is manufactured utilizing poly(ether ether ketone) (PEEK), hollow fiber membranes. The PEEK hollow fibers used in the contactor module construction are made using a patented process. We have performed numerous sets of tests to evaluate membrane contactor performance for CO2 and H2S removal from simulated natural gas streams. These tests were conducted with various types of membrane modules. Membrane properties and module design were optimized towards CO2 and H2S removal. Bench scale tests were conducted utilizing DEA as a solvent (30 wt% DEA). The measured overall volumetric mass transfer coefficient for CO2 removal for the membrane contactor equaled 1.1 (sec)-1, which is 15 times higher than the maximum mass transfer coefficient of a packed column. Based on the measured mass transfer coefficient it was estimated that one 8-inch diameter by 60-in long contactor module containing 1000 ft2 membrane surface area can treat 3.7 MMSCFD of feed gas containing 8.0 vol. % CO2 inlet concentration to 2.0 vol.% outlet CO2 concentration product gas. FLNG applications require higher product purity. For FLNG applications, the membrane contactor can be used to reduce the inlet CO2 concentration from 8% down to 50 ppm. The manufacturing process for the preparation of PEEK hollow fiber membrane modules was scaled up from the bench-scale cartridge sizes used in performance tests to larger membrane area size commercial cartridges with 1000 ft2 of membrane area.

I. INTRODUCTION

Carbon dioxide and often hydrogen sulfide are present in field gases and have to be removed to a specific level to meet different application requirements (typically <4 ppmv H2S and < 2% CO2 for U.S. pipeline tariffs , and < 50 ppmv CO2 for LNG). The alkanolamine absorption process is the state-of- the-art technology for this purpose, unless the acid gas concentration is very high, say >15 vol. %, in which case physical solvents may be preferred. These amines, such as DEA, MDEA, and specialty amines, are used as aqueous solvents to absorb H2S and CO2 from sour natural gas streams. The sour gas is introduced at the bottom of an absorber and flows up the tower countercurrent to a down flowing aqueous amine stream. Within the tower, the acid gases are chemically absorbed by the amine. This mass transfer occurs in the thin film of solvent formed as the solvent flows over the high-surface-area packing.

Conventional absorption towers in the amine sweetening system are large in size and heavy in weight. They pose operational challenges such as liquid channeling, flooding, entrainment, and foaming. The hybrid membrane/absorption process using the hollow fiber contactor (HFC) technology can replace these conventional packed or trayed columns with smaller, less expensive membrane modules, thereby avoiding all of these problems.

I.1 Background

Beginning in 1992, Aker (Kværner) Process Systems worked on the development of expanded polytetrafluoroethylene (ePTFE) membrane contactor technology for removal of CO2 from gas turbine exhaust offshore and from sour natural gas streams.1,2 This development work has initiated the possible use of gas/liquid membrane contactors (also called "gas absorption membranes") for a range of applications of interest to the international oil and gas industry. The ePTFE-based membrane contactor system was a technological success. However, several factors affected process economics including membrane module

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cost and pressure control system cost, etc. GTI has since entered in an agreement with PoroGen Corporation to study poly (ether ether ketone) or PEEK based membrane modules which, when used in the contactor system, overcomes the problematic factors. The PEEK modules incorporate proprietary manufacturing and surface-treating features that provide the functionality required for effective use in sour gas treatment.

While the overall chemical separation of CO2 or H2S remains the same between the conventional column-based process and the membrane contactor process, the contacting vessel significantly changes with gas/liquid membranes. The principle of gas absorption membrane using nano-porous, hollow fiber membrane modules offers several advantages over a conventional packed absorption tower. In the gas absorption membrane system, the gas flows inside the hollow fibers and the amine flows around the outside of the fibers. The membranes are super-hydrophobic and nano-porous, that is, the solvent will not wet the membrane pores, and the nano-sized pores will remain gas filled. This results in extremely low resistance to gas flow in the open pores. The mass transfer takes place at each pore along the length of the fibers. The separation driving force and component selectivity is set by the solvent/gas chemistry, essentially the same as in the tower.

One advantage of using a membrane contactor to separate the phases is that it becomes possible to eliminate the usual limitations of packed towers caused by flooding and entrainment of the liquid by the upward flow of gas. In the membrane absorber, the gas and liquid flow can be varied independently, and the contact area will then also be independent of the flow velocities as opposed to the behavior in a tower where the mass transfer area is varying with the liquid load. The hollow fiber membranes open up the possibility of a very high specific contacting area per unit volume for a membrane contactor (see Table 1). Practical considerations, such as pressure loss, limit the value to somewhere between 500 and 3,000 m2/m3. This is still larger than in a tower where values of 100-350 m2/m3 are common, and enables the possibility of reductions in contactor volume and weight.

Table 1. Gas-liquid contactor device surface area and volumetric mass transfer coefficient comparison

Gas‐liquid contactor Specific surface area, (m2/m3)

Volumetric mass transfer coefficient, (sec)-1

Packed column (Countercurrent) 10 – 350 0.0004 – 0.07

Bubble column (Agitated) 100 – 2,000 0.003 – 0.04 Spray column 10 – 400 0.0007 – 0.075 Membrane contactor 100 – 7,000 0.3 – 4.0

Other advantages of the membrane contactor which have been identified are elimination of foaming, avoidance of liquid maldistribution (channeling), and potentially reduced pick-up of contaminants and thereby decreased solvent degradation. Tidal- and wave-induced motion, as observed in floating platforms and ships, should not affect the performance of the Gas/Liquid Membrane contactor, whereas it does cause performance degradation by gas bypassing in conventional columns placed in such services.

I.2. PEEK Membrane Technology

PoroGen Corporation has developed a novel hollow fiber membrane technology based on the chemically and thermally resistant commercial engineered polymer poly (ether ether ketone) or PEEK3,4. The PEEK membrane material utilized in the proposed membrane contactor is a high-temperature engineered plastic that is extremely resistant to deterioration under the operating conditions encountered in typical gas absorption applications. It can withstand contact with most of the common treating solvents. GTI and PoroGen have developed a nano-porous, super-hydrophobic PEEK-based membrane contactor tailored for the membrane contactor/solvent absorption process in natural gas sweetening applications.

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The membrane contactor is constructed from super-hydrophobic PEEK hollow fiber membranes that constitute the novel, enabling feature of this technology. The hollow-fiber membrane is extremely hydrophobic with a water breakthrough pressure (differential pressure across the membrane fiber) greater than 600 psig. The technology developed by PoroGen has been commercialized for several gas separation applications such as hydrocarbon dew pointing for natural gas. Such a hollow fiber contactor is ideally suited for this application since the fibers can provide very high surface area to volume ratios and pressures on the bore side and the shell side can be maintained independently, which is not possible for conventional columns.

The PEEK membrane absorber module has been further tailored towards the specific needs of natural gas sweetening to provide improved mass transfer of acid gases from the gas phase to the solvent phase. The robust hollow-fiber membrane exhibits a high intrinsic CO2 permeance ( >1000 GPU, 1 GPU=1x10-6 (scm3)/(cm2 cm Hg sec)) while still providing an absolute gas/liquid inter-phase barrier. The contactor module is constructed using computer-controlled helical winding of the hollow fiber membranes and provides for a compact mass transfer device with high separation efficiency. PoroGen’s structured helical packing of hollow fibers breaks the liquid-side boundary layer and reduces concentration polarization. The productivity of the membrane contactor is a function of the mass transfer coefficient, which is controlled by the liquid interface resistance. Therefore, PoroGen’s PEEK membrane contactor modules provide the highest mass transfer coefficient by minimizing this liquid interface resistance. The PEEK membrane can withstand differential pressures up to 60 psi without solvent leakage or structural damage.

I.3 Mass Transfer Coefficient for Membrane Contactor

The transport of carbon dioxide through a membrane contactor is shown in Figure 1. Cg, Cgm, Cml, Clm and Cl are the CO2 concentration in the gas phase, at the gas-membrane interface, at the membrane-liquid interface, at the liquid-membrane interface and in the liquid.

Figure 1. CO2 concentration profile and resistance in series model in a membrane contactor

The CO2 mass transfer coefficient for a gas-liquid absorption process can be expressed as follows:

1𝐾

= 1𝑘𝑔

+ 1𝑘𝑚

+ 𝐻𝑎𝑑𝑖𝑚𝐸∙𝑘𝑙

(1)

Where K is the overall mass transfer coefficient [cm/s], kg is the mass transfer coefficient in the gas phase, km is the mass transfer coefficient in the membrane [cm/s], kl is the mass transfer coefficient in the liquid phase [cm/s], Hadim is the non-dimensional Henry’s constant, and E is the enhancement factor due to chemical reaction.

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The overall resistance to CO2 transport and the overall mass transfer coefficient have an inverse relationship. To maximize the mass transfer coefficient, the overall resistance must be reduced, which in turn entails reducing resistance of individual components contributing to the overall resistance. The resistance in the gas phase is typically very small and the resistance in the membrane phase is a function of membrane structure. The resistance in the liquid phase is a function of contactor module design, i.e. flow dynamics, and solvent characteristics.

II. OBJECTIVES

The objective of this study is to demonstrate the feasibility of using the hybrid membrane/absorption process for natural gas sweetening either to reach pipeline specifications or to reach LNG requirements. Laboratory tests using representative amine solvents such as DEA and aMDEA and using simulated natural gas (Nitrogen with varying amounts of CO2 and H2S at high pressures) were conducted to assess the performance of the membrane/absorption technology based on PEEK hollow fiber contactors. ProTreat™ simulation was used to size the column for two cases of interests and compared with the contactor process. The other objectives of this project are to develop the PEEK hollow fiber membrane and scale-up the membrane module fabrication process.

III. EXPERIMENTAL METHODS

The apparatus and procedures for the experiments are described in some detail below. This test unit has been used to conduct experiments on acid gas treating with activated MDEA and DEA.

The equipment, its simplified process flow diagram as shown in Figure 1, is set up in a Plexiglas enclosure that is under negative pressure due to an exhaust vent placed in the ceiling of the area. This ensures that leaks of small volume will be harmlessly exhausted to a roof vent system. The key equipment item is the membrane contactor itself. The apparatus has two main sections to handle the two process streams: A feed gas blending section and a solvent feed section.

Absorber(Membrane Contactor)

Mass Flow Controllers

N2/He

H2S

CO2

Water Saturator

P

Solvent Pump Storage Tank

HeaterTP

Solvent Flash and

Storage Vessel

Treated Gas

CO2

Figure 1. Simplified test loop process flow diagram

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Gas Feed Blending Section: Nitrogen and pure carbon dioxide are blended together, at controlled rates, to produce a feed gas of a desired composition and at a specified flow rate using a mass flow controller. A Fluitron diaphragm compressor is used to provide high-pressure (up to 1,100 psig) recycled test gases (mostly N2) up to 0.6 MMscfd (400 scfm) where the use of cylinder gases would be prohibitively expensive. Water vapor can be added to the feed-gas blend via the water saturator. This moisture could be present in the feed gas at levels of a few hundred to a few thousand ppm of water. To simulate this moisture content in the feed gas (that is, adding up to 1000 ppmv moisture), the planned approach is to direct the feed gas through a water saturator, at a controlled temperature. If needed for purge or for back-pressuring, nitrogen can also be directed from a position before the regulator to the outlet of the reactor.

Solvent Feed Section: Feed solvent to the process will come from one of two high-pressure pumps. Solvent will be loaded into the Solvent Feed Drum, and the Solvent Injection Pump will pressurize the solvent to test conditions, approximately 1000 psig. This is a Milton-Roy dual-piston pump, which has a nameplate capacity of 14 gallons per hour (gph) at 1500 psig discharge pressure. A flow rate indicator has been included on the discharge line, as well as a pressure transmitter.

Gas Product Section: The outlet gas from the membrane contactor goes into a high-pressure H2S scrubber and returns to the inlet of the Fluitron compressor.

Process Control / Data Acquisition: A PC-based version of the LabVIEW (National Instruments) program is employed for both process control and data acquisition.

The feed and product gases are analyzed by gas chromatography.

Two-inch laboratory size modules were leak tested at PoroGen and performance tested at GTI. The module performance was initially tested for CO2 removal utilizing DEA solvent system. The feed pressure was 500 to 1000 psig and the feed contained about 8% by volume CO2 with the remainder nitrogen. The CO2 was removed to generate product with either less than 2.0 vol% or less than 50 ppmv residual CO2 content. The gas-side pressure drop, the liquid-side pressure drop, and the lean and rich loading of the DEA solvent were also measured. The performance of the membrane contactor is summarized in the section below. Three types of PEEK membrane contactor were tested for H2S performance. Selective H2S removal was accomplished using MDEA as solvent.

IV. RESULTS AND DISCUSSION

IV.1 PEEK Membrane Development

The hollow fiber membranes are manufactured from the best in class commercial engineering plastic, PEEK. Porous PEEK hollow fibers used in preparation of super-hydrophobic membranes are manufactured by a high temperature melt extrusion process. The process is used commercially by PoroGen to prepare fluid separation membranes. PoroGen manufactures porous PEEK hollow fibers from blends of PEEK polymer with porogen polyether imide (PEI) following procedures described in US Patent 6,887,408 assigned to PoroGen. The process is illustrated schematically in Figure 2.

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Figure 2. Process for the preparation of nano-porous

PEEK materials (reagent bath monoethanolamine)

The following nano-porous PEEK hollow fiber substrate preparation parameters have been optimized for the natural gas sweetening application:

(1) PEEK hollow fiber morphology. Optimization of porous hollow fiber preparation procedures was carried out by varying the processing conditions in the spinning line. It has been found that the processing conditions have a significant effect on the fiber stability in contact with solvents. Although hollow fibers are solvent stable in all solvents tested, some small degree of swelling can occur with the most aggressive solvents leading to the deformation in the hollow fiber cartridge that, in turn, can result in cartridge failure. Processing conditions at low temperatures resulted in hollow fiber membranes with higher levels of swelling in solvents. Optimum processing conditions have been identified with the resulting fiber exhibiting good dimensional stability in contact with solvents. The experimental variables included the precursor blend composition, spinning temperature profile, extruder screw design, spinning speed, and draw ratio.

PEEK is a semi-crystalline polymer and it is critical to maximize the degree of crystallization to obtain optimal mechanical, thermal, and chemical resistance characteristics. The processing conditions were optimized to attain degree of crystalinity of about 34% in porous PEEK hollow fibers, which is identical to the virgin PEEK material. PEEK/PEI blends can form spherulitic or lamellar morphologies that can affect final pore structure as shown in Figure 3. Significant orientation of lamellar morphology occurs during spinning with increasing draw ratio as shown in Figure 4.

The processing conditions were systematically varied to obtain fiber morphology tailored towards contact with solvents (stability and non-wetting). The fiber compatibility with test solvents is discussed below.

(2) PEEK hollow fiber pore sizes. For successful operation of the contactor process, it is essential that: a) liquids are prevented from penetration into and passing through the membrane pores, and b) unimpeded transport of CO2 from the feed to permeate side can occur. The first requirement can be satisfied if the membrane surface is sufficiently oleophobic (very low surface energy) such that no absorption solutions can wet out and wick by capillary forces into the pores (requiring a contact angle between the liquid and solid phases of greater than 90°), and the surface tensions of the liquid phases are sufficiently high that the capillary penetration pressure of liquid into a pore is well in excess of the maximum pressure difference across the membrane that might be encountered in the operation. Liquid penetration into the pores will lead to a dramatic decrease in mass transfer coefficient. The critical penetration pressure is defined by the classical Kelvin Equation:

p = 2γcosθ/r (2)

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Figure 3. PEEK/PEI blend morphologies Figure 4. Uniaxial deformation during spinning

process (from top to bottom: stress vs. draw ratio curve; wide angle x ray pattern and chain alignment into crystalline

and amorphous regions)

where p is the pore-entry pressure, γ is the liquid surface tension, θ is the contact angle, and r is pore radius. The higher the surface tension of the liquid, the larger the contact angle (in excess of 90°), and the smaller the pore radius, the greater the intrusion pressure. There is a delicate balance between pore wettability and membrane mass transfer resistance. In order to have an unimpeded gas transport, larger pore sizes are better. On the other hand, in order to improve the non-wettability of the hollow fiber, smaller pore sizes are better.

Three types of porous PEEK hollow fiber membranes with different average pore size were prepared. The pore size was affected by polyimide selection and membrane preparation conditions. The average pore sizes based on the permporosimetry was 10 nm, 75 nm and 380 nm, respectively.

(3) PEEK hollow fiber surface modification: The super-hydrophobicity of the porous PEEK membrane was generated by surface modification with a functional perfluoro oligomer, such as PFC 504A/coE5 (containing reactive epoxy groups), commercially available from Cytonix Corporation. Prior to grafting with perfluoro oligomer the surface of the porous PEEK was first functionalized with ~ OH groups by reacting ketone groups in PEEK polymer backbone with monoethanolamine during the Reactive Porogen Removal (RPR) process. The functionalized porous PEEK was prepared in a single step RPR process during porous PEEK fiber preparation following the teachings of US Patent 7,176,273. Following porous structure formation, the functional ~ OH groups are reacted with perfluoro oligomers to form the oleophobic graft surface.

The hydroxyl groups as shown in Figure 5 are utilized as the anchor points for the subsequent grafting reaction with functional perfluoro hydrocarbon oligomer to render the pore surfaces oleophobic. The functional ~ OH groups are reacted with functional hydrophobic oligomers to form a hydrophobic graft surface. The grafting reaction with perfluorinated oligomer is illustrated schematically in Figure 6. Note that in Figure 6, Rf is a perfluoro hydrocarbon radical.

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O + H2NOH

NOH

Figure 5. Surface functionalization of porous PEEK with ~ OH groups during preparation of porous PEEK hollow fiber

+ RfRf

Hydrophilic Hydrophobic

OO

OH

N

OH

N

Figure 6. Preparation of hydrophobic PEEK membranes

Porous PEEK membranes are super-hydrophobic and do not wet out in contact with solvent systems. Porous PEEK membranes wet out with water at pressure above 600 psig and with isopropanol at pressure above 20 psig. The super-hydrophobicity is due to a combination of nanometer size surface pores, exceptionally uniform pore size distribution, and the perfluoro-hydrocarbon surface chemistry. The surface pore diameter is in the range of 1 to 5 nm. The combination of nanometer size pores and perfluoro-hydrocarbon surface chemistry generate the super-hydrophobicity via so called “Lotus effect.”5 The high contact angle and the non-wetting of porous PEEK membrane surface by solvents (ethanol liquid drop) is shown in Figure 7.

Figure 7. High contact angle and the non-wetting properties of porous PEEK membrane surface

IV.2 PEEK Membrane Module Development

Membrane modules for laboratory scale tests have been prepared. The hollow fiber membrane modules were of the four-port counter-current flow design. The contactor module design is directed to the counter-current flow configuration which is the most thermodynamically efficient. The design further takes into account gas side and liquid side pressure drops. The flow configuration and the general layout are shown in Figure 8.

F

F F

F F

F F

F F

F F

F F

F F

FF

F F

F F

F F

F F

F

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Figure 8. Laboratory scale contactor module design

The hollow fiber cartridges were formed by computer-controlled helical winding. The cartridge size was 2-inch diameter by 12-inch long and contained about 10 to 20 ft2 of membrane area (as measured on the outside diameter of the fibers). The cartridge was housed in a pressure vessel.

Membrane module design and construction have significant impact on the overall CO2 mass transport coefficient by minimizing liquid side resistance, maximizing the driving force and increasing the liquid side mass transport coefficient. For the conventional membrane modules designed for filtration applications, the flow conditions on the shell-side of the membrane can be generally ill-defined. However, for the membrane gas absorber, the flow conditions must be well defined on both sides of the membrane to achieve good mass transfer. Important design features of a module include the regularity of fibers (poly-dispersity and spatial arrangements of fibers), packing density and the relative flow directions such as parallel, i.e., concurrently, counter-currently, and cross-flows of the two phases. The liquid flow can either be on the bore or shell sides. PoroGen’s hollow fiber membrane module has been designed to operate with liquid on the shell side and gas flow on the bore side.

The following key design elements and development work have been carried out:

a) 4-port countercurrent flow design, enabling optimum driving force for the acid gas absorption;

b) computerized structural packing minimizing the absorption liquid flow maldistribution;

c) optimum fiber packing density to minimize the liquid pressure drop and optimize the liquid flow turbulence;

d) optimized winding patterns to promote the liquid-side gas mass transport; and

e) curved hollow fiber with enhanced gas phase mass transport.

The hollow fiber placement within the module was controlled through computer controlled helical fiber winding. The process generates a structured packing configuration minimizing channeling, bypassing, and minimizing concentration polarization. A wound cartridge with a controlled uniform structured packing is shown in Figure 9. The hollow fibers are arranged in a helical path, with the axis of the fibers running confluent to the principle direction of fluid flow. To enable the thermodynamically most efficient counter-current flow, the packing density in the cartridge must be uniform. In addition, flow bypassing, and entrance and exit effects must be minimized. The fiber packing density and packing uniformity were controlled to ensure an optimal flow distribution with minimal pressure drop on both the feed and the permeate sides.

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Figure 9. Helically wound structured hollow fiber cartridge

The design of commercial size membrane contactor module was completed. The contactor cartridge size is 8 in. diameter by 5 ft. long. The contactor cartridge will be housed in a 10 in. diameter flanged pressure shell designed for high-pressure operation (1000 psig feed pressure). The contactor will contain about 1000 ft2 of hollow fiber membrane area.

IV.3 PEEK Membrane Contactor CO2 Performance

The best performing membrane contactor results are summarized in the section below. This module was fabricated with hollow fiber PEEK membranes that had an intrinsic CO2 permeance of 1000 GPU measured using pure CO2 at 30°C. Test results and mass transfer coefficient calculations for this module are shown in Table 1.

Table 1. Calculated mass transfer coefficient

Inlet CO2, % Outlet CO2, % % Removal KGa, mol/(m3.hr.Kpa) KG,

cm/s KGa, 1/s

7.86 1.22 85.7 1609 0.0437 1.02 7.76 2.21 73.3 1721 0.0468 1.09 8.58 2.34 74.7 1788 0.0486 1.13 7.70 1.91 76.8 1858 0.0505 1.17

The gas-side flow pressure drop, the liquid-side flow pressure, and the lean and rich loading of the DEA solvent are shown in Table 2.

Table 2: Pressure drop and solvent loadings

Lean Loading, mol/mol Rich Loading, mol/mol ΔP, gas side, inH2O ΔP, Liq. side, inH2O

0.06 0.261 39.7 30.9

0.06 0.296 66.3 34.2

0.06 0.264 65.4 36.0

0.06 0.246 65.4 36.0

The overall volumetric mass transfer coefficient measured ranges from 1.02 to 1.17 (1/s). The industrial packed column generally has a volumetric mass coefficient range of 0.0004 to 0.07 (1/s). The highest membrane contactor mass transfer coefficient is 16 times greater than the cited maximum for a packed column.

Based on the measured mass transfer coefficient, it can be estimated than an 8-inch diameter, full-size commercial module (physical size: 10 inch outer diameter by 60 inch tall) with 1000 ft2 surface area could:

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• Treat 3.7 MMSCFD from 8.0 vol% inlet to 2 vol% outlet CO2 as projected from performance data with 30 wt% DEA at high pressure using lab scale 2 inch module, and

• To treat 500 MMSCFD of the same gas, 135 commercial modules are required.

IV.3.1 PEEK membrane contactor performance for ppm concentration CO2 in the product

Laboratory tests of a single-module configuration for CO2 removal utilizing a DEA solvent system were conducted to determine if a 50 ppmv product could be obtained. Test results indicate that we can reach about 800 ppmv within one single 10-inch length membrane contactor at 8 vol% inlet concentrations. A 10-inch module length did not provide a sufficient residence time for deep purification. Two 10-inch long modules in series were used to perform further tests. The results from these tests show that from 8 vol% inlet, a 200 ppmv CO2 concentration in the product stream is achieved in a combined 20 inch length contactor system. It was then decided to lower the inlet CO2 concentration to 1 vol% and repeat the test. The results from these tests show that a 50 ppmv product can be obtained.

IV.4 Comparison of Membrane Contactor with Conventional Packed Column

The membrane contactor process and the conventional packed column process were compared as to the size of the absorber and desorber. We have also performed HFC stripping tests and that such results were incorporated into the comparison although not reported here. The HFC process equipment size is estimated using experimentally measured mass transfer coefficients. The conventional packed column process equipment size is calculated using ProTreat®, an amine process simulation software from Optimized Gas Treating, Inc. The inlet gas conditions, compositions and the outlet product conditions and compositions are shown in Table 3 for the two cases compared.

Table 3. Gas conditions and compositions of the two cases

CO2 and H2S removal unit

Pipeline LNG Process conditions

Case 1 Case 2

Vapour fraction - 1.00 1.00 Temperature °C 40.00 40.00 Pressure psi-abs 1,015 1,015 Mass flow MMSCFD 250 250 Molecular weight -

Composition (same for Case 1 & 2)

CO2 mol% 8.00 8.00 H2O mol% 0.14 0.14 Nitrogen mol% 0.63 0.63 Methane mol% 77.50 77.50 Ethane mol% 6.00 6.00 Propane mol% 4.00 4.00 i-Butane mol% 1.00 1.00 n-Butane mol% 1.50 1.50 i-Pentane mol% 0.40 0.40 n-Pentane mol% 0.50 0.50 n-Hexane mol% 0.20 0.20 n-Heptane mol% 0.08 0.08

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n-Octane mol% 0.04 0.04 n-Nonane mol% 0.01 0.01 n-Decane mol% 0.00 0.00 H2S ppmv 60 0

Product Specifications

Case 1 Case 2

CO2 mol-% ≤ 2.0 ≤ 50 ppmv H2S ppmv < 4

max allowable ΔP psi 20 20 The results of the comparison are shown in Table 4.

Table 4. Comparison of process equipment sizes

Unit Operation Cases

1 2 Column

Absorber

Packing depth m 9.1 9.1

Diameter m 2.7 2.8

Volume m3 52 56 Regenerator

Packing depth m 12 12

Diameter m 3.4 4.1

Volume m3 109 158

Total Column Volume m3 161 214 Hollow Fiber Contactor

Diameter cm 20 20

Length m 1.5 1.5

Volume/module m3 0.05 0.05 Absorber

Number Modules # 67 330

Volume m3 3.4 16.5 Regenerator

Number Modules # 213 408

Volume m3 10.7 20

Total HFC Volume m3 14 37 Volume Reduction in Contacting Equipment Absorber

93% 71%

Regenerator 93% 90%

Generally, a hollow fiber contactor can achieve less than 4 ppmv H2S in the product stream along with a certain degree of CO2 removal. We performed some tests to study selective H2S removal, but refocused the HFC development program to CO2 removal. The selective test results indicate that, using MDEA as the solvent and with the same H2S removal percentage, CO2 removal is one-third as much, or equivalently CO2 slip is twice as much when using the HFC as compared with simulated trayed-column performance. This

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proves that the HFC is much more selective toward H2S when specific fibers are used for the fabrication of the HFC modules.

The comparison data shown in Table 4 indicate that the HFC is much smaller in size as expected from the much higher mass transfer coefficients. For treating pipeline specification gas, a more than 90% reduction in process equipment size can be realized for both the absorber and the desorber. For LNG specification gas (50 ppmv CO2), a 70% reduction in absorber and a 90% reduction in desorber size can be realized.

V. CONCLUSIONS

These encouraging initial results indicate that the HFC process should be effective in natural gas sweetening operations to obtain gases that meet either the pipeline specifications or the more stringent LNG specifications of less than 50 ppmv CO2. The measured mass transfer coefficient in the HFC is more than 16 times that of the maximum mass transfer coefficient observed in a packed column. The translates to much smaller operating unit sizes for the HFC as compared with the conventional column. A more than 90% reduction in equipment size was estimated when compared to a conventional column process sized using the ProTreat® process simulation software and using experimentally measured mass transfer coefficients to size the HFC.

The successful commercial deployment of this technology will enable large savings in capital cost, simpler construction and equipment transport logistics, improved reliability due to decrease or even elimination of foaming problems, more flexible capacity additions, and expand the range of concentrations to higher amounts of acid gases that can be considered for offshore processing.

VI. REFERENCES

1. Henrik Dannström, H., et al. Natural Gas Sweetening Using Membrane Gas/Liquid Contactors, presented at The 49th Annual Laurance Reid Gas Conditioning Conference, Norman, Oklahoma, February 21-24 (1999).

2. Herzog, H. and O. Falk-Pedersen, The Kvaerner Membrane Contactor: Lessons from a Case Study in How to Reduce Capture Costs, presented at the Fifth International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, August 13 - August 16 (2000).

3. US Patent 6,887,408, May, 2005.

4. US Patent 7,176,273, Feb., 2007.

5. Feng, et al., Adv. Mater., 2002, 14, p.1857.

LIST OF ACRONYMS AND ABBREVIATIONS

DEA: ....... Diethanolamine

ePTFE: .... expanded polytetrafluoroethylene

FLNG: ..... Floating liquefied natural gas

GPU: ....... Gas Permeation Unit (1 GPU=1x10-6 (scm3)/(cm2 cm Hg sec))

GTI: ......... Gas Technology Institute

HFC: ....... Hollow fiber contactor

LNG: ....... Liquefied natural gas

MDEA: .... methyldiethanolamine

aMDEA: .. activated methyldiethanolamine

PEEK: ..... poly (ether ether ketone)

PEI: ......... polyether imide

ppmv: ...... parts per million by volume

RPR: ....... Reactive Porogen Removal