gas separation

Gus Sep. Pur$ Vol. 9, No. I, pp. 35-43, 1995 Copyright cl 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 095&4214/95 $10.00 + 0.00

Field tests of membrane modules for the separation of carbon dioxide from low-quality natural gas

A. L. Lee, H. L. Feldkirchner, S. A. Stern*, A. Y. Houde”. J. P. Gamezt and H. S. Meyert

Institute of Gas Technology, Chicago, IL 60616, USA * L. C. Smith College of Engineering, Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13244, USA t Gas Research Institute, Chicago, IL 60631, USA

Received 10 January 1994; revised IO May 1994

A commercial-scale, single-stage, spiral-wound membrane system has been operated for approximately 20 months to upgrade low-quality natural gas from a well in East Texas. Throughout the test period the retentate product (“sales” gas) met pipeline specifications. Data were obtained on two membrane modules containing two types of asymmetric cellulose acetate membranes, one “standard” and the other one of higher density. A summary of the field test data shows the effects of the operating variables of pressure, feed flow rate, and CO2 concentration in the feed (from 3 to 25 mole percent). Concentrations greater than 6 mole percent were obtained by adding pure COP to the feed gas. In addition, computer models for the separation of gases under “perfect mixing” and cross-flow conditions were applied to the analysis of the field data. In general, the field test data were consistent with a flow regime which was intermediate between perfect mixing and cross-flow.

Keywords: cellulose acetate membranes; gas separations; CO2 removal; spiral wound membranes; natural gas; permeation

Introduction

Conventional natural gas processing has, in the past, included primarily absorption-type processes, using either chemical- or physical-type solvents (e.g. amine- or glycol-based systems) to remove CO,, H,S, and H,O’. Most of the “sour” natural gas produced in the lower 48 states contains less than 1 mole percent H,S and more than 5 mole percent CO,. Generally, pipeline specifications limit the CO, concentration to 2 mole percent and the H,S concentration to 4 ppm.

Recent data suggest that future discoveries of natural gas in the lower 48 states will be smaller, will produce lower-quality gas, and will be more remote than existing fields. Currently, many natural gas wells are shut in because of their low production rate and low quality (i.e. high CO, and/or H,S content). Therefore, it has become necessary to develop more efficient processes for upgrading low-quality natural gas than those presently available.

Membrane separation processes have been shown to be very effective for natural gas processing’-14. An

efficient separation of CO, and H,S from natural gas can be achieved by selective permeation through polymer membranes. The membranes utilized for this purpose are more permeable to CO,, H,S, and H,O vapour than to CH, and higher hydrocarbons. Therefore, the CH, is enriched in the high-pressure retentate stream without significant pressure loss, whereas CO,, H,S, and H,O vapour are concentrated in the low-pressure permeate stream. Available polymer membranes are highly selective to CO, and H,S relative to CH,, and reductions of the CO, concentration from 20-25 mole percent to the 2 mole percent pipeline specification are feasible. Whether or not the H,S content is simultaneously reduced to the pipeline specification of 4 ppm will depend on its initial concentration in natural gas. The membranes presently used for this application are made from cellulose acetate or other glassy polymerss~‘0,‘3~14.

Membrane separation processes have the important advantage of being energy-efficient. Moreover, membrane systems have been shown to be versatile in application because of their operational simplicity

Gas Separation & Purification 1995 Volume 9 Number 1 35

Field tests of membrane modules: A. L. Lee et al.

(no rotating equipment or solvents, etc.). Because they require no electrical power or water, membrane processes can operate essentially unattended in remote areas. Finally, in a recent economic study, it has been shown that hybrid membrane/amine processes may be more economic than conventional absorption processes in some ranges of feed flow rate and CO, concentration12. The use of hybrid processes also becomes necessary when the sour natural gas contains significant concentrations of H,S, perhaps higher than 5000 ppm13v14.

To obtain operating data that will help assess the technical and economic viability of membrane separation of CO, from low-quality natural gas, the Gas Research Institute (GRI) obtained a small, skid-mounted, single- stage membrane unit and field-tested it on a gas well in East Texas for approximately 20 months during 1990 and 1991.

The GRI-funded field tests were carried out cooperat- ively by Dallas Production, Inc. (DPI), Grace Membrane Systems (GMS), and the Institute of Gas Technology (IGT). A GMS test unit containing spiral-wound membrane modules and designed to treat 14200 Sm3/day (500 MSCF/day) of well-head natural gas that contained, on average, 6 mole percent CO,, 1070 kg H,0/106 Sm3 (67 lb H,O/MMSCF), and essentially no H,S, was installed at a DPI site in Trinity County, Texas. The feed gas pressure was 5270 kPa (750 psig). During the 573-day operating period the unit processed 2700 x lo3 Sm3 (95 000 MSCF) of natural gas. To obtain information on membrane performance with gases containing higher concentrations of CO, (up to 25 mole percent), additional tests were conducted using a liquid CO, system to increase the CO, concentration of the feed. The CO, concentration in the retentate product (the “sales” gas) remained at all times below 2 mole percent, and the H,O content remained below 112.1 kg/lo6 Sm3 (7 lb/MMSCF), i.e. within pipeline specifications. By reducing the number of membrane elements present from 6 to 4 in some tests, the recovery of CH, from the feed gas increased substantially and exceeded 90%.

Experimental procedure

Membrane unit

A schematic diagram of the GMS membrane unit used in the field studies is shown in Figure 1. The GMS unit fabricated for this programme was provided with more instrumentation than a standard one, because the objective of the programme was to develop a database for advanced processes for the removal of CO, from low-quality natural gas. A photograph of the metering system for the unit is given in Figure 2.

The membrane area in the GMS unit was sized to reduce the CO, content in 14 200 Sm3/day (500 MSCF/day) of natural gas at 5270 kPa (750 psig) containing 5.6 mole percent CO, to below the pipeline specification of 2 mole percent CO, in one of the permeators (or tubes). That area was sufficient to simultaneously reduce water vapour to meet the pipeline specification of

f-GJ& i _ _ _ _ _ _ _ _ _ _ _ _ _ _ . . _. . . _ . . . . _. . ., - Feed GFtS 500 M SCFD 750 sia 5.6 co2

$

Membrane Tubes I 20 psia 3O%CO*

Product ; L-e..- System Gas I__..._____.....____......___...__....._:

432 M SCFD 735 psla

2% co, 0‘ Less Membrane Skid 7#lMM or Less

Figure 1 Process flow diagram for membrane unit

Figure 2 Membrane unit metering system

112.1 kg/lo6 Sm3 (7 lb/MMSCF). The feed gas was pretreated to prevent liquid water and certain corrosion- inhibitor solvents from contacting the membrane (and thus to preserve the life of the membrane). The pretreatment skid comprised a coalescing filter, a guard bed (for solvent absorption), and a polishing (fine coalescer) filter. An off-skid heater was located downstream of the polishing filter (between the pretreatment skid and the membrane skid) to elevate the inlet gas temperature and reduce the relative humidity of the saturated feed gas. This was done to ensure that water and hydrocarbons do not condense on the membrane surface.

The membrane module consisted of two membrane tubes connected in parallel and the associated instrumentation. Each tube was capable of holding from one to six spiral-wound membrane elements. The dimensions of the elements are considered by the manufacturer to be proprietary information. Although only one tube was required for the above feed gas conditions, the two-tube unit allowed split-stream membrane comparison testing. The membrane elements were fabricated of two different density, asymmetric cellulose acetates, which are designated hereafter as Type 1 and Type II. One type of membrane was represented by GMS to be a “standard” density (Type I) and the other a higher density (Type II). During the test programme, the number of Type I elements installed

36 Gas Separation & Purification 1995 Volume 9 Number 1

varied between four and six. Early in the programme, one of the elements was removed to reduce the permeate flow and, thus, reduce the methane loss. Later, tests were also conducted with four elements so that the effect of membrane area could be studied further. Throughout the programme, three Type II elements were present.

Test procedure and data collection

Each membrane tube was provided with a feed and retentate orifice flow meter. The permeate flow rate was measured with a positive-displacement diaphragm meter. All flow meters were calibrated with an open-flow tester. Temperature indicators, pressure indicators, and gas sample connections were placed throughout the two-tube system. The compositions of the feed, retentate, and permeate streams were measured with a gas chromato- graph.

Results and discussion

Summary of field- test results

Throughout the operating period, changes were period- ically made in the type and number of membrane elements installed in each of the two tubes. Table I lists typical feed, retentate (“sales gas”), and permeate (“vent gas”) compositions for three different levels of CO, concentration in the feed gas. These results show that, even with a CO, content of over 20 mole percent in the feed, it was possible to meet the pipeline specification of 2 mole percent CO,. In fact, the CO, concentration in the retentate product remained below the pipeline specification throughout the tests.


The moisture content in the retentate also remained below its pipeline specification of 112.1 kg/lo6 Sm3 (7 lb/MMSCF).

Data correlations

The test data were correlated graphically. The following is a discussion of these correlations. However, the data were also compared for consistency with suitable computer simulations, which are presented in a later section.

Stage cut versus total pressure

Figure 3 shows the “total gas stage cut” for different components of natural gas as a function of pressure for membranes of Type I, with five membrane elements present in a tube. Also, although fewer data points are available for the Type I membrane with four membrane elements than for those with five elements, the data trends are similar. The number of membrane elements of Type I was changed during the programme to reduce the hydrocarbon losses (because there was limited use at this site for by-product fuel) and yet meet sales gas quality requirements. The “stage cut” is generally defined as the fraction of the feed stream allowed to permeate through the membrane, i.e. the permeate/feed ratio. It was found necessary to “force” the CO, balances for some tests to obtain a good data fit, especially for the data for low CO, concentrations in the feed. The field staff observed that the CO, concentration in the “sour” gas from the well typically varied by about f 1 mole percent out of an average concentration of about 5 mole percent. This meant that the CO, stage cut for

Table 1 Typical operating data for low, medium, and high feed gas CO:! contents8

1 o/23/91 Sf4~91 s/9/91

Feed Sales Vent Feed Sales Vent Feed Sales Vent

Pressure (psig)b 530 530 0.1 455 455 0.1 550 550 0.1 Temperature (‘F)’ 114 94 84 112 96 98 112 95 98 Flow rate (MSCF/day)d 101 63 38 113 72 41 112 57 56 Carbon dioxide (MSCF/day)d 5.3 0.2 5.0 14.4 1 .I 13.2 24 0.9 24.5 Methane (MSCF/day)d 88 56 31 89 63 26 80 50 30 Gas analysis (mol%)

Nitrogen 1.22 1.33 0.95 1.05 1.27 0.69 0.96 1.36 0.57 Carbon dioxide 5.23 0.35 13.18 12.72 1.51 32.12 21.34 1.53 43.75 Methane 86.67 89.66 82.14 78.97 87.97 63.98 71.42 88.54 53.48 Ethane 3.87 4.65 2.59 3.77 4.46 2.15 3.35 4.54 1.49 Propane 1.49 1 .ss 0.58 1.41 1.92 0.43 1.19 1.91 0.34 i-Butane 0.35 0.48 0.10 0.32 0.47 0.07 0.27 0.44 0.05 n-Butane 0.48 0.62 0.16 0.46 0.62 0.13 0.40 0.55 0.09 i-Pentane 0.25 0.37 0.07 0.27 0.35 0.07 0.21 0.30 0.05 n- Pentane 0.19 0.23 0.05 0.17 0.23 0.10 0.17 0.19 0.04 Cs and heavier 0.25 0.32 0.18 0.86 1.20 0.26 0.69 0.64 0.14

Gross heating value, dry (4 6O’F and 14.65 psiae 1039 1114 913 983 1132 719 880 1104 589 Specific gravity (Air = 1.000) 0.671 0.642 0.716 0.757 0.674 0.897 0.829 0.657 1 .ooo Membrane type’ I I I Number of elements 5 5 5

aThe “sales” gas is the retentate; the “vent” gas is the permeate bPressure, in kPa = (pressure in psig + 14.7) x 6.895 Temperature, in K = (temperature in “F - 32) x 1.8 + 273.2 dFlow rate, in Sm3/day = flow rate in (SCF/day)/35.3018 Weating value, in Btu/SCF x 1000 = 37.245 MJ/Sm3 ‘Type 1: Grace Membrane Systems, standard density cellulose acetate material


Field rests of membrane modules: A. L. Lee et al.

- HIGH FLOW RATE, 150-200 MCFD

- - - - LOW FLOW RATE, -100 MCFD

. HIGH CO, CONCENTRATION, >15 MOL%

0 MEDIUM CO, CONCENTRATION, lo-15 MOL%

0.5

0.4 0

0.3

0.2

0.1 I I I I

0 LOW CO, CONCENTRATION, ~10 MOL%

STAGE CUT, VENT GAS FLOW RATE

FEED GAS FLOW RATE 0.6

300 400 500 600 700 800

PRESSURE, PSIG

Figure 3 Total gas stage cuts for membrane Type I (5 membrane elements)

feed gases with low CO, content could vary by as much as 20%. For consistency, the CH, balances were also forced as necessary, but there was much less variability in these data because of the relatively high concentration of CH, in all streams.

The parameters of feed flow rate and CO, concentration in the feed are arbitrarily grouped in Figure 3 into ranges denoted as “low,” “medium,” and “high.” As can be seen from this figure, the data are generally consistent in that the stage cuts increase with increasing pressure and decrease with increasing feed flow rate. The scatter in the data is not unusual for field test conditions. It was not possible to obtain data at higher feed flow rates with medium-to-high CO, concentrations in the feed without exceeding the pipeline-specified limit of 2 mole percent CO, in the retentate. Therefore, the data are generally limited to lower feed flow rates and lower CO, concentrations. There was no indication of membrane deterioration with time, based on the field test data.

A substantial amount of data was also acquired for the Type II membrane (with three membrane elements). The same operating parameters and CO, concentrations in the feed were used with both the Type I and Type II membranes. In general, the stage cuts for the Type II membrane followed the same general dependence on pressure, feed flow rate, and CO, concentration in the feed as those for the Type I membrane. Stage cuts were substantially lower in the tests with the Type II membranes, but this was expected since only three

membrane elements were utilized. High CO, stage cuts were necessary to reduce the CO2 concentration in the retentate product to the pipeline specification of 2 mole percent.

Component stage curs versus membrane area

To show the effect of membrane area (four and five membrane elements with the Type I membrane, and three membrane elements with the Type II membrane) on the extent of CO,, CH,, and C,H6 separation, plots of the component stage cuts versus the number of membrane elements utilized are presented in Figures 4, 5, and 6, respectively. These figures show that the CO,, CH,, and C,H, stage cuts all increase at constant feed flow rate with increasing membrane area. While this results in a better CO, removal, it also increases the losses of CH, and higher hydrocarbons in the permeate (vent) stream. The component stage cuts also increase, as expected, with increasing pressure, because the partial pressures of the components increase.

It should be pointed out that the actual field test flow rates were generally much lower than the design rate (and oversized with respect to area) since the purpose of the tests was to obtain operating data over a wide range of conditions. Therefore, back-diffusion and perfect mixing were possible, and the methane loss in the permeate was generally higher than desired in commercial operation.

3 4 5 Membrane Area (Number of Elements)

Membrane Press.. I II Psig ~-

0 400

A . 500

0 . 600

v 7 700

Figure 4 Effect of membrane area on CO2 stage cut in a medium COP range and at high flow rates

05 I I i

% I

MEMB ANEII MEMBkANE I 0


Figure 5 Effect of membrane area on methane stage cut in a medium COs range and at high flow rates


Membrane I II

PWS., pslg

0 400 A * 500 q . 600 u . 700

, Membrane II j Membrane I


Figure 6 Effect of membrane area on ethane stage cut in a medium CO2 range and at high flow rates

Computer simulations

The characteristics of membrane processes for the separation of gas mixtures have been discussed elsewhere2~“~17. The following section presents an analysis of some of the data obtained during the field tests described above. The analysis was made by comparing these data with the results of computer simulations of the membrane separation process under consideration. The simulations assumed both “perfect mixing” and “cross-flow” conditions2*‘5p17 in the high- and low-pressure gas streams in the membrane modules.

The “perfect mixing” conditions were selected because they yield the most conservative estimates of separation performance, i.e. the lowest possible degree of separation achievable in a membrane separation “ stage”, and the highest membrane area requirement. Therefore, any tests yielding a lower separation than predicted for perfect mixing conditions would indicate the presence of pinholes or other defects in the membranes, inefficiencies in the process, such as leaks, or significant pressure drops in the modules (if not accounted for in the simulations). The cross-flow calculations were also made because the membrane modules utilized asymmetric cellulose acetate membranes. Pan” has shown that gas separation in asymmetric membranes occurs by cross-flow, even if the high- and low-pressure gas streams at opposite membrane surfaces flow cocurrently or countercurrently to one another.

Carbon dioxide is known to strongly plasticize (swell) cellulose acetate membranes, the degree of plasticization increasing as the partial pressure of the CO, is increased. It is also known that plasticization by CO, changes the membrane selectivity to this gas relative to other gases, e.g. towards CH,. Cellulose acetate can also be plasticized by higher hydrocarbons, but these are present in natural gas in relatively small quantities. Because plasticization by CO, would complicate the computer simulations, only the data obtained at the lowest CO, concentration (~4 mole percent) and lowest operating pressure (2515 kPa z 350 psig) were employed in the present study; these data were obtained at temperatures between 312 and 322 K (102” and 120’F).

The mathematical model of a membrane process for the separation of gas mixtures under “perfect mixing” conditions assumes that the gas stream on the high-pressure side of the membrane (the retentate) is mixed so rapidly that its composition at all points along the membrane is the same as that at the retentate outlet


point. The same assumption is made for the gas stream on the low-pressure side of the membrane (the permeate). The feed composition changes as a step function at the stage inlet. The model for separation under cross-flow conditions assumes that the retentate stream flows along the membrane surface and that the permeate stream is perpendicular to it’.’ s-l 7.

The present computer simulations were made assuming that the natural gas feed contained four components, namely, CH,, C,H,, CO,, and N,. Values of the separation factors for higher hydrocarbons relative to CH, were not available, and therefore the higher hydrocarbons were lumped with the C,H, in the simulations. The presence of H,O vapour was not taken into consideration because the feed to the membrane modules contained only 1070 kg H,0/106 Sm3 (67 lb H,O/MMSCF) (0.14 mole percent H,O). The pressure drop in the retentate streams was neglected because the field tests showed it to be on the order of only 14 kPa (2 psi); the pressure drop in the permeate stream was not measured.

The separation performance of the permeator tubes was difficult to analyse because several other items of proprietary information were unavailable, such as the membrane area per element and the permeability (or “permeance”) of the membrane to the different components of natural gas as a function of composition, pressure, and temperature. Therefore, the following procedure was adopted for the computer simulations:

(1) Three sets of field data obtained with the two tubes on given dates were taken as the basis for the computations. These data were for tube I(4) [test of 6/21,/91]; tube I(5) [test of S/20/91]; and tube H(3) [test of 8/23/91]; the numbers in parentheses indicate the number of membrane elements contained in the tube on the stated dates. These data met the above-mentioned criteria of CO2 concentration (~4 mole percent) and total pressure of 2515 kPa (350 psig).

(2) Surprisingly little information is available in the open literature on the gas permeability of either homogeneous (dense) or asymmetric cellulose acetate membranes, particularly since the latter are being used for upgrading natural gas on an industrial scale. The published data are summarized below.

Puleo et ~1.‘” reviewed and analysed in 1989 the data reported in the literature on the permeability of homogeneous cellulose acetate membranes to several pure gases, including CO, and CH,. These data were obtained at a low pressure.

Pan” reported the results of laboratory-scale separations of CO, from two different multicomponent gas mixtures by means of asymmetric hollow-fibre membranes made of cellulose acetate. These data are not germane to the present analysis because of the very high CO, or H,S contents of the laboratory gas mixtures. Pan2’ made the interesting observation that the membrane permeability to CO, and H,S increased strongly with increasing pressure when measured with the pure gases, but that no such pressure dependence was present when the measurements were made with gas mixtures.

Funk and Li” reported the results of field test studies for the purification of very sour natural gas containing about 29.3 mole percent CH,, 16.9 mole percent H,S, and 44.9 mole percent CO,, the balance being higher hydrocarbons and other organic compounds. The



separation of the acid gases (CO, and H,S) was performed with asymmetric cellulose acetate membranes.

Donohue et aL21 studied in the laboratory the permeability of asymmetric cellulose acetate membranes to pure CO, and CH,, as well as the separation of three COJCH, mixtures containing 2.04, 30.6, and 70.6 mole percent CO, with these membranes. The studies were made over a wide range of pressures.

Most pertinent to the present analysis are the data of Spillman and Cooley” and Schell et ~f.~~, who reported natural gas processing data obtained from field test studies with GMS spiral-wound modules.

On the basis of the data in references 19, 21, and 22, the following average permeability properties of cellulose acetate membranes were used for the computer simulations:

p(C0,) = 9 x 10m5 cm3(STP) s-’ cm-’ cmHg-’ CO,/CH, Selectivity: 20 C,,/CH, Selectivity: 0.40 N,/CH, Selectivity: 1

where HC designates hydrocarbons higher than CH,, but mostly C,H,.

(3) The membrane areas per module were estimated by a trial-and-error procedure for “perfect mixing” conditions only, to obtain the permeate and retentate compositions which best matched the field data.

Comparison of computer simulations with field test data

The computer simulations for “perfect mixing” and cross-flow conditions are compared with the selected field tests in Tables 2-7. Cross-flow always yields a better separation and requires a lower membrane area than “perfect mixing”2s1 5-1 7. Tables 2-7 show that the concentrations of CO,, CH,, N,, and higher hydrocarbons in the retentate and permeate streams obtained from the field tests generally fall between the correspond- ing values calculated for “perfect mixing” and cross-flow conditions. The slight deviations from these results for some gases in some field tests can be attributed to experimental errors and/or the values assumed for the CO, permeability and for the various selectivities of the cellulose acetate membranes in the computer simulations. For example, the value selected for the selectivity to “higher hydrocarbons” relative to CH, represents more closely the C,H&H, selectivity, because no data were available for hydrocarbons higher than C,H,.

The consistency between the field data and the computer simulations indicates that the membranes did not have any serious defects. However, it is not known how the membrane modules performed at higher pressures and higher CO, concentrations when the

Table 2 Comparison of field tests and computer simulations for tube l(4)a

Test of 6/21/l 991

Composition (mole percent)

Retentate (sales gas) Permeate (vent gas)

Component Feed Perfect-mixing Cross-flow Field tests Perfect-mixing Cross-flow Field tests

Carbon dioxide 4.8 1.9 1 .o 1.9 20.2 24.9 22.7 Methane 87.1 89.2 90.0 89.2 76.1 71.7 73.4 Higher hydrocarbons 7.0 7.3 7.8 7.7 2.7 2.5 3.1 Nitrogen 1.2 1.2 1.2 1.2 1.0 1 .o 1 .o

Operating temperature: 318 K (112’F) Feed pressure: 2650 kPa (370 psig) Permeate pressure: 102 kPa (0.1 psig) Stage cut: 0.154 “The number of membrane elements in the tube is indicated in parentheses


Test of 6/27/1991




Carbon dioxide 21.9 4.6 0.3 1.7 45.2 51.2 49.4 Methane 71.5 85.3 89.9 88.4 52.1 46.7 47.9 Higher hydrocarbons 5.7 8.4 8.7 8.6 1.0 1.7 2.2 Nitrogen 0.9 1 .I 1.1 1.2 0.7 0.6 0.5

Operating temperature: 317 K (111 ‘F) Feed pressure: 4720 kPa (670 psig) Permeate pressure: 102 kPa (0.1 psig) Stage cut: 0.426 7he number of membrane elements in the tube is indicated in parentheses




Test of 8/20/l 991




Carbon dioxide 3.8 1.5 0.8 1.5 16.3 20.1 16.2 Methane 86.7 88.1 88.7 89.2 79.3 75.7 79.2 Higher hydrocarbons 8.2 9.2 9.3 8.0 3.3 3.1 3.7 Nitrogen 1.2 1.2 1.2 1.3 1.1 1 .o 0.9

Operating temperature: 319 K (114°F) Feed pressure: 2480 kPa (345 psig) Permeate pressure: 102 kPa (0.1 psig) Stage cut: 0.154 The number of membrane elements in the tube is indicated in parentheses


Test of 9/09/l 991




Carbon dioxide 21.3 3.8 0.05 1.5 38.8 42.5 43.7 Methane 71.4 85.1 88.3 88.5 57.7 54.5 53.5 Higher hydrocarbons 6.4 10.1 10.5 8.6 2.7 2.3 2.2 Nitrogen 0.9 1 .l 1 .l 1.4 0.7 0.7 0.6

Operating temperature: 318 K (112°F) Feed pressure: 3890 kPa (550 psig) Permeate pressure: 102 kPa (0.1 psig) Stage cut: 0.500 “The number of membrane elements in the tube is indicated in parentheses

Table 6 Comparison of field tests and computer simulations for tube ll(3)a

Test of 8/23/l 991




Carbon dioxide 3.9 2.1 1.7 2.1 22.0 26.3 23.5 Methane 87.5 88.8 89.2 88.3 74.3 70.3 73.4 Higher hydrocarbons 7.4 7.9 7.9 8.3 2.7 2.5 2.4 Nitrogen 1.2 1.2 1.2 1.2 1.0 1 .o 0.7

Operating temperature: 318 K (112°F) Feed pressure: 2740 kPa (382 psig) Permeate pressure: 102 kPa (0.1 psig) Stage cut: 0.091 “The number of membrane elements in the tube is indicated in parentheses

membranes were significantly plasticized by CO,, Table 8 lists the membrane areas estimated from the because of the lack of data required for the computer computer simulations. These areas must be viewed as simulations. Nor is it known whether the membrane only approximate because they depend entirely on the elements met the GMS design specifications. Finally, assumed CO, permeability of the membranes. The values although there was no detectable deterioration in of the CO, permeability of cellulose acetate membranes performance, there may have been internal leakage in the reported in the literature vary by a factor of two or more. unit that we are not aware of. Both the field tests and the computer simulations



Table 7 Comparison of field tests and computer simulations for tube ll(3)a

Test of 9/l 1 /I 991




Carbon dioxide 22.9 5.4 0.6 2.1 49.7 57.1 55.4 Methane 70.1 84.8 89.2 88.4 47.6 40.8 43.1 Higher hydrocarbons 6.0 8.7 8.99 8.2 2.0 1.5 1 .o Nitrogen 1 .o 1.2 1.3 1.3 0.7 0.6 0.5

Operating temperature: 315 K (107°F) Feed pressure: 5020 kPa (714 psig) Permeate pressure: 102 kPa (0.1 psig) Stage cut: 0.395 “The number of membrane elements In the tube is indicated in parentheses

Table 8 Estimated membrane areas of tubes used in field tests=

Feed flow Feed pressure rate

Module Date bskt) (MSCFiday)

l(4)b 6/21/91 370 171 l(5)b 8/20/91 345 156 ll(3)b 8123191 382 143

Stage cut

(0)

0.158 0.154 0.091

CO, in feed (mole

percent)

4.8 3.8 3.9

Membrane area

P2)

Per Total element

935 234 944 189 427 142

Temperature range: 318-319 K (112-l 14’F) %omputer simulations assumed “perfect-mixing” conditions bTh e number of membrane elements in the tubes is indicated in parentheses

show that the CO2 concentration of 3-25 mole percent and the moisture content of 1070 kg/lo6 Sm3 (67 lb H,O/MMSCF) in the natural gas feed were reduced under all test conditions to or below the pipeline specifications of 2 mole percent CO, and 112.1 kg/lo6 Sm3 (7 lb H,O/MMSCF), respectively.

Conclusions

Membrane processing can provide a pipeline-quality natural gas from high-CO,-content (up to 25 mole percent) natural gases. Methane losses, in this work, could be reduced to less than 10% with a single-stage unit by reducing the number of membrane elements used. By adding a second stage in series, the losses could probably have been reduced even further. Long life, without measurable deterioration in the membrane performance, ageing, or hysteresis, has been demon- strated. The sales gas water content averaged 48.0 kg/lo6 Sm3 (3 lb/MMSCF), and thus met the specification of 112.1 kg/lo6 Sm3 (7 lb/MMSCF) for feed gas water contents that averaged 1070 kg/lo’ Sm3 (67 lb/MMSCF). The membrane system was shown to be reliable, with no loss of production because of membrane performance during the 573-day operating period. For complete data from this project, please see the GRI Topical Report24.

Acknowledgements

In addition to GRI funding, the technical direction, the guidance in project planning and the active participation

of Dr. K. Woodcock of GRI in this project are gratefully acknowledged. The cooperation and assistance of Messrs. D. Smith, D. Hamilton and D. Sheetz of DPI, as well as the cost-sharing of DPI, are greatly appreciated. Finally, the technical assistance of Messrs. L. Mologne, D. Mirdadian, and W. Detloff of GMS, as well as the cost-sharing of GMS, who provided the membrane elements, is also greatly appreciated.

Initially, Dr. R. Zabransky was IGT’s principal investigator on this project and was responsible for setting up much of the early calculation and data reduction procedures. He also was involved in the early work on the specification and installation of the GMS unit. The test data were obtained at the Trinity County Texas well by the IGT Hitchcock, Texas, staff, Mr. J. Anhaiser and Mr. C. Hayden, under the supervision of Dr. P. Randolph. Ms. S. Wasserberg assisted in data reduction and computations. The daily data collection and the operation of the well and the membrane unit were done by Mr. J. Vondra of DPI.

References

I Kohl, A.L. and Riesenfeld, F.C. Gas Purification 4th Edn., Gulf Publishing Co.. Houston, TX (1985)

2 Stern, S.A. New Developments in Membrane Processes for Gas Separation Sy/hrlic Membrunes (Ed. M.B. Chenoweth) MM1 Press S~fmp Srr Harwood Academic Publishers, New York, NY (1986) 5 t-37

3 Schendel, R.L. Using Membranes for the Separation of Acid Gases and Hydrocarbons Chern Eng Prvg (1984) SO(5) 39

4 Cooley, T.E. and Detloff, W.L. Field Tests Show Membrane Processing Attractive C&m Eng Pray (1985) 81 45



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Fournie, F.J.C. and Agostini, J.P. Permeatton Membranes can Efficiently Replace Conventional Gas Treatment Processes J Pet Technoi (1987) 39 707 Babcock, R.E., Spillman, R.W., Goddin, C.S. and Cooleg, T.E. Natural Gas Clean-Up: a Comparison of Membrane and Amine Treatment Processes Energy Prog (1988) S(3) 135 Spillman, R.W. and Detloff, W.L. Membrane Gas Treatment is Economic Am Oil Gus Rep (1988) 31(10) 36 Lee, S.Y., Minhas, B.S. and Donahue, M.D. Effect of Gas Composition and Pressure on Permeation Through Cellulose Acetate Membranes AIChE Svmu Ser (1988) 841261) 93-101 Spillman, R.W. Economics of’Gas Separation Membranes Chem Eng Pray (1989) 85(l) 41 Scheli, W.J., Wensley, C.G., Chen, M.S.K., Venugopal, KG., Miller, B.D. and Stuart, J.A. Recent Advances in Cellulosic Membranes for Gas Separation and Pervaporation Gas Sep Purif (1989) 3 162 Funk, E.W. and Li, N.N. Purification of Natural Gas by Membranes Gas Separutiorr Technaloyy (Eds E.F. Vansant and R. Dewolfs) Elsevier, Amsterdam (1990) 355-372 McKee, R.L.. Changela, M.K. and Reading, G.J. CO, Removal: Membranes plus Amine H~drocurhon Procc.~s (1991) 70 63 Bbide, B.D. and Stern, S.A. Membrane Processes for the Removal of Acid Gases From Natural Gas, I. Process Configurations and Optimization of Operating Conditions J Met-&r Sci (1993) 81 209 Bbide, B.D. and Stern, S.A. Membrane Processes for the Removal of Acid Gases From Natural Gas, II. Effects of Operating Conditions. Economic Parameters, and Membrane Properties J Mendw &i (1993) 81 239

15 Stern, S.A. and Walawender, W.P. Analysis of Membrane Separation Parameters Sep Sci (1969) 4 123 -

16 Stern. S.A. Gas Permeation Processes Industrial Proce.r-sinu Wirh Membranes (Eds S. Loeb and R.E. Lacey) Elsevier Scientific, New York, NY (1972) Ch. 13, 279-339

17 Koros, W.J. and Chern, R.T. Separation of Gaseous Mixtures Using Membranes Hundhook of Sepurcrtion Process Technology (Ed. R.W. Rousseau) Wiley-Interscience, New York (1987) Ch. 20. 8622953

18 Pan, C.Y. Gas Separation by Permeators With High-Flux Asymmetric Membranes AIChE J (I 983) 29(4) 545 552

19 Puleo. A.C., Paul, D.R. and Kelley, S.S. The Effect of Degree of Acetylation on Gas Sorption and Transport Behavior in Cellulose Acetate J Memhr Sci (1989) 47 301

20 Pan, C.Y. Gas Separation by High-Flux, Asymmetric Hollow- Fiber Membranes AIChE J (1986) 32(12) 2020-2077

21 Donobue, M.D., Minhas, B.S. and Lee, S.Y. Permeation Behavior of Carbon Dioxide-Methane Mixtures in Cellulose Acetate Membranes J. Memhr Sci (1989) 42 197

22 Spillman, R.W. and Cooley, TX. Economic Considerations in Membrane Gas Separation Process Design Puperpresenfedut the AIChE Spring .&leering, Houston, TX (April 1987)

23 Schell, W.J., Wensley, C.G., Cben, M.S.K., Venugopal, K.G., Miller, B.D. and Stuart, J.A. Recent Advances in Cellulosic Membranes for Gas Separation and Pervaporation Gas Sep Pur[f (1989) 3 1622169

24 Lee, A.L. and Feldkirchner, H.L. Development of a Database for Advanced Processes to Remove Carbon Dioxide From Subquality Natural Gas GRI Topiul Report No. GRI-93/O-747 (June 1993)


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