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C H A P T E R 12

F U T U R E T R E N D S O F MEMBRANE R E A C T O R S

INORGANIC M E M B R A N E S AND

Inorganic membranes and their uses as membrane reactors have just grown from an infant stage, particularly the latter. They are currently limited in the choice of membrane material, pore size distribution, element shape, end-seal material and design and module configuration. But these limitations mostly reflect the degree of maturity in the technology development. As more research and development efforts are directed to these areas as witnessed worldwide today, inorganic membranes are expected to be more technically feasible and economically viable as separators and reactors in selected applications.

12.1 ECONOMIC ASPECTS

While technical data of inorganic membranes and inorganic membrane reactors are generated at an accelerated pace, very little information on their economics has been published. This can be attributed to two major factors: (1) Inorganic membrane technology is an emerging field such that new developments in its preparation or process development as separators or reactors can drastically change the capital investments and operating costs required; (2) Even in the more established applications, the process economics is in general fairly competitive and well guarded as a trade secret and has not been much discussed in the open literature.

Thus, it is not easy to obtain reliable cost data for separation processes, let alone catalytic reaction processes, using inorganic membranes. Some general guidelines, however, have been provided for separation processes in isolated cases and will be summarized in this chapter. Understandably no definitive economics related to inorganic membrane reactors has been presented in the literature due to the evolving nature of the technology.

12.1.1 Cost Components

Like all other chemical processes, the separation processes by inorganic membranes have two major cost issues: capital investments and operating costs. Capital costs are affected by the membrane area required (which in turn are determined by the permeability and permselectivity), compression or recompression energy requirements as dictated by the operating pressure, piping and vessels, instrumentation and control, and any pretreatment requirements (depending on the nature of the feed material and the membrane). The operating costs are determined by the required membrane replacement

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frequency and costs, membrane regeneration (e.g., backwashing, cleaning, etc.), system maintenance, pumping energy, labor, depreciation and any feed pretreatment.

There are two seemingly not so obvious cost elements that should be considered. One is capital and operating costs related to any compression or recompression requirements. The other element is product losses [Spillman, 1989].

Recompression may be an issue, for example, in the case of hydrogen separation. In the majority of cases, most of the hydrogen from a hydrogen-containing feed gas mixture appears in the permeate which is on the lower pressure side of the membrane. Most applications, however, require the hydrogen so obtained to be at high pressures for subsequent processing. Thus, in a case like this, recompression-related costs must be included for an equitable comparison and these costs sometimes are critical to the economic feasibility of the gas separation process.

In a more complete membrane process optimization or cost comparison of alternative membrane processes, the cost associated with any loss of the valuable gas component(s) needs to be considered as another operating expense just like any other utility expense. For example, in the case of the removal of carbon dioxide from natural gas (methane), there will be some amount of methane in the permeate (albeit in a small quantity) due to imperfect rejection of methane by the membrane. Since methane is, in this case, the desirable product component, its presence in the permeate represents a loss or an expense. The value of the loss should be included in any cost comparison.

The basic guiding principle for the economics of membrane or membrane reactor processes is the operating cost per unit of the desired product. If the unit cost is higher than those of the competitive separation or separation/reaction processes, economic justification will be difficult. The operating cost of a membrane unit strongly depends on its permeate flux and permselectivity. The permeate flux is, in turn, essentially proportional to the permeability and the available permeation area and inversely proportional to the membrane thickness. Among these factors, the available permeation area is the easiest one to effect the economy of scale in membrane processes, using either organic or inorganic membranes. Chan and Brownstein [ 1991 ] demonstrated that as the membrane filtration area increases from 100 to 1000 m 2 the average operating cost per unit filtration area is decreased by 1/3.

Even available cost information can not be easily generalized. However, for rough estimates in scaling up an inorganic membrane system, cost data from other similar applications using the same type of membrane and equipment may be applied with caution. For example, some cost data from the applications of ceramic membranes in the dairy industry has been utilized as a basis for cost estimating fish processing applications also using ceramic membranes [Quemeneur and Jaouen, 1991].

Future Trends 571

12.1.2 General Guidelines for Various Cost Components

Based on limited available literature information [e.g., Spillman, 1989; Chan and Brownstein, 1991; Muralidhara, 1991; Quemeneur and Jaouen, 1991; Johnson and Schulman, 1993], some guidelines or assumptions that are general in nature for ceramic membranes may be developed:

(1) The average service life of a ceramic membrane is approximately 3 to 5 years compared to 1 to 3 years for an organic polymer membrane although the service life can greatly depend on the module design and the nature of the feed streams (e.g., fouling propensity and corrosion tendency). The replacement cost of an inorganic membrane may be close to twice that of an organic membrane currently but is expected to be lower as the demand increases in the future. The unit cost of an inorganic membrane is highly sensitive to the production volume.

(2) Maintenance cost is 5 to 10% of capital investments or approximately $0.05 per 1,000 liter liquid processed.

(3) Straightline depreciation is usually assumed over a 5 to 10 year period.

(4) The total system cost is significantly larger than the membrane module cost, typically 3 to 5 times as much.

(5) For scale-up, the general power law applies. This can be expressed in terms of the available membrane permeation area:

C2/C1 = (A21A1) m (12-1)

where C and A are the membrane or system cost and the available membrane permeation area, respectively, and the exponent m is 0.9 for the membranes and 0.67 for the systems. An illustration of this rule of thumb has been given by Chan and Brownstein [1991]. They made a cost estimate for the case of increasing the fdtration area from 100 to 1000 m 2. This tenfold increase in the throughput results in an increase of capital cost by only approximately 5 times. Part of the reason for this economy of scale is that at a low capacity the quantities of required instrumentation and control devices are not proportionally reduced and thus may constitute a significant portion of the capital costs. An alternative approach to costing membrane systems for scale-up is based on the volumetric flow rate of the permeate:

C2/C1 = (V2/V1) n (12-2)

where C and V are the membrane element or system cost and the maximum volumetric flow rate of the permeate, respectively, and the exponent n is 0.32 [Lahiere and Goodboy, 1993]. An example of the general trend described by Eq.

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(6)

(12-2) is given in Figure 12.1 which shows the decreasing ceramic membrane system cost with increasing module size for microfiltration applications. An assumption implied in the above equation is that the density of the process streams is constant.

General breakdown of the operating costs (excluding return on investment) is given in Table 12.1. The depreciation cost is a strong function of the system capacity and the accounting practice and therefore can vary significantly from one case to another.

9

~'811 I I~ ~ (Modules, Elements/Module respeclNely)

_=. ~/V~ ba )o.~ :~ Cost a = Cost b +_. 8.1% y s Assumes constant liquid & solid densities

E 0 3 -

0 2 - >

_= 0

r r - 1 -

0 0.0

1 l A 1

1 ~ 225 3OO

Maximum m 3 of Perrneats Water/Hour @ 20*0

Figure 12.1 General trend of ceramic membrane element or system cost as a function of the volumetric permeation rate [Lahiere and Goodboy, 1993]

Not explicitly included in Table 12.1 is the cost associated with any pretreatment required. For the extreme cases where the feed stream needs to be treated prior to entering the membrane system to facilitate membrane separation and prevent frequent shutdown, the pretreatment costs may be quite significant. Flux enhancement schemes are critical to continuous and consistent operation of membrane separation systems. They can add significant expenses to the total operating costs. For example, it has been reported that backflushing can reduce fouling and result in a cost savings of $0.41 per 1,000 liter of processed liquid stream, mostly through savings in the membrane replacement costs [Muralidhara, 1991]. In the absence of any prior cost knowledge for

Future Trends 573

extrapolation, the above cost-related guidelines may be used with caution for other applications employing ceramic membranes.

TABLE 12.1 Distribution of operating costs of inorganic membrane systems

Cost component % of total cost Cleaning products 4-8 Depreciation 10-40 Power 10-15 Labor 10-20 Maintenance 3-10 Membrane replacement 10-20

12.1.3 High-Temperature Gas Separations and Membrane Reactors

When the application temperature increases as in many gas separation and membrane reactor applications, additional capital as well as operating costs associated with fluid containment can be expected. For many such high-temperature applications, inorganic membranes have the distinct advantages over other separation processes including organic membrane separation in the savings of recompression costs. Quite likely for those cases where recompression of the permeate or the product stream is needed, the recompression costs may be close to or more than the operating costs of the membrane. As a preliminary estimate for the cost of gas separation in the petroleum refining industry (e.g., hydrogen recovery) using inorganic membranes, the unit cost of 13r per million standard cubic feet (Mscf) may be used [Johnson and Schulman, 1993].

When combining the membrane and the reactor into an integrated system, cost savings may be significant. This difference can be attributed to the convection and other transport modes in the separation steps by membranes in contrast to the traditional diffusion mode by other separation techniques. For example, the use of membrane reactors instead of the traditional reactors for bioengineered products can reduce the operating costs by as much as 25% [Chan and Brownstein, 1991].

For high-temperature processes where inorganic membranes may be employed, the potential energy savings are considerable. Humphrey et al. [ 1991] made an estimate on the energy savings that may be realized if inorganic membrane reactors are used in a number of large-volume dehydrogenation and dehydration reactions. Potential industrially important dehydrogenation applications include ethylene from ethane, propylene from propane, styrene from ethylbenzene, hydrogen from hydrogen sulfide, butadiene from butene and finally benzene, toluene and xylene (BTX) from cycloalkanes with their possible energy savings in the descending order. The energy savings in the above cases primarily come from reduced reactor preheat and elimination

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of distillation recovery of unconverted reactants. Target dehydration reactions are ketene from acetic acid and diethylether from ethanol. The combined energy savings of the above dehydrogenation and dehydration reactions total about 0.309 quad (1015 Btu) per year for the U.S. alone (see Table 12.2). The authors assumed that the energy values are

TABLE 12.2 Potential energy savings via membrane reactor technology

Application

Dehydrogenation:

Styrene from ethylbenzene

Ethylene from ethane

Propylene from propane

Butadiene from butene

BTX from cycloalkanes

Hydrogen from hydrogen sulfide

Dehydration:

Reasons for energy reductions

Total Energy U.S. national savings production energy (Btu/lb (billion savings product) lb/yr) ~quad/yr)

Reduced reaction 4,095 8.13 0.033 temperature, elimination of ethylbenzene recovery

Reduced reaction 4,655 34.95 0.163 temperature, elimination of ethane recovery

Reduced reaction 2,794 20.23 0.057 temperature, elimination of propane recovery

Reduced reaction 4,375 3.09 0.014 temperature, elimination of the butene recovery

Eliminating need to recycle 1,128 9.32 0.011 (via compressor) hydrogen, reduced reaction temperature

Producing hydrogen as a 60,000 0.49 0.029 by-product

Ketene from acetic Reduced reaction acid temperature, elimination of

recovery of ketene

Diethylether from Elimination of an ethanol azeotropic distillation tower

Total energy savings

(Adapted from Humphrey et al. [1991])

1,781 0.64 0.001

65,000 0.012 0.001

0.309

Future Trends 575

1,000 Btu/scf for natural gas, 3,412 Btu&wh for electricity, 1,000 Btu/lb for steam and 60,000 Btu/lb for hydrogen and 100% of the reactants are converted to products. The last assumption may not be validated in the foreseeable future. Nevertheless, the magnitude of the potential energy savings is reasonable.

Humphrey et al. [1991] further analyzed the amount of energy that can be saved by partially or completely replacing distillation operations with membrane technologies which encompass both gas separation and membrane reactor applications. A significant portion of the estimated savings will be derived from the usage of inorganic membranes. Given in Table 12.3 are the potential energy savings progression due to the use of membrane technologies as they evolve and also the estimated energy savings per installation. The potential energy savings are substantial and the savings per installation become much larger if distillation for difficult separations is completely replaced by the membranes. In their estimates, assumptions were made that: (1) distillation-membrane hybrid systems will become materialized within fifteen years (some are within five or ten years); (2) complete replacement of distillation for difficult separations (where low relative volatility is involved) with membranes will take fifteen years (applicable to new plants); and (3) one-half of the targeted membrane reactor applications for dehydrogenation and dehydration reactions can be realized within fifteen years (mostly applicable to new plants).

TABLE 12.3 Energy savings of membrane technologies

Quads/year saved Technology at end of Applications

Distillation-membrane hybrid systems

5yrs 10yrs 15yrs

0.02 0.03 0.07 Ethanol and isopropanol separations- all applications realized within five years. Olefins, miscellaneous hydrocarbons, water- separations - one- half of applications realized within ten years and all within fifteen years

Estimated energy savings per installation

34%

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TABLE 12.3 - Continued

Quads/year saved Technology at end of

Membranes to replace distillation

5yrs 10yrs 15yrs

0 0 0.18

Membrane reactors 0 0 0.16

(Adapted from Humphrey et al. [ 1991])

Applications

Estimated energy savings per installation

To replace 84% distillation for difficult separations. Olefins, miscellaneous hydrocarbons, water- oxygenated hydrocarbons, and aromatic separations. All applications realized within fifteen years. Applicable to new plants.

Dehydrogenation 83% and dehydration reactions. One-half applications realized within fifteen years. Mostly applicable to new plants.

12.2 POTENTIAL MARKETS

As stated earlier, the field of inorganic membranes for separation and reaction is evolving and both process and product R&D can significantly change the production costs, process economics or pricing. Therefore, any projection of the market sizes in the years ahead can be speculative.

However, based on certain assumptions of some technological breakthroughs, the market sizes of inorganic membranes have been estimated [Business Communications Co., 1994] and an example is shown in Table 12.4. It can be seen that ceramic membranes dominate the market of inorganic membranes currently and in the next

Future Trends 577

decade. At the present, many inorganic membranes are largely used in the food and beverage sector. But greater growth is expected in other applications. It is estimated that the average annual growth rates in the U.S. alone within the next ten years of inorganic membrane usage in the food/beverage, biotechnology/pharmaceutical and environmental sectors are 13, 19 and 30%, respectively [Business Communications Co., 1994]. By the year 2003, the expected market size in the environmental sector will almost match that in the industrial processing sector.

The above market projection hinges on the assumption that critical technical hurdles will be removed over time. Some of the breakthroughs required before the market of inorganic membranes can reach the estimated size are discussed below.

TABLE 12.4 Market sizes of inorganic membranes in the U.S. and worldwide

Year U.S. market ($million) Inorganic Ceramic

1989 1993 39 27 1994 1998 112 1999 2003 288 249

World market ($million) Inorsanic Ceramic 32 19

108

546 -440

12.3 MAJOR TECHNICAL HURDLES

It is obvious that some major technical issues are related to the preparation of more permselective and robust inorganic membranes. In addition, system packaging and installation and operating knowledge also await to be advanced to realize full potentials of inorganic membranes for both separation and reaction applications. While breakthroughs in one of the above developments can accelerate those in the others, only co-current advancements in all fronts can facilitate the acceptance of inorganic membranes by potential users. History of the membrane technology has shown that the process of gaining acceptance to replace other separation technologies with membranes is often accompanied by persistent demonstration of values and continuous technology evolution.

12.3.1 Synthesis of Inorganic Membranes with High Permselectivities and Permeabilities

Dense palladium and palladium alloy membranes have been repeatedly demonstrated to show extremely high selectivities of hydrogen and certain solid electrolyte membranes

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made of metal oxides such as stabilized zirconia exhibit very high permselectivities of oxygen. However, they all suffer from low permeabilities which are directly related to throughput of a separation or reaction/separation process. In fact, their permeabilities are often at least an order of magnitude lower than those of porous membranes. Some developmental efforts are being contemplated to improve the permeation fluxes of dense inorganic membranes. One pursuit is to deposit a very thin dense inorganic membrane layer on a porous support. Similar to the concept of a porous composite membrane, this approach segregates two of the most important functions of a membrane into two separate layers: an integral, defect-free permselective layer (membrane) and a non- permselective porous support layer for mechanical strength. The thin membrane layer assumes most of the permeate flow resistance, but, due to its very small thickness, a relatively high permeability is still possible for a given transmembrane pressure difference. The challenge, however, remains in the manufacturing of a very thin and yet defect-free layer on a porous support. The quality of the porous support surface becomes an even more critical issue for dense membranes that deposit on the support.

To make an inorganic membrane an economically attractive separation medium or reactor, a high permselectivity imparted by the membrane to the feed components is not only desirable but necessary in many cases. Size-selective separation is generally expected to be favored for achieving a high permselectivity. To this end, it has been suggested that inorganic ultramicroporous membranes with mean pore diameters smaller than approximately 0.8 nm are needed [Armor, 1992].

Inorganic membranes with pore sizes in that range are still a great challenge particularly from the standpoint of defect-free membranes which are essential in many separation and membrane reactor applications. The synthesis and assembly of these membranes will continue to be a major hurdle before successful commercial implementation of inorganic membrane gas separation and reactors can be realized. Some proof-of-concept studies have been conducted successfully on the preparation of these ultramicroporous inorganic membranes. For example, as mentioned in Chapter 3, zeolite membranes have been prepared by precipitation of zeolite inside the pores of mesoporous inorganic membranes. Key questions, however, still remain as to how the membranes can be produced and assembled consistently on a large scale.

12.3.2 High Packing-Density Inorganic Membranes

Current commercial inorganic membranes come in a limited number of shapes: disk, tube and monolithic honeycomb. Compared to other shapes such as spiral-wound and hollow-fiber that are available to commercial organic membranes, these types of membrane elements have lower packing densities and, therefore, lower throughput per unit volume of membrane element or system.

There have been some attempts to address this issue. However, the size of the inorganic fibers or spiral wound unit is still quite larger than that of the organic counterparts. To be

Future Trends 579

more competitive, the packing density of inorganic membranes preferably exce, exls 1,000 m2/m 3.

12.3.3 Prevention or Delay of Flux Decline

Permeation flux of a membrane declines with run time as a result of concentration polarization or fouling in separation processes or coking in catalytic reaction processes. Fouling is frequently observed to decrease the flux of porous organic or inorganic membranes. This problem occurs most often in liquid-phase systems across various industries from chemical to petroleum to pharmaceutical applications. Dehydrogenation reactions are the most promising application area for inorganic membrane reactors; however, the removal of hydrogen also tends to promote coking in the membrane reactors. The resulting coking not only can deactivate the catalyst but also block membrane pores.

In addition to slowing down the fouling rate on a membrane, timely regeneration of a slightly fouled porous membrane is also very important and necessary in many cases. Flow dynamics of the permeate as well as the foulant passing across the membrane surface and through the membrane layer plays a key role. Furthermore, experimental studies and theoretical analyses on backflushing (or backpulsing) improve the design and efficiency of membrane regeneration systems. The operating conditions and the feed compositions in a membrane reactor have great influences over the coking propensity. Thermal and/or chemical cleaning are also vital in preserving the service life of an inorganic membrane. Care needs to be exercised to address the issue of chemical compatibility between the membrane material and the cleaning chemical and conditions.

Despite the aforementioned efforts, membrane flux decline due to fouling continues to be a major operational issue. Attempts have been made to modify inorganic membranes, mostly their surfaces, to render them less prone to foulant adsorption. One of the frequently encountered fouling problems in biotechnology and food applications is protein adsorption. In membrane reactor applications which are largely associated with hydrocarbons, carbonaceous deposits pose as one of the operational problems.

Novel designs may be required to further prolong the run time cycle between the thermal and chemical cleaning cycles.

12.3.4 Fluid Containment at High Temperatures

Critical to both gas separation and membrane reactor applications, fluid leakage and any potential re-mixing of the separated species have to be avoided. The problems could arise if pin-holes or structural defects exist or if the ends of the membrane elements or the connections between the membrane elements and assembly housings or pipings are not properly sealed.

580 Chapter 12

Many methods have been proposed to address this issue (see Chapter 9). Beside thermal and chemical resistances of the sealing materials, other issues need to be considered as well. One such important issue is the mismatch of the thermal expansion coefficients between the membrane element and the sealing material or joining material. While similar material design and engineering problems exist in ceramic, metal and ceramic- metal joining, developmental work in this area is much needed to scale up gas separation units or membrane reactors for production. The efforts are primarily performed by the industry and some national laboratories.

12.4 OUTLOOK OF INORGANIC MEMBRANE TECHNOLOGY

Twenty some years after the massive installations of porous inorganic membranes for production of enriched uranium started, commercial porous alumina membranes were introduced into the marketplace and became the first major commercial inorganic membrane. It has been about a decade since the market entry that inorganic membranes are starting to become a contender in selected liquid-phase separation applications. Early applications were focused on the food and beverage industries with encouraging successes. Later application developments spread into other uses such as produced water, waste or recycle water treatment and clean air filtration. Inorganic membranes are also being seriously considered for future petroleum and petrochemical processing. This is evident by a number of major chemical and ceramic companies currently involved in marketing as well as developing new porous inorganic membranes.

An anticipated major application of inorganic membranes is high-temperature gas separation which requires, in addition to acceptable membrane materials and geometries, compatible sealing materials and methods for the ends of the membrane elements and the connection between the elements and assembly housings or pipings. Moreover, within the range of commercial and developmental inorganic membranes available today, Knudsen diffusion appears to be the dominant separation mechanism and offers only limited permselectivities in many gas separation requirements. To take advantage of other potentially useful transport mechanisms for gas separation purposes, membranes with pore sizes finer than 3 to 4 nm range or with controlled pore modifications are needed. Promises have begun to appear in bench developments of these ultramicroporous membranes.

No commercial installations of inorganic membrane reactors are in operation today. This technology has the most potential in high-temperature reversible dehydrogenation, hydrogenation and dehydration reactions. Their potential payoffs are widely viewed as very high and are considered high-priority research and development areas by government agencies. However, the high risks associated with the current status of the technology keep the industry from major R&D investments at this time. On the contrary, the breadth and depth of academic research in this area have increased immensely in recent years.

Future Trends 581

Traditionally, acceptance of organic membranes as a substitute for other separation processes has been slow. A major reason for this hesitation is the general uncertainty associated with the scale-up of the membrane separation process relative to that of the well established bulk separation processes such as distillation, adsorption and others. This mindset is likely to prevail for some time before critical mass of the acceptance of inorganic membranes occurs, particularly for the inorganic membrane reactor technology. The ongoing gradual acceptance of inorganic membranes in several liquid- phase applications will be followed by that in gas- and vapor-phase separations which may be five to ten years away from key breakthroughs. The inorganic membrane reactors will probably not be a significant unit operation until one or two decades from now. This, of course, is contingent upon the development of inorganic membranes capable of making precise gas separations at a cost effective permeation flux level and the resolution of the technical hurdles previously described.

12.5 SUMMARY

Like many other large-scale chemical processes, the economics of inorganic membrane separation and reaction processes is very sensitive to capital investments and operating costs. While little accurate cost data is available for extrapolation, some guidelines on various cost components have been extracted from the literature for rough cost estimates. Caution should be exercised to use the data for comparison purposes.

While it is generally believed that high-temperature gas separation and catalytic reactors using inorganic membranes can benefit from significant energy savings, reliable cost information in these areas is practically not available, probably due to the emerging nature of the technology.

Based on certain assumptions of some technological breakthroughs, the market size of inorganic membranes have been projected to reach about six million dollars (US) worldwide by the year of 2000. Ceramic membranes are expected to continue to dominate the inorganic membrane market within the next decade. While the food and beverage applications currently are the largest usage of inorganic membranes, the environmental applications will grow to become a major force within the next decade.

Breakthroughs are needed to overcome three major technical hurdles: synthesis of inorganic membranes with high permselectivities and permeabilities, prevention and delaying of permeate flux decline due to fouling or coking, and fluid containment at high temperatures. Some key efforts are identified.

Finally, the current status of the inorganic membrane technology is summarized for an overall perspective. The future is speculated based on that perspective to provide a framework for future developments in the synthesis, fabrication and assembly of inorganic membranes and their uses for traditional liquid-phase separation, high- temperature gas separation and membrane reactor applications.

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REFERENCES

Armor, J.N., 1992, Chemtech 22, 557. Business Communications Co., 1994, "Inorganic membranes: markets,

technologies, and players" Report No. A6GB-112R. Chan, K.K., and A.M. Brownstein, 1991, Ceram. Bull. 70, 703. Humphrey, J.L., A.F. Selbert and R.A. Koort, 1991, Separation technologies - advances

and priorities, Final report (Report No. DOE/ID/12920-1) to U.S. Dept. of Energy. Johnson, H.E., and B.L. Schulman, 1993, Assessment of the Potential for Ref'mery

Applications of Inorganic Membrane Technology -- An Identification and Screening Analysis, Report DOE/FE-61680-H3 for U.S. Department of Energy.

Lahiere, R.J., and K.P. Goodboy, 1993, Env. Progr. 12, 86. Muralidhara, H.S., 1991, Key Eng. Mat. 61&62, 301. Quemeneur, F., and P. Jaouen, 1991, Key Eng. Mat. 61&62,585. Spillman, R.W., 1989, Chem. Eng. Progr. 85, 41.