Gas separation systems

One unique appHcation area for PSF is in membrane separation uses. Asymmetric PSF membranes are used in ultrafiltration, reverse osmosis, and ambulatory hemodialysis (artificial kidney) units. Gas-separation membrane technology was developed in the 1970s based on a polysulfone coating appHed to a hoUow-fiber support. The PRISM (Monsanto) gas-separation system based on this concept has been a significant breakthrough in gas-separation... 

Figure 4-12. Oil and gas separation system for a flooded screw compressor. (Courtesy of Sullaii)...

There are a number of industrial gas separation systems that use the selective permeability of plastics to separate the constituents. In design problems relating to such applications, the designer must consider the environmental conditions to determine whether the materials having the desired properties will withstand the temperatures and physical and chemical stresses of the application. Frequently the application will call for elevated temperatures and pressures. In the case of uranium separation, the extreme corrosivity of the fluorine compounds precluded the use of any material but PTFE. The PTFE... 

Liquid/gaseous detoxification systems, based on hydrogen peroxide, 14 64-65 Liquid-gas separator system, in sulfonation systems, 23 552 Liquid halogen fluorides, 13 128 Liquid heat-transfer media, 12 83 Liquid helium... 

Membrane gas-separation systems have found their first applications in the recovery of organics from process vents and effluent air [5]. More than a hundred systems have been installed in the past few years. The technique itself therefore has a solid commercial background. Membranes are assembled typically in spiral-wound modules, as shown in Fig. 7.3. Sheets of membrane interlayered with spacers are wound around a perforated central pipe. The gas mixture to be processed is fed into the annulus between the module housing and the pipe, which becomes a collector for the permeate. The spacers serve to create channels for the gas flow. The membranes separate the feed side from the permeate side. 

A single train oil-gas separation system for the subject crude would have vessels about 12 ft in diameter by 45 ft long. Although not unreasonable compared with the space and cost associated with two or three trains, a single-train concept may not perform at the 95% uptime which Is... 

The first paper. Challenges Associated With the Design of Oil-Gas Separation Systems for North Sea Platforms" by Penick and Thrasher, discusses and reviews the different design aspects, requirements, and constraints associated with oil and gas separation facilities in the North Sea. Specifications of crude-oil vapor pressure and gas dewpoint are reviewed from a pipeline standpoint, and potential solutions and design approaches to meet the separation objectives are presented. 

CHALLENGES ASSOCIATED WITH THE DESIGN OF OIL-GAS SEPARATION SYSTEMS FOR NORTH SEA PLATFORMS... 

Crude oil specifications may be either for offshore tanker loading or for delivery to an oil pipeline at the platform. Crude oil specifications are usually defined relatively simply, through limitations cn vapor pressure and on BS W (basic sediment and water) content. BSMf la normally limited to a nominal percentage, such as 0.5X, and meeting this specification is outside the scope of this paper. The oil-gas separation system in Dost cases does not significantly affect whether or not the oil will meet the BSAW specification, since for those oils where this is a problem special emulsion treating is required independent cf the oil-gas separation system. 

Crude oil vapor pressure is the specification which most influences the design of oil-gas separation systems. For offshore tanker loading, the oil may be limited to a vapor pressure In the range of 8 to U pounds KVP (Reid Vapor Pressure). RVP refers to a standard method of vapor pressure testing utilizing a specific test cylinder assembly. and determined at a temperature of 100°F. RVP is (not Identical with TVP (true vapor pressure), which is the actual vapor pressure exerted by a liquid in equilibrium with a vapor at any given temperature. 

Gas specifications will be inqportant only if the gas is to be delivered to a gas pipeline system. If the gas is to be injected in the producing field the only usual critical requirement is to dehydrate the gas adequately to prevent hydrate formation anywhere in the system. The gas pipeline specification which most Influences the design of oil-gas separation systems is the hydrocarbon dewpoint limitation. This is usually expressed as a maximum dewpoint temperature at a specified pressure. For onshore gas pipelines in the USA end Europe this specification may be in the range of 32°F (0°C) at 1000 paia (68 atmospheres), which is adequate to prevent condensation of liquids in the pipelines in the normal range of onshore pipeline operating pressures from 900 to 1000 psl. In the USA this specification is seldom iiqposcd on producers and is controlled with pipeline facilities. 

It is apparent from the preceding discussion of objectives that two factors are most significant in the design of oil-gas separation systems. These are the vapor pressure of the crude oil, and gas pipeline considerations which influence the hydrocarbon dewpoint of the produced gas. Some further discussion of these two factors may be helpful. 

The principles discussed in ihis paper may be helpful in charting the direction of studies for the design of North Sea oil-gas separation systems. 

Table 1.1 shows two developing industrial membrane separation processes gas separation with polymer membranes (Chapter 8) and pervaporation (Chapter 9). Gas separation with membranes is the more advanced of the two techniques at least 20 companies worldwide offer industrial, membrane-based gas separation systems for a variety of applications. Only a handful of companies currently offer industrial pervaporation systems. In gas separation, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the feed mixture the membrane permeate is enriched in this species. The basic process is illustrated in Figure 1.4. Major current applications... 

The three factors that determine the performance of a membrane gas separation system are illustrated in Figure 8.12. The role of membrane selectivity is obvious not so obvious are the importance of the ratio of feed pressure (p ) to permeate pressure (pt) across the membrane, usually called the pressure ratio, [Pg.317]

Figure 8.12 Parameters affecting the performance of membrane gas separation systems...

The relationship between pressure ratio and selectivity is important because of the practical limitation to the pressure ratio achievable in gas separation systems. Compressing the feed stream to very high pressure or drawing a very hard vacuum on the permeate side of the membrane to achieve large pressure ratios both require large amounts of energy and expensive pumps. As a result, typical practical pressure ratios are in the range 5-20. 

Another factor that affects membrane system design is the degree of separation required. The usual target of a gas separation system is to produce a residue stream essentially stripped of the permeable component and a small, highly concentrated permeate stream. These two requirements cannot be met simultaneously a tradeoff must be made between removal from the feed gas and enrichment in the permeate. The system attribute that characterizes this trade-off is called the stage-cut. The effect of stage-cut on system performance is illustrated in Figure 8.15. 

Both techniques shown in Figure 11.20 increase the complexity of the separation process significantly, and neither has advanced to a commercial process. The focus of much of the recent work on facilitated transport has been to produce membranes that are inherently stable and can be used in conventional gas separation systems. Laciak has recently reviewed this work [38],... 

Even though most chemical purification methods are not carried out at low temperatures, they are useful in several cryogenic gas separation systems. Ordinarily water vapor is removed by refrigeration and adsorption methods. However, for small-scale purification, the gas can be passed over a desiccant, which removes the water vapor as water of crystallization. In the krypton-xenon purification system, carbon dioxide is removed by passage of the gas through a caustic, such as sodium hydroxide, to form sodium carbonate. 

Gas-separation manager includes both vapor recovery and gas-separation systems. Vapor recovery handles the recovery of valuable condensable components from a gas stream or the removal of undesired components since they are corrosive, toxic, polymerizable, have a bad odor, etc. Gas separation deals with the recovery of recycled gaseous reactants, as well as with the delivery of purified products and byproducts. Douglas [6] recommends the following heuristics for placing the vapor-recovery system ... 

Brine Staging Velocity past the membrane is important. If too low, polarization is excessive, local O rises, and rejection declines. Fouling occurs faster. If too high, pressure losses are higher than they need be, and the osmotic pinch is premature. Since the volume of feed declines continuously, the hydraulic design needs periodic rearrangement. This is commonly done as shown in Fig. 22-64, sometimes known as a Christmas tree. This design is commonly used where the fluid is pumped once, as in RO, NF, and gas-separation systems, but not where recirculation is practiced, as in ultrafiltration. 

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