Prism membrane systems

Figure 8.21 Simplified flow schematic of the PRISM membrane system to recover hydrogen from an ammonia reactor purge stream. A two-step membrane system is used to reduce permeate compression costs...

Figure 8.22 Photograph of an Air Products and Chemicals, Inc. PRISM membrane system installed at an ammonia plant. The modules are mounted vertically...

Advanced Prism membrane systems for cost effective gas separations, http //www. airproducts.eom/NR/rdonfyres/81FB384C-3DB5-4390-AED0-C2DB4EEDDB18/0/ PrismPGS.pdf. 

C. Aitken, K. Jones and A. Tag, PRISM Membrane Systems for Cost Efficient Natural Gas Dehydration, GPA Technical Meeting, 1998. 

The main advantages of the Prism separator system are simplicity, ease of operation, and low maintenance. Reference [962] compares membrane and cryogenic separation units for a large ammonia plant. 

A simple water scrubber is used as a preliminary stage to the Prism separators to remove all but traces of the methanol from the purge gas which might otherwise damage the membrane material. In order to avoid residual water condensation in the membrane system, the feed gas is then heated to some 10 °C above the water dew point and then fed to the Prism separators. The permeate gas, i.e. the gas that has permeated the membrane, consists of a mixture containing some 90% of the hydrogen in the feed gas and has the following composition ... 

As previously mentioned, the first widespread commercial application of membranes in GS was the separation of hydrogen in the ammonia purge stream, by using Permea Prism T systems. Hydrogen recovery is applicable to several processes, divided into three main categories ... 

Currently, PRISM membranes provide an attractive alternative to traditional glycol dehydration systems (Figure 14.8) based on simple process designs, lower costs, and the other benefits listed below. These benefits become even more pronounced as the industry produces natural gas from very remote locations [55]. An offshore membrane system for Shell Nigeria was designed to dry 600000NmVh of natural gas from an inlet dew-point of 41 °C to an outlet dew-point of 0°C at 38 bar. It was designed and built by Petreco, an Air Products PRISM Membranes licensed partner. Other plants were installed in Italy and Holland. 

Figure 1.5 Applications of membrane systems. (Source Prism Gas Separation Systems, Monsanto Chemical Co., St. Louis, MO Air Products and Chemicals, Inc., Allentown, PA.) Gaseous components and systems include argon, helium, hydrogen, carbon monoxide/syngas, nitrogen, oxygen, CO,-removal, H,S-removal, dehydration. Capacity for nitrogen recovery is up to 35,000 SCF/hr for 97% purity, and about one-tenth of this for 99+% purity.

Development work has also investigated alternative asymmetric membrane systems, including (a) an ultra thin nonporous film laminated to a much thicker microporous backing (which may be a different material) and (b) a very thin nonporous film applied as a coating to a thicker microporous substrate (Stem, 1986). A complex membrane structure reportedly used in the Monsanto Prism separator is a skinned asymmetric hollow fiber of polysulfone coated with a thin film of silicone mbber (about 1 micron thick). The polysulfone skin (about U. 1 micron thick) is the active separator, while the silicone mbber serves to. seal any defects in the base membrane without affecting the intrinsic permeability of the membrane (Koros and Chem, 1987). 

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... 

In order for membranes to be used in a commercial separation system they must be packaged in a manner that supports the membrane and facilitates handling of the two product gas streams. These packages are generally referred to as elements or bundles. The most common types of membrane elements in use today include the spiral-wound, hollow fiber, tubular, and plate and frame configurations. The systems currently being marketed for gas separation are of the spiral-wound type, such as the SEPAREX and Delsep processes, and the hollow-fiber type such as the Prism separator and the Cynara Company process. 

The purge gas is water scrubbed at 135 -145 bar, reducing the ammonia concentration to less than 200 ppm. The scrubbed purge gas is heated to 35 °C and sent directly to the Prism separators. Trace concentrations of ammonia and water vapor in the gas stream pose no problem to the membrane. Therefore, a dryer system is not required. 

The polyimide membrane is reported to be capable of operating at temperatures up to 150°C compared to upper limits of 100°C for PR ISM separators and 60°C for cellulose acetate.46 Permeabilities increase with temperature while selectivities normally drop. The second way to overcome the lower permeabilities is to operate the Ube system at higher temperatures than possible with polysulfone or cellulose acetate. Even at the high temperatures, the polyimide selectivity will remain high enough for the abovementioned hydrogen separations. Thus, one expects the Ube system to be competitive with PRISM separators for many hydrogen applications. 

The cellulose ester systems have some application in hydrogen separations, but are more limited in capability due to temperature limitations. Operation at higher temperature minimizes the need for stream pretreatment to remove components which may attack the membranes. The PRISM separators and the Ube systems are superior to the cellulose esters in this respect. 

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