Single-stage membrane unit

The design of single stage membrane units depends on the specific application... 

This application shows that it is possible to meet the stringent German TI Air standards with a single-stage membrane unit. But this is also an example for the potential of the improvement of the process economics. If it is possible to recycle the retentate the following savings are possible ... 

The single-stage membrane unit becomes equivalent to a so-called flash vaporization. The flash vaporization calculation itself is straightforward, with the vapor and liquid phases assumed at equilibrium, and is presented in a number of references." " The limits correspond to the dew-point and bubble-point calculations for vapor-liquid equilibrium, which are special or limiting cases for the flash vaporization calculation. It is the object, therefore, to adapt the membrane calculation to the techniques for the flash vaporization calculation and thereby take advantage of the relative simplicity of the latter. 

Membrane units are highly attractive for very small production wells (<5 million scfd) due to their low installation and operation costs. Simple, single-stage membrane design is used at this scale where the membrane unit separates CO2 from the raw NG to achieve pipeline specification however, 10%-15% methane is also lost in the permeate stream, which is often flared [2,24]. For small-scale systems (5-40 million scfd), two-stage monbrane systems are used to reduce methaue loss. In this gas flow range, amine and membrane systons compete with the final choice depending... 

Koyuncu et al. [56] presented pilot-scale studies on the treatment of pulp and paper mill effluents using two-stage membrane filtrations, ultrafiltration and reverse osmosis [56]. The combination of UF and RO resulted in very high removals of COD, color, and conductivity from the effluents. At the end of a single pass with seawater membrane, the initial COD, color and conductivity values were reduced to 10-20 mg/L, 0-100 PCCU (platinum cobalt color units) and 200-300 ps/cm, respectively. Nearly complete color removals were achieved in the RO experiments with seawater membranes. 

The allowable current density—normality ratio for electric membrane operation has been approximately doubled by an improved tortuous path spacer with strap turbulence promoters and by operation at higher pressures up to 60 p.s.i. As a result, twice as much water can now be demineralized per square foot of membrane utilized and/or greater demineralization achieved per pass in electric membrane units. One practical result of this development is a new continuous-flow, two-stage single-stack demineralizer which will provide 93% demineralization at a capacity of 5000 gallons per day and 72% demineralization at a capacity of 30,000 gallons per day. These units produce from 67 to 150% more water per unit membrane area than previously used automatic batch-recirculating units and are far simpler in construction and operation. 

As a general rule, gas separation by membranes is most attractive in applications where a product purity of 95% or lower is acceptable or the feed flow-rate is not too high. As the required purity approaches 100%, the membranes become less cost effective than other separation processes. This is particularly true with single-stage units. For more stringent applications, some traditional separation processes are preferred or required to integrate with the membrane system. 

One of the earliest (and largest) membrane plants for FOR was at the SACROC unit in West Texas, which started up in 1984. (The hollow Hber membrane units are owned and operated by Cynara, a subsidiary of Dow.) In this process, the purified CO2 stream from the membranes is further treated with hot potassium carbonate before reinjection into the oil field. A single-membrane stage is used followed by multiple banks of membrane permeators in parallel, thus plant performance can be optimized under varying feed conditions by adjusting the number of permeators in operation. (Over tbe years tbe CO2 content increased from 0.5 vol% up to a level of -60 vol%.) The Cynara membrane system would successfully process 70 million SCFD of gas containing 40-70 vol% CO2 around 1990 [122]. 

Single membrane units can be evaluated based on their geometry and operation conditions. Zolandz and Fleming [4] provide a good description for gas permeation systems and models for design purposes. Sender [5] discusses the use of cascades (or staging) for various series and/or parallel sets of membrane modules. 

A list of RO apphcations is given in Table 1.9. Several case studies are discussed in Chapter 3. RO plays a major role in providing potable water, defined as <1000 mg/1 total dissolved solids (TDS) by the W.H.O. or <500 mg/1 TDS based on U.S. EPA criterion. Desalination of seawater (50—100 bar g 35,000 mg/1 TDS) and brackish water (15-25 bar g 1000—10,000 ppm TDS) are the largest applications of RO. The highest rejection RO membranes are those that can make potable water from seawater using multi-staged, single-pass RO units. 

The single-pass RO unit is a three-stage (1 1 1) array designed to produce purified water at an overall recovery of 75%. The TFC RO membrane average rejection is 98%. The permeate conductivity should be less than 10.0 pS/cm. When the AP across the membrane exceeds the maximum allowable value, the RO unit is taken out of service to clean the membranes. The frequency of cleaning the RO unit is typically 3-6 months for softened RO feed water. 

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