Membrane inlet configuration

From outward appearance membrane contactors look similar to other membrane devices. However, functionally the membranes used in contactors are very different. They are mostly nonselective and microporous. Membrane contactors can be made out of flat sheet membranes and there are some commercial apphcations. Most common commercial membrane contactors are, however, made from small-diameter microporous hollow fiber (or capillary) membranes with fine pores (illustrated in Figure 2.1) that span the hoUow fiber wall from the fiber inside surface to the fiber outside surface. The contactor shown as an example in Figure 2.1 resembles a tube-in-sheU configuration with inlet/outlet ports for the shell side and tube side. The membrane is typically made up of hydrophobic materials such as Polypropylene, Polyethylene, PTFE, PFA, and PVDF. 

In the cross-flow operation, the inlet feed stream entering the module at a certain composition and it flows parallel to the membrane surface. The composition of the stream changes along the module, and the stream is separated into two parts a permeate stream and a retentate stream. Flux decline is relatively smaller with cross-flow and can be controlled and adjusted by proper module configuration and cross-flow velocities. 

We have two options either include an inlet flow on the permeate side of the membrane, or set the exit flow rate. From the mathematical perspective the two amoimt to the same thing, thus they get us out of the bind. Physically, they are reasonable as well. We can certainly configure a mass flow controller and a pump that would keep the flow out of the system constant, but the inlet flow is easier to do experimentally, and therefore we will include this. 

For an inlet temperature of 640 K, the fixed bed exhibited runaway. With the membrane reactor, profiles from inlet temperatures of 630 and 640 K showed no hotspots, and the inlet temperature eould be increased to 670 K without runaway. The yield to desired products was lower, however, due to the reduced reaction rates in the 02-metered membrane reactor. So for the same inlet conditions and catalyst volume, the increased temperature control of the membrane reactor did not improve seleetivity enough to overcome the decreased conversion typically seen in the reactant feed configuration. 

Another improvement in the MR configuration that leads to a further reduction of the VI was proposed by Barbieri et al When the feed mixture enters the WGS stage it has a high CO content, such as in the case of the streams coming out from coal gasification, the traditional MR configuration does not allow the best exploitation of the whole membrane area because of the low H2 partial pressure at the inlet of the MR. For this reason the authors proposed the membrane only in the second part of the catalytic bed (Figure 12.12). 

This chapter presents the simulation of a button cell data reported by Liu et al. [38]. The model parameters derived here are further used in the performance analysis presented in the later chapters. A schematic representation of button cell is given in Fig. 6.1. The configuration is basically a concentric cylindrical assembly intercepted by the membrane electrode assembly (MEA). The fuel and air inlet are through the inner cylindrical pipe, which reaches above the anode and cathode. The product gas outlet is through the concentric space between the inner and outer cylinder. 

Numerous calculations have been carried out to reproduce the experimental data reported by Liu et al. [38]. Since the experimental report does not give a detailed description of the flow configuration, the simulation study assumes the inlet fuel pipe to be 7 mm in diameter and ends 5 mm above the anode. The anode is a 20 mm in diameter and 0.5 mm thick porous membrane. The parameters used for the calculation are given in Tables 6.1 and 6.2. 

Fiaschi and co-authors assessed a power plant based on this OTM-CPO concept, where H2 produced after CPO, WGS and CO2 absorption is used to raise the temperature of the vitiated air exiting the membrane reactor up to the turbine inlet temperature. Overall efficiencies of 44.4-44.6% were obtained, roughly 10% points less than that of a reference combined cycle without CO2 capture. The main limitation of this configuration is perhaps the low CO2 capture rate (72-77 %), due to the low conversion of CH4 in the membrane reactor. 

An important technique used in the bioartiflcial liver is based on the immobilization of the cells on the inner or outer surface of hollow fibers made of porous material. This configuration allows oxygen and other nutrients to be supplied to cells by means of a liquid stream different from the blood or plasma stream. The nutrient stream flows in the membrane side where cells are fixed, whereas blood plasma flows in the other side. The flow scheme can be co-current or counter current but usually the co-current scheme is adopted. The module is a bundle of hollow fibers contained in a cylindrical shell. A main problem to study is the level of O2 concentration in the cells at the outlet of the feed stream actually, this section is the most critical, owing to oxygen consumption along the fiber from the inlet to the outlet section. 

Microfiltration (MF) is a membrane filtration in which the filter medium is a porous membrane with pore sizes in the range of 0.02-10 pm. It can be utilized to separate materials such as clay, bacteria, and colloid particles. The membrane structures have been produced from the cellulose ester, cellulose nitrate materials, and a variety of polymers. A pressure of about 1-5 atm is applied to the inlet side of suspension flow during the operation. The separation is based on a sieve mechanism. The driving force for filtration is the difference between applied pressure and back pressure (including osmotic pressure, if any). Typical configurations of the cross-flow microfiltration process are illustrated in Fig. 2. The cross-flow membrane modules are tubular (multichannel), plate-and-frame, spiral-wound, and hollow-fiber as shown in Fig. 3. The design data for commercial membrane modules are listed in Table 1. 

There are other important physical features in this flow configuration. At the inlet region of the membrane, z > 0, the filtration flux Vs is determined only by the resistance of the membrane, R (from equation (6.3.136a)), AP and p ... 

Unlike conventional reactors, MRs have two compartments separated by the membrane and thus are characteristic of at least three inlets/out-lets for the feed and product streams. They are designed and fabricated based on the membrane configuration and the application conditions. 

---