Energy conversion membranes membrane reactors

Cyclohexane dehydrogenation, similar to other dehydrogenation reactions, is an endothermic and equilibrium-limited reaction, which means that its conversion is restricted to thermodynamics and enhances with temperature. An increase in temperature means higher energy consumption and an enhancement in side reactions and coke formation. Considering the fact that hydrogen removal from the reaction side brings about an increase in conversion, a membrane reactor is a potential candidate for this reaction. 

Damle, A.S., Separation of Hydrogen and Carbon Dioxide in Advanced Fossil Energy Conversion Processes using a Membrane Reactor, 2002 Pittsburgh Coal Conference, Pittsburgh, PA, September 2002. 

Jansen, D., W. Haije, M. Carbo, V. Feuillade, J.W. Dijkstra, and R. Brink, Advanced membrane reactors for carbon-free fossil fuel conversion, ECN GCEP Project Presentation, GCEP Energy Research Symposium, Stanford, September 2006. 

J. R.H., Koukou, M.K. and Papayannakos, N. (1997) Water gas shift membrane reactor for C02 control in IGCC systems Techno-economic feasibility study. Energy Conversion and Management, 38 (9999), S159-S164. 

The use of a membrane reactor in steam reforming has several advantages. Because of the lower temperature operation, the energy consumption of the process is reduced which results in lower emission of C02. The lower temperature also requires less expensive catalyst, tubing and other reactor materials. Since hydrogen of sufficient purity is produced directly from the reformer, the downstream shift conversion can be omitted. Moreover, the dimensions of the C02 removal and final purification units can be reduced. Hence, significant savings in equipment costs can be expected. 

In addition to this increase in conversion, other benefits can be expected when using a membrane reactor. The same yields can be achieved at lower temperatures, leading to energy savings and reduced catalyst deactivation (one of the major problems of alkane dehydrogenation), increased selectivities when temperature-promoted side reactions exist or when the permeating species arc involved in these side reactions. Moreover, the formation and separation of products in the same unit leads to a reduction in capital costs. 

The permeation rate and its profile along the membrane reactor length can substantially determine the reactor performance. Moreover, to increase the reaction conversion, a lower permeate concentration on the permeate side is often adopted. This has implications on energy consumption and downstream separation costs. These issues will be addressed here. 

There is one more group of operations in the figure. Among the operations are facilitated transport, active transport, membrane reactors, medical membrane devices, and membrane energy conversion systems. Although the techniques in question are still under basic research they are available on the market, but their marketability is rather low. 

By appropriately designing the membrane reactor, the possibility of decreasing the reactor volume to a given, required capacity with respect to that of a conventional unit or conversely increasing the capacity given the reactor volume is equally important. In addition, the energy balance can be improved considerably using membrane reactors, as reported by several authors. 

The role of the membrane reactor is obvious. The membrane module Ml is important because it decreases the total amount of hydrogen flowing through Rrf this in turn decreases the ratio of reactor pressures P ,/Pdh, conversions (X / X ,) and temperatures Tj,) remaining the same. M3 is also important from an energy efficiency point of view, since it allows one to keep the pressure at 17 at the level of the hydrogen partial pressure at R (rather than the total pressure P/i) thus reducing the load on compressor C2. 

A membrane cell recycle reactor with continuous ethanol extraction by dibutyl phthalate increased the productivity fourfold with increased conversion of glucose from 45 to 91%.249 The ethanol was then removed from the dibutyl phthalate with water. It would be better to do this second step with a membrane. In another process, microencapsulated yeast converted glucose to ethanol, which was removed by an oleic acid phase containing a lipase that formed ethyl oleate.250 This could be used as biodiesel fuel. Continuous ultrafiltration has been used to separate the propionic acid produced from glycerol by a Propionibacterium.251 Whey proteins have been hydrolyzed enzymatically and continuously in an ultrafiltration reactor, with improved yields, productivity, and elimination of peptide coproducts.252 Continuous hydrolysis of a starch slurry has been carried out with a-amylase immobilized in a hollow fiber reactor.253 Oils have been hydrolyzed by a lipase immobilized on an aromatic polyamide ultrafiltration membrane with continuous separation of one product through the membrane to shift the equilibrium toward the desired products.254 Such a process could supplant the current energy-intensive industrial one that takes 3-24 h at 150-260X. Lipases have also been used to prepare esters. A lipase-surfactant complex in hexane was used to prepare a wax ester found in whale oil, by the esterification of 1 hexadecanol with palmitic acid in a membrane reactor.255 After 1 h, the yield was 96%. The current industrial process runs at 250°C for up to 20 h. 

The same group [2.354] has also recently reported on the performance of a membrane reactor with separate feed of reactants for the catalytic combustion of methane. In this membrane reactor methane and air streams are fed at opposite sides of a Pt/y-A Os-activated porous membrane, which also acts as catalyst for their reaction. In their study Neomagus et al. [2.354] assessed the effect of a number of operating parameters (temperature, methane feed concentration, pressure difference applied over the membrane, type and amount of catalyst, time of operation) on the attainable conversion and possible slip of unconverted methane to the air-feed side. The maximum specific heat power load, which could be attained with the most active membrane, in the absence of methane slip, was approximately 15 kW m with virtually no NO emissions. These authors report that this performance will likely be exceeded with a properly designed membrane, tailored for the purpose of energy production. 

E. Johannessen, K. Jordal. Study of a H2 separating membrane reactor for methane steam reforming at conditions relevant for power process with CO2 capture. Energy Conversion Management 2005, 46(7-8), 1059 1071. 

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