Miodrag N. Tekić

Static turbulence promoter in cross-flow microfiltration of skim milk☆

The efficiency of cross-flow microfiltration processes is limited by membrane fouling and concentration polarization leading to permeate flux decline during operation. The objective of this study was to investigate the influence of static turbulence promoter on permeate flux during skim milk microfiltration and the potential application of this arrangement for an industrial development. Experimental investigations were performed on 100 nm ceramic membrane using the Kenics static mixer as a turbulence promoter. The insertion of the Kenics static mixer caused improvement of permeate flux of more than 700% at the same feed flow rate. Although the hydraulic dissipated power was significantly increased (about 6 times), the decrease of more than 25% in specific energy consumption was obtained by the use of the static mixer. Moreover, the presence of the static mixer in the membrane tube provided the similar permeate flux values at about 5 times lower cross-flow velocity, thus reducing the energy consumption for more than 80%. Additional energy saving was achieved using the Kenics static mixer with the aspect ratio (ratio of element length to mixer diameter) greater than 1, especially at the volumetric concentration factors greater than 2.

Membrane fouling during cross-flow microfiltration of Polyporus squamosus fermentation broth

The application of microfiltration to biological systems is hindered by membrane fouling that results in a decrease in the filtrate flux and solute transmission with time. In this work, the effects of transmembrane pressure, cross-flow feed velocity, biomass structure and feed composition, on membrane fouling during cross-flow microfiltration of Polyporus squamosus fermentation broth, were investigated. The results of cross-flow trials with 0.2 μm aluminum oxide membrane showed the existence of the optimal operation transmembrane pressure and cross-flow velocity, in respect of membrane fouling, for examined range of operation conditions. It was noticed that the process of membrane fouling was moved from a predominantly surface layer phenomenon to internal membrane fouling as the cross-flow velocity was increased. Comminution of the fungal biomass prior to microfiltration to reduce particle size showed a beneficial effect on the transient flux. The steady-state flux and the transmission of solutes were not significantly affected by the biomass comminution. The observations of the filtrate flux and the transmission of solutes indicated a decrease in the rate of fouling with decreasing content of soluble components with large molecular weights in the feed.

Membrane-based ethanol extraction with hollow-fiber module

Abstract The high energy requirements of the traditional separation of ethanol from fermentation liquors by distillation led us to seek a new energy-efficient separation method. Several alternatives, including absorption, molecular sieves, membrane separation processes, and liquid-liquid extraction processes, have been proposed and investigated (I). One of the most investigated separation techniques during the past few years has been membrane-aided solvent extraction (2–5). This dispersion-free solvent extraction process, which uses microporous membranes, overcomes such shortcomings of conventional liquid extraction as flooding and loadings. On the other hand, a technique with microporous hollow fibers may provide high mass transfer per unit volume since hollow-fiber modules contain an enormous surface area.

Kenics Static Mixer as Turbulence Promoter in Cross-Flow Microfiltration of Skim Milk

The efficiency of cross-flow membrane filtration processes is limited by membrane fouling and concentration polarization. The question of membrane fouling and membrane pore blocking during microfiltration is much more important than in the traditional ultrafiltration, not only for the maintenance of acceptable flux but also for the adequate recovery of the permeate components. The objective of this study was to demonstrate that use of a static mixer as turbulence promoter results in enhanced cross-flow microfiltration of skim milk. Experimental investigations were performed on 50-nm and 100-nm ceramic tubular membranes. The use of a static mixer provided a significant reduction of membrane fouling and an increase of more than 700% in permeate flux for both membranes compared with that obtained without a static mixer at the same feed flow rate. The similar flux enhancement indicates that surface layer resistance dominates the overall fouling resistance. Although the power consumption was significantly increased by using a static mixer, a decrease of more than 25% in specific energy consumption for both membranes was achieved with static mixer as compared to arrangement without static mixer in experiments performed at the same cross-flow velocity.

Applicability of a double-membrane reactor for thermal decomposition of water : a computer analysis

A theoretical study of applicability of double-membrane reactor for direct thermal decomposition of water is presented. The analysis is based on an isothermal, steady state, plug flow reactor model. One of the two membranes was supposed to be hydrogen permeable, and the other one, oxygen permeable. The simulations were performed for T=2000 K, it being the upper limit of temperatures of practical interest. With a high vacuum in separation zones, the double-membrane configuration theoretically enables complete conversion providing high values of Damkohler number and total rate ratio. When a sweep gas is introduced into separation zones, significant water conversions can also be provided by a countercurrent single membrane reactor, but considerably lower than those obtained in a double-membrane reactor. The double-membrane reactor seems to be a promising solution for the water splitting reaction, deserving experimental investigations.

Applicability of two-membrane reactors for reversible gas phase reaction. Effects of flow patterns and inerts

The objective of this study is a comparative analysis of single and two-membrane reactor performances for isothermal reversible gas phase reaction. The effects of flow patterns (ideal mixing, cocurrent and countercurrent plug flow) and the presence of inert components were investigated. It is shown by simulation that for the pure reactant feed in absence of inerts, the performance of a two-membrane reactor is not significantly affected by the flow patterns, providing the pressure ratio is kept close to zero. Concerning the conversion efficiency in the case when the reactant is the slowest permeating component, the advantage of a two-membrane reactor is evident, it being least significant for countercurrent plug flow. In the presence of inerts in the separation zone, the advantage of a two-membrane reactor is maintained, while it is diminished by increasing inert flow rate in the reaction zone.

Maximal extent of an isothermal reversible gas-phase reaction in single- and double-membrane reactors; direct thermal splitting of water

Abstract The maximal reactant conversions in isothermal double-membrane reactors are discussed theoretically, in comparison to single-membrane reactors. A method for calculating the limiting conversion is proposed and demonstrated on the elementary reversible gas-phase reaction aA=bB+cC. The criteria for approaching the limiting conversions are presented, in terms of values of dimensionless model parameters. The complete conversion of reactant can be realized in double-membrane reactors concerning the reactions with low equilibrium conversion and realistic process parameters. As to the applicability of double-membrane reactor model that assumes reaction equilibrium, that approximation could be reasonable only when the maximal attainable conversion is lower than the complete conversion of reactant. To demonstrate the validity of the conclusion that double-membrane configuration is superior to the single-membrane configuration considering the maximal attainable conversion, the results of simulation of thermal water splitting are presented.

Non-isothermal two-membrane reactors for reversible gas phase reactions

Abstract The performance of a non-isothermal two-membrane reactor for reversible chemical reactions in gas phase has been analyzed by numerical simulation. The analyzed reactions were of the form: aA = bB + cC . Two membranes, that are permeable to all the components of the reaction mixture, are supposed to be the most permeable to one of the two reaction products, satisfying the condition of reverse products permselectivities. The reactant is taken to be the slowest permeating component. A negative temperature influence on the permeabilities of components has been assumed. Co-current plug flow pattern has been accepted. It has been shown that it is possible to enhance reactant conversion above that of a conventional reactor for both endothermic and exothermic reversible reactions, including adiabatic and non-adiabatic case. By using a two-membrane reactor, considerable lowering of feed temperatures is enabled for an endothermic reaction. For endothermic reactions, there is the optimum feed temperature, whereas for exothermic reactions, the higher the temperature, the lower is the attained conversion. In reactor design, the optimal external heat exchange for both endothermic and exothermic reactions can be determinated.

Applicability of two membranes for improving performance of a membrane reactor

Abstract The applicability of the two-membrane reactor concept is demonstrated on a simple perfectly mixed reactor model. The two-membrane reactor has the advantage over the single membrane one, when both high conversion and mutual separation of products are required. By introducing the second membrane, the overall process efficiency (high conversion and high separation of components) could be improved, if the reactant was the slowest of the permeating components.

A simple analytical analysis of the performance of a two-membrane reactor

Abstract In a previous study [1] the concept of a two-membrane reactor was demonstrated on a simple perfectly mixed reactor model. The model equations were solved numerically and it was shown that the two-membrane reactor had a better performance than a single-membrane reactor. A simple analytical analysis of the problem is presented in this paper. The results are in accordance with those obtained in previous work [1].