Carl A. Koval

A Taylor vortex reactor for heterogeneous photocatalysis

Abstract Heterogeneous photocatalysis is a promising advanced oxidation process for water purification. One of the limitations in photocatalysis has been inherently low photoefficiencies. Recent studies show that the photoefficiency of formate decomposition can be increased through the use of controlled periodic illumination. In this study, a Taylor vortex reactor was designed, constructed and tested as a novel photocatalytic reactor that incorporated illumination and dark recovery times via fluid mixing. A combination of the vortex motion and the limited light penetration depth into the photocatalytic slurry allowed the photocatalyst particles to move into and out of the illuminated portion of the reactor. The photoefficiency in the reactor increased by nearly a factor of three with the optimal conditions of an inner cylinder rotation rate of 300 rpm and an unusually high weight loading of 10 g/l of TiO 2 photocatalyst.

Products, Intermediates, Mass Balances, and Reaction Pathways for the Oxidation of Trichloroethylene in Air via Heterogeneous Photocatalysis.

Studies of the photocatalytic reaction of a solution of trichloroethylene in the air and in contact with UV-irradiated titanium dioxide have produced conflicting reports in regard to the composition of the product mixture. This paper resolves these discrepancies by reporting the results of experiments designed to identify and quantify intermediates, products, and reaction pathways. Mass balances are closed in differential and integral modes to ascertain the effects of factors such as the extent of conversion, feed composition, and photon energy on the composition of the product stream

Design methodology for a membrane/distillation column hybrid process

Abstract Hybrid processes consisting of a distillation column and a membrane separation module have gained some importance in the recent years due to their attractive capital and operating costs and potential in saving energy. For several applications hybrid processes proved to be superior to single separation processes. The aim of an optimal hybrid design should be to take advantage of the specific benefits of both separation methods. This paper introduces some guidelines and general rules which can be applied to distillation/membrane hybrid processes. The results of numerical calculations dealing with an olefin purification hybrid process using a distillation column and a facilitated transport vapor permeation membrane in different configurations are introduced and discussed. A McCabe-Thiele based approach is used to analyze different process configurations and find some short-cut methods to determine the optimal operating conditions for each configuration. Both new column design and augmentation of an existing distillation column by a membrane module are subjects of this paper.

Transport mechanism of carbon dioxide through perfluorosulfonate ionomer membranes containing an amine carrier

Abstract A new facilitated transport model for CO 2 through ion-exchange membranes containing a diamine complexing agent was developed. The diamine ion behaves as a mobile carrier for CO 2 . Although the morphology of the ion-exchange membrane affects carrier transport, the effect of morphology on ionic carrier transport is not clear. The Nernst-Planck equation and the penetration model were employed in this modeling study. The electrical double layer effect and friction effect in the ion-exchange membrane was also considered. In the membrane, there are two kinds of counter ions (NH 3 + -R-NH 2 and NH 3 + -R-NH 3 + ), CO 2 and NH 3 + -R-NHCOO − (carbamate ion). The carbamate ion can be treated as a neutral molecule because it has both plus and minus charge. Commercial Nafion 117 (N117) and heat treated Nafion117 (HN117) were used as ion-exchange membranes. The water content of N117 and HN117 was 16 and 45%, respectively. Nafion has cluster channels which were filled with water, and HN117 has a larger cluster channel size than N117. Monoprotonated ethylenediamine was used as a carrier. Mobile counter ion diffusivities were measured by membrane conductivity. Carbon dioxide diffusivity was determined from transport measurements in a nonreactive Nafion membrane. The diffusivity ratio of carbamate ion to CO 2 was estimated by the group contribution method which is effective in aqueous solutions. We estimated a friction effect for the carbamate ion which reduces the carbamate ion diffusivity ratio in the cluster channel. For the HN117 membrane case, experimental results and simulations were in good agreement when we used the diffusivity ratio which was estimated from the group contribution method. The counter ion diffusivities, which are restricted by electrical forces, are the rate limiting step for CO 2 transport through large clusters. For the N117 case, we must consider the friction effect, and when we use a small carbamate diffusivity ratio, simulations and experimental results agreed well. The diffusivity of the carbamate ion, which is the largest molecule in the membrane, is the rate limiting step for transport through small cluster channels. This model can explain the permeate-side CO 2 pressure effect as the permeate-side CO 2 pressure seriously reduces the facilitation effect.

Analysis of a membrane/distillation column hydrid process

Abstract Distillation of low relative volatility mixtures is highly energy intensive and can require tray numbers greater than 150. Two design methods for a hybrid process are given to minimize the tray number in a distillation column that contains a membrane unit: the minimum area method and the Smokers equation method. The Smokers equation method is used to study the effect of different membrane variables on the tray number. The optimal position of the membrane unit on the distillation column is close to the feed to the distillation column. The number of trays in the distillation column does not decrease indefinitely with an increase in the membrane surface area due to the decreased selectivity. Maximum reduction in tray number of the distillation column occurs when the ration of the membrane feed rate to the membrane area is close to 0.1 mol/m2 s. An analytical expression for optimal placement of a membrane unit on a distillation column is given for a special case using the minimum area method. Finally, simulations were performed to prove that both methods are equally valid.

Evidence of critical illumination and dark recovery times for increasing the photoefficiency of aqueous heterogeneous photocatalysis

Abstract A hypothesis is proposed and tested for increasing the photoefficiency of heterogeneous photocatalytic reactions on TiO 2 particles. Increasing the photoefficiency is vital for the economic scale-up of photocatalysis for decomposing organic compounds in water. An open-channel reactor was designed to test the following rationale. There is a critical illumination time during which the absorbed UV photons generate the oxidizing species on the particle surface. These species react further to oxidize the organic material in the water. This latter step can take place in the dark. After a critical dark recovery time the particle is illuminated again. Using this light—dark cycling, the photoefficiency for formate decomposition increased by 500% when the illumination time was 72 ms and the dark recovery time was 1.45 s.

Facilitated transport separation of benzene and cyclohexane with poly(vinyl alcohol)-AgNO3 membranes

Abstract Facilitated transport membranes of poly(vinyl alcohol) containing Ag(I) ions as the carrier were prepared and characterized using ultraviolet-visible spectroscopy (UV-Vis), elemental analysis (ICP-AE), scanning electron microscopy (SEM), and SEM(EDXR). Transport experiments were performed using benzene/cyclohexane feed mixtures having volume ratios of 8 2 and 2 8 (v/v). Permeation rates for benzene ranged 0.5 – 5.5 kg μm m −2 h −1 and benzene/cyclohexane selectivities ranged 51–84. Effects on membrane performance of Ag(I) content, counter ion used, hydration, and degree of cross-linking were investigated.

Design of combined membrane and distillation processes

Abstract The performance of three hybrid membrane and distillation processes is compared theoretically, using the separation of propylene and propane as a representative case. For each system configuration the effect of the main design variables is illustrated through parametric studies. Results dealing with both design of new processes and retrofit of existing distillation-based separation processes are presented. The results show qualitative trends which may serve as guidelines for design of hybrid membrane and distillation processes. If the membrane is placed parallel to the column, the optimal position for the membrane feed stream is close to the column feed plate, which represents a potential pinch point in the column. The optimal membrane cut rate for this configuration is generally close to the molefraction of propylene in the membrane feed stream. The comparison of the systems performances indicates that placing the membrane in parallel or on the bottom stream of the column gives the best performance of the hybrid process, both in terms of compressor duty and in terms of installed membrane area.

Electrochemical separation and concentration of <1% carbon dioxide from nitrogen

Low energy separations for <1% CO 2 gases would benefit gas treatment and CO 2 sequestration. Theoretically, electrochemical pumping can separate and concentrate CO 2 from the atmosphere or other gases with < 1% CO 2 at significantly lower energy cost than current systems. Principles of electrochemical pumping for CO 2 separations are discussed and results for both organic solvent and ionic liquid working fluid systems are presented. Due to the large quantities of gases requiring processing during the separation/concentration of < 1% CO 2 gases, this work looked at solvents with negligible vapor pressures, specifically propylene carbonate and the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Other important parameters, as illustrated by the data and models presented, are low CO 2 solubility in the solvent, high CO 2 carrier solubility, CO 2 binding constants, and the CO 2 carriers electrochemistry. Reported is the electrochemical pumping of CO 2 from 0.5% (in nitrogen) to 100%, a 200-fold increase in partial pressure, using the CO 2 carrier 2,6-di-tert-butyl-1,4-benzoquinone in a propylene carbonate solution. The ratio of CO 2 moles pumped per electron mole was 0.43. The models determined the optimal CO 2 solubility in the solvent and the required redox swing in the CO 2 binding constants of the carrier.

Olefin separation using silver impregnated ion-exchange membranes and silver salt/polymer blend membranes

Olefin separations through silver containing carrier membranes were investigated. A AgBF4/Nafion blend membrane and Nafion 117 membrane exchanged with Ag+ were used. A dry AgBF4/Nafion blend membrane showed butene selectivity over n-butane with dry feed butene pressures below the dissociation pressure. This result is evidence that there was some hydration of the Ag+ so the complexation was a gas-liquid type reaction. Moreover, the selectivity decreased with time, and after eight days of operation, the butene flux approached the n-butane flux even though the butene feed pressure was above the dissociation pressure. The AgBF4/Nafion blend membrane showed a high facilitation effect for C4 separation with low humidity in comparison to Nafion ion-exchange membranes which require high humidity. A 1-butene flux of 8 × 10−8 mol/cm2 s and 1-butene selectivity of 50 over n-butane was measured with the AgBF4/Nafion blend membrane from a 1-butenen-butane 8020% feed mixture at 86 kPa total presure with only 20% relative humidity at 50°C. The membrane showed a 1,3-butadiene selectivity of 2 over 1-butene, and the total flux was greatly reduced by the presence of 1,3-butadiene. C6 unsaturated hydrocarbon transport through these membranes was also investigated. Both AgBF4/Nafion blend membranes and Nafion ion-exchange membranes showed selectivity for 1,5-hexadiene over 1-hexene. The Nafion ion-exchange membrane showed better performance for the C6 separation. Heat treated Nafion 117 containing silver ions produced a higher separation factor and flux than obtained with a normal silver form Nafion 117 membrane.