Pervaporasyon ile İPA/Su ve MTBE/Metil alkol azeotropik karışımlarının ayırılması

Abstract

Temel işlemlerin enerji kullanımını azaltma gerekliliği membran ile ayırmanın, özellikle de pervaporasyonun, klasik metodlara alternatif olarak ortaya çıkmasına sebep olmuştur. İnsan sağlığını ve çevreyi korumak için, yağ ekstraksiyonunda zehirli olan hekzanın yerine zehirli olmayan İPA (izo propanol) kullanımı ve benzinde kurşunlu bileşikler yerine zehirli olmayan MTBE (metil tersiyer bütil eter) kullanımı, gereklilik göstermektedir. Bu çalışmada İP A/Su ve MTBE/Metil Alkol (MA) azeotropik karışımlarının, harmanlanmış ve çapraz bağlı PVA/PAA (poli vinil alkol ve poli akrilik asit) membran kullanarak pervaporasyonla ayrılması amaçlanmıştır. Çalışılan azeotropik karışımlar için PVA'nın PAA kullanılarak harmanlama ve çapraz bağlama ile modifiye edildiği membranda; en uygun PVA/PAA oranının tespit edilmesi amacı ile; dört farklı PVA/PAA oranı için öncelikle saf çözücüler (İPA, Su, MTBE, MA) ve çeşitli konsantrasyonlarda^ sıvı karışımlarının (İP A/Su, MTBE/MA) sorpsiyon deneyleri sabit sıcaklıkta yapılmış; daha sonra üç farklı sıcaklık ve değişik besleme konsantrasyonları için pervaporasyon deneyleri yapılmıştır. PVA/PAA oranlan, sıvı besleme konsantrasyonu ve çalışma sıcaklığının, pervaporasyon karakteristikleri olan akı ve seçiciliğe olan etkileri incelenmiştir. Hazırlanan membranlann yoğunluklarının hesaplanmasıyla, homojen membranlann elde edildiği anlaşılmıştır. Saf çözücü-membran arasındaki etkileşim parametreleri Flory-Huggins teorisine göre belirlenmiştir. Membranlann çözücü ilgileri sıralandığında öncelikle su, sonra MA, daha sonra da İPA gelmiştir. Membranların MTBE ilgisinin ise çok az olduğu görülmüştür. Her iki azeotropik karışım için PVA/PAA oranındaki PAA içeriğinin artışı ile çözünürlüğün azaldığı, sorpsiyon seçiciliğinin ise arttığı görülmüştür. Pervaporasyon sonuçlan sorpsiyon sonuçlarına paralel çıkmış, aynı şekilde PVA/PAA harmanlama oranındaki PAA içeriğinin artması ile pervaporasyon akısının düştüğü, pervaporasyon seçiciliğinin de yükseldiği saptanmıştır. Besleme karışımında geçen madde konsantrasyonun artmasıyla çözünürlük ve akı artmış, sorpsiyon ve pervaporasyon seçicilikleri düşmüştür. Çalışma sıcaklığındaki artış akının artmasına, seçiciliğin azalmasına neden olmuştur. Sorpsiyon ve pervaporasyon seçicilikleri yardımı ile difüzyon seçicilikleri belirlenmiş, kanşımlann pervaporasyonla ayırılmasında sorpsiyonun etkin olduğu anlaşılmıştır. Hazırlanan membran ile, MTBE/MA azeotrop karışımı için İPA/Su azeotropik karışımından daha yüksek seçicilikler ve daha yüksek akı değerleri elde edildiğinden; PVA/PAA çapraz bağlı membranın MTBE/MA sistemi için daha fazla seçici-geçirgen olduğu söylenebilir.

Separation of various mixtures especially liquid mixtures, is a very necessary unit operation in industry. For liquid mixtures having components with similar boiling ranges and, azeotropic liquid mixtures however, such conventional separation techniques are energy intensive and add considerably to the cost of the final product. With the advent of the necessity of reducing the energy requirements of unit operations, membrane separation has been recognised as an alternative to the conventional methods. Pervaporation, a promising separation method is regarded as one of the most versatile membrane separation processes because of its economical advantage, expectations for new features on the separation and its successful commercialisation despite its short history in the industry. It is an energy efficient way of separating liquid mixtures that are difficult to separate by conventional means such as distillation, adsorption, liquid-liquid extraction, and fractional crystallisation. In this study, for the purposes of membrane material development for pervaporation separation, poly (vinyl alcohol) (PVA) was blended with a low molecular weight of poly (acrylic acid) (PAA) and then crosslinked by heat treatment. The membranes are evaluated for the separation of water from iso propanol (IP A) and methanol (MA) from methyl tert-butyl ether (MTBE) by pervaporation. Both IP A/Water mixtures and MTBE/MA mixtures are azeotropic mixtures. The recent developments in pervaporation technology, are encouraging, and could make a significiant impact on the use of IPA as an extraction solvent in the oilseed industry. In a typical oilseed extraction facility, the amount of IPA and water vapor condensates collected from desolventization of oilseed marc (meal containing some solvent), and from evaporation of miscella, could easily reach 400-4000 tons per day. Improvements in membranes and module design resulting in higher productivity rates, could significantly reduce capital equipment costs and will allow very large volumes of solvent to be recovered. MTBE is produced by the reaction of methanol with isobutylene in the liquid phase over a strongly acidic ion-exchange resin catalyst. The reaction is rapid and selective, but is limited by equilibrium conditions. Therefore, it is desired to improve the conversion by using excess of methanol. Excess concentrations of methanol up to about 20 % of the stoichiometric amount are generally used to achieve high conversions. The use of excess methanol, however, causes a purification problem because methanol forms minimum-boiling azeotropes with MTBE at a composition of 14.3 wt % methanol at 760 mmHg. Presently the reactor effluent is first separated by a debutanizer column into a bottom MTBE at the overhead of the column. XVI Subsequently methanol is washed out by water, and the water/methanol mixture is then distilled to recover methanol for recycle. This conventional separation process is both capital and energy intensive. Pervaporation has been considered as an alternative separation technique. It may not be practical to separate completely the entire reactor effluents by pervaporation, but a hybrid distillation- pervaporation process may be very attractive. In this case, pervaporation is used only in a limited area of separation such as for breaking the azeotrope. PVA is known as one of the popular synthetic polymers which have higher hydrophilicity. The characteristics are ascribed mainly to the hydroxy group in the side chain, and the polymer is soluble in water. In order to apply this polymer to practical use, for example, to permselective membranes, some kind of crosslinking is necessary to prevent the dissolution. PVA can be well adapted for dehydration, but it has poor stability in aqueous mixtures. Since PVA is water soluble, it is easily swollen, the mechanical strength and the resistance to water has to be improved. There are two methods of treatment to improve the stability in aqueous solution: crystallization and crosslinking. Altough the membrane with the crystallization pretreatment can be used in aqueous mixtures at low temperature, it still has the problem of stability at high temperature. PVA contains many hydroxy groups and is a very "tight" membrane that is caused by high degree of inter-and intramolecular hydrogen bonding. When PVA membrane was used for separation alcohol and water, it was found that altough the flux was very low the membrane possessed a high degree of selectivity toward water. A useful procedure to improve the permeability characteristics of PVA membrane and yet maintain the selectivity lies in modification of the polymer. The modification of chemical structure in the polymer membrane can be achieved through crosslinking reaction to improve its separation properties. PVA is known as a semi crystalline polymer. Crystalline regions in PVA are generally considered to be impermeable to solvents such as water, the crystallites are then obstacles and the penetrant molecules have to pass round them. Therefore, the crystalline structure of the films should have notable influence on the transport properties. If the diffusive transport only in the amorphous phase, selectivity will not be significantly affected by crystallinity, but the permeability will be expected to be the highest possible with a completely amorphous film. It seems very difficult to prevent crystallization without altering the permeation selectivity and permeability. Crosslinking is the most effective way for preventing crystallization. Polymer blending is also a way to prevent crystallization. In developing a polymeric membrane material for pervaporation, commercially available polymers and polymer films are considered first. However, the separation properties may be quite unsatisfactory. Therefore much research effort is aimed on the development of new "tailor-made" materials by synthesis of new polymers or by modification of existing polymers. Another method is the blending of polymers. This technology is very interesting due to the low development costs and the capability to develop new membrane materials. The polymer blend concept can be utilized to improve the permeation rate of a polymer membrane which is very selective for one component of a mixture to be separated but with a low permeation rate. By combining this polymer with a suitable second polymer with a higher sorption capability for the feed mixture than the former, a range of different materials with an improved permeation rate can be developed. xvu The pervaporation technique has proved to be a very effective separation method for extracting water from water soluble organic solvents. Regarding polarity and molecular size, methanol is quite similar to water. Accordingly, hydrophilic membranes have been chosen for this investigation. And alcohols and other organic solvents which are miscible with water are hardly soluble in PVA with the exception of methanol. Because of molecular size and molecular shape, IPA shows different sorption behaviour in PVA. Thus, IP A/Water mixtures can be separated by PVA membranes Methanol belongs to the organic molecules which are rather similar to water concerning the size and ehe polarity. Since PVA membranes proved to be very effective in separating water from organic solvents by pervaporation, it might be expected that methanol can also be extracted from other organic solvents, which are larger and less polar. PVA membrane absorbs solvent molecules the better the smaller in size and the more polar such as water in IP A/Water mixture, and methanol in MTBE/MA mixture. Usually the component with the highest solubility in the membrane material permeates prefentially. PVA has a high permselectivity and mechanical stability, PAA has also very high solubility for polar solvents. Swelling can be reduced by means of crosslinking. Reduction of swelling can also be achieved by blending. In this study, PVA is blended with PAA and furthermore this blend membrane is crosslinked covalently through an ester linkage formation between a hydroxyl group of PVA and carboxyl group of PAA Crosslinking reaction time and temperature are as in the investigation of Rhim's (1993). Polymer blending was performed by a solution method. Both component polymers were separately dissolved in water. Aqueous 10 wt % PVA solutions were prepared by dissolving preweighed quantities of dry PVA in pure water and heating them 100°C under reflux and moderate stirring the solution for at least 6 h. Aqueous 10 wt % PAA solutions were prepared by dissolving preweighed quantities of dry PAA in pure water and moderate stirring the solution for at least 1 h at room remperature. Then two polymer solutions were mixed together by varying each component composition to form a homogeneous solution for 24 h at room temperature. Homogeneous membranes were cast onto a flotal glass plate using a casting knife with predetermined drawdown thickness. The membranes were allowed to dry in air at room temperature during 2-3 days, and completely dried membranes were then peeled off The dried blended membranes were heated in a thermosetted oven with nitrogen convection for lh at 150°C. PVA was crosslinked by this heat treatment using PAA through the reaction between the hydroxyl group in PVA and the carboxylic group in PAA The thickness of the resulting membranes was in the range of 30 to 50 um. In order to obtain more information on the separation mechanism in pervaporation, equilibrium sorption experiments were carried out. The dry strips of polymer films were immersed in a closed bottle containing either water, IPA, MA, MTBE or a mixture of these solvents. The bottle was placed in a thermostated oven. After the swelling equilibrium state was reached, the strip was removed from the bottle, and put into a closed tube after the surface liquid was quickly removed with tissue papers and the swollen membrane was weighed. The sorbed liquid was distilled XV1U out of the sample by a vacuum apparatus. The composition of the distilled sorbate was analyzed by gas chromatography equipped with a thermal conductivity detector. The overall solubility (Q) is calculated from the weight of the swollen and dry polymer sample, and is expressed in units of grams of sorbed liquid per 1 gram of dry polymer. The degree of swelling (DS) is also obtained. The sorption selectivity (ctsorp) is determined from the composition of the distilled liquid, and defined in the same way as the pervaporation selectivity. After the determination of sorption selectivity, the component solubilities of solvents in membranes (Qwater, Qipa, Qma, Qmtbe ) were also calculated. The density of the membranes (pp) was measured with a buoyancy technique. A well dried membrane sample was first weighed in air. Thereafter it was held in iso- octane at constant temperature and its weight was measured in that medium. The volume of the sample can be calculated from the weight difference of both measurements; dividing it by the density of iso-octane. From the weight in air and the volume, the density is calculated. The binary interaction parameters between the liquid components and the polymer, (xu and X23), were assumed to be concentration independent and were calculated from the single liquid sorption and density measurement. The pervaporation experiments were performed at different temperatures and different liquid feed mixture concentrations for four type membranes. Membrane was installed in the pervaporation cell. The effective membrane diameter was 6 cm. The feed temperature was kept constant. The feed was circulated through the pervaporation cell from a feed tank by a pump with a rate of 2 1/h. The pressure at the downnstream side was kept approximately 1 x 10"1 m bar. The permeate as collected in cold traps cooled by liquid nitrogen. The composition of the collected permeate was determined by gas chromatography equipped with a thermal conductivity detector and, Chromosorb column. Pervaporation flux (J, kg/m2h) was determined by measuring the weight of liquid collected during a certain time in the cold trap at the steady state. The fluxes of different membranes were normalized to the membrane thicknesses. The pervaporation selectivity (apv) is defined the concentrations in the feed and in the permeate. According to the solution-diffusion model the pervaporation selectivity is determined by the differences in solubility and diffusivity of penetrants in a membrane. Therefore by comparing the pervaporation selectivity and the sorption selectivity, the influence of diffusion can be deduced in terms of a diffusion selectivity (a^f ). The diffusion selectivity value was calculated in this way. The solubilities (g/g) that obtained from sorption experiments for EPA and water pure solvents at 40°C were given in Table 1. Table 1. Water and D? A Solubilities (g/g) at 40°C XIX The solubilities (g/g) that obtained from sorption experiments for MTBE and MA pure solvents at 25°C were given in Table 2. Results of pervaporation experiments for IP A/Water mixtures at 40°C, 50°C, 60°C were given in Table 3. Results of pervaporation experiments for mixtures MTBE/MA at 25°C, 35°C, 45°C were given in Table 4. From the sorption experiments it was observed that the solubility of the membranes could be adjusted by controlling the PVA/PAA ratio in the blends. As the PAA content in the blends increases the affinity for water in the IP A/Water mixture and MA in the MTBE/MA mixture decreases, and thus the solubilities of these solvent decreases. Swelling of the membranes increased with increasing liquid feed mixture concentration. The sorption selectivity increased with increasing amount of PAA and decreasing liquid feed mixture concentration. Furthermore it was observed that the separation characterictics of the blend membranes could easily be controlled by adjusting the blend composition. As the PAA content in the blends increased, fluxes decreased but selectivities increased. In addition, a strong influence of the feed mixture composition on the separation characteristics was also observed. With increasing feed mixture composition, flux increases, but selectivity decreases. The temperature of the feed liquid showed a favorable effect on the separation performance of the systems studied. The flux increased with temperature, while the selectivity decreased. XX The diffusion selectivity was evaluated by comparing the pervaporation selectivity with the sorption selectivity, it could be deduced that the prefential sorption dominated the pervaporation selectivity in the studied membrane-liquid systems. Azeotropic mixtures have been industrially separated by means of extractive distillation techniques. On the other hand, separation by membranes is one of the most promising process as an energy-efficient technology. Capital costs of industrial scale membrane plants generally increase almost linearly with growing plant capacities. They hardly allow any substantial economy of scale due to their modular construction design. Distillation and rectification plants, however, offer a very significant economy of scale. Their specific capital costs generally decrease with increasing plant capacity. In a cost comparison of competing membrane and distillation technologies, the specific capital dependent costs of distillation and rectification plants will decrease with larger plant size and increase with smaller plant capacities while the specific capital dependent costs of the pervaporation plants practically remain constant. For large combined systems of distillation and pervaporation (hybrid system), cost optimization also will lead to a design where a maximum separation duty is performed by distillation while pervaporation just will serve to break the azeotrope. Distillation serves for concentrating the solvent mixture to subazeotropic concentration and also for separating impurities. Pervaporation is used for breaking the azeotrope. In this study IP A/Water and MTBE/MA azeotropes were broken by pervaporation. Pervaporation has been considered as an alternative separation technique. It may not be practical to separate completely any entire reactor effluents or it may not replace any distillation column in petrochemical industry. Pervaporation should be used in combination with a conventional separation technique. Studied azeotropic mixtures can also be separated by a hybrid distillation- pervaporation system, economically.