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Dehydration of Industrial By-Product Solutions for Recycling via Pervaporation/Adsorption Hybrid Process Emre Bukusoglu, Halil Kalipcilar, and Levent Yilmaz Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04306 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018
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Industrial & Engineering Chemistry Research
Dehydration of Industrial By-Product Solutions for Recycling via Pervaporation/Adsorption Hybrid Process Emre Bukusoglu*, Halil Kalıpçılar and Levent Yılmaz Chemical Engineering Department, Middle East Technical University, Üniversiteler Mahallesi Dumlupınar Bulvarı No:1, 06800, Ankara – Turkey Keywords: Pervaporation, liquid phase adsorption, polymeric membranes, solvent dehydration, printing industry, byproduct. ABSTRACT: Dehydration of the used solvents of the printing and packaging industry is required in order to reuse or increase the commercial value of the solvents. In this study, a pervaporation/liquid phase adsorption hybrid process for dehydration of a low value by-product solution obtained from a real operation was examined. The by-product solution contained water varying in a range 1-11.5% wt and the majority of the rest is ethanol and isopropanol with minor components such as ethyl acetate and methoxy propanol. We measured the flux and separation factor of two commercial membranes as a function of process parameters such as feed/membrane module temperature, feed flow rate, and permeate-side pressure in addition to the feed water and ethyl acetate concentration. We observed that higher flux and separation factors at high temperatures and low permeate side pressures. PERVAP 2201 membranes showed higher flux with a reasonable separation factor towards water. PERVAP 2211 showed higher water separation factor but lower flux compared to PERVAP 2201. We performed concentrated-mode experiments to simulate a long-term use (~week) of the membranes, which clearly demonstrated a successful removal of water from the by-product solution below 98.5% wt. Finally, application of liquid phase adsorption on the pre-dried solution (using pervaporation) revealed a successful dehydration to 0.04% wt water.
1. INTRODUCTION Recovery and reuse of waste and by-product stream of the used materials in industries gained importance recently as the common aim is to decrease the product cost. Finding an environment-friendly and cost-effective method to separate and purify the used materials in the process is the main concern of the design of the recovery systems. One specific example for these processes suffering from high amounts of solvent waste is the packaging-printing industry. Solvents, mainly ethanol, isopropanol, ethyl acetate are used in the process either to dilute or tune the viscosity of the ink, which is to be applied on the packages. If the printing and packaging plant has a solvent recovery facility, the solvent evaporated from the ink during the printing process is dragged with air and fed to adsorption beds. The adsorbed solvent and small amount of water is stripped from the bed by nitrogen and is condensed to obtain a solvent solution. The solvent mixture is then fed to a train of three distillation columns where a mixture of ethanol, water and isopropanol (about 1 ton daily) is collected from the top product of the second distillation column. The economic value of this solvent solution increases about 5 folds if the water content is reduced below 0.1% by weight, and it can be recycled back to the process. Pervaporation is a membrane separation method, which is gaining importance in separating the minor components from the liquid solutions.1–4 It is superior over the traditional methods such as distillation, adsorption, extraction considering its
low energy requirement, no vapor-liquid equilibrium limits (azeotrope), process continuity, easy scale-up, no need for additional chemicals, no emission and easy mounting to the current process in the working ranges it is applied. In pervaporation, a specific membrane is in contact with the liquid feed where either vacuum or purge inert gas is applied to the other side of the membrane, retentate side, to create potential gradient for mass transport. Depending on the type of the membrane, components of the feed solution is selectively separated by diffusion through the membrane and evaporation from the other side. As a result of this simple method, high purity of solvents could be obtained even from azeotrope forming solutions since pervaporation is not affected by the vapor-liquid equilibrium. Successful separations of the solvents such as, ethanol, isopropanol, tetrahydrofuran and methyl ethyl ketone using commercial membranes have been reported in the literature.5– 15 These studies were conducted mostly using synthetic feed solutions as well as with the solutions obtained directly from the industry. As a result of these studies, it was shown that pervaporation processes can successfully be applied directly to the industry. Recent studies also investigate the pervaporative dehydration of the systems including reactive media. In a particular study, Truong et al. showed that it is possible to selectively separate water from a solution containing ethanol, water, ethyl acrylate and acrylic acid using PERVAP 1201 membranes of
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Sulzer Chem-tech.16 The permeate water composition observed in this study was about 99% by weight which shows a promising application of pervaporation systems to industry. Beyond these studies, dehydration of the solvents is an active and dynamic research field. Recent studies have investigated the development of new materials and pervaporation process parameters for the dehydration of alcohol mixtures.17–26 As the activity of the compound in the feed decreases upon removal of that component from the feed, the flux of that compound through the membrane decreases. Therefore, the time required to enrich the feed solution using pervaporation increases. To overcome such difficulties, hybrid processes have been developed which combine the advantages of pervaporation with those of the traditional separation processes. For example, one of the most studied hybrid pervaporation separation technique is the combination of distillation with pervaporation. Sommer and Melin27 classified the hybrid process schemes and listed some use areas of these separation sequences. Another candidate for a hybrid process is the use of adsorption in combination with pervaporation that combine the advantage of adsorption on the removal of the compound of lower concentrations. Herein, we propose using a combination of liquid phase adsorption with pervaporation process for the dehydration of the alcohol solutions obtained from printing and packaging industry. The applicability of pervaporation in combination with liquid phase adsorption for the dehydration of the industrial byproduct solution obtained from a local packaging and printing company were investigated in this study. The effects of pervaporation process parameters such as feed flow rate, temperature, feed water and ethyl acetate content and permeate side pressure on the performance of hydrophilic PERVAP 2211 and PERVAP 2201 membranes of Sulzer Chem-tech® for the dehydration of the solvent mixtures containing mainly ethanol, isopropanol, water and ethyl acetate were investigated. The targeted water content of the solvent mixture is below 0.1% wt.
2. EXPERIMENTAL
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Table 1. Composition of industrial and synthetic solutions used during the pervaporation experiments Component
Content (wt%)
Water
11.5
Ethanol
58.5
Isopropanol
29.3
Ethyl Acetate
0.7
Others
Trace
2.2. Pervaporation experiments. The home-made pervaporation setup used in this study is illustrated in Scheme 1. A centrifugal pump (Cole Parmer model 75225-05) was used to circulate feed solution between feed side of the membrane module and the feed tank. Temperature of the feed solution was set using a water bath (Polyscience Model 8201) and the temperature of the module was controlled using a home-made oven installed around the membrane module (rectangular shaped membrane module having 148 cm2 active membrane area). Vacuum was applied to the permeate side of the membrane using a rotary vane pump (Edwards RV3) where the pressure was controlled using an Edwards ADC Active Digital Controller equipped with Edwards Pirani Gauge and a solenoid valve. The experimental parameters used in this study are listed in Table 2. Table 2. Experimental parameter ranges used in the experiments Parameter
Range
Temperature
40 - 77°C
Permeate Side Pressure
2 - 10 torr
Feed Flow Rate
250 - 1600 mL/min
Feed Water Content
1 - 11.5 wt%
Feed Ethyl Acetate Content 0.7 - 10 wt%
2.1. Materials. The membranes used in this study were commercially available polymeric membranes PERVAP 2211 and PERVAP 2201 that were purchased from Sulzer Chem-tech, Germany. PERVAP 2201 membranes are capable of an operation at a maximum long-term temperature of 95°C and can tolerate up to 50% of water in feed mixture. Similarly, PERVAP 2211 can operate at a long-term temperature of 100°C and feed water content must be below 40%. Pervaporation experiments were carried out using a byproduct solvent solution obtained from the solvent recovery process of a local packaging and printing industry. The main components of this solution were ethanol, isopropanol, water and ethyl acetate and the composition is given in Table 1. In addition, chemicals such as methoxypropanol can be found in the solution in trace amounts (lower than 0.1%). Feed from a stock solution obtained from the real process at the composition shown in Table 1 was used in experiments unless otherwise indicated.
There are two parallel permeate traps connected to the module in order to operate the system continuously and not to disturb the system during sampling. The permeate vapor was collected in the permeate traps as condensed phase using liquid nitrogen. Each experiment lasted until steady state was reached, determined from three consecutive flux measurements with similar values. Separation performances of the pervaporation membranes are reported as flux and separation factor. Flux (Ji) was calculated by Equation (1), where mp,i is the amount of material i collected from permeate in unit time per unit area of a membrane,
Ji =
m p,i A.t
(1)
Separation factor (ßi) was calculated using Equation (2) where xi and yi are the mass fractions of the component i in permeate and feed, respectively.28
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