High frequency backshock effect on ultrafiltration of selected polysaccharides

Transcription

1 High frequency backshock effect on ultrafiltration of selected polysaccharides Inês Rosinha a, Maria Norberta de Pinho a, Oscar Rubio b, Manuel Pinelo b, Gunnar Jonsson b a Chemical Engineering Department, Instituto Superior Técnico, Lisbon, Portugal b Chemical Engineering Department, Danmarks Tekniske Universitet, Lyngby, Denmark Hollow fiber ultrafiltration module performance in separation of polysaccharides was studied using a system with compressed air to backflush the permeate stream. The most important goals of this work were to develop a fast and effective cleaning of the membrane module and select the best conditions for backflush in order to maximize the permeate flux. The combined use of hot water (50 C) and the backshock system revealed the best recoveries. The backshock system besides being used for membrane cleaning was also used during the operation since allows to rise up the permeate flux and get various separations of polysaccharides with different Molecular Weight. A two layer experimental design was performed. For the first layer of experimental design the factors chosen were retentate flow rate and transmembrane pressure and the response was permeate flux. The optimum flux was obtained at transmembrane pressure of 1.0 bar and retentate flow rate at 162 L/h. In terms of rejection, it was revealed a reduction of it with an increase of transmembrane pressure. The second layer of experimental design was performed at the optimum conditions of the previous layer. The factors chosen were Backshock time and Time of backshock cycle and the response permeate flux. The optimum flux was obtained at Time of backshock cycle 15 seconds and Backshock time of 1.25 seconds. Concerning the rejection, the variation of backshock conditions allowed obtaining dynamic separations which mean the influence of the backshock in separation of mixture of Molecular Weights of the same polysaccharide. Keywords: Ultrafiltration, hollow fibers module, backshock, rejection, permeate flux, polysaccharide. Introduction Ultrafiltration is a pressure driven process for the separation of suspended solids and solutes of high Molecular Weight ( Da) are retained, while water and low Molecular Weight solutes pass through the membrane [1]. The membrane pore size has a range of 1 to 100 nm [2]. In dead-end filtration, the flow direction is perpendicular to the filter medium [3]. In the cross-flow mode, one stream of the feed flows tangentially to the membrane surface, and there are two streams leaving the membrane module with one for the retentate flow and the other for the permeate flow. In most applications, the accumulation of the rejected particles or molecules is so severe that dead-end operation becomes impractical and crossflow operation has to be adopted [4]. Crossflow generates a shearing force and/or turbulence along the membrane. This reduces the deposition and increases the efficiency and life span the membranes. However, in most cases, the filtrate flux decreases with the time without the foundation of a visible formation of cake, despite the feed tangential velocity. A higher crossflow velocity (shear) generates higher drag on particles and improves the flux [5]. Like any separation processes, the membrane separation processes can be evaluated by two important parameters, efficiency and productivity. The productivity is characterized by the permeate flux, which indicates the rate of mass transport across the membrane. In general terms, the local mass transport of a component i through a membrane element is related to its concentration on the feed side, and the permeate side,. The retention factor R i of a component i can be defined and used as a measure of performance [4]. =1,, (1) where C p,i and C R,i are the concentration of component i in the permeate and the retentate. Usually there is only one specie, microparticle or macromolecule, of interest, and the observed rejection will only be referred to the concerned species. Often the permeate flow rate is much less than the retentate flow rate in a single pass, hence the change of concentration in the retentate is not significant. The rejection can then be conveniently calculated by [4]: =1 (2) Membrane fouling Fouling manifests itself as a decline in flux with time of operation, and its strictest sense the flux 1

2 decline occurs when all operating parameters, such as pressure, flow rate, temperature and feed concentration, are kept constant. Membrane fouling influences the economic viability of the separation process. Accordingly, understanding and minimization of membrane fouling is crucial for the effective use of this technology. Factors such as flow conditions, pretreatment membrane properties, cleaning agents and cleaning performance are important for membrane fouling reduction. Fouling of membranes by chemical and biological contaminates transported in the feed can significantly reduce the energy efficiency and cost effectiveness of membrane applications [6]. Concentration polarization A layer is formed near the surface of the membrane, whereby the solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane, and its concentration is lower than that in the bulk fluid. On the other hand, the concentration of the non-permeating component increases at the membrane surface. A concentration gradient is formed in the fluid adjacent to the membrane surface. This phenomenon is known as concentration polarization and it serves to reduce the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity [7]. Such a concentration build up will generate a diffusive flow back to the bulk of the feed. After a given period of time steady-state conditions will be established where the solute flow to membrane surface due to convection flow will be balanced by the solute flux through the membrane plus the diffusive flow from the membrane surface to the bulk. A concentration profile has now been established in the boundary layer. Steady-state conditions are reached when the convective transport of solute to the membrane is equal to the sum of the permeate flow plus the diffusive backtransport of the solute (Noble 1995): + = (1) Membrane Cleaning The main goals of membrane clean-up are to restore the original permeate flow rate by removal of a flux-inhibiting fouling layer from the membrane surface and to remove precipitated small molecular weight substances (e.g. salts) from within the membrane structure [8,9]. The methods for cleaning membranes after fouling has occurred are based on hydraulic, mechanical, chemical, and electrical methods. The method used depends upon the separation process and the configuration of the module [10]. - hydraulic cleaning of the membrane is achieved with back flushing of permeate through the membrane. - mechanical cleaning such as forcing foam balls down the tubes in tubular modules. - electrical cleaning methods use an electrical pulsing which results in the movement of the charged species away from the surface. - chemical cleaning methods include the use of strong acid and basic solutions or oxidizing agents. Hydraulic cleaning A way to minimize fouling is to use an in situ cleaning technique such as backflushing. This technique is particularly effective in minimizing external fouling, since the external fouling layer is lifted off the membrane by the reverse flow and swept out by the crossflow [11]. The process is carried out by reversing the direction of flow of the permeate at a pressure which can be as large as the filtration pressure. This effectively dislodges the foulant from the membrane and restores the flux to a value close to the initial (or previous high) value. Back flushing is carried out repeatedly at regular intervals and leads to a saw-tooth type of flux behavior. The reverse flow can be realized by using pressurized air, water or permeate. The average flux rate generally still shows a gradual decline with time and membrane cleaning will still be necessary [10,11]. Chemical cleaning Some foulants can be detached by hydraulic means such as filter backwashing. Most foulants can be removed by chemical means. It has a profound impact on performance and economics. The chemicals used as cleaning agents should release and dissolve the foulant, keep the foulant in dispersion and solution, not attack the membrane (and other parts of the system) and disinfect all wetted surfaces [12]. Experimental Experimental set-up The system is composed of seven main components such as: hollow fiber module of type bore-side feed, feed tank, pump, flowmeter, backshock system and thermostat as shown in Figure 1. 2

3 1 Computer. 2 Feed Tank. 3 Pump. 4 Membrane module. 5 Flowmeter Figure 1 - Experimental Set-up with backflushing system Table 1 Membrane modules properties Membrane Type Material Membrane Area (m 2 ) Water Permeability Coefficient (L p ) (L/(m 2 h.bar)) 1 Hollow fiber MF Poly(ether sulfone) ,97 2 Hollow fiber UF Poly(ether sulfone) Hollow fiber UF Poly(ether sulfone) The temperature of the feed solution is controlled by a thermostat which recirculates the hot water into the external jacket of the tank. The feed solution is pumped from the feed tank to the membrane module. At the inlet of the membrane, there is a manometer which measures the pressure in this point (P in ). Three modules of hollow fibers of ultrafiltraation, made of Poly(ether sulfone) with 0.1 m 2 of membrane area, were used: one microfiltration membrane and two ultrafiltration membranes which properties are summarized in Table 1. Cleaning Methods Figure 2 shows the results of water permeability tests before the cleaning and after cleaning with sodium hydroxide and DIVOS solutions. It is verified that both cleaning improved the membrane performance. However, cleaning with DIVOS solution provided a better cleaning comparing to sodium hydroxide solution. For instance, at transmembrane pressure of 0.3 bar, cleaning with DIVOS the flux was 68.5 L/(m 2 h) which was better than the cleaning with sodium hydroxide solution (58 L/(m 2 h)). Comparing to initial water permeability test it can be found that the permeate flux improved in approximately 62% with DIVOS cleaning. Methods to clean between the experiments were also investigated. This cleaning technique must be quick, simple and recover the performance of membrane and therefore to guarantee that each experiment is carried out from the same starting point. After an experiment with Dextran T500, the cleaning of the system was studied by using three methods. 3

4 Flux (L/m 2 /h) Water permeability test Before cleaning After cleaning NaOH After cleaning with DIVOS Transmembrane pressure (bar) 0.35 Figure 2 - Results of water permeability tests before the cleaning and after cleaning with sodium hydroxide and DIVOS solutions performed with Membrane 1. The Method A consisted in to clean the system with distilled water at 40 C by circulating through the permeate side for 15 min by closing valve V 1 and opening valves V 3 and V 4 and afterwards circulate for 15 min through the feed side by opening valve V 1 and closing valves V 3 and V 4. The Method B consisted in to clean with distilled water at environmental temperature by circulating through the feed and permeate sides for 20 min by opening valves V 1, V 3 and V 4. This method was adapted from the literature [13]. The Method C consisted in running the system with distilled water at 50ºC through the feed side during 2h. This method also involve a backflush of the permeate. The backshock has the duration (BS) of 0.25s and a frequency (TBBS) of 5 s. By analysis of Figure 3, it is possible to verify that it was not possible to recover the previous performance of the membrane with methods A and B. Although the Method C took more time to clean the membrane surface, it was possible to recover its performance. The hot temperaturee and backshock effect combined could release the foulants within the pores and from the membrane surface. The Method C was adoptedd to clean the membrane for all the experiments perform during this project. With this method it was always possible to recover the previous membrane performance throughout the project. This method was also used for first cleaning to get the maximum performance for the other two membranes used in this project. This method was chosen because it was very difficult and took lots of time to remove the DIVOS solution from within the system. Analytical methods In this study, methods to analyze the collected samples were also investigated. One of methods, Reducing Sugars + Hydrolysiss (RS+H), is an adaption of two methods used in biotechnology [14, 15]. This method is an attempt to find a fast method to evaluate the membrane rejection. The other method consists in using the Size Exclusion High-performance liquid chromatography (SE-HPLC or SEC). Method Reducing Sugars + Hydrolysis This method is a method to quantify the solute concentration and the Mean Molecular Weight in permeate samples in very short time comparing to the Size Exclusion High-performance liquid chromatography (SE-HPLC or SEC). This quantitative strategy is the combination of two methods: hydrolysis of the polymer [15] and estimate reducing sugars in solution [14]. The first stage for this method is to split a sample in two. One part of the sample will be used only to determine the concentration of molecules of Dextran in solution using p-hydroxybenzoic Acid Hydrazide analysis for reducing sugars. The other will be under acid hydrolysis conditions in order to hydrolyze Dextran molecules and afterwards using p-hydroxybenzoic Acid Hydrazide analysis method for reducing sugars will be determined the concentration of D-Glucose. By combining both analytical results, it is possible to determine the Mean Molecular Weight of Dextran in solution. 4

5 140 Water permeability tests Flux (L/m2/h) Transmembrane pressure (bar) Figure 3 - Results of water permeability tests after each cleaning method after Dextran T500 experiment. From the first sample whichh contains the Dextran, the concentration of reducing group will be determined: C D (mol polymer/l). From the second sample which contains Dextran hydrolyzed, the concentration of reducing sugars will be determined (it is considered that Dextran was totally hydrolyzed in D-Glucose molecules): C G (mol Glucose/L). The Mean Molecular Weight of Dextran in solution can be calculated by this equation: = _ + where _ is the molar mass of the repeating unit (162,16 g/mol) and is the molar mass of water. Some experiments with the Reducing Sugars Method were made. Table 2 shows the results of one of these experiments. The calibration curve is (3) Calibration Curve - Dextran T500 represented in Figure 4. From Table 2 it can be verified that all the values of concentrations estimated by the linear regression are larger than the ones that were expected. There are two possible explanations that could explain that the values of absorbance are higher for Dextran samples than for Glucose standards for the same theoretical concentration. The first explanation is related to the time that the samples are inside of the oven. Since there are two different substances on the microplate (Glucose standards and Dextran samples), the time that samples and standards need to be inside the oven may be different for the same concentration due to the reaction time. The second explanation is related to the potential interactions between Dextran and the reactant. The Dextran molecule could split during the analysis. If there is split of polysaccharide chains it could explain the fact of higher values for Dextran samples. Absorbance y = x R² = Glucose E E E E E E E-06 Concentration (M) Figure 4 - Calibration curve for experiment with Dextran T500. 5

6 Table 2 - Results of the assay with Dextran T500. Dextran T500 Theoretical Concentration of Average Concentration Calibration Absolute Relative solution (M) Absorbance Curve (M) Error Error (%) 4.04E E E E E E E E E E E E E E E E E E E E E Additionally, it could exist some interactions between the reactant and other repeating unit beside the reducing group of the chain. This technique is not the most appropriate to determine the Molecular Weight in Dextran solution because not only it does not allow getting a distribution of Molecular Weight in a solution it is also not a very accurate method. Since this method was not the most accurate, all the samples of this project were analyzed by the Size Exclusion High-performance liquid chromatography (SE-HPLC or SEC). Experimental design After the setting up the system and verify that there was an effect on the separation by the backshock, a two layer experimental design was performed. The software used was MODDE 7.0, the model chosen was Response Surface Modelling (RSM) since provides detailed modeling and optimization using quadratic and cubic models. The design chosen was central composite face-centered, CCF, quadratic model, design in 11 runs. For the first set of experiments the variables studied (factors) were transmembrane pressure and retentate flow rate. These parameters were chosen in order to study their effect into the flux through the membrane and fouling at the membrane surface, since both parameters have influence on them. In this case the regression model is: measurement scales of the factors, but have been re-expressed to relate to the coded -1/+1 unit. The Figure 5 represents the model of the first experimental design. From the figure it is possible to see a relevant effect of transmembrane pressure on the increasing permeate flux and the membrane fouling. Although the increase of transmembrane pressure favors an increase of the permeate flux. Variations of the retentate flow rate did not have a substantial influence of the permeate flux, especially at low transmembrane pressure values. That could be ascribable to a limited effect of an increasing turbulence in reducing fouling during filtration. When the transmembrane pressure is low, variations of the retentate flow rate do not affect the permeate flux, probably due to the low levels of fouling and concentration polarization caused by low transmembrane pressure. = ( ) ( ) (4) In this equation, the regression coefficients are scaled and centered. This means that they are no longer expressed according to the original Figure 5 - Model for the first experimental design. 6

7 As the transmembrane pressure increase, the fouling and concentration polarization become more important, and increases in the retentate flow rate can contribute to reduce it, resulting in increases of the permeate flux. The Figure 6 shows the results of the rejection of the membrane from samples of each assay analyzed by HPLC. It can be verified for transmembrane pressure values of 0.2 and 1bar, while the flow rate (F) increases the rejection in general diminishes first and increases afterwards. For the value of 0.6 bar this is not verified. The fact that the rejection diminishes for medium value of flow rate could be explained by two possible explanations: there is a counterbalance effect between the flow and the permeate flux or actually there is no variation by changing the conditions and then, those results are not significant. The first explanation can be explained by the following facts. When there is an increase of the flow rate it is expected that the boundary layer thickness is reduced by the shear at the membrane surface. Consequently, concentration polarization profile does not have very sharp shape. However if the flow rate increases, the flux through the membrane also increases favoring a higher concentration at the membrane surface. Therefore, if the turbulence is not enough to minimize the boundary layer thickness, the concentration at the membrane surface will be still large and the rejection of the membrane will diminish. The second explanation can be explained by the fact that for a small drop (only around 5-10% of variation) that is verified could be related to some variations on the system conditions along the experiments such as the membrane surface, since the experiments were perform according to an order instead of randomly. The second set of experiments was performed under the conditions with which was possible to get highest permeate flux value determined at the first layer of experimental design. The variables studied were: Backshock time and Time of backshock cycle. These parameters were chosen in order to study their influence on the flux through the membrane and material accumulation at the membrane surface, since a backshock system has as main goal to diminish the effect of the concentration polarization and the potential fouling which influences the rejection. In this case the regression model is: = ( ) (5) From Figure 7, it is possible to conclude that when the Time of backshock cycle value is small there is a sharp reduction of the flux while the backshock increases. In fact, while the time backshock increases more of the accumulated volume in the shell will be flushed into the retentate stream. Flow L/h Flow L/h Flow L/h Figure 6 - Membrane rejection for the different assays of the first experimental design. BS corresponds to Backshock time value and TBBS corresponds to the Time of backshock cycle. 7

8 For that reason, the volume accumulate will be drop and it is necessary more time to fill the shell up again. Therefore, if the system does not establish permeate stream quickly, the permeate flux will decrease.on the other hand, when the Time of backshock cycle (TBBS) value is large there is an increment of the flux while the backshock increases. Presumably, the larger is the backshock the concentration profile at the boundary layer will be destroyed more efficiently; it will allow a larger flux through the membrane. In this case, there is no influence by the time necessary to establish the permeate stream since the time between cycles is large and it can be establish easily before the following cycle starts. The Figure 8 shows the results of the rejection of the membrane from the sample of each assay analyzed by HPLC. It can be verified that while the Backshock time (BS) increases the rejection in general also increases. This happens due to the flushing away of the solute which is accumulated at the membrane surface and consequently the disturbance to the boundary layer concentration profile. Therefore, the concentration in permeate flux will be lower. In fact, the rejection increases around 30% from an experiment which there was no backshock for an experiment where the Backshock time was around 1.25 seconds. Regarding the Time of Backshock cycle (TBBS) it can be seen that for cycles of 5 seconds the rejection values are very high and its curvilinear profile is very sharp. This fact is due to the destruction of the concentration profile at the boundary layer and it cannot be fully reestablish before the next cycle starts. Concerning the cycles of 10 and 15 seconds it can be verified that their rejection profile are less sharp. In fact the time of each cycle allows reestablishing the boundary layer and therefore the rejection diminishes for molecules of small Molecular Weight. Figure 7 - Model for the second experimental design. Figure 8 - Membrane rejection for the different assays of the second experimental design. BS corresponds to Backshock time value and TBBS corresponds to the Time of backshock cycle. 8

9 In general the differences on the rejection for molecules of higher molecule weight are not influenced by the time of the backshock cycle (TBBS). Conclusions On the topic of cleaning strategies, several methods with different conditions until determine the most efficient and fast method were tried. In order to evaluate the cleaning of the membrane, the parameter chosen was the water permeability. Two substances recommended to clean membranes of ultrafiltration were used, Sodium Hydroxide and DIVOS D124. These chemical cleanings revealed not to be the best methods since they were not always efficient and it was necessary to wash the chemical from the system, which sometimes had consumed as much time as the cleaning itself. The best cleaning method consisted in running the system for 2 hours with hot Distilled Water (50 C) using the backshock system: Time of backshock system (TBBS) of 5 seconds and Backshock time of 0.25 seconds. During this project an analytical method was founnd which could provide a fast conclusion about the experiments in order to make the decisions for the next set of experiments. The method named Reducing Sugars + Hydrolysis (RS+H) can be considered as inaccurate and inconclusive. Indeed, there was a great disparity between the concentrations experimentally determined and the theoretically known. In fact, the method of Size Exclusion High-Performance Liquid Chromatography (SE-HPLC) revealed to be the most appropriate and accurate to analyze the samples collected from the permeate stream although it is a method which takes too much time to analyze the samples. The analysis of results of the first layer of experimental design reveals that the transmembrane pressure has relevant effect on the increasing permeate flux and the membrane fouling. When the transmembrane pressure is low, variations of the retentate flow rate do not affect the permeate flux, probably due to the low levels of fouling and concentration polarization caused by low transmembrane pressure. At higher transmembrane pressure values, the fouling and concentration polarization become more important, and increases in the retentate flow rate contribute to reduce it, resulting in increases of the permeate flux. Concerning to the rejection, there is a decreasing of the rejection values while the transmembrane pressure increases. This is related to the fact that the increasing of the accumulation of solute at the membrane surface which causes a diffusion of the molecules through the membrane. From the second layer of experimental design it is possible to conclude that when the Time of backshock cycle (TBBS) value is small there is an abrupt reduction of the flux while the backshock increases. This fact is due to the great amount of the accumulated volume in the shell will be flushed into the retentate stream and the time to replenish it again is higher than the Time of backshock cycle (TBBS). On the other hand, when the Time of backshock cycle value is large, the concentration profile at the boundary layer will be destroyed more efficiently which will allow a larger flux through the membrane. Regarding the rejection, it can be concluded the increment of the Backshock time (BS) causes a rejection increase which is related to the flushing away of the accumulated solute at the membrane surface and consequently the disturbance to the boundary layer concentration profile. Another conclusion, the Time of Backshock cycle it can be seen that for the cycles of 5 seconds the rejection values are very high and its curvilinear profile is very sharp, because after the destruction of the boundary layer it is not possible reestablish it before the following cycle. For large cycles it can be concluded that their rejection profile are less sharp due to the fact that the time of each cycle allows reestablishing the boundary layer and therefore the rejection diminishes specially for molecules of small Molecular Weight. In general the differences on the rejection for molecules of higher Molecular Weight are not influenced by the time of the backshock cycle (TBBS). The project provides interesting information to the biotechnology where the filtration of mixture of polysaccharides with different Molecular Weight is a common practice. This information can be used for instance for modeling the membrane process. References [1] Nath, K. (2008) Membrane Separation Processes. 1 st Ed. New Deli: Prentice- Hall of India Private Limited [2] Mulder, M. (1997) Basic Principles of Membrane Technology. 2 nd Ed. Kluwer Academic Publishers [3] Roh, S., H. 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