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Proceedings of the 3rd (2011) CUTSE International Conference Miri, Sarawak, Malaysia, 8-9 Nov, 2011
Dehydration of Ethanol on Zeolite Based Media Using Adsorption Process Fouad R. H. Abdeen, Maizirwan Mel, Maan Al-Khatib and Azlin Suhaida Azmi Department of Biotechnology Engineering International Islamic University Malaysia Kuala Lumpur, Malaysia
[email protected]
Abstract — —Fossil Fossil fuels diminution and the following increase in fuels price have directed researchers towards producing fuel ethanol from biological materials. The main challenge encountered in such production process is the removal of large excess amount of water within the produced ethanol. Distillation, though is an energy extensive process, is usually used to produce ethanol up to 95% purity. Production of higher purity ethanol is usually a major challenge due to the formation of an azeotrope. In this study, a small adsorber bed apparatus was constructed and used to purify ethanol up to 99.5%. The apparatus consists
of fluid delivery system, adsorber bed subsystem, storage and sampling unit and adsorption column where adsorbents like zeolite can be packed. The apparatus is designed to be packed and repacked several times and with various types of adsorbents. 3A zeolites are used as adsorbent materials in this study. 3A zeolites proven to be efficient in removal of water from ethanolwater azeotrope since their pore size is less than 0.3nm which allows only water to adsorb to the inner large surface area of zeolite. An optimization process was performed for the dehydration process manipulating three process parameters, namely; feed concentration, feed flow rate and adsorption temperature. Optimum set was determined to be at 95 % feed concentration, 200 ml/min flow rate and 25 ºC adsorption temperature. Validation of the optimum set resulted in the production of ethanol of purity higher than 99.5% and w ith 91 % efficiency of recovery. Keywords- Ethanol Dehydration, Adsorber Bed, 3A Zeolite Molecular Sieves.
I. INTRODUCTION Ethanol or ethyl alcohol, C2H5OH, is conventionally produced by catalytic hydration of ethylene with sulphuric acid. A process which makes ethanol regarded as a petroleum product. However, the continuous depletion of petroleum has directed ethanol producers towards finding other possibilities for ethanol production. The current most potential way of producing ethanol is the microbial fermentation of agricultural crops and/or wastes. Ethanol (bio-ethanol) is regarded as the most bio-fuel to be used in transportation either as a fuel or as a gasoline enhancer. Consequently, there has been an increasing demand for fuel
ethanol production in several countries worldwide. Different governmental regulations and strategies are being made by dedicating a great concern for the production of fuel ethanol [1]. Several countries including Brazil, United States, Canada, Japan, India, China and Europe have been implementing strategies to increase their market use of fuel ethanol [2]. In the industrial production of ethanol, whether by chemical or biological process routes, the raw product is generally a dilute aqueous solution. Ethanol yield of a biological production process is ofusually 5% toby10% by weight only. Further concentration the ethanol traditional distillation processes usually is used to produce an azeotrope containing about 5% water by weight [3]. The recovery of ethanol to dryness in excess of the azeotropic composition is normally achieved by azeotropic or extractive distillation processes. However, such separation processes are energy intensive [4] [5]. Therefore, there is a high demand for non-distillation methods that economically produce anhydrous fuel ethanol. To produce ethanol at a high level of dryness, adsorption process on zeolite material has proven to be ideal. There have been several researches on adsorption of water from ethanolwater mixture using using zeolite m media. edia. Most of these rresearchers esearchers studied the adsorption of ethanol in vapor phase and/or liquid phase using some common commercialized zeolite material [6]. The effects of feed flow rate, feed concentration and adsorption temperature are among the interesting factors that are examined. In fact, dehydration by adsorption on 3Å zeolite is known to have the advantage that the micropores are too small to be penetrated by alcohol molecules. Thus, water, in the water ethanol mixture, is adsorbed without being competed with the ethanol molecules in the liquid phase [7]. Therefore, dehydration of ethanol by adsorption on 3Å zeolite requires little energy input compared to other methods [8]. The aforemen aforementioned tioned dehy dehydration dration process is believed to have high adsorbent productivity and is often capable of producing very pure product [9].
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In fact several studies have emphasized the fact that 3Å zeolite is efficient in dehydrating ethanol [7] [8] [10]. However, only few studies have highlighted the effect of different controlling parameters in the packed column system on the dehydration process. Nevertheless, it is well recognized scientifically and practically that different sets of process parameters will yield ethanol with different purities or produce pure ethanol with different efficiencies. Hence, there is a real need for evaluating the different performance of an adsorber apparatus using different set of controlling paramete parameters. rs. This study presents the dehydration process as an alternative to the existing conventional methods. This study also aims to determine the actual effects of different operating parameters on the efficiency of adsorption of water on 3Å zeolite through experimental works system mainly in terms of product recovery and enrichment. II. THEORETICAL BACKGROUND In the theoretical background of the dehydration of ethanol, it is important to examine studies concerning ethanol water separation techniques, molecular sieves technology, fixed bed adsorption, adsorbers design and fabrication, and different adsorption processes used for ethanol dehydration. A. Ethanol Water Separation Techniques
Ethanolwith water is usually done using processes eachseparation process being recommended at a different specific composition. The very first ethanol concentrati concentration on process after completion of fermentation is usually done by normal distillation. However, after concentrating ethanol up to 95 % an azeotrope is formed. Formation of azeotrope forced ethanol producers to work for finding other customized distillation processes or processes other than distillation. Distillation is the most widely used technique to separate mixtures of liquids, the separation being based on the difference in boiling temperature of the components. Consecutive heating up and condensing of the mixture will make the component of higher boiling point more concentrated in the liquid while the component of lower boiling point is more concentrated in the vapor [11]. Distillation as such cannot be used for the separation of azeotropes as they the same in theprocess vapor andis liquid phase. A have special type composition of distillation conventionally used to separate components of azeotropes. A process which is usually called extractive distillation or azeotropic distillation. Such extractive distillation processes make use of a third component called entrainer and is well known of having several unfavorable side effects [12] [13]. Azeotropic distillation is one of the common methods used in ethanol-water azeotrope breaking. In azeotropic distillation, a distillation column is designed in a way that allows the solvent used to be fed over the feed tray [14]. The bottom product then can be collected as a dry ethanol. It is worth mentioning here that selecting a good entrainer is very important to conserve energy. A nonvolatile solvent, which is
used in the azeotropic distillation process, usually appears in the bottom product [15]. Another process also used is the salt extractive distillation. In this process salt is used as a non-volatile material which is fed near the top tray of the distillation column [16]. Solution flows then downward the distillation column allowing salt to be removed as the bottom product. However, on top of the column, dry ethanol can be collected [17]. In fact, azeotropic distillation is a very energy-consuming process and the use of an entrainer might cause unwanted impurity in the product side streams. Therefore, other techniques should be and considered to overcome these disadvantages. Pervaporation is introduced as an alternative to extractive distillation to separate azeotropes. It is a combination of membrane permeation and evaporation. It has the advantage to be less energy consuming and more environmental friendly than distillation [18]. In pervaporation the separation is not based on the relative volatility of the components in the mixture, but only depends on the relative affinity of the components for the membrane [19]. The driving force of the pervaporation process is the pressure difference that is usually over the membrane. Such pressure difference is applied by keeping the feed side of the membrane under atmospheric or higher pressures while applying a vacuum on the permeate side of the membrane. The main principle here is that the partial pressure of permeating matter is maintained lower than the equilibrium vapor pressure. Hence, components permeating through membrane will evaporate [18]. Pervaporation membranes vary according to the material they are fabricated from. Two main types of pervaporation membrane are, organic membranes, also called polymeric membranes, and inorganic membranes, also called ceramic membranes. A key disadvantage of the polymeric membranes is their lack of solvent and temperature stability [20] [20] [21]. A number of water-permeable polyvinyl alcohol membrane modules compose the pervaporator. Various materials were suggested by researchers to be used as pervaporators such as polyacrylic acid, amine copolymers, polyaniline, and others [22]. Although pervaporation is considered as an energy efficient method for dewatering ethanol, it still needs some energy when applying the pressure and the vacuum pressure. In addition, it is also costly in terms of membrane fabrication. Hence there should be a better method to dehydrate ethanol other than pervaporation. Another important separation technique usually used to separate fluid mixtures is adsorption. As early as the eighteenth century, varieties of porous solids have been recognized for their ability to reversibly adsorb large volumes of vapor and liquid [23]. The process by which a component of a mixture is being adsorbed to a surface and then washed away is technically called adsorption. Compared to distillation, which
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is often used for bulk separation; adsorption is usually used for removal of trace impurities. In the ethanol-water azeotrope separation process, adsorption has been used in many different ways. One of the most common adsorption processes used to dehydrate ethanol was used on 3Å molecular sieves suggested first by [24]. In the aforementioned study, the researcher has described only one adsorption process; using zeolite molecular sieves to selectively remove water from aqueous ethanol. Other researchers have used adsorption process on various
always shown that both compounds are highly adsorbed to ordinary adsorbents [38]. Competition between ethanol and water in adsorption to standard adsorbents has directed researchers towards finding materials that can preferentially adsorb water without ethanol in order to dehydrate ethanol. One of the most potential materials for the aforementioned purpose is 3Å molecular sieves since it has a very small pore diameter (3 angstrom) [6]. There are few citations on the adsorption of water and ethanol on 3Å zeolites (a type of 3Å molecular sieves).
materials including [25], molecular crystalline sieves silica polymorph (Silicate)activated [26] [27],alumina and carbon [28]. However, most of the recent researches have shown great interest in using 3Å zeolite molecular sieves for ethanol dehydration [5] [6] [8] [10] [29]. One of the most common processes used to dehydrate ethanol is the pressure swing adsorption (PSA) process. This process has been followed by researchers utilizing a 3 Ǻ molecular sieve that preferentially adsorbs water [30] [31]. The PSA process is usually performed utilizing an adsorber bed column which is usually referred to as packed adsorption process [32]. The packed bed adsorption process starts with the production (adsorption) step followed by the regeneration (desorption) step. Adsorption is carried out first which ends with producing ethanol with lower water concentration. This
However, of researches done fuel in this areaethanol have proven the ability of some 3Å zeolites to produce grade in either gaseous phase [7] [8] or liquid phase [29]. The special feature that 3Å zeolite are privileged after is being able to separate ethanol form water based on the differences in molecular size as well as polarity. It is worth mentioning here that although water and ethanol are both highly polar compounds, water is slightly more polar than ethanol [39]. The molecular diameters of water and ethanol are 2.6Å and 5.2Å, respectively [40]. Since 3Å zeolite are 3 Angstrom in diameter, the smaller 2.6Å water molecules will be adsorbed the inner pores of the 3Å zeolites while the larger 5.2Å ethanol molecules will not penetrate the zeolite pores. Hence, 3Å zeolite is believed to have great ability to adsorb the smaller and more polar water molecules in the water-ethanol mixture.
process isbedfollowed by the regeneration (desorption) adsorber which aims at preparing the bed to be usedofforthea second adsorption process [31] [33].
C. Apparatus Designed as Fixed Bed Adsorbers Adsorbers
The liquid phase adsorption with a 3Å zeolite was investigated experimentally by [10]. Ethanol can also be dehydrated by adsorption in the vapor phase with solid agents such as cornmeal, cellulose or cornstarch [34]. B. Molecular Sieves Zeolite Technology
The term molecular sieves is used to describe microporous solids which are used as adsorbents in fluids purification and separation processes. The major and most abundant class of microporous sorbents is zeolites. Zeolites have been introduced to the industry of separation and purification since the year 1954. Zeolites are known as crystalline porous materials that can have customized properties by varying their structure and composition [35]. Compared to existing other adsorptive zeolites are regarded as the only crystallinematerials, materials with an organized microporous structure. The presence of aluminium in the structure granted zeolite unique features such as having strong acid sites at their surface [36]. Since early 1950s, Zeolites have been used in different applications including air conditioning, refrigerating, and laundry detergents as well as other separation and purification processes in chemical and petrochemical industries [37]. By 1960s zeolites have been extensively used in several natural gas drying applications [35]. The main problem faced when breaking water-ethanol azeotrope by adsorption on molecular sieves that ethanol and water are both highly polar compounds. Thus it has been
Adsorption is usually performed using different processes and different types of adsorbers. The determination of the type of the adsorption process to be used decides the various features of the adsorber apparatus. For instance, in a temperature swing adsorption process it is usually needed to provide the adsorber apparatus with a heater [41]. Design of fixed bed adsorbers is controlled by the fact that the adsorber should be able to perform the two main steps in each adsorption process namely; adsorption step and regeneration step [42]. In any adsorption process, whether pressure swing adsorption (PSA) or temperature swing adsorption (TSA), an adsorption bed column is needed. The adsorption bed column contains the adsorptive material through which the feed fluid passes [43]. In this study, it is required to design an adsorber apparatus for ethanol dehydration. Many researchers have shown that it is possible for an ethanol dehydration process to be performed using adsorber apparatus which consists mainly of the following parts [7] [8]: 1. Fluid delivery system: This consists of a gas cylinder to flush the system with, either nitrogen or carbon dioxide, an ethanol storage tank and a number of flow controllers and pumps. 2. Adsorber bed column: which is used to contain the adsorptive material and through which the ethanol passes. 3. And a product storage and sampling unit.
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Adsorber bed column is the essential part over which researchers differ in their designs. In most of the cases the adsorber column were always designed by maintaining the length to diameter ratio more than 10:1 [7] [8] [29]. According to [29], which is one of the recent studies in this field has used an adsorber column of 915 mm long and 50.7 mm in diameter. In another study done by [7], an adsorber column of 597 mm long and 48.2 mm in diameter has been used. The adsorber column is usually provided with two layers of glass beads placed below and above the zeolite material to ensure proper flow distribution [29]. Other design considerations such as pipes and fittings diameter and tanks capacities can vary with respect to the column size. Size can also be determined by calculating the maximum bed capacity and maximum product to be purified by each run. D. Adsorption Processes for Ethanol Dehydration Dehydration
Fixed bed adsorption for ethanol dehydration is usually carried out as either a temperature swing adsorption (TSA) or a pressure swing adsorption (PSA). Both processes, PSA and TSA consist of the two main steps adsorption and desorption. Nevertheless, PSA and TSA differ mainly in the driving force used in the desorption/reg desorption/regeneration eneration step [44]. In a PSA process both adsorption and regeneration steps are usually carried out under constant temperature. As for the PSA separation process, it is reported that researchers have been using relatively high pressure during the adsorption and a lower or vacuum duringpressure, the regeneration steppressure [45]. in some cases, is employed On the other hand, TSA process operates at constant pressure (atmospheric pressure) during adsorption and regeneration. And in the same analogous way to the aforementioned PSA it employs low temperatures during adsorption step and higher temperatures during the regeneration step [46]. Ethanol dehydration process involves the removal of water as impurity from ethanol. Water adsorbed to adsorbents in the adsorption step should be later desorbed in the regeneration step. In practical applications, both PSA and TSA processes have been used for the purpose of ethanol dehydration. In the PSA process water is adsorbed at higher pressure and desorbed at lower/vacuum pressure. Whereas in TSA water is adsorbed at lower temperature and desorbed at higher temperature. PSA has been employed for ethanol dehydration purpose by many researchers. PSA has been reviewed in the works of a wide number of researchers such as [5], [10] and [31]. In addition to the work mentioned previously, PSA has been employed for ethanol dehydration by [7] and [47]. Reference [7] has used PSA for ethanol dehydration under the following conditions: 1. Regeneratio Regenerationn step: zeolite materials have been regenerated at a temperature in the range of 220-240 ºC. Absolute pressure is set in the range 6-10 kpa and nitrogen flow rate is 200 ml/min. Regeneration continues until water is no longer detected in the effluent composition.
2. Adsorption step: carried out in the vapor phase at a temperature of 167 ºC and a pressure of 65 psia. The flow rate was manipulated in the range of 13-40 g/hr. The whole PSA process took 30 hours. The general PSA processes are usually carried out in the same steps mentioned above with slight changes in the adsorption and desorption/regeneration conditions. TSA is usually performed in the same steps also but with pressure kept constant during the whole adsorption process. PSA has been claimed by many researchers to be energy efficient since itusually involvesheating no heating. However, in thetime actual PSA process is needed at the of regeneration. Many researchers have proven that it is necessary to heat up the molecular sieves at the time of regeneration [30] [36]. Thus, PSA cannot be considered to be more energy saving than TSA process since both processes include heating in the regeneration step. TSA is verily preferred in the ethanol dehydration process due to the fact that it is more suitable to regenerate/desorb strongly adsorbed components. It has been proven by researchers that a slight change in the temperature results in a large change in the equilibrium of liquid/gas-solid adsorption [7]. Hence, in the ethanol dehydration process it is recommended to use a TSA process to ensure a complete and efficient regeneration for the adsorbents used. III. MATERIALS AND METHODS Before beginning with the experimental work, all materials and chemicals were made ready. The main materials required for these studies are; 3Å zeolite materials, ethanol of different concentrations, adsorber bed apparatus items and hydrometer. In addition, some basic simple materials are also required such as glassware (including flasks, volumeter), syringes and ethanol storage containers. The following lines discuss the main materials used in this study as well as methods followed. A. Materials and Chemical Preparation
Since the concern of this study is to dehydrate ethanol to have less than 1% water content, the feed ethanol used in this study was a mixture of water and ethanol only. Pure ethanol of 99.5% was ordered from industry prior to preparation of ethanol of different purities. This pure ethanol was diluted to prepare ethanol of two different concentrations. The lower feed ethanol concentration was 85 % while the higher feed ethanol concentration was 95 %. A hydrometer is an instrument that is usually used to measure density of liquids. A hydromete hydrometerr is conventi conventionally onally made of glass cylindrical stem having a bulb at the bottom. The bulb is usually filled with mercury or lead shot to enable it to float in the upright position. The main principle hydrometers are based on is that the weight of floating body is equal to the quantity of liquid it displaces [48]. Hydrometers usually have graduated scale that facilitates the reading of the concentration of the liquid being examined. In this study an ethanol concentration hydrometer was ordered and used to monitor the purity of produced ethanol by each run. Compared to other methods such as gas chromatography,
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hydrometer use is much easier, faster and still can give precise concentration of the ethanol-water liquid mixtures. Prior to hydrometer use in monitoring produced ethanol, several samples of ethanol-water mixtures of different composition were prepared by diluting 99.5 % ethanol. The hydrometer has shown to be very precise in reading 0.5 % difference of the ethanol concentration in the ethanol-water mixture. The most important material of concern in this study is zeolite material to be packed in the adsorption column. There
Glass beads are usually placed on top and in the bottom of the column to ensure flow distribution.
When fabricating adsorption column, the length to diameter ratio should be at least 10:1 (L/D 10).
•
•
≥
Following are the equations used to perform some calculations to determine the different features of the adsorber bed apparatus.
m
V = bed
are a variety of zeolites which usually in separation processes. However, it has beenare proven thatused the most suitable zeolite materials to be used for the purpose of dehydrating ethanol are 3Å zeolites. This type of zeolites has 3 angstrom pore size which allows the adsorption of the small water molecules (2.6 Å) without competition from the larger ethanol molecules (5.2 Å) [40]. TABLE I.
ZEOLITE PROPERTIES AS PROVIDED BY THE MANUFACTURER
Typical Properties
Unit
Value
mm
Bead Size Bulk Density *Equilibrium Water Adsorption Capacity ( C )
1.6-2.5
Kg/m g/m
≥700
%wt (g of water/g of adsorbent)
≥21
eq
* C is measured as gram of water adsorbed per gram of adsorbent eq
In this study, zeolite Materials has been ordered from Jianlong Chemical, China. Properties of zeolite materials are shown in Table I. Ordered zeolites are classified as 1.6-2.5 mm diameter 3Å bead zeolites. Table I shows that bulk density of 3Å zeolites used in the study is about 700 Kg/m3 and that its water adsorption capacity is 21 wt%. These two properties of zeolites were used in some calculations involving bed capacity and amount of ethanol produced per run. B. Equations and Design Considerations
The design and fabrication of any apparatus is usually controlled by a set of equations and considerations. From the literature made in this study the most important considerations used in the design and fabrication of an adsorber bed apparatus are the following. •
The apparatus should have all necessary items such as fluid deliver system, adsorber columns, storage tanks, valves and pipes.
•
When using temperature swing adsorption (TSA) as an adsorption process, band heaters are suitable to be used to jacket the column. In addition, the pipe line in contact with the hot surface should be made from material that can stand high temperature such as stainless steel.
•
To control flow rate easily it is recommended to use a sensitive flow meter that can detect small flows.
= ρ bed V bed
bed
(1)
Length x Cross Sectional Area
C = C eq mbed
(2) (3)
Where m is the mass of the zeolite used in the adsorption experiment ρ bed is the bulk density of the zeolite used in adsorption bed
V bed is the volume of
the adsorption column
C is adsorber bed capacity measured as mass of water adsorbed by bed C eq is
equilibrium water adsorption capacity
Another equation required to perform calculations is an equation finding mass of ethanol-w ethanol-water ater mixture to the be used prior to full saturation of feed the zeolite materials used in bed column. This mass can be calculated by dividing mass of water the bed can adsorb by the percentage of water in the ethanol water mixture. Equation 3.4 provides the needed relation. mf.ethanol = C ÷ ÷ Water%
(4)
Where mf.ethanol is
per run
maximum mass of ethanol-water mixture used
Water% is percentage of water in the ethanol-water mixture used as feed C. Experimental Procedure
After all calculations were made, the first step in the experimental procedure was to pack the column with the zeolite material. Before adding the zeolite material, layers of 2 mm glass beads were placed bellow and above the adsorbent to ensure good flow distribution in the adsorption step and to control bed height height [29]. Initially, the lower and tthe he upper layers of the glass beads were 100 mm and 62 mm deep, respectively,, as shown in Figure 1. respectively The zeolite packing required 3 steps of processes. First 100 mm depth of glass beads were placed at the bottom of the column, second the amount of zeolite was added, and finally a 62 mm depth of glass beads were added at the top. Once, packing was completed, the experimental procedure was carried out in two main steps:
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Regeneration step: In this step, the bed temperature was raised to 200°C and a small flow of nitrogen was used to purge the adsorber bed. This step was carried out under atmospheric pressure and nitrogen was flowing at 200 ml/min.
Since normal distillation processes can purify ethanol up to 95%, this study focuses on varying feed concentration in the range of 85-95 wt% ethanol. As for feed flow rate and adsorption temperature, they were varied in the ranges of 200-600 ml/min and 25-50 ºC, respectively. The aforementioned parameters were manipulated to optimize the adsorption process. Ethanol purity and efficiency of recovery are the two process responses in the optimization study. D. Data Analysis
Analysis of data was carried out by monitoring produced ethanol purity and calculating efficiency of recovery. The ethanol purity was measured using hydrometer which is designed to read ethanol concentration in the ethanol-water mixture. However, efficiency of recovery was calculated using (5). Eff R =
V c EtOH conc. × (V f − V d )
(5)
Where EffR is efficiency of recovery measured in percentage EtOH conc. is
Figure 1: Adsorber Bed Column •
percentage
Adsorption step: The adsorber bed was fully
regenerated before The bed temperature wasthesetstart withoftheeach helpexperiment. of band heaters surrounding the column and the heater in the ethanol feed tank. After that an ethanol-water mixture was flowing through the adsorber bed in the column. Once this is achieved, an adsorption step was completed. Another regeneration step will start. The above steps were repeated cyclically for different sets of adsorption process parameters (feed concentration, feed flow rate and adsorption temperature) as shown in Table II. TABLE II.
concentration of feed ethanol measured in
SET OF EXPERIMENTS PERFORMED WITH DIFFERENT PARAMETERS Parameter Adsorption Temperature
Run
Feed Concentration (wt %)
Feed Flow Rate (ml/min)
1
85
200
25
2
85
200
50
3
85
600
25
4
85
600
50
5
95
200
25
6
95
200
50
7
95
600
25
8
95
600
50
(ºC)
V is
volume of pure ethanol collected at the end of the dehydration process (litres) c
V f is
volume of feed ethanol-water ethanol-water mixture (litres)
V d is
volume of ethanol-water mixture drained from the adsorber column (litres) E. Statistical Analysis and Optimization Method
Statistical analysis of this study is performed by manipulating three process parameters to optimize the performance of the fabricated adsorber apparatus. The three parameters namely are; feed concentration, feed flow rate and adsorption temperature. Each parameter has been used in two levels, high and low. The different parameter variations are shown in Table II. The analysis of the process was carried out using Design Expert v6 Software. Two-Level Factorial Design was used to design the experimen experiment.t. Three parameters of the adsorption process were analyzed within a certain range divided into 2 values. In other words, the design was held with 3-factors 2levels category which consists fully of 8 runs as shown in Table II. From every experiment, the ethanol purity (response 1) and efficiency of recovery (response 2) were measured and calculated. Data was analyzed using the statistical analysis features of the Design Expert v6 software.
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IV. RESULTS AND DISCUSSION
A. Adsorber Bed Apparatus Design and Fabrication Fabrication
Adsorber bed apparatus fabricated in this study was designed and used for producing anhydrous fuel ethanol. In conjunction with this, the key factors affecting the ethanol dehydration (water adsorption) process were varied. In this study three main parameters affecting adsorption process, namely; feed concentration, feed flow rate and adsorption temperature were manipulated. Therefore it is required to design adsorber bed apparatus that enables varying the three abovementioned parameters. Feed concentration can be manipulated easily by preparing ethanol of different concentrations to supply to the feed storage tank. However, varying feed flow rate and adsorption temperature requires mass flow controllers and heaters provided to both storage tank and adsorption column. The most important item and the first to be fabricated in the adsorber bed apparatus is the adsorption column. As mentioned earlier in this study, the length to diameter ratio should not be less than 10:1 (L/D 10). The column used in the present study is 762 mm long 50.8 mm in diameter made of 304 stainless steel material. Figure 1 shows the dimensions of the fabricated column. In order to determine the characteristics and sizes constraints of the adsorber bed apparatus different items some ≥
calculations wereofmade. All calculations were made (1)in– (5). The results the important calculated terms areusing shown Table III. TABLE III.
IMPORTANT CALCULATED TERMS Percentage of Ethanol in Feed Mixture
Term
V
bed
m
85 %
(litres)
bed
95 %
The system also contains: • Two electric diaphragm pumps: the first was used to pump out the low concentration ethanol from the sample tank and give the flow pressure through the adsorption column to the product tank. The second was used for recycling the product in the case of not achieving targeted purity. • Tube fittings and connections: all tube fittings and connections were 1 '' of 304 Stainless Steel. 4
B. Ethanol Purity and Efficiency of Recovery Results
Responses of concern in this study have been determined as ethanol purity and efficiency of recovery. Ethanol purity is a measure of the ethanol percentage in the product ethanol. On the other hand, efficiency of recovery is a measure of how efficient is the adsorption process in recovering ethanol from water-ethanol mixture using the fabricated adsorber apparatus.
1.2161
(Kg)
Figure 2 shows that the apparatus consists mainly of the three following subsystems: 1. Fluid delivery system: This consists of nitrogen and ethanol-water mixture reservoir with heater. Nitrogen was supplied by nitrogen cylinder tanks (for the purpose of flushing the system during regeneration). Ethanol-water mixture was stored in stainless steel tanks provided with a heater so that ethanol can be heated and purified at different temperatures. The size of the tank is 5 litres since the maximum amount of ethanol produced per run does not exceed 4.78 litres based on calculations. The flow of nitrog nitrogen en and ethan ethanol-water ol-water mixtu mixture re was controlled by mass flow controllers. 2. Adsorber bed: The bed was construct constructed ed using a 762 mm long 50.8 mm in diameter 304 stainless steel cylinder (which can be packed with different materials several times). The wall thickness of the cylinder was 6 mm. Two band heaters of a power of 3 kW were attached around the cylinder to set and maintain the temperature in the adsorber bed. 3. Product storage and sampling unit: The produced ethanol was kept in a stainless steel tank (5 litre in capacity) and sampling valve was employed to collect the sample at regular intervals during the adsorption experiments to analyze them using hydrometer.
0.8513
C (Kg of water)
0.1788 V f (litres)
1.33
4.78
Volume of pure EtOH (litres)
1.563
5.03
Calculations have shown that the maximum amount of ethanol to be purified in each run is 4.78 litres. Therefore, feed and product containers will not be more than 5 litres in capacity. Based on the previous calculations and literature review, an adsorber bed apparatus has been fabricated as shown in Figure 2. Adsorber bed apparatus shown in Figure 2 was fabricated with the help ENK Bioscience Sdn. Bhd.
Ethanol purity has been determined using a hydrometer designed to read the ethanol concentration in the ethanol-water mixture. The results obtained for each of the 8 runs is shown in Table IV. From the table it is obvious that run 1 and run 5 have both produced ethanol of purity 99.5%. Therefore, the second objective of this study has been achieved. Data presented in Table IV show that ethanol purity has varied in the range 97 - 99.5 % when using 85 % ethanol as feed ethanol-water mixture. Moreover, the range of ethanol purity results was 97.5 % - 99.5 % when using 95 % ethanol as feed. Both of the aforementioned ranges are close to each other and can give a wider overall range of 97 – 99.5 %.
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Figure 2: Ethanol adsorber bed apparatus for ethanol dehydration.
TABLE IV.
ETHANOL PURITY AND EFFICIENCY OF RECOVERY RESULTS Parameter
Resp. 1
Resp. 2
Feed Conc. (wt %)
Feed Flow Rate (ml/min)
Ads. Temp.
EtOH Purity
EtOH Purity
(ºC)
(wt %)
(wt %)
1
85
200
25
99.5
2
85
200
50
97.5
79
3
85
600
25
98.5
77
Run
*Eff R R (%)
4 5
85 95
600 200
50 25
97 99.5
83 82
6
95
200
50
99
91
7
95
600
25
98
89
8
95
600
50
97.5
94
*EffR stands for efficiency of recovery ca lculated using (5)
Efficiency of recovery is a measure of how efficient is the adsorber bed apparatus and the adsorption procedure in recovering pure ethanol from ethanol-w ethanol-water ater mixture. Efficiency of recovery was calculated using (5).
Table IV shows that efficiency of recovery ranges from 7794 %. When examining results according to purity of feed ethanol used it can be noticed that 85 % feed ethanol is much less in efficiency of recovery than 95 % feed ethanol. Efficiency of recovery using 85 % feed ethanol has ranged from 77-83 %. On the other hand efficiency of recovery using 95 % feed ethanol had a relatively higher range of 89-94 %. The difference in efficiency of recovery between 85 % feed ethanol and 95 % feed ethanol runs might be due the less amount of feed ethanol used in the case of 85 % feed ethanol. The volume of 85 % feed ethanol used per run was usually two litres; whereas, the volume of 95 % feed ethanol used per run is 5 litres. C. Statistical Analysis and Optimization Results
Statistical analysis was performed by keying in all the data in Table IV into 3-factors 2-level full factorial design using the Design Expert v6 software. The effect of each process factor on the two responses of interest was noticed and recorded in Table V. Table V summarizes the effect of each process parameter on each response. It is obvious that the effect of feed concentration and adsorption temperature on efficiency of recovery has been determined to be in conformity with their effect on ethanol purity. Since feed concentration is directly related to ethanol purity and efficiency of recovery, and adsorption temperature is inversely related to both of them as well. However, feed flow rate has been proven to be inversely related to the more important response (ethanol purity) while being directly related to the relatively less important response
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(efficiency of recovery). Thus, running an experiment using feed ethanol of higher purities has a positive effect on both, ethanol purity and efficiency of recovery. TABLE V. PARAMETERS EFFECT ON ETHANOL PURITY AND EFFICIENCY OF RECOVERY Parameter
Feed Concentration
Number
Feed Conc.
Flow Rate
Temp.
Eth. Purity
Eff R
1
94.71
196.69
25.40
99.5
90.4691
Effect on Efficiency of recovery
Directly related
Directly related
Effect reduces at low temperature due to interaction Inversely related
No significant interaction
2
94.74
176.63
28.90
99.5001
90.1124
Directly related
3
95.00
158.12
32.85
99.4998
90.0067
No significant interaction Inversely related
No significant interaction Inversely related
4
94.94
162.76
31.82
99.4998
90.0418
5
94.50
189.21
26.47
99.5002
90.0885
Effect reduces at high feed concentration due to interaction
No significant interaction
6
94.84
171.20
30.06
99.5
90.115
7
94.31
197.16
25.06
99.5002
90.0225
8
94.64
190.39
26.43
99.4998
90.2748
As for the optimization results, optimization criteria have been set in the Design Expert v6 software by indicating the range for each of the aforementioned process items (two responses and three parameters) and by indicating the weight for each process response. Ranges of process parameters were not very different from those shown in Table VI. TABLE VI. CONSTRAINTS MADE FOR OPTIMIZATION PROCESS USING DESIGN EXPERT V6 SOFTWARE Name
Goal
Lower Limit
Upper Limit
Importance
Feed
is in
90
95
3
Conc.
range
Flow
is in
100
500
3
Rate
range
Temp.
is in
25
40
3
BasedExpert on the criteria and constraints, Design v6 aforementioned software has generated eight possible solutions. All solutions are shown in Table VII which indicates that the most applicable solution is solution number 7. The chosen solution, with approximately 95 % feed concentration, 200 ml/min flow rate and 25 ºC adsorption temperature, is exactly identical to the set of parameters values used in run number 5 as indicated in Table IV. According to the results generated by the software, this set of parameters (indicated in solution number 7) is predicted to produce ethanol of 99.5 % purity and at an efficiency of recovery up to 90 %. This predicted result is also approximately similar to that obtained in run 5 as shown in Table IV. D. Validation of Optimized Parameters
range 99.3
100
5
Purity
is target = 99.5
EffR
is in
90
100
3
Eth.
TABLE VII. SOLUTIONS OBTAINED FROM OPTIMIZATION PROCESS USING DESIGN EXPERT V6 SOFTWARE
Effect on Ethanol Purity
Flow Rate
Adsorption Temperature
constraints made for the optimization process are shown in Table VI.
range
On the other hand, as for the ethanol purity response the target has been set to be in the narrow range of 99.3%-100%. In the same way efficiency of recovery target has been set to be in the range of 90%-100%. A higher weight of five stars has been given to ethanol purity response. Whereas, a three stars weight was given to the efficiency of recovery response. The
Statistical analysis and optimization process results have shown that parameters set of 95 % feed concentration, 200 ml/min flow rate and 25 ºC adsorption temperature, is the optimum set for the ethanol dehydration process. This set of parameters is believed (once repeated) to produce ethanol of 99.5% purity and at an efficiency of 90 % and more. To validate the results obtained in the optimization process, the optimum set has been repeated three more times under same environmental conditions. The results of ethanol purity and efficiency of recovery of the three validation runs are scheduled in Table VIII.
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TABLE VIII. ETHANOL PURITY AND EFFICIENCY OF RECOVERY RESULTS (VALIDATION RUNS) Parameter
Resp. 1
Resp. 2 Ethanol Recovery Efficiency (Eff R R) (%)
Feed Conc. (wt %)
Feed Flow Rate (ml/ min)
Ads. Temp.
EtOH Purity
(ºC)
(wt %)
1
95
200
25
99.5
90
2
95
200
25
99.5
88
3
95
200
25
99.5
91
Run
The results shown in Table VIII demonstrate that the optimum set decided by the optimization process is applicable and repeatable. Repeating the process three different times have resulted in the same values for both ethanol purity and efficiency of recovery. The three runs have shown that ethanol purity is maintained at 99.5%. However, there is an insignificant change in the efficiency of recovery value. V. CONCLUSIONS AND RECOMMENDATIONS adsorber apparatusethanol. was designed and fabricated for the An purpose of bed dehydrating The apparatus consists mainly of a fluid delivery system; adsorption column jacketed with band heaters and storage tanks. The fabricated apparatus has performed as a good adsorber bed apparatus for ethanol dehydration purposes. It is also believed that the fabricated apparatus can be used for various other dehydration processes. Having an adsorption column jacketed with band heaters, the adsorber bed apparatus is capable of dehydrating any liquid with the aid of any adsorptive material. In other words, by changing the type of adsorptive materials used in this study (zeolite), it is possible to use the aforementioned apparatus for different applications other than ethanol dehydration. It will be possible to use the fabricated apparatus for the purpose of dehydrating oils for example. Moreover, the results have shown that the fabricated apparatus performed as anupexcellent dehydrator. Ethanol hashas been concentrated to 99.5 %ethanol purity and with an efficiency of recovery up to 91 % applying the TSA (temperature swing adsorption) using the fabricated apparatus. In addition, experiments have proven that the fabricated adsorber bed apparatus has successfully promoted the use of zeolite materials several times by the periodical regeneration step after each adsorption step. Statistical analysis has shown that three process parameters (feed concentration, feed flow rate and adsorption temperatur temperature) e) are all significant for both responses; ethanol purity and efficiency of recovery. Statistical analysis has also shown the different effects of the different parameters on the two responses of interest as summarized in Table V.
Optimization process was performed using the Design Expert v6 software. Optimum set of parameters was determined to be at feed concentration of 95%, a flow rate of 200 ml/min and an adsorption temperature of 25 ºC. As for validation of results, optimum set was repeated three times. The three validation runs has proven that, at the aforementioned set of parameters, it is possible to maintain ethanol purity at 99.5% and efficiency of recovery up to 91%. For further improvements and optimizations in the performance of any adsorber bed apparatus, it will be useful to have more control over the feed flow rate. More control over feed flow rate would be obtained using a miniature pump instead of the diaphragm pump. Miniature pumps help providing the adsorption column with a slow and steady flow which increases the purity of the product and the efficiency of recovery. As for the ethanol dehydration process, many improvements can be made. Based on the results obtained by this study, it is advisable that interaction between adsorption temperature and feed concentration should be studied in more depth. This could be achieved by manipulating process parameters at a wider range of adsorption temperature and feed concentration. Also, it is strongly recommended that dehydration of ethanol using adsorption process on zeolite materials should be performed for feed concentration not less than 90%. This T his is due to the very low efficiency of recovery values accompanied with the low feed concentration. ACKNOWLEDGMENT We would like to thank all the staff of the Kulliyyah of Engineering of the IIUM for the helpful preparation of all necessary laboratories to carry out this study. Special thanks are due to Sukiman Sengat and Ikhwan Khuzairi for their technical assistance. Special thanks are also due to ENK Bioscience for their help and useful cooperation in the development and fabrication of the adsorber bed apparatus. And finally a warm gratitude is due to the Research Management Center in the International Islamic University Malaysia for their financial support in funding this project. REFERENCES [1] Prasad, S., Singh, A., and Joshi, H. C. (2007). Ethanol as an alternative fuel from agricultural, industrial and urban residues. Resources, Conservation and Recycling, 50(1), 1-39. [2] Mussatto et al. (2010). Technological trends, global market, and challenges of bio-ethanol production. Biotechnology Advances, 28, 817–830. [3] Ginder, H., and William, F. (1983). Method of removing water from ethanol. United states. Retrieved from http://www.freepatentsonline.co http://www .freepatentsonline.com/4407662 m/4407662.html. .html. [4] Carmo, M. J., Adeodato, M. G., Moreira, A. M., Parente, E. J., and Vieira, R. S. (2004). Kinetic and thermodynamic study on the liquid phase adsorption by starchy materials in the alcohol-water system. Adsorption, 10(3), 211-218. [5] Sowerby, B., and Crittenden, B. D. (2001). An experimental comparison of type A molecular sieves for drying the ethanol-water azeotrope. Gas Separation and Purification, 2(2), 77-83.
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