Decolorisation of Wastewater By Adsorption On Activated Carbon In A Liquid Solid Fluidized Bed
One of the major problems concerning textile wastewater is colored effluent. The discharge of color waste is not only damaging to the aesthetic nature of the receiving streams but also toxic aquatic life. In addition, color interferes with the transmission of sunlight into the stream and therefore reduces photosynthetic action. The color in the effluent is mainly due to unfixed dye. The concentration of unused dyes in the effluent depends upon the nature of dyes and dyeing process underway at the time (McMullan, et al., 2001). Inefficiency of dyeing process results in 10-25 % of all dye stuffs being lost directly to the wastewater (Perineau, et al., 1982). Although the textile dyes contribute only a small portion of the total volume of discharged wastewater after the dyeing process, yet they make it deeply colored (McKay, et al., 1985). Considerable work has been carried out on the removal of dye from wastewater (Perineau, et al., 1982; McKay, et al., 1985; Gupta, 1985; Khattri, 2000; Low, et al., 2000; Liversidge, et al., 1997; Choy, et al., 1999, Asilian, et al., 2006). Water insoluble dyes (e.g. disperse and vat dyes) generally exhibit good exhaustion properties i.e. most of the dye bonds to the fiber and have been reported to be removed by physical means such as flocculation. When effluents containing these classes of dyes are discharged to a conventional sewage treatment works most of the color is removed by adsorption on biomass. However, since the introduction of water soluble dyes (reactive dyes), which are extensively used in the industry, conventional biological treatment processes such as the primary and secondary treatment systems are no longer able to achieve adequate color removal. The color of reactive dyes is due to the presence of N=N azo bonds and chromophoric groups. These dyes are first absorbed on the cellulose and then react with fiber by forming covalent bond between the dye molecule and the fiber. After fixation of the dyes on the fiber, about 10–50% of the initial loading is present in the dye bath effluent which gives rise to a highly colored effluent. Because of non-biodegradability due to chemical structure and molecular size they can create a problem in the environment. It is necessary, therefore, to use tertiary treatment to remove color before discharging the wastewater into natural streams.
Adsorption appears to offer the best prospects over all the other techniques of dye removal at tertiary stage (Robinson, et al., 2001; Kamel, et al., 1991; Keith, et al., 1999; McKay, 1981). From literature survey, it is revealed that a number of biological adsorbents have been investigated for removing reactive dyes, these include amongst others; maize cob, wood and rice hull (Low et. al 1997). It is known that activated carbon is a versatile adsorbent because of its sufficient surface area, pore volume, high degree of surface reactivity and exhibits reasonable adsorption capacities for decolorants in aqueous solutions. Dye adsorption is enhanced because of the presence of mesopores together with micropores in the activated carbon. There are some commercially available active carbons but these are expensive (Bhattacharya et. Al 1984, Singh and Rawat 1994, McKay et. al. 1986, Khare et al. 1987).
Now one aspect is that what technology is to be used for removing color. As more and more stringent regulations for industrial effluent deposition into water bodies are being applied, selection of a treatment method with high removal efficiencies for dyes and less operational problems is becoming inevitable. Liquid effluent treatment is commonly done in fixed beds for large-scale operation. However, fluidized bed or mechanically agitated suspensions are preferred for medium/small-scale ones. Another aspect to be considered is that wastewaters always contain suspended solids (e.g., fibers, waxes and gums), which clog fixed beds, requiring frequent backwashing or fluidization to dislodge the foreign material. Besides, fluidized bed technology can overcome the setbacks of the fixed bed treatment process by increasing adsorbent surface area, efficient mixing, and better contact of adsorbent and adsorbate. Typical problems encountered in fixed bed are dead zones, gas/vapor pockets, and channeling. In addition, pressure drops in fixed bed increases with the increase in flow rate causing high pumping cost compared to fluidized bed where pressure drop remains almost constant with increase in flow rate.
Textile dyeing industry is an important labor-based, export orientated sector in Bangladesh. The full flourishment of this industry is significant for the country’s economy. In the dyeing industries, above 30-60 L of water are consumed per kg of cloth dyed and large quantities of the effluents are released during processing. It amounts to about 16% of the total water consumed in the factory (Denold 1984, Namasivayam et. al. 1994). Most of the factories have no effluent treatment plants. They are sometimes directly drained to the rivers or canals causing severe water pollution and in other cases they are treated to some extent but not enough vigorously, so that all the dissolved colored materials can be decreased below the tolerable limit. Human and ecological health concerns have prompted the government to require textile effluent discharges to have increasingly lower color and nitrogen levels. The removal of dye in an economic fashion, however, remains an important problem. Thus, despite being aware of the problem, many textile manufactures have failed to adequately remove azo dye compounds from their wastewaters. So in Bangladesh context an efficient but cost effective method for removal of colored materials from effluent is crucial. The current practice of a very few local industries who have undertaken tertiary stage water treatment is to use low grade activated carbon as single use adsorbent in a fixed bed and discard the spent for further use as fuel. To make such practice economically viable low-cost adsorbent as well as maximum utilization of it are necessary.
While the research for low cost adsorbent and indigenously prepared activated carbon is going on (G. Mckay et. al. 1986, Mohammad et al, 2006), the present study is focusing on the performance of fluidized bed of activated carbon in decolorizing tertiary stage waste water.
1.1 Objectives with Specific Aims and Outcome
The objectives of this study are to investigate the followings:
Ø Physical properties and adsorption characteristics of commercially available activated carbon
Ø Performance of a fluidized bed of activated carbon in removing colored materials from water
Ø Mass transfer characteristics of the fluidized bed
With these specific aims in mind physical properties and equilibrium isotherm of granular activated carbon collected from Graphics Textiles Ltd. Sreerampur, Savar, Dhaka, Bangaldesh were investigated. The decolorizing performance of the fluidized bed is investigated by generating breakthrough curves. The breakthrough curves were obtained for dyed water prepared in the laboratory as well as industrial effluent collected from …at tertiary stage. Finally, based on the isotherm and breakthrough curves mass transfer analysis of the fluidized bed was carried out.
Color is one of the characteristics of an effluent which is easily detected and readily traced back to source. Most of the dyes are stable to biological degradation. Color affects the nature of the water and inhibits sunlight penetration into the stream and reduces photosynthetic action. The primary concern about effluent color is not only its toxicity but also its undesirable aesthetic impact on receiving waters. Non-biodegradable nature of most of the dyes reducing aquatic diversity by blocking the passage of sunlight through the water represents serious problems to the environment. In some cases, dyes in low concentration are harmful to aquatic life. Since many dyes have adverse effect on human beings, the removal of color from the effluent or process has appeared of importance for ensuring healthy environment. It is pointed out that less than 1 ppm of dye content causes obvious water coloration [Allen et.al 1998].
From an environmental point of view, for removal of dyes from wastewater, lots of research works have been done in the past and still are going on at present on stability of liquid solid of both fixed bed and fluidized bed technology. The reviews of some earlier research works related to the present investigation are presented below:
2.1 Color removal efficiency
The textile finishing industry generates a large amount of wastewater. Wastewaters from dyeing and subsequent rinsing steps form one of the largest contributions to wastewater generation in the textile industry. Because dyes are almost invariably toxic, their removal from effluent stream is ecologically necessary. Reactive dyes pose the greatest problem in terms of color, which is exacerbated by the dominance of cotton in today’s fashion industry. The human eye can detect concentrations of 0.005 mg/L of reactive dye in water, and therefore, presence of dye exceeding this limit would not be permitted on aesthetic grounds (Pierce, 1994). After the reactive dyeing process is complete, up to 800 mg/l of hydrolyzed dye remains in the bath (Steankenrichter and Kermer, 1992). Fixation rates for reactive dyes tend to be in the range of 60–70% although the values tend to be higher in dyes containing two reactive groups (Carr, 1995). Therefore, up to 40% of the color is discharged in the effluent from reactive dyeing operation resulting in a highly colored effluent. An additional problem is that the reactive dyes in both ordinary and hydrolyzed forms are not easily biodegradable, and thus, even after extensive treatment, color may still remain in the effluent. The conventional processes such as coagulation, flocculation and biological methods adopted for decolorizing effluents containing reactive dyes are no longer able to achieve an adequate color removal.
2.2 Adsorbents for color removal
Adsorption methods have been invariably successful to decolorize textile effluents but according to the Santhy (2005) this application is limited by the high cost of adsorbents. The removal efficiency of activated carbon prepared from coir pith towards three highly used reactive dyes in textile industry was investigated. Batch experiments showed that the adsorption of dyes increased with an increase in contact time and carbon dose. Maximum decolorization of all the dyes was observed at acidic pH. Adsorption of dyes was found to follow the Freundlich model. The column experiments using granular form of the carbon (obtained by agglomeration with polyvinyl acetate) showed that adsorption efficiency increased with an increase in bed depth and decrease of flow rate. The bed depth service time (BDST) analysis carried out for the dyes indicated a linear relationship between bed depth and service time. The exhausted carbon could be completely regenerated and put to repeated use by elution with 1.0 M NaOH. The coir pith activated carbon was not only effective in removal of color but also significantly reduced COD levels of the textile wastewater.
G. McKay et. al (1985) removed color by using some low cost materials. The low-cost materials can be used once and discarded by burning them. A variety of materials are reported in the literature for adsorption of different pollutants. Tree bark, coal, cotton waste, clay, hair etc., have been reported to adsorb different pollutants like heavy metals, pesticides, phosphates and sulphates, viruses etc. Since low-cost materials have been tried to remove different types of pollutants in different ionic forms, experiments were conducted to assess the feasibility of six low-cost materials to adsorb various types of dyes. He represents a number of low-cost materials (teakwood bark, ricehusk, coal, bentonite clay, hair and cotton waste) have been used as adsorbents for dyestuffs in aqueous solutions. Four red and four blue dyes have been studied; each color group consisted of an acidic, a basic, a disperse and a direct dye. The equilibrium isotherm for each dye-adsorbent system was determined and an adsorption capacity from zero to 200 mg dye g-1 of adsorbent was obtained. In general basic dyes adsorbed to a greater extent than the other dye classes but no single characteristic of the dye or adsorbent seemed responsible for such dye-adsorbent interactions and adsorption capacities.
M. EL Guend et.al (1986) has been studied the adsorption of four dyestuffs, namely, Basic Blue 69 (BB69), Basic Red 22 (BR22), Acid Red 114 (AR114) and Acid Blue 25 (AB25), onto bagasse pith. Bagasse pith is a cheap, abundant waste product from the sugar industry in Egypt and was found to have the following monolayer equilibrium saturation capacities: 158, 77, 23 and 22 mg dye/g pith. The effects of pith particle size range and dye solution temperature were studied.
Ahsan Habib et.al (2006) used tuberose sticks as an adsorbent for the removal of dyes present in industrial effluents. Methylene blue was selected as a model dye as an attempt to use waste tuberose sticks as an adsorbent for the removal of dye from wastewaters. The use of low-cost and eco-friendly adsorbents has been investigated as an ideal alternative to the current expensive methods of removing dyes from wastewater. Methylene Blue was used as model compound. The effects of contact time, initial dye concentration (20, 30, 40, 50 mg/L), pH and adsorbent dosages have been studied at 25 °C. The equilibrium time was found to be 30 min for all the dye concentrations. A maximum removal of 80% was obtained at pH 11.0 for an adsorbent dose 50 mg/50 mL of 40 mg/L dye concentration. Adsorption increased with increase in pH. Maximum desorption of 50% was achieved in water medium at pH 2.0.
There is a growing interest in using low cost, commercially available materials for the adsorption of dye colors. A wide variety of low cost materials, such as clay minerals, bagasse pith, wood, maize cob and peat are being tried as viable substitutes for activated carbon to remove dyes from colored effluents. That’s why Nassar and Magdy (1996) studied the adsorption of three basic dyes (basic yellow, basic red and basic blue) from an aqueous solution on palm-fruit bunch particles. The experimental results indicate that maximum adsorption capacities of the palm-fruit bunch particles were found to be 327 mg yellow dye per gram of adsorbent, 180 mg red dye per gram of adsorbent and 92 mg blue dye per gram of adsorbent. A comparative case study, based on the adsorption capacity alone, has shown that the costs of the adsorbent required are 1.9%, 4.4% and 7.1% respectively, compared with the case of commercial activated carbon granules.
According to the Nigam et. al (2000) the release of dyes into the environment constitutes only a small proportion of water pollution, but dyes are visible in small quantities due to their brilliance. Tightening government legislation is forcing textile industries to treat their waste effluent to an increasingly high standard. Currently, removal of dyes from effluents is by physio-chemical means. Such methods are often very costly and although the dyes are removed, accumulation of concentrated sludge creates a disposal problem. There is a need to find alternative treatments that are effective in removing dyes from large volumes of effluents and are low in cost, such as biological or combination systems. This article reviews the current available technologies and suggests an effective, cheaper alternative for dye removal and decolorization applicable on large.
2.3 Adsorption in Fixed bed
In terms of fixed bed adsorption one of the more successful simple modeling methods is the bed depth service time (BDST) model of hutchins (1973). This model assumed a linear relationship between the bed depth and the service time required for a chosen percentage content of impurity to reach the selected breakpoint in the bed. This model was applied to several fixed bed studies for the adsorption of various dyestuffs onto chitin by McKay et al.4 with considerable success. However, the model does not consider the mass transport kinetics of the adsorption process and therefore is limited in accuracy to the data under investigation and great care must be adopted in extending this model to predict design data.
Consequently, Mckay, Blair and Gradner (1986) has been developed a model. The model is based on external mass transport a pore diffusion, which is controlled by an effective diffusion coefficient. The model has been tested using experimental data obtained for the adsorption of Acid Blue 25 on chitin. Chitin has the ability to adsorb substantial quantities of dyestuffs from aqueous solutions. Hence, it may be a useful adsorbent for effluent treatment from textile mills. Mckay, Blair and Gradner (1984) investigate also the design procedures for batch and continuous fixed bed adsorption columns have been investigated for four dyestuffs. Batch type process are usually limited to the treatment of small volumes of effluent, but small adsorbent particle sizes may be used hence large external surface areas are available for mass transfer. Fixed bed systems, however, would sustain high pressure drop losses if fine adsorbent particles were used, but they have an advantage because adsorption depends on the concentration of solute in the solution being treated. The adsorbent is continuously in contact with fresh solution; hence the concentration in the solution in contact with a given layer of adsorbent in a column is relatively constant. Conversely, the concentration of solute in contact with a given quantity of adsorbent is continuously changing due to the solute being adsorbed.
Walker and Weatherley (1997) shown the reduction in effluent color produced by acid dyestuffs. This work involved the treatment of industrial wastewater from a nylon-carpet printing plant in Northern Ireland which currently receives no treatment and is discharged straight to sea. As nylon is particularly difficult to dye, acid dyes are required for successful coloration, but they cause major problems with the plant’s effluent disposal. Granular activated carbon Filtrasorb 400 was used to treat this effluent in a fixed-bed column system. Breakthrough curves from the fixed-bed column were shallow, even at low flow rates, which indicated a large mass transfer zone and inefficient use of adsorbent. Decrease in adsorbent particle size and decrease in linear flow rate produced a better bed performance. The bed depth service time (BDST) model proved effective for comparison of column variables, with calculated BDST constants providing a useful indication of bed performance. The BDST model also gave good approximation in predicting a bed performance using the relationships postulated by Hutchins (1973).
On the other side Arvind Varma and Dmitrios Chatzipoulos (1994) investigates the aqueous-phase adsorption and desorption of toluene in Filtrasorb-300 (F-300) activated carbon fixed-bed adsorbers at 25°C under a wide range of operating conditions. Process dynamics were described successfully using a homogeneous surface diffusion model with external mass transfer and a surface diffusion coefficient that increases with surface coverage. The model also accounted for irreversible toluene adsorption on F-300. The adsorption isotherm parameters, the surface diffusion coefficient and its dependence on surface concentration were determined independently in batch adsorption studies. The value of the external mass transfer coefficient as a function of the Reynolds number was determined by fitting the adsorption breakthrough curves. The fraction of irreversible toluene adsorption as a function of initial surface loading was found from the desorption breakthrough curves. Use of these independently measured equilibrium and transport parameters in the model permitted the successful description of experimental rates of toluene adsorption and desorption in F-300 fixed beds under a variety of operating conditions.
2.4 Adsorption on Activated Carbon: Comparative studies in a fixed bed and fluidized bed
For removal of color at tertiary level treatment adsorption on a fixed or packed bed of activated carbon is much popular, due to the properties of activated carbon of high organic color removal capacity. Though with activated carbon both organic and inorganic dyestuff can be adsorbed but it is more efficient in removing the organic color as it is an organic substance itself. The dyestuff or colored materials get adsorbed on the surface by external mass transfer and when the surface gets covered with adsorbate, the adsorption rate decreases as internal mass transfer is slower due to complicated process path of inside molecular structure. So after a certain period the activated carbon loses the ability of further adsorption and then it has to be either regenerated, which is still a difficult and expensive process or it has to be replaced by new activated carbon.
There are some difficulties faced in the operation of fixed or packed bed, such as:
Clogging, channeling, gas/vapor pocket and dead zone may be created inside the bed, which reduces the flow and make it nonuniform, thus decreases adsorption rate. More over large pressure drop and non uniform temperature has to be often faced in fixed bed operation. These set backs of fixed bed operation can be overcome by fluidized bed operation.
Advantages of using Fluidized bed adsorption process over fixed bed adsorption:
1 Adsorption surface area increases due to fluidization, more free adsorption sites become reachable to the adsorbate
2 Efficient mixing and good contact between adsorbent and adsorbate is ensured
3 Pressure drop is almost constant and thus required pumping energy does not vary
4 Temperature in the system can assumed to be uniform or constant
5 No clogging or channeling takes place
6 Dead zone, gas/ vapor pocket creation inside the bed can also be avoided.
2.5 Adsorption in Fluidized bed
Liquid effluent treatment in fluidized beds has received limited attention as adsorption onto activated carbon is commonly done in fixed beds for large-scale operation. But for the advantages of applying fluidized bed technology over fixed bed have drawn attentions of researchers for several years and research is still going on the industrial application of this process.
Adsorption with activated carbon is widely employed for the removal of organics in water purification. Several adsorber configurations are possible for treatment with activated carbon; these include batch vessel, continuous flow stirred tank, fixed bed, moving bed, and fluidized bed [e.g. Gulp et al. (1978)]. Traditionally, the treatment of choice has been packed-bed adsorption due to the ease and reliability of this operation. Nevertheless, several problems, such as excessive head loss, air binding, and fouling with biological and particulate matter are associated with packed-bed operation. These problems are significantly reduced in fluidized-bed adsorption; hence, Veeraraghavan, Fan and Mathews (1989) have focused on this mode of operation. Under certain conditions, the breakthrough time in a fluidized-bed adsorber is considerably shorter than that in a comparable fixed-bed adsorber. This phenomenon is probably due to the appreciable macro scale or axial mixing occurring in the solid and liquid phases of the fluidized bed. An axial dispersion model has been adapted to characterize the fluidized bed adsorber. The model takes into account the effects of axial mixing in the solid and liquid phases, mass transfer resistance in the laminar fluid boundary surrounding an individual adsorbent particle, and diffusional resistance within the particle. The model has been solved numerically to simulate the performance of a laboratory-scale adsorber. The results of the simulation closely represent experimental observations over wide ranges of the influent flow rate, fluidized bed height and adsorbent particle size.
Gordan Mckay (1988) studied the fluidized bed adsorption of pollutants onto activated carbon. The pollutants were phenol, p-chlorophenol and sodium dodecyl sulphate, and the effects of the flow rate of the solution and of the pollutant concentration have been studied. He have shown the correlations between the external mass transfer coefficient kf and the liquid-phase Reynolds number and Analysis of the kinetic data seems to indicate that both external and internal mass transfer coefficients are rate controlling.
Correa et al (2006) developed a fluidized bed system for adsorption of phenol from aqueous solutions. This work is related to removal of phenol from wastewaters by adsorption onto polymeric resins, a current alternative to activated carbon. A closed circuit, bench-scale liquid fluidized bed system was developed for this purpose. Phenol aqueous solutions with initial concentrations in the range of 0.084 to 0.451 kg/m3 were used to fluidize small permeable capsules of stainless steel screen containing a commercial resin at 308 K. Experiments were carried out using a fluidizing velocity 20% above that of the minimum fluidization of the capsules. Typically, 30 passages of the liquid volume circulating through the bed were required to reach a quasi-equilibrium concentration of phenol in the treated effluent. A simple batch adsorption model using the Freundlich isotherm successfully predicted final phenol concentrations. Suspended solids, often present in residual waters and a common cause of fixed bed clogging, were simulated with wood sawdust.
Kargi and Eyiisleyen (1995) investigated the kinetics of biological removal of COD and nitrogen from a synthetic wastewater in a fluidized bed operating in batch mode. Synthetic wastewater consisted of diluted molasses, urea, KH2PO4, and MgSO4, resulting in COD/N/P = 100/l0/2. The fluidized bed contained sponge particles surrounded by stainless-steel wires as support particles for microorganisms, The system was operated with different initial COD and nitrogen concentrations, and COD-nitrogen consumption profiles were obtained. From the initial slopes of these curves, the initial rates of COD and nitrogen consumption and kinetic constants were determined. The system operated under a COD limitation with no dissolved oxygen limitations. The kinetic analysis for COD removal has shown a Monod type of kinetics with possible inhibition on the Ks term. Nitrogen removal rate data indicated an inhibition for nitrogen concentrations above 1,200 mg/l
Wang et. Al (1996) analyzed the performance of granular activated carbon in a liquid-solid fluidized bed to remove phenols. Although the use of activated carbon for treatment of water and wastewater has received a lot of interest, the fundamental kinetic study of the adsorption process should be considered in advance. For an efficient adsorption process, the kinetic study provides the rapid removal of pollutants from solution and the adsorption equilibrium is the ultimate capacity for adsorption. Several physical configurations of adsorbers are available for activated carbon adsorption, e.g. batch vessel, continuous stirred tank, fixed bed and fluidized bed. Fluidized-bed operation has the advantage that the particles are in continuous motion with efficient mixing of the fluid, but suffers from particle attrition and erosion. Successful design of a fluidized-bed adsorption column requires the prediction of the effluent concentration time profile, i.e. the breakthrough curve.
2.6 Kinetics of adsorption on activated carbon
Adsorption on activated carbon is one of the most effective and dependable technologies currently available for the treatment of drinking water and wastewaters contaminated with low concentrations of hazardous compounds (Irvin 1993). The role of activated carbon in effective water pollution control is well established. The rate of adsorption on activated carbon from a liquid phase is rather slow. The removal of man-made pollutants often is the predominant goal of adsorption by granular activated carbon in waste water treatment. As a cost consideration, granular activated carbon is much cheaper than the powdered form (Wang, R. et. Al 1996).
Liquid-phase adsorption has been shown to be highly efficient for removal of colors, odors, organic and inorganic matter from process or waste effluents. Activated carbons (granular or powdered) are widely used adsorbents because of their excellent adsorption capability for organic pollutants (Juang et. al. 2000). Fettig and Sontheimer  investigate the adsorption kinetics of a variety of single substances on activated carbon is investigated a low liquid-phase concentrations. Experimental data they have obtained by the mini column method show the influence of dissociation and molecular weight of the adsorbates o their external mass transfer behavior. Modeling of mini column breakthrough curves proves the internal mass transport resistances have to be taken into account at low concentrations as well as high. A differential fixed-bed reactor technique was used to determine intraparticle mass transport parameters. For strongly adsorbate substances, diffusion in the adsorbend phase proves to be prevailing, whereas for weakly adsorbable or high molecular weight assorbates, pore diffusion also contributes to the internal mass transport. However the surface diffusivities depend on the carbon loading in an adsorbate-specific manner.
Kannan and Sundaram  have been using activated carbon which is come from various indigenously prepared activated carbons from agricultural wastes and to compare their adsorption capacity for the removal of methylene blue under optimum experimental conditions. The effects of various experimental parameters have been investigated using a batch adsorption technique to obtain information on treating effluents from the dye industry. The kinetics of adsorption were found to be first order with regard to intra-particle diffusion rate. The adsorption capacities of indigenous activated carbons have been compared with that of the commercial activated carbon. The results indicate that such carbons could be employed as low cost alternatives to commercial activated carbon in wastewater treatment for the removal of colour and dyes.
Sakoda, Nomura and Suzuki (1996) used activated carbon membrane which is to be used in water treatments was developed and the decolorization of the coke furnace wastewater was successfully demonstrated as a model case. The activated carbon membrane was prepared by carbonizing poly-vinylydenchloride (PVdC) and poly-vinylalcohol (PVA) microspheres aggregating on and within a ceramic pipe. The membrane developed in this work was suspected to have a bidispersed structure, which made it possible to play the roles of both a porous membrane having the molecular weight cut-off of about 10,000 and an activated carbon bed where the dissolved organics with low molecular weight could be adsorbed. The activated carbon membrane developed in this work appears to be useful for compact water treatment processes.
Mansi(1996) using sawdust as an adsorbent in a fixed bed adsorber for decolorizing wastewater. Fuller’s earth and bauxite were found to be successful as adsorbents for color removal on a laboratory scale, but considerable flow problems were encountered in a fixed bed system. Mc.kay et. al. investigate the removal of basic astrazone blue from effluents using silica gel as an adsorbents. Investigations by Mckay et. al. were made to determine whether cheap, commercially available materials hold promise in the treatment of wastewater. Their initial findings indicated that peat wood has a high adsorptive capacity for dyes and is relatively cheap. The cheapness of the adsorbent means that regeneration is not necessary and the spent adsorber can be burned. The batch adsorption is usually limited to the treatment of small volumes of effluent, whereas a fixed bed flow systems has an advantage because the solute concentration is changing continuously while the solute is being adsorbed.
Hak Lee et. al (2006) have been used zinc chloride treated indigenous activated carbon. The adsorption of colored compounds from the textile dyeing effluents of Bangladesh on granulated activated carbons produced from indigenous vegetable sources by chemical activation with zinc chloride was studied. The most important parameters in chemical activation were found be the chemical ratio of ZnCl2 to feed (3:1), carbonization temperature (450–465 ?C) and activation time (80 min). It is observed that adsorption of reactive dyes by all sorts of activated carbons is higher than disperse dyes. It is explained that activated carbon, because of its acidic nature, can better adsorb reactive dye particles containing large number of nitrogen sites and –SO3Na group in their structure. The use of carbons would be economical, as saw-dust and water hyacinth are waste products and abundant in Bangladesh.
Granular activated carbon is the most popular adsorbent and has been used with great success (McKay, 1982), but is expensive. Consequently, new materials as chitin (McKay, 1982), silica gel (McKay et al., 1980), natural clay (El-Geundi, 1991, 1993a, b), bagasse pith (McKay, 1998) are being studied. A very limited amount of information is available on the use of natural zeolites as a method for dye removal (Meshko et al., 1999). So Meshko studied the adsorption of basic dyes from aqueous solution onto granular activated carbon and natural zeolite using an agitated batch adsorber
S. J. Allen (1999) describes the adsorption capacities for anionic reactive dyes, namely Remazol Yellow, Remazol Red and Remazol Black B were determined using Filtrasorb 400 activated carbon. F-400 was selected because of its high adsorption capacity for a large number of contaminants in aqueous solutions. Competitive adsorption for reactive dyes in a mixture was investigated to test the efficiency of the activated carbon for purifying real effluent containing dyes. Chemical surface properties for F-400, including surface acidity, surface basicity, H+ and OH- adsorption capacities as well as the zero point of charge of the carbon (pHZPC) were estimated. pHZPC is a critical value for determining quantitatively the net charge (positive or negative) carried on the activated carbon surface during adsorption of reactive dyes.
Ahsan Habib et. at. (2006) have been used tuberose, the low-cost and ecofriendly adsorbent as an ideal alternative to the current expensive methods of removing dyes from wastewater. Methylene Blue was used as model compound. The effects of contact time, initial dye concentration (20, 30, 40, 50 mg/L), pH and adsorbent dosages have been studied at 25 °C. The equilibrium time was found to be 30 min for all the dye concentrations. A maximum removal of 80% was obtained at pH 11.0 for an adsorbent dose 50 mg/50 mL of 40 mg/L dye concentration. Adsorption increased with increase in pH. Maximum desorption of 50% was achieved in water medium at pH 2.0.
McKay, Blair and Gardner (1983) studied the intraparticle diffusion processes for the adsorption of dyestuffs onto chitin. The amount of dye adsorbed per gram of chitin has been plotted against the square root of time. Sometimes two and even three linear regions are apparent on the root time plots indicating a possible branched pore mechanism. The controlling mechanisms are due to macropores and micropores in the chitin particle creating rapidly and slowly diffusing regions. McKay, Otturburn and Sweeney (1979) have been used Sorbsil silica as an adsorbent and the rate of adsorption of Astrazone Blue, a basic dye, on Sorbsil Silica has been studied.
Yang and Sun using peat–resin particle as a adsorbent for the adsorption of basic dyes from aqueous solution. Peat, as an adsorbent, is porous and rather complex material, containing lignin and cellulose. Recently, peat has been used to remove some pollutants (such as heavy metals, dyes and oil) from aqueous solution [ Coupa et. al.1976, McKay et. al. 1980, Couillard 1921 and Couillard 1994]. Many studies showed peat could effectively remove the dyes from aqueous solution. Peat can effectively remove the pollutants from solution and is inexpensive. However, when raw peat is directly used in wastewater treatment, there are many limitations, such as low chemical stability and mechanical strength, leach of fulvic acid from peat and difficult regeneration. In order to overcome these limitations, they prepared the modified peat–resin particle by mixing modified-peat with polyvinyl alcohol (PVA) and formaldehyde. The modified peat–resin particle contains polar functional groups, such as alcohols and acids. Both modified peat and resin in particle can adsorb the dyes from solution. Finally they analyze the adsorption isotherm and kinetics experiments of basic dyes (Basic Magenta and basic Brilliant Green) were conducted and the different kinetic models were used to analyze adsorption processes of basic dyes on modified peat–resin particle.
2.7 Mass transfer in fluidized bed in case of wastewater treatment
Mass transfer in fluidized beds has been studied because of its fundamental importance in many engineering operations. Many experimental studies of the mass transfer in fluidized beds have been reported in the past 40 years (Garim et. at. 1999). Packed bed has disadvantages of low mass-transfer coefficient, large pressure drop, bed clogging, and therefore, periodic operation. It is a semi-batch process with low water treatment capacity. Fluidized bed partially overcomes these problems. Fluidization of bed increases the mass transfer coefficient and therefore, water treatment capacity. It also partly eliminates bed clogging. But, it is still a semi batch process and needs periodic operation to replace the saturated adsorbent with fresh adsorbent in the bed (Kishore and Verma 2005).
Wright and Glasser (2000) have developed a model and solved to describe phenol adsorption in a fluidized bed. Liquid-solid mass transfer, adsorption and hydrodynamic effects were taken into account. The model was examined for both pore and homogeneous diffusion. Parametric sensitivity analysis showed that superficial velocity and particle radius had the largest effects on breakthrough behavior for all conditions. The effect of axial dispersion, film mass transfer and solid diffusion coefficients were less significant contributors to breakthrough at all expansions and bulk phase viscosities. The simulation results for pore diffusion were affected more significantly by changes in superficial velocity and particle radius than the simulation results for homogeneous diffusion. The performance of the fluidized-bed adsorption unit was limited by intraparticle mass-transfer effects, especially at high degrees of bed expansion.
Kishore and Verma (2005) developed a counter current multi-stage fluidized bed ion exchanger to study mass transfer during the continuous removal of dissolved anions from wastewater using commercially available resin. OH ion is used as an example in the study. A higher removal efficiency in the multi-stage fluidized bed than in a single-stage fixed and fluidized bed is demonstrated. The experiment shows progressive fluidization on a stage, smooth flow of resin across the stage and transfer of resin from one stage to the other. In each stage of the fabricated four-stage perspex made column, a downspout has been provided to facilitate the downward flow of resin on to the next stage, while water flows counter currently upward through the mesh of the stage. In addition, provision has been made to adjust the down comer height on the stage without disturbing the operation with the aid of the rack and pinion arrangement. The experimental variables in the multi-stage column operated under steady state includes the flow rates of water and resin, feed concentration, stage height and the number of stages. A mathematical model is also developed for determining the key parameters that affect the overall mass transfer in the multi-stage continuous counter current column. In general, number of stages and diffusional resistance on the resin side control the extent of separation in the column
Mckay, G. et. al (1989) have investigated the external mass transfer during adsorption of various pollutants onto activated carbon. He explores a wide range of experimental studies which are reported for the adsorption of phenol and p-chlorophenol onto activated carbon–Type Filtrasorb 400–in an agitated batch adsorber. A model has been used to determine the external mass transfer coefficient for the systems and the effects of several experimental variables have been investigated: these include agitation, initial pollutant concentration, carbon mass, carbon particle size and solution temperature. The mass transfer coefficient has been correlated in terms of the dimensionless Sh/Sc0.33 against each variable. The Sherwood number relates the external mass transfer coefficient k/ to particle radius, R, and molecular diffusivity. The Schmidt number, Sc, is the ratio of kinematic viscosity, v, to molecular diffusivity. A few results are also reported for the adsorption of sodium dodecyl sulphate and mercuric ions onto activated carbon.
According to Mc.kay et. al (1989) several steps can be used to explain the mechanism of solute adsorption onto an adsorbent. However, for the purposes of the present work the overall adsorption process is assumed to occur using a three step model:
(i) Mass transfer of solute from the bulk solution to the particle surface;
(ii) Adsorption of solute onto sites;
(iii) Internal diffusion of solute via either a pore diffusion model or a homogeneous solid phase diffusion model.
Throughout the present work it has been assumed that step (ii) is rapid with respect to the other two processes and therefore is not rate limiting in any kinetic analysis. Consequently, the two controlling factors are film mass transfer and internal mass transfer. The development of models based on two such mass transport steps is quite complex, requiring a coupling equation and its subsequent solution. Initially, therefore simplifying assumptions were made and attempts to describe the adsorption processes in terms of either a film mass transfer coefficient of an internal diffusion mass transfer parameter have been undertaken. That’s why Mckay, Bino and Altamemi (1989) determined a single resistance model which has been developed enabling the external mass transfer coefficient.
Mckay, Blair and Gardener (1984) have developed Two-Resistance Mass Transfer Model for the Adsorption of Various Dyestuffs onto Chitin. The mass transfer model is based on the assumption of a pseudoirreversible isotherm and two resistances to mass transfer. These are external mass transfer and internal pore diffusion mass transfer. The rate of adsorption of dyestuffs onto chitin can thus be described by an external mass transfer coefficient and a pore diffusion coefficient.
In 1988 McKay, Geundi, and Wahab described two resistance mass transfer model for describing the adsorption of four dyes from aqueous solutions onto bagasse pith, a waste product from the sugar industry. The dyes studied are Basic Blue 69, Basic Red 22, Acid Blue 25 and Acid Red 114 and the system variables are initial dye concentration and pith mass. A method has been presented for the prediction of concentration decay vs time. The model is based on external mass transfer and pore diffusion and enables the external transport coefficients and the effective diffusivities to be determined. Constant mass transport coefficients were obtained for each dye-pith system to correlate the effects of varying the initial dye concentration and pith mass.
2.8 Adsorption isotherms
The distribution of dye between the adsorbent and dye solution, when the system is at equilibrium, is important to obtain the capacity of the granular activated carbon for the dyes.
A number of equations exist which enable the equilibrium data to be correlated and two most frequently used, for dilute solutions, are the Langmuir
and Freundlich isotherms equations which will discussed here. The parameters of these equations are very useful for predicting adsorption capacities and also for incorporating into mass transfer relationships in the design of contacting equipment.
These two equations are used more than 99% of the time to describe the equilibrium adsorption of solutes to activated carbon. The reason is simple: in almost every case, one of these two equations fits the data quite well. Thus, there is no need for more elaborate isotherm equations, particularly those involving two or more parameteres.
Adsorption is the accumulation of atoms or molecules on the surface of a material. This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the adsorbent’s surface. It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. The term sorption encompasses both processes, while desorption is the reverse process.
Adsorption is present in many natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, synthetic resins, and water purification. Adsorption, ion exchange, and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column.
Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding).
Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials.
The first mathematical fit to an isotherm was published by Freundlich and Küster (1894) and is a purely empirical formula for gaseous adsorbates,
where q* is the quantity adsorbed per unit weight of the adsorbent, P* is the pressure of adsorbate and A and n are empirical constants for each adsorbent-adsorbate pair at a given temperature. The function has an asymptotic maximum as pressure increases without bound. As the temperature increases, the constants A and n change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface.
184.108.40.206 Freundlich equation
The Freundlich equation or Freundlich adsorption isotherm is an adsorption isotherm, which is a curve relating the concentration of a solute on the surface of an adsorbent, to the concentration of the solute in the liquid with which it is in contact. There are basically two well established types of adsorption isotherm: the Freundlich adsorption isotherm and the Langmuir adsorption isotherm
The Freündlich Adsorption Isotherm is mathematically expressed as
p = Equilibrium pressure of adsorbate
C = Equilibrium concentration of adsorbate in solution.
A and n are constants for a given adsorbate and adsorbent at a particular temperature
220.127.116.11 Langmuir equation
The Langmuir equation or Langmuir isotherm or Langmuir adsorption equation relates the coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature. The equation was developed by Irving Langmuir in 1916. The equation is stated as:
? or theta is the fractional coverage of the surface, P is the gas pressure or concentration, ? alpha is a constant.
The constant ? is the Langmuir adsorption constant and increases with an increase in the binding energy of adsorption and with a decrease in temperature.
The equation is derived starting from the equilibrium between empty surface sites (S * ), particles (P) and filled particle sites (SP)
The Equilibrium constant K is thus given by the equation:
Because the number of filled surface sites (SP) is proportional to ?, the number of unfilled sites (S *) is proportional to 1-?, and the number of particles is proportional to the gas pressure or concentration (P) the equation can be rewritten as:
where ? is a constant
? = ?(1 ? ?)P
? = P? ? P??
? + P?? = P?
?(1 + P?) = P?
The Langmuir equation is expressed here as:
where K = Langmuir equilibrium constant, C* = aqueous concentration (or gaseous partial pressure), = amount adsorbed, and = maximum amount adsorbed as c increases.
The equilibrium constant is actually given by :
The Langmuir equation can be fitted to data by linear regression and nonlinear regression methods. Commonly used linear regression methods are: Lineweaver-Burk, Eadie-Hofstee, Scatchard, and Langmuir.
The double reciprocal of the Langmuir equation yields the Lineweaver-Burk equation:
A plot of (1/) versus (1/c) yields a slope = 1/(K) and an intercept = 1/. The Lineweaver-Burk regression is very sensitive to data error and it is strongly biased toward fitting the data in the low concentration range. It was proposed in 1934. Another common linear form of the Langmuir equation is the Eadie-Hofstee equation:
A plot of () versus (/c) yields a slope = -1/K and an intercept =. The Eadie-Hofstee regression has some bias toward fitting the data in the low concentration range. It was proposed in 1942 and 1952. Another rearrangement yields the Scatchard regression:
A plot of (/c) versus () yields a slope = -K and an intercept = K. The Scatchard regression is biased toward fitting the data in the high concentration range. It was proposed in 1949. Note that if you invert the x and y axes, then this regression would convert into the Eadie-Hofstee regression discussed earlier. The last linear regression commonly used is the Langmuir linear regression proposed by Langmuir himself in 1918:
A plot of (c/) versus (c) yields a slope = 1/and an intercept = 1/(K). This regression is often erroneously called the Hanes-Woolf regression. The Hanes-Woolf regression was proposed in 1932 and 1957 for fitting the Michaelis-Menten equation, which is similar in form to the Langmuir equation. Nevertheless, Langmuir proposed this linear regression technique in 1918, and it should be referred to as the Langmuir linear regression when applied to adsorption isotherms. The Langmuir regression has very little sensitivity to data error. It has some bias toward fitting the data in the middle and high concentration range.
There are two kinds of nonlinear least squares (NLLS) regression techniques that can be used to fit the Langmuir equation to a data set. They differ only on how the goodness-of-fit is defined. In the v-NLLS regression method, the best goodness-of-fit is defined as the curve with the smallest vertical error between the fitted curve and the data. In the n-NLLS regression method, the best goodness-of-fit is defined as the curve with the smallest normal error between the fitted curve and the data. Using the vertical error is the most common form of NLLS regression criteria. Definitions based on the normal error are less common. The normal error is the error of the datum point to the nearest point on the fitted curve. It is called the normal error because the trajectory is normal (that is, perpendicular) to the curve.
It is a common misconception to think that NLLS regression methods are free of bias. However, it is important to note that the v-NLLS regression method is biased toward the data in the low concentration range. This is because the Langmuir equation has a sharp rise at low concentration values, which results in a large vertical error if the regression does not fit this region of the graph well. Conversely, the n-NLLS regression method does not have any significant bias toward any region of the adsorption isotherm.
Whereas linear regressions are relatively easy to pursue with simple programs, such as excel or hand-held calculators, the nonlinear regressions are much more difficult to solve. The NLLS regressions are best pursued with any of various computer programs.
2.9 Fluidized Bed
A fluidized bed is formed when a quantity of a solid particulate substance (usually present in a holding vessel) is placed under appropriate conditions to cause the solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids; such as the ability to free-flow under gravity, or to be pumped using fluid type technologies.
The resulting phenomenon is called fluidization. Fluidized beds are used for several purposes, such as fluidized bed reactors (types of chemical reactors), fluid catalytic cracking, fluidized bed combustion, heat or mass transfer or interface modification, such as applying a coating onto solid items.
2.9.1 Properties of fluidized beds
A fluidized bed consists of fluid-solid mixture that exhibits fluid-like properties. As such, the upper surface of the bed is relatively horizontal, which is analogous to hydrostatic behavior. The bed can be considered to be an inhomogeneous mixture of fluid and solid that can be represented by a single bulk density.
Furthermore, an object with a higher density than the bed will sink, whereas an object with a lower density than the bed will float, thus the bed can be considered to exhibit the fluid behavior expected of Archimedes’ principle. As the “density”, (actually the solid volume fraction of the suspension), of the bed can be altered by changing the fluid fraction, objects with different densities comparative to the bed can, by altering either the fluid or solid fraction, be caused to sink or float.
In fluidized beds, the contact of the solid particles with the fluidization medium (a gas or a liquid) is greatly enhanced when compared to packed beds. This behavior in fluidized combustion beds enables good thermal transport inside the system and good heat transfer between the bed and its container. Similarly to the good heat transfer, which enables thermal uniformity analogous to that of a well mixed gas, the bed can have a significant heat-capacity whilst maintaining a homogeneous temperature field
Fluidized beds are used as a technical process which has the ability to promote high levels of contact between gases and solids. In a fluidized bed a characteristic set of basic properties can be utilised, indispensable to modern process and chemical engineering, these properties include:
? Extremely high surface area contact between fluid and solid per unit bed volume
? High relative velocities between the fluid and the dispersed solid phase.
? High levels of intermixing of the particulate phase.
? Frequent particle-particle and particle-wall collisions
Fluidization is defined as an operation through which fine solids are transformed into a fluid like state through contact with either a gas or a liquid.
18.104.22.168 The Phenomenon of Fluidization
When a fluid is pumped upward through a bed of fine solid particles at a very low flow rate the fluid percolates through the void spaces (pores) without disturbing the bed. This is a fixed bed process.
If the upward flow rate is very large the bed mobilizes pneumatically and may be swept out of the process vessel. At an intermediate flow rate the bed expands and is in what we call an expanded state. In the fixed bed the particles are in direct contact with each other, supporting each other’s weight. In the expanded bed the particles have a mean free distance between particles and the particles are supported by the drag force of the fluid. The expanded bed has some of the properties of a fluid and is also called a fluidized bed.
First, we consider the behavior of a bed of particles when the upward superficial fluid velocity is gradually increased from zero past the point of fluidization, and back down to zero.
At first, when there is no flow, the pressure drop zero, and the bed has a certain height. As we proceed along the right arrow in the direction of increasing superficial velocity, tracing the path ABCD, at first, the pressure drop gradually increases while the bed height remains fixed. This is a region where the Ergun equation for a packed bed can be used to relate the pressure drop to the velocity. When the point B is reached, the bed starts expanding in height while the pressure drop levels off and no longer increases as the superficial velocity is increased. This is when the upward force exerted by the fluid on the particles is sufficient to balance the net weight of the bed and the particles begin to separate from each other and float in the fluid. As the velocity is increased further, the bed continues to expand in height, but the pressure drop stays constant. It is possible to reach large superficial velocities without having the particles carried out with the fluid at the exit. This is because the settling velocities of the particles are typically much larger than the largest superficial velocities used.
Fig 1: Typical curves for a liquid-solid fluidized bed of particles of approximately uniform size.
Now, if we trace our path backward, gradually decreasing the superficial velocity, in the direction of the reverse arrows in the figure, we find that the behavior of the bed follows the curves DCE. At first, the pressure drop stays fixed while the bed settles back down, and then begins to decrease when the point C is reached. The bed height no longer decreases while the pressure drop follows the curve CEO. A bed of particles, left alone for a sufficient length of time, becomes consolidated, but it is loosened when it is fluidized. After fluidization, it settles back into a more loosely packed state; this is why the constant bed height on the return loop is larger than the bed height in the initial state. If we now repeat the experiment by increasing the superficial velocity from z