Factors Affecting Small Scale Testing - Term Paper Example

?Filtration Filtration is defined as the process of separating a fluid-solid mixture whereby most of the fluid passes through a porous barrier which also retains most of the solid particles that are present in the mixture. This porous barrier is known as the filter medium or the septum. It may be made of cloth, paper, a screen or a bed of solids. In filtration, the liquid or fluid that passes through the filtration medium is called the filtrate. There are several ways of classifying filtration and filters. These are; (a) By the driving force. There are several methods through which the filtrate induced may flow through the filter medium. This may be by gravity (hydrostatic head), pressure being applied upstream of the filter medium, reduced pressure or vacuum being applied downstream the filter medium or through centrifugal sedimentation. (b) Filtration mechanism This may be through cake or clarifying filtration. Cake filtration is when the solid gets stopped at the surface of the filter medium where it piles upon one another to form a cake. Also known as depth or filter medium filtration, clarifying filtration is when the solids get trapped within the pores of the filter medium. (c) By objective The objective of the filtration process may be either dry solids or clarified liquid or both. The filtration process may be either intermittent or continuous. When classifying filters, they are first divided into either cake or clarifying filters. They are then classified depending on the driving force and then finally into either batch or continuous classes. Continuous filtration testing and scale up In continuous filtration, it is first assumed that the resistance of both the filter cloth and the filter drainage is insignificant as compared to the resistance of the filter cake. It is also assumed that both specific cake resistance and pressure drop remain constant throughout the filtration process. Factors affecting small scale testing Pressure or Vacuum Most continuous filters use vacuum as the driving source for filtration. However, pressure as the driving force may be required if the feed slurry is saturated, hot or contains a very highly volatile liquid, and or near its boiling point at atmospheric pressure. Pressure filtration may also be used where the cake moisture that is required is lower than that can be obtained when using vacuum. Cake discharge A practical filter application is one which produces a cake that is thick enough to discharge. There are minimum acceptable cake thicknesses that are required for discharge for various types of filtration systems and discharge mechanisms. Therefore, when running small scale tests, the experimenter should decide early during the test program the applicable type of discharge then later tailor the data collected in a way that it will fit the physical requirements of that unit type. Feed slurry temperature An increase in the feed slurry temperature decreases the viscosity of the liquid phase. The overall result of this is an increase in the filtration rate and a decrease in the cake moisture content. Cake thickness control At times, the rate of cake formation with bottom feed type filters may be rapid enough in such a way that it creates a cake that is too thick for subsequent operations. The cake thickness can be adjusted by simply adjusting the bridge blocks found in the filter valve so as to decrease the effective submergence, through reduction of the slurry level in the vat, and also by reducing the vacuum level in the portion where the cake forms. Representative samples It is essential for the sample to be used as the representative of the slurry in full scale plants to be tested under the conditions prevailing in the process. If the slurry has a temperature that is different from the ambient temperature, subsequent heating or cooling may change the distribution of the particle size. The age of the sample may also influence the particle size significantly. If an effect is likely, the bench scale testing should be at the laboratory site or at the plant on fresh materials. Free solids concentrations An increase in the feed solid concentration results in an effective increase in the solid filtration rate. This assists in the formation of a homogenous suspension while thereby causing a decrease in the cake moisture content. Pretreatment chemicals Addition of flocculating agents (either natural or synthetic polymers or organic chemicals) is a common method of treatment which provides improvement in the washing rate, filtration rate or the final cake moisture content. This is attained by modifying the slurry. Cloth blinding Continuous filters use a type of medium in the filtration process. This medium is usually in contact with the solid hence causing a danger in occurrence of medium blinding (blockage of the fabric). The filter medium to be used should be as open as possible and also capable of maintaining the required filtrate clarity. Filtrates that are excessively dirty should always be avoided since these solids may damage the internals of the filter or even cut the fabric yarn. Homogenous cake In order for accurate tests to be achieved and for the filter to operate optimally, there has to be the formation of a homogenous cake as well as the maintenance of a homogenous suspension. Agitation of sample When carrying out bottom feed tests, all slurries to be used during the tests must be agitated using the hand (if the characteristics of the slurries permit). Alternatively, a wide spatula may be used for agitation after it is determined that the spatula is capable of providing the needed agitation. This is done in order to confirm whether or not the solid particles are around the edges of the container as well as to find out the degree of agitation that is required in order to maintain suspension of the solids. Use of hot air or steam Although both steam and hot air can be used effectively for drying, the applications are difficult and they make the testing procedures both difficult and specialized. Steam application reduces the cake moisture by about 2 to 4 %. Procedure for carrying out small scale tests When carrying out bench-scale tests, there are two techniques that may be used as discussed below. These are bottom-feed and top-feed testing and pre coat testing procedure. Apparatus There are several variations of equipment that may be used for carrying out small scale tests. However, they all have features that are similar to the equipment described below. The apparatus mostly used are: Metal Dam Clamp Pipe Vacuum flask Vacuum pump Vacuum regulation valve Vacuum gauge Ball valve (Top feed leaf test set up) Procedure Bottom feed test procedure The apparatus should be set up as shown in the following figure. 1. Select a test leaf capable of giving the best results and fit it into the test leaf then use silicon to seal the back of the leaf and side of the dam. 2. Crimp the test leaf and turn on the vacuum pump while regulating the vacuum level using the bypass valve. 3. Use a wide spatula or the hand to agitate the slurry until a homogenous suspension is maintained. 4. Release the crimped hose and start the timer simultaneously in order to start cake formation. Maintain agitation during the cake formation process while moving the leaf as required in order ensuring that solids do not settle in any part of the container. 5. At the end of the cake formation period, remove the leaf from the slurry and note the time. Tilt, shake and rotate the leaf until all the filtrate has reached te drainage outlet. 6. Measure a quantity of wash fluid and apply it onto the cake (if t is to be washed) and record the time the fluid takes to disappear from the surface of the cake. Moreover, in order to prevent the cake from being gouged, pour the wash fluid onto a deflecting baffle. 7. Carry on with the various operations in the sequence that is already predetermined. 8. Record all the pertinent information such as temperature, vacuum level, during each of the operations. 9. Finally, measure and record the following; Filtrate volume and weight (where appropriate) Thickness of the cake Final temperature of the cake Discharge characteristics e.g. rolls, sticks to media etc. 10. If the associated solution has little or no dissolved solids, it is appropriate to dry the total cake sample (for runs that involve only cake dewatering). Top feed Procedure The operations in top feed tests are similar to that of bottom feed test with the only difference been that the leaf is not immersed into the slurry. The apparatus are arranged as shown in the following figure. The slurry can be transferred to the leaf from a beaker if the particles contained in the slurry do not settle rapidly. However, if the particles settle rapidly, an Erlenmeyer flask (most probably one made from plastic) can be used. When using this technique, the slurry is swirled inside the flask until it gets suspended completely then it is inverted abruptly over the leaf. Pre-coat testing procedure The procedure of running pre coat tests is also similar to that of bottom feed tests with the only difference being that the leaf must first be coated using a bed of diatomaceous earth, perlite or other inert solids. The grade of precoat materials that retains solids to be filtered without any major penetration is selected using trial and error. The most appropriate beds for selection are those that are relatively thin of 1 cm to 2 cm. After selecting the proper grade, bench scale tests should be carried out using precoat beds which have the same thickness as is expected in the full scale unit. Below is a figure showing the set up of apparatus during pre coat testing. Data Correlation In basic filtrations, data is correlated through the application of several simplifying equations that are valid for many but not all equations. The following is a summary of the applied correlations. Dry Cake Weight vs. Thickness The dry cake weight is converted to the weight of the dry cake per unit area per cycle (W) then these values are plotted as a function of cake thickness as shown below. Dry solids or Filtrate rate The rate of filtration is expressed in terms of dry solids or the filtrate volume is plotted as a function of time on a log-log paper as shown below. Effect of time on Flocculated slurries The filterability of flocculated slurries decreases with time as shown below. Running a series of leaf tests repetitively and at frequent intervals on flocculated slurry helps in establishing the rate of degradation. Scale-up factors In order for a scale up factor to convert a rate calculated from bench scale data to design rate data for use in commercial installations, it must incorporate distinct factor for the following; (a) Scale up on rate The filtration rates that are calculated from bench scale data ought to be multiplied by 0.8 for the commercial units that do employ continuous washing of the filter medium and a scale up factor of 0.9 for those that employ continuous washing of the filter medium. (b) Scale up on cake discharge The experimenter judges the percent of the expected cake discharge and factors the design rate consequently. (c) Scale up on actual area The filtration surface is divided into various sections using some impervious separators. Materials that form a thin and impervious cake will not form across the dividers and hence the actual area will be less than the nominal. Overall scale up factor In order to determine the final design filtration rate, the bench scale filtration rate is multiplied by each of the above scale up factors. BATCH FILTRATION The filtration flow is determined by the pumping mechanism of a filtration system. This acts as the basis for these three categories; i. Constant pressure filtration The mechanism of actuation is compressed gas maintained at a constant pressure. ii. Constant rate filtration The mechanism used is positive displacement pumps. iii. Variable pressure and variable rate filtration This technique uses a centrifugal pump whereby the discharge rate decreases as the pressure increases. Pressure tests Leaf tests The performance of leaf test filters is simulated using a bomb filter. A small leaf (50.8 mm by 50. mm) covered with a filter medium and suspended in a cell that is large enough to hold the sufficient slurry is used. The slurry may also be agitated slightly. Tests should be carried out at several pressures in order to identify the compressibility of the cake solids. Plate and frame tests They should be conducted only if the use of a filter press plant is anticipated. It is advisable to carry out several confirmation tests unless the slurry is filtering very rapidly. Compression-Permeability tests They come as substitutes for leaf tests. They are advantageous for use where the solids are compressible. Scaling Up test results Results from small scale tests can be determined in two ways; as dry weight of solids or as volume of the filtrate per unit of area per cycle. FILTER MEDIA The filter media is required in any filtration system in order to retain the solids. Selection of the right type of medium is essential as it determines the success of the operation. Medium selection in cake filtration involves optimization of the following factors; 1. The ability to quickly bridge solids across its pores after starting the feed. 2. Should not blind easily. 3. Low resistance to filtrate flow 4. Highly resistant to chemical attack and mechanical wear 5. Able to discharge cake cleanly and easily 6. Minimum cost The filtration media can be made from a variety of materials as discussed below. Woven fiber fabrics They are the most common fabrics for use in cake filtration. The weaves may be made from any textile, natural or synthetic fiber. Metal fabrics or screens They are available in an array of weaves that may be made of nickel, brass, copper, stainless steel or other alloys. Pressed felts and cotton batting These filtration materials are used to filter out gelatinous particles from spinning solutions, paints and other viscous liquids. Filter papers They have a wide range of thickness, permeability and strength. They however require a back up plate that is perforated for support. Rigid porous media Are available in sheets/plates and tubes. Polymer membranes They are used in fine particle filtrations. FILTRATION EQUIPMENT Cake filters They accumulate visible quantities of solids on the surface of the filter medium. They are used when the product in the operation is solid, filtrate or both. Batch cake filters Nutsche filters This is a tank with a perforated or porous bottom that acts a filter medium or that supports a filter medium. The slurry is loaded into the filter vessel and separation occurs through gravity flow. Horizontal plate filter It consists of several horizontal and circular plates stacked in a cylindrical shell. Filter press Are simple, low cost filters that are the most used in the world. The two most popular designs are flush plate and plate and frame design. Liquid bag filters They utilize one or more perforated tubes that are supported by a tube sheet. A cylindrical filter bag sealed on one end gets inserted into the perforated tube. A flange is then put on the open end of the filter bag to prevent leakage. External cake tubular filters Consists of vertical tubes supported by a filtrate chamber tube made of wire cloth sheet positioned in a vertical cylindrical vessel. Pressure leaf filters Consist of a flat filtering element supported in a cylindrical pressure cell. The filtering elements (leaves) have filtering surfaces on both sides. These filters may be either horizontal or vertical depending on the shell axis orientation. Centrifugal discharge filter The horizontal top surface filter plates can be mounted on shaft that acts as both a filtrate discharge manifold and as a drive for centrifugal removal of the cake. The main advantage of using centrifugal discharge filters is that they make it possible for the cake to be removed without opening it. Continuous cake filters They are used in situations where the slurry is greater than 5 L/min, the slurry concentration is greater than 1% and the particles have a diameter greater than 0.5 nano meters. Rotary drum filters They utilize a rotary valve arrangement to facilitate the filtrate removal and to allow easy introduction of air where blowback is necessary. Scrapper Discharge filter The filtration medium is caulked into grooves in the drum grid and a scrapper blade that facilitates cake removal. The scrapper blade directs the cake into the discharge chute. String discharge filter Consists of a system of endless strings that pass around the filter drum and they lift the cake off when leaving contact with the drum. The movement of the strings is guided by rollers. Removable media filters The filter medium can be removed and reapplied easily as the drum rotates. This makes it possible for complete discharge of the cake and also offers regenerative washing of the filter medium to prevent blinding. Continuous pressure filters They consist of conventional drum filters that are enclosed in pressure filter vessels. Filtration occurs when the vessel is pressurized. Continuous pre coat filters Are operated as either vacuum or pressure filters. They are not continuous but they have a log batch cycle. Disc filters This is a vacuum filter that consists of a number of discs attached vertically and at intervals on a continuously rotating shaft. Horizontal vacuum filters They are classified as either rotary circular or belt type units. They offer flexibility of choice of the thickness of the cake, washing time and the drying cycle. Horizontal-Table and Pan Filters These are revolving annular tables that are divided into sectors where the top surface acts as a filter medium. Tilting pan filters This is a modification of the table filter whereby the each sector is an individual pan that is inverted for easy discharge. Filter thickeners These are devices that remove a portion of the liquid from the slurry in order to increase the concentrations of solids in the suspension. Clarifying filters These are filters used to separate liquid mixtures containing small solid quantities. Granular media filters These types of filters are mainly used clarification and operate as either pressure or gravity filters. Gravity filters depend on the difference in elevation that exists between the inlet and the outlet. This difference in elevation is very essential as it provides the force required to push the liquid through the granular media. CASE STUDY This case study is intended to show how data from the correlations discussed earlier in this report as well as how knowledge of the various physical characteristics of a certain filter can be utilized in order to determine the size of a filter and its filtration cycle. The case study all involves a disc, a horizontal belt filter and a drum belt. CASE STUDY - Sizing a Disc Filter Introduction This experiment was carried out with the main objective being to determine the size of the filter and the vacuum system capacity that is required in order to dewater 15 metric tons per hour (mtph) of dry solids and produce a cake that contains at least an average of 25 wt % moisture content. The most appropriate equipments for use during this experiment were selected from the following table. Figure showing the typical equipments for cycle design. The following are the physical factors of the equipments chosen and used during the experiment. (i) A maximum effective submergence of 28% (ii) The maximum portion of the filter cycle that is available for dewatering is 45%. (However, for high submergence versions, trunion seals are required with their use being limited to specific applications). The following scale up factors was used; On rate scale up factor is 0.8 On area scale up factor is 0.8 On discharge scale up factor is 0.9 However, the on discharge scale up factor (0.9) can also be increased to 0.97. This is based on previous experience or to a factor of 0.95 if the total filter area is based on the effective area of the disc that is measured. When calculating the filter size, the following procedure was followed: 1. Choose cake thickness. The cake thickness in this experiment was chosen as 1.5cm. This thickness was chosen as it was slightly thicker than the minimum value among the other values listed in the figure below. Figure showing the minimum cake thickness for discharge 2. Choose the cake weight The cake weight was chosen to be 20 kg. This was determined with the help of the graph below which is the weight of the dry cake per unit area plotted against the cake thickness. A graph of dry cake weight against the cake thickness. 3. With the aid of the following figure, the form time, f, was established to be 1.20 minutes. Figure showing the dry cake weight against the time. 4. The following figure was used for the simplified correlating factor of the moisture content. At an average moisture content of 25 wt %, the correlating factor was chosen as 0.04 % Figure showing the cake moisture correlation. The Dry time, d, was then calculated and gotten as 0.80 minutes. This value was gotten by multiplying the correlating factor (0.04) by the weight of the dry cake (20) as shown below; Dry time = d = 0.04 ? 20.0 = 0.80 min. 5. Calculate CT (Cycle time) on the basis of both form time and dry time in order to determine the one which is controlling. This was calculated as shown below; CT form = 1.20 / 0.28 = 4.29 mpr (min per rev.). CT dry = 0.80 / 0.45 = 1.78 mpr. This result therefore shows that it is the cake formation rate that is controlling and also that a cycle time of 4.29 minutes per revolutions should be used. 6. Calculate the overall scale up factor. This is calculated based on the previously presented factors and the calculation is as follows; 0.8 ? 0.8 ? 0.9 = 0.58 7. Design the filtration rate. This is calculated as shown below; Design filtration rate = (20.0 / 4.29) (60 ? 0.58) = 162 kg / h ? m2 8. Calculate the area that is required in order to filter dry solids weighing 15 metric tons per hour. Area required = 15 ? 1000 / 162 = 92.6 m2 Practically, the choice would be the nearest commercial size of the filter that corresponds to the calculated area. 9. Calculate the dry time. This dry time is 45% of the cycle time. Therefore, Dry time = 45% of 4.29 = (45 / 100) * 4.29 = 1.93 minutes This dry time (1.93 minutes) is longer than the required dry time. It is therefore reduced to 1 minute through proper bridging in the filter valve. 10. During the 1.00 minute drying time, the average gas flow rate was found to be 2.41 m3 / m2 ? min. This was gotten through graphical integration from the following figure. Figure showing air flow through cake. 11. Calculate the volume of air flowing during the dry period. This is = 2.41 ? 1.00 = 2.41 m 3 per m 2 per cycle. 10% was also added in order to tolerate for the evacuation of the drainage passages. Therefore, the Total flow = 2.65 m 3 / m 2 ? cycle. 12. Establish the required vacuum pump capacity. This was calculated as follows; Required vacuum pump capacity = Total Flow / Cycle Time = 0.62 m3 / min ? m2of the total filter area After establishing the required vacuum capacity (= 0.62 m3 / min ? m2of the total filter area), a pressure drop within the system should be allowed when specifying the vacuum pump. Observations Based on the experiment and the calculations, it was concluded that an area of 92.6 m 2 was required in order to filter dry solids weighing 15 metric tons per hour. Therefore, putting this into practical use would involve finding a commercial size of the filter that corresponds to this area (92.6 m 2). From the calculations, it was also clearly observed that the vacuum pump capacity required is 0.62 m3 / min ? m2 of the total filter area (92.6 m 2). This implies that a filtration system with a vacuum system capacity of 0.62 m3 / min ? m2 of the total filter area is required in order to effectively dewater 15 metric tons per hour (mtph) of dry solids and produce a cake that contains at least an average of 25 wt % moisture content. References Robert H. Perry, Don W. Green (2008). Perry's Chemical Engineers' Handbook. New York: Mcgraw Hill. Read More