PRODUCTION OF PARTICULATE MATERIALS

Transcription

1 Reproduced with permission from Chemical Engineering Progress (CEP), November Copyright 2004 AIChE. Improve Solids Handling During Thermal Drying Herman Purutyan, John W. Carson and Thomas G. Troxel, Jenike & Johanson Ensuring reliable solids flow during thermal drying operations is a challenging task. Try these tips for handling slurries, pastes or wet granular materials, and achieve a dried product with the desired characteristics. PRODUCTION OF PARTICULATE MATERIALS is a key technology for most chemical processes. Statistics for industrial processes suggest that over 50% of products manufactured by the world s major chemical companies involve solid particles. Research continues to show that operating efficiencies for particulate processes are significantly below those of plants that handle fluids (1). One of the areas that causes process inefficiency is unreliable flow of solids into or through dryers. In the chemical process industries, production of solid or particulate materials often begins with either simple washing or more complex steps that involve crystallization, precipitation, wet agglomeration or other means to produce the desired particle characteristics. The resulting product takes the form of a slurry, paste or wet granular material that, in almost all cases, must undergo solid-liquid separation, followed by thermal drying. Slurry is often pumped into a centrifuge, filter or press to remove the bulk of the liquid before thermal drying. However, for the drying process to be successful, a reliable transfer of thickened slurry, paste or wet cake from the solid-liquid separation system to the dryer, and reliable solids flow without interruption or buildup through the dryer, is required. This step is probably the most difficult to execute properly during thermal drying operations. In this article, the authors examine some of the most common solids-handling issues in drying processes and present practical solutions for successful dryer operation. Since the flow properties of bulk solids usually become worse with higher moisture contents, problems often arise during the handling of the wet cake upstream of the dryer, feeding of the solids into a dryer that may be at a positive or a negative pressure, and flow of solids through the dryer. Handling the wet cake Solid-liquid separation, as well as thermal drying, can be accomplished either continuously or in batches. In batch processes, surge storage often in the form of a hopper or a bin with a minimum capacity of batches must be provided between the separation step and the drying step, which must be able to discharge the wet cake reliably. When the drying step is performed on a continuous basis, the uniformity of the discharge rate from the surge bin also becomes critical, as dryer efficiency is often closely tied to the uniformity of the solids feed rate. Surges in solids flow can result in incomplete drying, whereas loss of feed to the dryer can result in both overdrying the material, which may cause a chemical or physical change, or damage to the dryer due to overheating. Thus, in many cases, the primary design goal of the surge system is to provide a uniform, controlled feed to the downstream dryer. In general, wet materials are prone to flow problems. Feed hoppers, conveyors and chutes are much more likely to accumulate solids, plug or exhibit erratic flow when handling wet solids than when handling dry 26 November 2004 CEP

2 solids. Flow stoppages caused by arching or ratholing, and loss of surge capacity due to material buildup on the walls, are very common in many of these systems. If flow from the surge bin stops, the entire process will come to a standstill. Problems in this part of the system often result in one or two operators continuously manning the hopper, poking and prodding the material to prevent solids plugging. Even in systems where solid-liquid separation is a continuous process that may not require a surge bin, the buildup of solids in transfer chutes, feeders and conveyors causes serious problems. Pluggage can cause material to back up into the filter, press, or centrifuge, resulting in system shutdown. In this case, operators may be enlisted to man the chutes and surrounding equipment to ensure that solids keep moving. Impact of dryer pressure on solids flowrate Flash, rotary and fluidized-bed dryers almost always operate at a higher or lower pressure than the upstream feeding system. Therefore, in almost all applications where there is a continuous feed, pressure isolation is required between the feed and the ambient conditions in the dryer. This usually requires a rotary valve or other means of sealing against the pressure differential, such as a sealing screw, standpipe, eductor or double-gate valve. Materials containing a significant amount of fines (particles smaller than 100 mesh) often exhibit a flowrate limitation not seen with coarser materials. This limitation, caused by two-phase flow effects, occurs when gas movement within the voids between particles results in body forces that are high enough to retard flow. Such behavior arises in systems that operate without a pressure differential across the solids feeder, but it is exacerbated when solids are fed into a higher-pressure environment. Since many of the materials fed into dryers are fine, achieving the desired flowrate is a challenge unless the opening is sized appropriately, based on the properties of the material. Gas pressure gradients acting opposite to the direction of solids flow can initiate the formation of an arch when handling cohesive solids in a hopper. Even if the hopper outlet is large enough for the solids to flow reliably when no gas is present, it is possible for arching or erratic flow to develop if the gas flows upward through the hopper from downstream equipment. Problems are less severe when feeding solids into a dryer at lower pressure. Since pressure isolation devices can be costly, there is often an incentive to minimize their size. However, this may result in flow problems, particularly with cohesive solids and sticky cakes. Difficulties with wet-cake handling can be avoided if the equipment designs are based on the flow properties of the particular material handled. These properties can be used to design hoppers that provide reliable solids flow and prevent buildup and flow interruptions. Flow property information can also be used to size pressure-isolation devices to achieve uninterrupted flow at the desired rates. Ensuring uniform flow through gravity dryers Gravity dryers are silos or bins in which drying is accomplished by injection of ambient or conditioned gas, which usually flows countercurrently to the solids flow. These dryers often provide efficient heat transfer by achieving intimate contact of drying gases and solid particles. Additionally, these dryers can reduce capital expenditures by doubling as storage or surge capacity, and reducing the required footprint for a drying operation. Intimate contact between the gas and solid particles requires that the residence time of the solid particles be uniform and that the gas be distributed uniformly across the vessel. In gravity dryers that operate in a batch mode, a batch is placed in the vessel, the gas is turned on for the required amount of time, then the gas is either turned off or left on while the contents are discharged. In such dryers it is critical to ensure that the gas injection is done in a manner to ensure a uniform upward gas velocity across the cross section as low in the vessel as possible. Also, it is important to determine how the flow properties will change while the material is at rest during the drying cycle. Flow properties of most materials change when left under consolidation without any movement. Appropriate flow properties, including the effect of storage at rest, must be measured and used as a design basis in order to avoid product hang-up following the drying cycle. Gravity dryers can also be used in a continuous mode, where solids move continuously through the vessel and the gas is always flowing. In these dryers, in addition to ensuring that the gas is distributed uniformly, it is also critical that the solid particles have a uniform residence time in the vessel, which requires that the velocity profile of the flowing solids be as uniform as possible. There are two primary flow patterns that can develop in a chute of a bin or a silo funnel flow and mass flow. Both patterns are shown in Figure 1. In funnel flow, an active flow channel forms above the outlet, with non-flowing material at the periphery. Obviously, this would be a poor flow pattern for a continuous flow vessel that is used as a dryer. In mass flow, all of the material is in motion whenever any Funnel Flow Moving Stagnant Mass Flow Figure 1. Two flow patterns that can occur in a bin or silo are funnel flow and mass flow. CEP November

3 is withdrawn from the hopper. Material moves toward the outlet from the center as well as from the periphery. This does not imply or require that all the particles are flowing at the same velocity, especially particles that are in the converging section of the hopper. Material flows more slowly at the walls than at the centerline of the hopper, due to friction at the walls. In some cases, a velocity differential may be beneficial (e.g., for in-bin blending). In other cases, the most economical design that achieves mass flow may result in a substantial velocity profile, even though it would meet the general design requirement of providing reliable flow everywhere in the vessel. In dryer applications, a uniform solids velocity is essential to enforce uniform residence time. This usually requires a design that not only provides mass flow, but also provides a low velocity gradient in the hopper. In continuous dryers, the amount of gas that can be injected may be limited, since too much countercurrent gas can upset solids flow. In order to maintain a uniform residence time in the vessel, fluidization of the bed must be avoided. Gas injection at too high a rate can create local fluidization and regions of high solids velocity, resulting in incomplete drying, or excessive gas usage to make up for process inefficiencies. Flow properties of solids The application of scientific principles and measured flow properties of bulk solids in the design of solids handling equipment has evolved over the last century to a reasonably well developed state. Today a number of standardized test methods are available to characterize properties of bulk solids (2, 3). These flow properties are very Figure 2. External view of the original zirconium sulfate wet-cake transfer chute below the belt press. Figure 3. Original wet cake transfer chute design with a constant-pitch, constant-diameter screw in a V-trough. Flow occurs over the first pitch of the screw, only. useful in designing reliable handling equipment for dryers. Some of the most useful flow properties are cohesive strength, wall friction, compressibility, permeability and critical chute angles (4). For instance, in contact bed dryers, which use counterflowing air or other gas for drying, flow properties of the solids play a critical role in determining the design needed to Figure 4. Modified wet cake transfer chute design with twin mass-flow screw feeder in a U-trough. Flow occurs from the entire chute outlet. achieve the desired combination of residence time, gas velocity, pressure drop and solids velocity profile. An understanding of the flow properties of wet materials will greatly increase the chances for successful startup of a new drying system or retrofit of a problematic existing system Case studies Wet-cake handling. In a zirconium sulfate processing plant, wet cake is discharged from a belt press into a chute that functions as a surge hopper to meter the material into a downstream flash dryer (Figures 2 4). On first inspection, it appeared that the design of the chute and feeder were fairly conservative, since the chute had vertical walls and mated directly to a screw conveyor that covered the entire length of the chute. However, even with what appeared to be a relatively safe design, this system experienced constant plugging problems at startup. Examination of the flow properties of the wet cake provided an explanation for this behavior, as well as information for correcting it. The solids flow properties are summarized in Tables 1 and 2, and respectively displayed in Figures 5 and 6. The material had significant cohesive strength and could easily have formed an arch over the width of the chute. But why would this occur in a vertical chute? Arching is a phenomenon that usually occurs in converging hoppers. One of the flaws in the system was that the feeder attached to the bottom of the chute had a V-trough. This added a small, but, significant, convergence at the bottom of the chute, and provided the support necessary for an arch to form. Another design flaw was the use of a constant-pitch conveying screw, which, in this application resulted in flow only through a very short length of the chute. The designers may have expected the system to function as a chute and conveyor, but they did not consider that each time the belt press emptied, it would fill the chute to a depth of several feet. In this mode of operation, the chute must be designed to function as a bin and feeder November 2004 CEP

4 The simplest solution to the system s flow problems was to replace the screw with one that was designed to feed over the entire length of the chute. In order for this approach to work, the material would have had to be able to flow through the converging portion of the feeder trough above the screw. Review of the cohesive strength data indicated that this was not a likely solution because the cake was capable of forming an arch over a slot with a width measuring 40% wider than that the width of the chute. The changes needed to make the system work reliably are shown in Figure 4. The existing V-trough was replaced with a standard U-trough and a twin screw that feeds from the entire outlet area of the chute. The modifications were implemented and the system has been working reliably for over three years. Continuous gravity dryer. One of Dow Chemical s North American facilities produces a flaked material. At a certain point in the process, the flakes exhibited excess surface moisture, which had to be removed. Dow s engineers investigated whether two unused blending silos could be converted to gravity dryers in which hot, dry gas would be fed countercurrently to the flowing material. Modification of the existing blending silos would be much less costly than the installation of a commercially available dryer, and would have a shorter turnaround time. Process requirements dictated that the vessels dry the flakes uniformly to less than 1,000 ppm of water, thus removing approximately 10% of water by weight. The drying also had to be done in less than 60 min to assure product stability. Engineers evaluated the feasibility of retrofitting the blending silos to meet Dow s requirements. The basic dimensions of the dryer were determined from the process requirements and the materials flow properties, which were Shear stress, lb/ft L, mill finish, 4 h Epoxy coating 316L, mill finish 316L, 2B finish Pressure normal to the wall, lb/ft 2 Figure 6. Wall friction data for zirconium sulfate at 37.5% moisture. measured. The design of the vessel and the gas introduction system were selected to keep the superficial gas velocity low enough to prevent the flakes from becoming locally fluidized and/or airborne. The engineers used heat-balance calculations to estimate the amount of drying air needed to remove the required amount of moisture. Design limits for gas velocities were based on the permeabilities of the flakes, which were Strength, lb/ft hours at rest, 140 to 72 F No storage at rest, 140 F Figure 5. Cohesive strength vs. consolidating pressure for 37.5% moisture zirconium sulfate during continuous flow and after 4 h storage at rest. Table 1. Calculated minimum outlet dimensions for mass flow storage bins. Circular Outlet, ft Consolidating pressure, lb/ft 2 Slot Width for Wedge or Transition Hopper, ft No storage at rest, 140 F Gravity loading gravity loading h at rest, 140 F to 72 F Gravity loading gravity loading >10.9 >5.4 This data applies to only the sample tested, and should not be taken as representative of zirconium sulfate in general. The dimension given for a slotted outlet applies when the length of the slot is at least 3 times its width. The values for 1.25 gravity loading represent conditions of excess consolidation that may occur if vibration or other forces exist that consolidate the solids to a greater extent than do the solids weight. The increase in strength indicates that using vibrators with this material will not be an effective way to maintain reliable flow, and may make problems worse. Table 2. Calculated maximum hopper wall angles (from vertical pose) for flow to occur at wall. Conical hopper, Flat sides of wedge or deg transition hopper, deg No storage at rest Epoxy coating Type 316L stainless steel (SS), no storage h at rest, 140 F to 72 F Type 316L SS, 2B finish, no storage 4 16 Type 316L SS, 2B finish, no storage This data applies to only the sample tested, and should not be taken as representative of zirconium sulfate in general. The dimension given for a slotted outlet applies when the length of the slot is at least 3 times its width. Type 316L SS with a 2B finish showed no increase in friction after storage at rest. CEP November

5 measured over a wide range of gas flowrates. The units have been successfully operating for over four years. In fact, this project received a Dow Technology Center Award because of its use of significant new technology and savings in capital expenses of over $1 million. Batch gravity dryer. During the production of sugar, it is necessary to condition (or dry) the sugar before packaging to prevent caking and the formation of lumps before the product reaches customers. Drying is accomplished in a conditioning silo through the introduction of dry air at about 1 ft 3 /min per ton of sugar storage capacity and reduces the sugar s moisture from approximately 0.06% (upon entering the silo) to %. Sugar-conditioning silos vary widely in size and capacity due to different plant production capacities. A typical silo consists of a 23-ft-dia. by 130-ft-tall flat-bottomed cylinder. A 16- ft-dia. inverted cone is centrally located at the base of the silo. Conditioning air is introduced through over one hundred 2-in.-dia. holes covering the surface of this cone. An exhauster blower located outside the silo pulls a vacuum at the top, which assists the flow of the conditioning air up through the sugar mass. Discharge occurs through 12 small cones equally spaced in the annulus between the bottom of the center cone and the silo walls. The bottom of each discharge cone has a stainless steel pipe connected to it, and all of the pipes terminate in a single collection hopper. There is typically no means of controlling or monitoring the discharge of solids through each of the twelve pipes. Silos like this are usually built in pairs, with one silo operating in conditioning mode and the other operating as a storage vessel for the conditioned sugar before the sugar is loaded into a shipping or packaging silo. Numerous problems often occur in sugar conditioning silos of this type: Lumps often form, requiring the silos to be emptied out and cleaned at regular intervals perhaps once a year or more often. Discharge from the various outlets is not reliable or controlled,which necessitates constant poking and prodding to keep the sugar flowing. It is not uncommon to find the stainless steel pipes severely dented by operators. A thick layer of sugar often builds up on the silo walls, causing further quality-control problems. Vertical cracks develop in the reinforced concrete silo walls, allowing ingress of moisture, loss of conditioning air and loss of insulation, resulting in condensation inside the silo and concern about the silo s structural integrity. Such challenges are common when a funnel-flow silo design is used. Even though the conditioning air is introduced only while the silo is being filled and no discharge is taking place, a funnel flow pattern upon discharge results in non-uniform sugar flow and stagnant regions that can cake up. Furthermore, the walls of multiple outlet silos usually experience non-uniform pressure around their circumference. This imposes severe bending moments on the walls, which can lead to cracking in reinforced concrete silos and severe distortion in metal silos. The solution to problems like these is to use a mass flow design, which eliminates dead regions and promotes a first-in-first-out discharge pattern. Special designs of the conditioning air inlet can efficiently introduce and distribute the air throughout the sugar mass. Even though the air flowrate is high, the sugar flowrate through the outlet is not affected, since the air is turned off prior to commencing discharge. Designs of this type have been successfully implemented at several plants in North America and Europe over the last 15 years. CEP Literature Cited 1. Merrow, E. W. Linking R&D to Problems Experienced in Solids Processing, Chemical Engineering Progress, 81 (5), pp (May 1985). 2. American Society of Testing and Materials (ASTM) D , Standard Test Method for Shear Testing of Bulk Solids Using the Jenike Shear Cell, ASTM (2000). 3. American Society of Testing and Materials D , Standard Shear Testing Method for Bulk Solids Using the Schulze Ring Shear Tester, ASTM (2002). 4. Carson, J. W., and Marinelli, J., Characterize Bulk Solids to Ensure Smooth Flow, Chemical Engineering, 101 (4), pp (April 1994). HERMAN PURUTYAN is vice president of Jenike & Johanson (One Technology Park Drive, Westford, MA 01886; Phone: (978) ; Fax: (978) ; hpurutyan@jenike.com). Since joining the firm in 1991, he has designed reliable handling systems for a wide range of materials for the food, pharmaceutical and chemical industries. He lectures frequently on the subject, has published numerous articles on the field of bulk solids handling and is the holder of two patents. Purutyan received his bachelor s and master s of science in mechanical engineering from Worcester Polytechnic Institute in Worcester, MA, and his MBA from Babson College in Wellesley. He is a member of the ASME Structures for Bulk Solids Committee. JOHN W. CARSON is president of Jenike & Johanson ( jwcarson@jenike. com), where he has been active in research, consulting and management of the company. Carson has written more than 90 articles dealing with solids flow, including bin and feeder design, flow of fine powders, design of purge vessels, and structural failures of silos, and lectures extensively on the topic of fine powder storage and flow of solids. He received a BS in mechanical engineering from Northwestern Univ., and a PhD in mechanical engineering from Massachusetts Institute of Technology. Carson is a member of AIChE, ASME, ASCE, ASTM International, and is a founding member of AIChE's Powder Technology Forum. THOMAS G. TROXEL is vice president of Jenike & Johanson ( tgtroxel@slo.jenike.com).troxel has been involved in many aspects of the firm s consulting and research activities on a wide range of projects, including flow-properties testing, modeling, blending, pneumatic conveying and fluidization. He has been a major force behind the firm s expansion of service in the areas of mechanical design engineering and supply of custom built equipment. Troxel has published numerous articles and papers in the field of bulk solids handling, and lectures frequently on the subject. Troxel has a BS in engineering from California Polytechnic State Univ. (San Luis Obispo, CA) November 2004 CEP