Phosphoric Acid Plant Reaction Cooling System Design

Transcription

1 By Eric Ramella 40 th Annual AIChE Clearwater Conference June 2016

2 Table of Contents Abstract Introduction Background Reaction Cooling System Vacuum Cooler Cooler Circulation Pump Reactor Cooler Precondenser Cooler Condenser Cooler Vacuum System Troubleshooting Conclusion References

3 Abstract The phosphoric acid production reaction between sulfuric acid and phosphate rock is exothermic. This heat must be rejected in order to maintain a stable temperature within the reactor. The reactor cooling system design influences phosphogypsum morphology, filtration rates, and P 2 O 5 losses. The operating temperature of the reactor determines which of the three types of phosphogypsum crystals are formed: Anhydrate, Dihydrate, or Hemihydrate. The reactor cooling system design methodologies have improved throughout history out of necessity due to cost reduction efforts to realize economies of scale benefits through increased plant capacities, and in turn, increased heat loads. Current design practice maintains reactor temperature through the use of a low level vacuum cooler. Proper design of the reaction cooling system is necessary to promote plant stability, to provide operational control, to minimize maintenance, and to maximize profitability. This report will describe and evaluate the purpose and evolution of reactor temperature control, current practices, associated equipment, potential problems, and design solutions. 2

4 1. Introduction The typical phosphoric acid plant (PAP) production utilizes the wet process reaction between the calcium phosphate in the phosphate rock and sulfuric acid. This exothermic reaction, combined with the heat of dilution of sulfuric acid, produces a heat load which must be dissipated in order to provide the desired reaction chemistry. The following reaction equations show the Dihydrate variation of the core two-step reaction used for most industrial scale phosphoric acid production: Step 1: Ca 3 (PO 4 ) 2 + 4H 3 PO 4 + 3H 2 O 3CaH 4 (PO 4 ) 2 H 2 O Step 2: 3CaH 4 (PO 4 ) 2 H 2 O + 3H 2 SO 4 + 3H 2 O 3CaSO 4 2H 2 O + 6H 3 PO 4 Overall Dihydrate Reaction: Ca 3 (PO 4 ) 2 + 3H 2 SO 4 + 6H 2 O 3CaSO 4 2H 2 O + 2H 3 PO 4 + Heat In the Hemihydrate variation of the phosphoric acid reaction, the following chemistry applies: Overall Hemihydrate Reaction: Ca 3 (PO 4 ) 2 + 3H 2 SO H 2O 3CaSO H 2O + 2H 3 PO 4 + Heat While phosphoric acid is produced in either case, the type of phosphogypsum crystal formed varies according to operating temperature. A lower relative operating temperature results in the formation of the Dihydrate phosphogypsum crystal structure. A higher relative operating temperature leads to the formation of the Hemihydrate or the Anhydrate phosphogypsum crystal structures. Of the three possible reaction routes, only the Dihydrate and Hemihydrate are utilized in industrial scale production of phosphoric acid. Figure 1 illustrates the typical relationship between acid strength, gypsum (aka Calcium Sulfate) crystal formation type, and operating temperature. The figure also indicates the typical reactor operating points in the Dihydrate and Hemihydrate process designs respectively. The control of reactor temperature is particularly important in the Dihydrate process where the normal operating point is close to the transition line. 3

5 Hemihydrate Temperature ( C) 60 Dihydrate % P 2 O 5 Figure 1: Calcium Sulfate Crystallization Morphology The quality and consistency of gypsum crystals produced, whether Dihydrate or Hemihydrate, impact filter operation. The acid viscosity also plays a role. These factors combine to affect the filtration rate, the frequency of filter cleaning required, and the percentage recovery of P 2 O 5. These characteristics help define the plant operating factor and the rock cost per ton of product, a primary source of OPEX for the plant. The plant operating factor limits the plant production rate, thereby determining the achievable annual revenues of the plant. For various reasons, the Dihydrate process is currently the most commonly used phosphoric acid production process worldwide. Reactor temperature control represents one of several key factors that determine the quality of crystal growth. Other factors include excess sulfate level, rock particle size distribution, surface area, acid impurities, retention time, acid concentration, and viscosity. The control of reactor temperature must not only be on an average temperature basis, but should also include maintaining a nearly homogenous temperature profile throughout the reactor. This means eliminating hot spots or points in the reactor where temperature is drastically higher than the surrounding fluid. The formation of hotspots within the reactor will lead to inconsistent crystal formation. This phenomenon is minimized through proper recirculation 4

6 and agitation design within the reactor itself, independent of the method of heat transfer utilized. For the purposes of this report, it is assumed the reaction will take place in an annular reactor design. Though the temperature is considered nearly homogenous, the ideal crystal growth environment requires a solubility swing which must come from a temperature gradient within the reactor. In fact, a truly homogenous temperature in the reactor would provide poor gypsum crystal formation. The varied, yet controlled temperature gradient is created through the use of a disengagement zone via external cooling and also careful positioning of rock and sulfuric acid introduction to the reactor. The reactor cooling system design process incorporates the consideration of several concepts. Identifying and understanding the implications of the phosphate rock composition is critical. The magnitude of the heat load itself must be established based on the nameplate capacity of the reactor. The desired reaction route must be selected to determine that since the Hemihydrate process generates less heat relative to the Dihydrate process. The design must facilitate the transfer of heat load as well as adequate circulation and mixing. The heat load may be dissipated through air cooling or evaporative cooling. The design may also need to account for the recovery of cooling media in the case of evaporative cooling. The reactor control should be maximized and scaling minimized. Further, the use of evaporative cooling creates the potential for P 2 O 5 losses which must be mitigated. The optimal cooling system should be designed to maximize plant profitability by viewing it as a whole system rather than individual components. While certain equipment is required to operate the wet process phosphoric acid plant, such as a reactor, other equipment is subject to owner preference. The specific type of equipment may also be subject to variation based on owner input. A responsible plant design should consider the capabilities and comfort level of the owner s operations and maintenance staff. The design should also reflect any geographic or political limitations which may exist such as proximity to the ocean or local environmental regulations. The design process requires technical expertise, but often underestimated is the importance of communication throughout the project. The design engineer must provide this valued service to the client along with the technical knowledge necessary to properly design the plant. This communication must not only include the ability to clearly express concepts and recommendations to the owner, but also to listen and interpret how the owner s input should impact design decisions. The design engineer must know the right questions to ask early in the project to lay the groundwork for staying within the schedule. It is also the responsibility of the design engineer to display confidence and competence in order to gain and maintain the owner s trust since technical expertise is only translated into the design if this trust exists. Nothing hinders the success of a project s design optimization and timely completion more than the engineer s inability to communicate effectively. 5

7 2. Background The air cooling method used to control reactor temperature consists of pulling large quantities of air through the reactor with high capacity exhaust fans. The fans discharge to the fume scrubber which removes fluoride from the vapor stream prior to discharge to atmosphere. Historically, this was the method of choice due to the limited CAPEX and OPEX requirement compared to evaporative cooling. The air cooling method has been implemented successfully for smaller reactor sizes and is still used today in some plants. The evaporative cooling method includes the use of a more complex system of equipment which takes advantage of the heat of vaporization of water to efficiently dissipate large heat loads. The system consists of a series of equipment which vaporizes water from the reactor slurry stream and then transfers heat load out of the system. The air cooling method has practical limitations when attempting to dissipate large heat loads. The operating factor for the air cooling method is less than the evaporative cooling method which would lower plant revenues to the point where the economics favor the evaporative cooling method despite any advantage in terms of CAPEX and OPEX. The seasonal and/or geographical climate limitations associated with air cooling may limit plant production rates as does the more frequent maintenance problems associated with operating high capacity fans in extreme operating conditions. Overall, the evaporative cooling method is more appropriate for larger reactor nameplate capacities. In last few decades, the design trend is towards larger equipment. The economies of scale benefit to having increased reactor nameplate capacities has made evaporative cooling the preferable means of controlling reactor temperature. The CAPEX per ton of P 2 O 5 produced is significantly lowered as reactor nameplate capacities increase. The savings can be more than 20% less CAPEX per ton of P 2 O 5. At the same time, OPEX and revenues per ton of P 2 O 5 produced follow a linear trend in relation to plant production levels. Therefore, the overall plant profitability may be greatly improved. In following along with the previously mentioned design trends, this report focuses on the evaporative cooling method used for larger sized Dihydrate reactors, approximately 1000 metric tons of P 2 O 5 per day or more. This current reaction cooling system design trend includes the use of a Vacuum Cooler which interacts with the reactor through the circulation stream provided by the Cooler Circulation Pumps as shown in Figure 2. This flow discharges from the attack tank, passes through the Vacuum Cooler, and returns to the maturation tank providing cooling and an external source of reactor circulation. The reactor design incorporates internal features which promote internal reactor circulation and mixing. The evaporated water discharges from the Vacuum Cooler and is transferred to a Cooler Precondenser where any entrained acid as well as some heat and evolved fluoride are recovered to the filter wash stream. The remaining vapor stream continues on to the Cooler Condenser where the remaining majority of the evaporative load is recovered through condensation. The condensate is discharged to the sea, pumped to a cooling tower, or pumped to a cooling pond depending on geographic limitations, environmental regulations, and owner preference. 6

8 Cooling Water Supply To Atmosphere Process Water Vapor Vapor Optional Circulation COOLER PRECONDENSER COOLER CONDENSER Reactor Slurry to Vacuum Cooler VACUUM COOLER Reactor Slurry Return Optional Makeup Water Blowdown to Filter Wash COOLER CIRCULATION PUMP REACTOR Cooler Precondenser Seal Tank Cooler Condenser Seal Tank Cooling Water Return Figure 2: Typical Phosphoric Acid Plant Reaction Cooling System The optimal reaction cooling system design should provide operational benefits including: Production rate stability Higher P 2 O 5 recovery Lower filter wash heating requirement Improved Fluorosilicic Acid (FSA) recovery Optimal temperature and excess sulfate profile in the Reactor Minimized scaling Compliance with environmental regulations 7

9 3. Reaction Cooling System 3.1 Vacuum Cooler The Vacuum Cooler operates under vacuum at about 275 mmhg abs to lower the boiling temperature of the water in the reactor slurry in order to provide evaporative cooling while maintaining an optimal temperature environment for crystal growth. The vessel construction is rubber lined carbon steel with acid brick overlay on the bottom up to slightly above the operating liquid level. The vessel wall and floor thickness is designed to accommodate the stress created when it is full of liquid due to the potential for upset conditions. The vessel bottom should be sloped toward the outlet to minimize solids settling. The distribution nozzles should be designed to maximize spreading of the liquid and increasing the liquid surface area to maximize evaporation capacity. The Vacuum Cooler may be operated at a high or low elevation level. The low relative operating elevation level has inherent advantages over the high level alternative. The high level requires a high head centrifugal pump whereas the low level allows for the use of a low head axial flow pump. The power consumption is much lower for the low head pump alternative given the large flow rate required. Further, the sheer in the high head pump may cause crystal breakage which leads to lower filtration rates. In recent years, the design trend has been geared entirely towards low level vacuum cooling for these reasons. The carryover of acid in the exiting vapor stream is one of the biggest challenges with operating the Vacuum Cooler. The high evaporation flow rate necessary to dissipate the reactor heat load can lead to a high upward vapor velocity. In addition, vacuum pressure fluctuations may also cause pullovers. Though acid entrainment in the exiting vapor stream is somewhat unavoidable, the diameter of the unit should be as large as the owner s budget and the plant layout will tolerate to minimize potential P 2 O 5 losses. The down leg of the Vacuum Cooler forms scale over time during normal operation. The temperature drop in the reactor slurry pushes it into a state of supersaturation causing spontaneous nucleation of phosphogypsum crystals; the larger the temperature differential, the higher the scaling rate. This scaling in the Vacuum Cooler down leg eventually causes the liquid level to build to an undesirable level within the unit. The down leg diameter should be slightly over-sized to extend intervals between cleanings and minimize down time. The additional annual production time tends to pay for any additional costs as a result of the larger pipe size. The design should seek to optimize the balance between pipe cost, pump power cost, and plant operating factor. 3.2 Cooler Circulation Pump The flow rate through the Vacuum Cooler dictates the temperature differential required to dissipate the reactor heat load. A large temperature differential creates scaling and control problems while too small a differential causes pump design to exceed practical constraints as well as economic feasibility. The Cooler Circulation Pump design ideally maintains a 2.5 C temperature differential by providing the appropriate high flow rate. 8

10 The pump type must satisfy the need for a high flow rate while tolerating a low available NPSH. The axial flow pump design is ideally designed to provide high flow rates under these conditions. The pump may be positioned upstream or downstream of the Vacuum Cooler in a push or pull configuration respectively. Historically, the installation of the pump downstream of the Vacuum Cooler has led to NPSH problems, gas entrainment, decreased flow rates, higher temperature differential, increased scaling, and more shut downs. For these reasons, the Cooler Circulation Pump is recommended to be installed in the reactor upstream of the Vacuum Cooler. The axial flow pump suction may be horizontal or vertical based on owner preference. The horizontal design receives some NPSH advantage over the vertical design with sufficient submergence. During shutdown, the horizontal design requires an isolation valve and draining outside the reactor whereas the vertical design is self-draining within the reactor. The horizontal design creates the potential for unintended reactor draining if not properly designed and installed. Overall, both design types demonstrate successful operation at existing plants around the world. 3.3 Reactor The external circulation provided by the Cooler Circulation Pump and the cooling provided by the Vacuum Cooler facilitate the transfer of heat load out of the reactor, but they do not fully create a controlled temperature and excess sulfate profile within the reactor. The Reactor design must also include an internal circulation which optimizes mixing and minimizes hot spots. The Jacobs reactor incorporates an annular tank design, as shown in Figure 3, to optimize flow patterns within the reactor. Internal overflows, underflows, and agitation enhance the ability to control reactor temperature and excess sulfates. These flows add to the effect of the external circulation to help improve crystal quality which minimizes water soluble (W.S) P 2 O 5 losses in filtration, and to help improve excess sulfate control which minimizes citrate insoluble (C.I.) or soluble (C.S.) P 2 O 5 losses from unreacted rock or co-crystallization respectively. The normal operation of the reactor involves managing foaming due to reaction with organics in the rock and carbon dioxide release in a calcium carbonate side reaction. As mentioned in the Cooler Circulation Pump section, gas entrainment can cause decreased Cooler Circulation Pump flow rates which leads to a higher temperature differential in the Vacuum Cooler, increased scaling, and increased downtime due to more frequent cleaning. It may even lead to pump cavitation and excessive wear on internal parts. Further, the presence of too much reactor foam may cause excessive carryover leading to P 2 O 5 losses. Attempts to minimize defoamer expense may actually lead to an overall loss in profitability due to the net negative impacts on the reaction cooling system. In contrast, not all rock contains the organics to justify defoamer use. Therefore, the owner should carefully consider the need for defoamer, the proper type, and the proper dose to the reactor. 9

11 Figure 3: Jacobs Reactor Design 3.4 Cooler Precondenser The Cooler Precondenser serves multiple roles in the production process. One role is that of a carryover safety net which recovers acid entrainment lost from the Vacuum Cooler. A second role is that of a heat exchanger which recovers some energy to heat the blowdown that becomes the filter wash. The unit also acts as a fluoride scrubber. In serving these purposes, the overall P 2 O 5 recovery of the plant is improved, the filter wash heating requirement is lowered, and the fluoride emissions are limited. The Cooler Precondenser is optional and not required to operate a phosphoric acid plant. With proper design, operation, and maintenance, it can improve overall plant profitability and emissions control for the reasons previously mentioned. If Fluoride recovery is not a concern, P 2 O 5 carryover could be recovered in a more economical way through the use of a spray duct and a separation elbow. For the purposes of this report, it is assumed that the owner determines the operational benefits of a Cooler Precondenser outweigh the unit cost. The Cooler Precondenser system design includes the primary vessel, a seal tank, and a circulation pump. The vessel is FRP or rubber-lined carbon steel construction. It is sized to provide a diameter and upward vapor velocity which eliminates entrainment according to the relevant Souder-Brown factor. The diameter of the down comer pipe is sized using the appropriate self-venting Froude number based on experience to eliminate unwanted liquid retention and increased liquid level within the unit. A conservative approach is best when sizing the down leg to extend time between cleanings due to scaling. A circulation pump discharges a water circulation rate to spray nozzles within the vessel. A low chloride water source is suitable in order to use the blowdown as filter wash, thereby returning recovered components to the process. The nozzle spray pattern is designed to 10

12 maximize coverage and heat transfer surface area while not creating additional risk of entrainment. The nozzle installation may be multilevel and offset to optimize NTUs. The circulation rate may be as high as 10 times the blowdown to also optimize heat transfer and fluoride recovery efficiency. The circulation pump transfers a portion of the flow rate to the filter wash tank inlet providing some or all of the filter wash water makeup. A once-through design may be used to lower pump costs, but at the expense of other benefits. The higher recirculation flow design creates improved heat transfer efficiency and fluoride scrubbing benefits that would not be realized fully in a once-through design. 3.5 Cooler Condenser The Cooler Condenser recovers the majority of the heat load by condensing most of the water vapor created in the Vacuum Cooler. The goal of the Cooler Condenser design is condensation efficiency. In this way, the discharge to atmosphere meets the fluoride content limitations in the vapor stream as required by environmental regulations and the valuable water resource is recovered in most cases. The condensation may be accomplished using process water or sea water depending on geographic or regional constraints and owner preference. Unlike the Cooler Precondenser blowdown, the Cooler Condenser blowdown does not impact the filter water balance. This provides the flexibility to either isolate the stream from directly affecting the process acid or to send it out to sea. The cooling water consumption costs may be significantly decreased by using sea water if local environmental regulations tolerate FSA discharge to sea. For regions where FSA discharge to sea is prohibited or impossible, the process water used to provide condensation interacts with a cooling tower or cooling pond which dissipates the heat load through evaporation. The Cooler Condenser vessel design, as with the Cooler Precondenser design, must account for interaction with FSA rich streams by using rubber lined carbon steel or FRP construction. However, given the difference in purposes between the two units, the Cooler Condenser operates with a smaller required vessel diameter and a much higher liquid flow rate. The unit is typically sized for about 125 gpm per square foot of cross sectional area. While the diameter may be smaller relative to the Cooler Precondenser, the height of the Cooler Condenser vessel is usually larger. As with the Cooler Precondenser design, the down leg pipe is recommended to be sized to provide a self-venting Froude number. The Cooler Condenser system also consists of a primary vessel and a seal leg. However, the seal leg typically gravity drains to a trench and on to a sump or the sea rather than being pumped as in the case of the Cooler Pre-condenser system design. 3.6 Cooler Vacuum System The Cooler Vacuum System design should include a vacuum pump or a steam ejector system. The barometric condenser down leg may be used to create vacuum when the noncondensable load is low such as in the case of some concentration system designs. However, in the case of the flash cooler, the carbon dioxide and fluoride content in the rock creates a non-condensable load much larger than the barometric down leg can typically 11

13 handle reliably. The use of this method for reaction cooling system leads to inadequate and/or unstable vacuum pressure in the system. The vacuum pump and the steam ejector will both provide sufficient vacuum pressure in the system through the use of different mechanisms. The vacuum pump converts electricity into vacuum pressure. The steam ejector uses steam energy to create vacuum pressure. The units may be incorporated individually into the design or combined in series. When a steam ejector is placed upstream of a vacuum pump, it will lower the inlet vacuum pressure design requirements of the vacuum pump. Since the steam ejector requires less footprint and less CAPEX, this may optimize the vacuum system. The typical vacuum pump is capable of pulling much more vacuum than is necessary in the Vacuum Cooler. While too little vacuum leads to limited production, too much vacuum leads to pullovers in the Vacuum Cooler. A vacuum pressure regulating valve can be a critical tool used to minimize these pullovers. A properly sized vent valve helps the operator to maintain a stable system vacuum pressure and to prevent P 2 O 5 losses. The vacuum pump seal water can be provided by process water or sea water. If process water is used, it can be set up on a closed loop system which conserves water but may operate at a higher relative temperature which decreases pump capacity. However, if run through a small cooling tower, that capacity effect can be eliminated. The vacuum pump is sensitive to water quality, so careful monitoring of water quality is critical in a closed loop system. A once through system may maximize pump capacity and eliminate quality concerns in comparison if the water source is cool enough. If the water source is too hot, this alternative would not be viable. The vacuum pump alternative provides more vacuum pressure consistency and reliability than the steam ejector alternative, but at significantly higher capital cost. The system must be designed with appropriate interlocks to protect the vacuum pump in case of an upset condition. The steam ejector design requires less capital cost, but may subject downstream piping or an FRP condenser to temperatures above their respective ratings. The steam ejector system is also subject to fluctuations in the steam pressure which may lead to pullovers in the Vacuum Cooler and P 2 O 5 losses. In addition, seasonal temperature fluctuations may impact the performance of both and potentially lower production rate capability if not designed appropriately. However, the vacuum pump design can be adjusted to higher capacity after installation more easily whereas the steam ejector design is less adaptable once installed. The decision between the alternatives may also depend on the owner s intentions with regards to steam usage and availability. If the overall chemical complex steam system design is geared towards maximizing power production in the turbine generator area, and steam supply is limited, the vacuum pump alternative may be most appropriate. However, if the overall complex design creates an excess steam supply, then perhaps the steam ejector system provides the most economically favorable solution. 12

14 4. Troubleshooting Process Symptom(s): Cooler Circulation Pump rates decrease or stop Likely Root Cause(s): 1.) Scale build up in Vacuum Cooler down leg 2.) Excessive reactor foam 3.) Insufficient NPSH 4.) Low vacuum decreases cooling and increases temperature triggering interlock Design Solution(s): 1.) Clean and/or resize down leg 2.) Increase defoamer rates and/or quality 3.) Adjust from pull to push configuration if applicable OR Clean Reactor down leg compartment scale OR adjust reactor level OR check level instrument calibration 4.) Inspect vacuum system for leaks and repair as needed Process Symptom(s): Increased scale formation in Vacuum Cooler down comer pipe Likely Root Cause(s): 1.) Change in rock composition 2.) Higher reactor slurry temperature differential across Vacuum Cooler 3.) Poor sulfate control Design Solution(s): 1.) Increase pipe size to extend time between cleanings AND/OR eliminate non-vertical sections of pipe run as much as possible 2.) Increase flow rate of reactor slurry to Vacuum Cooler through resizing pump, additional defoamer, AND/OR adjust from pull to push configuration if applicable. 3.) Install appropriate sulfate control model 13

15 Process Symptom(s): Decreased filtration rates Likely Root Cause(s): 1.) Crystal breakage 2.) Higher reactor slurry temperature differential across Vacuum Cooler 3.) High reactor slurry viscosity from rock impurities 4.) Poor sulfate control 5.) Poor crystal growth Design Solution(s): 1.) Lower reactor agitator rpm OR Switch to Low Level Vacuum Cooler with low head axial flow pump rather than high head centrifugal pump 2.) Increase flow rate of reactor slurry to Vacuum Cooler through resizing pump, additional defoamer, AND/OR adjust from pull to push configuration if applicable 3.) Monitor/control rock quality through blending of various rock qualities 4.) Install appropriate sulfate control model 5.) Add aluminum to reactor to increase Al 2 O 3 /P 2 O 5 ratio based on testing Process Symptom(s): Cooler Circulation Pump wetted parts wearing out quicker than desired Likely Root Cause(s): 1.) Erosion from reactor slurry solids 2.) Corrosion from chlorides or fluorides Design Solution(s): 1.) Increase alloy grade of wetted parts with higher chromium 2.) Increase alloy grade of wetted parts with higher Pitting Resistance Equivalent Number (PREN) AND/OR control slurry oxidation state by adding reactive silica 14

16 Process Symptom(s): Low vacuum or no vacuum from Vacuum Pump Likely Root Cause(s): 1.) System leaks 2.) Hub key damaged preventing rotor from turning 3.) Belt broken preventing shaft from turning 4.) Insufficient Cooler Condenser water Design Solution(s): 1.) Check and/or seal system 2.) Replace hub 3.) Replace belt 4.) Increase Cooler Condenser water flow rate Process Symptom(s): Brick lining failure Likely Root Cause(s): 1.) Improper installation or materials (ie. flat surface, asphalt layer, no air pockets, quality check, etc) 2.) Scale build up overloading adhesion 3.) Lack of inspection and maintenance program Design Solution(s): 1.) Reevaluate materials and install according to proper procedures 2.) Clean scale and avoid thick scale build up 3.) Implement proactive inspection and maintenance program 15

17 Process Symptom(s): Rubber lining failure Likely Root Cause(s): 1.) Improper installation or materials (ie. surface preparation, wrong rubber, curing method, quality check, etc.) 2.) Lack of inspection and maintenance program Design Solution(s): 1.) Reevaluate materials and install according to proper procedures 2.) Implement proactive inspection and maintenance program 16

18 5. Conclusion The heat load created by the exothermic wet process phosphoric acid production reaction must be transferred out of the plant or used elsewhere within the plant to maintain control of reactor temperature. The reactor slurry temperature level and control dictates gypsum morphology and the consistency of gypsum quality which impacts equipment design decisions, filter performance, plant production level stability, and plant profitability. The ideal phosphoric acid reaction cooling system design must address the needs and preferences of the owner while adapting to the particular geographic and political limitations associated with the plant location. It must also fit the capability of the owner s operations and maintenance staff and/or economically justify any costs associated with a learning curve necessary to grow their experience level in order to utilize new design approaches. Further, since the rock source varies from owner to owner, rock analysis and preliminary pilot plant testing are critical to properly designing the plant. Global competition in the fertilizer industry continues to create the need for owners to lower production costs per unit of product in order to maintain profitability. The trend towards a realization of economies of scale benefits through larger reactor sizes has led to the need for the use of evaporative cooling systems in many cases. Further, maximizing the recovery of P 2 O 5 becomes more critical as sources of quality rock become scarce. Due to the practical limitations of historical approaches used for smaller plant sizes, these systems now typically involve the use of equipment including a Vacuum Cooler, a Cooler Precondenser, a Cooler Condenser, and a Cooler vacuum system. A whole system and open-minded approach to design allows for the maximization of plant profits for each individual owner. The design engineer is responsible to initiate and maintain effective communication throughout the project in order to accomplish this objective within the project timeline. 17

19 6. References [1] Becker,P. Phosphates and Phosphoric Acid: Raw Materials, Technology, and Economics of the Wet Process Second Edition, New York, NY: Marcel Dekker, Inc., 1989 [2] Krebs, D & Bourillion, F. Vacuum Cooling Evaporator, Low Level Flash Cooler Pump, AIChE Central Florida Section, 37 th Annual International Phosphate Fertilizer & Sulfuric Acid Technology Conference, 2013 [3] Waters, P & Byrd, J. Optimum Recirculation Rates in Phosphoric Acid Production, AIChE Central Florida Section, 37 th Annual International Phosphate Fertilizer & Sulfuric Acid Technology Conference, June 2013 [4] Messrs, G., Lizee, S. & Orenga, M. Rhone-Poulenc Wet Process Phosphoric Acid Air Cooling and Energy Conservation, AIChE New Orleans Meeting, New Concepts in Phosphoric Acid Manufacturing Symposium, 1981 [5] Barloy, M., & Dafanna, Dr. Update of the Zimphos Phosphoric Acid Unit in Zimbabwe, AIChE Central Florida Section, 21 st Annual International Phosphate Fertilizer & Sulfuric Acid Technology Conference, May 1997 [6] Sloley, A. Effectively Removing Droplets: Various Factors Affect the Choice and Operation of Liquid/Vapor Separators, Accessed January 2016 [7] United Nations Industrial Development Organization (UNIDO) and International Fertilizer Development Center (IFDC). Fertilizer Manual, Dordrecht, The Netherlands: Kluwer Academic Publishers, 1988 [8] Hutter, K., Samuelson, J., Walters, M. & Earl, C. Revamp of the J.R. Simplot Phosphoric Acid Plant, AIChE Central Florida Section, 19 th Annual International Phosphate Fertilizer & Sulfuric Acid Technology Conference, 2013 [9] Partin, D., When Nameplate Is Not Enough Expanding Phos Acid Capacity with Defoamers, Phosphates International Conference and Exhibition, Paris, France, 2005 Special thanks to the following Jacobs personnel for reference interview contributions based on their combined 120+ years of experience: James Byrd Manager of Process Engineering Department, Lakeland FL office Paul Waters, P.E. Senior Process Engineer, Lakeland FL office Elton Curran Senior Process Engineer, Lakeland FL office Dharmeshkumar Desai Senior Process Engineer, Lakeland FL office Jim Hebbard, P.E. Senior Process Engineer, Lakeland FL office 18