Optimum Recirculation Rates in Phosphoric Acid Production

Transcription

1 Optimum Recirculation Rates in Phosphoric Acid Production Prepared by: Paul S. Waters, P.E. James Byrd Clearwater AIChE June 2013

2 INTRODUCTION Integral to modern phosphoric acid reaction circuits is the concept of recirculation rates. Recirculation of reactor slurry is critical in optimizing regular crystal growth and controlling sulfates. Filtration rates and P 2 O 5 losses are correlated to both crystal growth and sulfate control. In most plants, at least part of the reactor recirculation is passed through a vacuum cooler to maintain optimum slurry temperature. The balance between circulation through the cooler and through the reactor outside the cooler is critical for proper crystal growth. Optimum excess sulfates, the primary parameter governing reactor operation, and regular crystal growth are a function of source ore and the physical reaction circuit. Therefore, the optimum recirculation rate is not equal for all phosphate ores or reactors. This paper discusses parameters affected by reactor recirculation, consequences of deviation and means to optimize a given reaction circuit. DESCRIPTION Quite simply put, reactor recirculation is the quantity of reactor slurry circulated rather than passed on to the filter feed. More typically, a recirculation ratio or rate is referenced, which is the quantity circulated divided by the filter feed rate. Ratios reported in industry vary from 20 to 100. Measurement of the circulated acid is not always straight forward and will be dependent on the reaction system. < The simplest to measure is the external circulation. In a usual configuration, this will be the flash cooler feed minus the filter feed. There are some systems in industry that employ circulation pumps outside of the flash cooler circuit. Still other circuits, such as the Rhone-Poulenc and Isothermal, do not have external circulation. More difficult to measure is internal circulation. In some circuits, such as the Jacobs and Rhone-Poulenc, additional circulation is achieved through agitation. With an open annular circuit, a flywheel effect can take place if the agitators and reactor geometry are properly sized. This effect can result in significant circulation benefits and has been reported to equal the external circulation in some systems. Other forms of internal circulation can result from Couch pumps and geometric configuration of the reaction circuit. External and internal recirculation is additive. Total measure is possible through testing and review of equipment data. There are different approaches, but the change in concentration of a traceable additive with respect to time can provide data that can be analyzed to this end. The effort is not trivial in deriving the internal rate both in practice and in data analysis. Even so, there are qualitative results from such testing that can provide insight to this parameter. 2

3 OPERATING CONSIDERATIONS Reactor recirculation is one of the more critical operating parameters in the phosphoric acid reaction circuit. The parameter has direct effects on sulfate control and filtration rates. Process Engineers in industry need to understand this effect by correlating data in optimizing operations. Not all recirculation rates will be ideal between facilities. Dependencies include rock source, reaction technology, specific reactor volume and operating philosophy. Arguably, the single most important tool an operator has during the course of running a phosphoric acid plant is a measure of excess sulfates. Usually, there is a target the operator attempts to achieve. Controlling the excess sulfates in achieving this target then becomes the Operator s challenge. But understanding the reasons why sulfate control determines the course of an Operator s success merits discussion. Sulfate ions from sulfuric acid dissociation react with calcium in the rock to form gypsum in the reactor slurry. The kinetics of this reaction is relatively fast, but not instantaneous, and is somewhat different depending on the rock source and dissolution rates. During the course of this reaction, the P 2 O 5 in the rock slurry becomes soluble and forms phosphoric acid, the liquid phase of the reactor slurry containing gypsum solids. Concerns herein are losses of P 2 O 5, or overall yield. An excess of sulfate is targeted in the reactor to drive the reaction through completion. Thus, unreacted rock losses, or citrate insoluble losses, are typically small. Phosphate, or P 2 O 5, can also penetrate the gypsum lattice resulting in co-crystallized losses, or citrate soluble losses. These types of losses can be more significant, but can be minimized by targeting a higher excess sulfate, see Figure 1. The danger with high sulfate targets is that the sulfate ions can surround, or coat the rock and not react with available calcium. This results in a runaway effect with excess sulfates climbing at high rates which lowers yields and reduces filtration rates. Low filtration rates further lowers yields as water soluble losses increase. This is liquid in the gypsum cake post filtration which contains P 2 O 5. Thus, loss of sulfate control adversely affects both production and overall yield. Each ore source will have an ideal sulfate target which maximizes recovery. Operators tend to run their targets somewhat lower than this ideal in order to mitigate variations in calcium feed against the potential of the runaway coating effect. The better control the operator has over the reactor and excess sulfates, the closer the Operator can safely operate to the ideal target. The Process Engineer should be evaluating the target on a routine basis to determine the optimum for the given circuit. 3

4 Figure AVERAGE ALL TESTS DORR- OLIVER PILOT PLANT 1 1 TOTAL SULFATE AS % H2SO4 IN 30-32% P2O5 ACID 10 In addition to recovery, the sulfate target should maximize regular crystal growth. Control over crystal growth is how the operator will maximize throughput on the filter, or filtration rates. Regular crystal growth is determined by a myriad of factors including temperature, time, viscosity, concentration, particle size distribution, available surface area for growth versus nucleation and importantly, impurities in the acid. However, as referenced, the most important determining factor for gypsum crystal growth in a phosphoric acid reactor is excess sulfate management. Recirculation rates are a powerful tool for the Operator. The higher the rate, the more forgiving the system against sulfate upsets. This occurs for a variety of reasons. From a theoretical perspective, addition of a stoichiometric quantity of sulfuric acid to provide the excess sulfatess for a given target should move the solution past the supersaturation point. This causes spontaneous nucleation. Too much spontaneous nucleation will skew the gypsum particle size distribution too low for adequate filtration, regardless of the crystal shape. By increasing the circulation for an equal addition of sulfuric acid, the solution will move across the saturation curve to some lesser degree of the supersaturation point as the overall excess sulfate concentration is somewhat less. It is control of supersaturation in the reactor that results in the optimal amount of spontaneous nucleation such that the overall gypsum particle size distribution maximizes both filtration rates and yields. 4

5 REACTOR TECHNOLOGIES A generic gypsum crystallization curve is shown in Figure 2. This crystallization curve plots fixed gypsum crystallization rates against percent weight concentrations of free Ca and SO 4 ions in 30% P 2 O 5 wet process phosphoric acid. The lower bolded line is the saturation line below which no crystallization occurs. The upper bolded line is the supersaturated line above which spontaneous nucleation occurs. Crystallization growth occurs between the bolded lines as a function of available surface area rather than nucleation. The optimum operating point for a phosphoric acid reactor is between the bolded lines. However, in the course of rock dissolution and sulfuric acid introduction, the operating point will move. This movement is dictated by reactor type, agitation and feed location. Figure 2. Weight % CaO Figure (2)(5) - CaSO4 Crystallisation W eight % SO4 Figure 3 follows rock dissolution and reaction for a Jacobs Reactor. For the system analyzed in Figure 3, dye tests indicated the internal recirculation was half the external recirculation. In other circuits, internal recirculation in a Jacobs Reactor has been measured at equal rates to the external recirculation. The internal recirculation in a Jacobs Reactor is a function of power input to unit volume, impeller type, reactor modifications and internals design. Point A represents the return from the cooler seal tank. At this point it is mixed with incoming rock slurry. The theoretical mixture should go to point B, but rock dissolution takes a couple of minutes giving the slurry time to reach the sulfuric and return acid introduction and the gypsum formation reaction competes for Ca ions stunting the breach into the supersaturation zone as shown to point B1. Dissolution continues to point C. The introduction of sulfuric and return acid mixes rapidly and returns the acid to point D. As the sulfuric acid and calcium ions continue to react through the remainder of the reactor, the slurry moves back to point A. Point F 5

6 represents desaturation in the maturation circuit. Desaturation is an important consideration in scale prevention downstream of the reaction circuit. The Jacobs reactor is well suited to use the Jacobs high yield mode to reduce cocrystallized, citrate soluble, losses below 2% of rock fed. Because the sulfuric acid is well mixed with recycled acid and the addition point can be well separated from the rock addition point avoiding high local concentration. The Jacobs reactor can achieve 97.5% recovery of P 2 O 5 fed. These results were achieved in a commercial plant on western rock finely ground for overland pipe transport. The Jacobs reactor allows the addition of a small amount of rock to desulfate the acid if required for downstream products. Figure 3. Weight % CaO Figure (3)(6) Jacobs - Crystallisation C B1 F B A D Recirculation - External - 40:1 - Internal - 20:1 Total - 60:1 Rock Dissolution mins Sulfuric Reaction - 2 mins Filter Feed - 26 mins Weight % SO4 6

7 Figure 3a. Figure (3a) Jacobs Reactor REACTOR 5 6 VACUUM COOLER 4 COOLER FEED 1 COOLER SEAL FILTER FEED Slurry to Filter Sulfuric Acid/ Recycle Acid 3 2 ADJUSTMENT ADJUST- MENT Rock Slurry Figure 4 represents a similar profile for a multi-compartment reactor. Jacobs experience and data analysis has shown the inherently reduced recirculation rate necessitates this type of reactor to run at lower relative sulfate targets. The reason is due to rock particle occlusion resulting in sulfate control instability. Lower recirculation affects the ability of the system to mitigate feed variations because supersaturation is amplified. Note that the curve in Figure 4 is the same curve in Figure 3, but the operating points are different because of the necessity to target lower sulfates. Because of this, sulfuric acid addition will push the system into supersaturation. Higher recirculation rates run the risk of pushing the reactor into plug flow, which could result in a more exaggerated swing into supersaturation zones in addition to creating hypersaturation zones in the reactor. Use of an Advisory Control System is strongly recommended for multi-compartment reactors for these reasons. These types of reactors typically report fish eyes, or large rock particles coated with calcium sulfate which build up in the reactor, particularly in the overflow compartments. These buildups often require cleanups more frequently than once per year. 7

8 Figure 4. Weight % CaO Figure (4) (7) - Prayon Crystallisation C B1 B A Recirculation - External - 40:1 - Internal - 0:1 Total - 40:1 Rock Dissolution mins Sulfuric Reaction mins Filter Feed - 62 mins D F Weight % SO4 Figure (3) Prayon Reactor System Figure (4a) Prayon Reactor System 8

9 Figure 5 represents the rock dissolution plot for a single tank reactor. The central agitator creates pumping power equal to about a recirculation ratio of around 330:1. As a result, the rock, sulfuric and return acid are mixed very quickly resulting in very small changes in concentrations. The obvious advantage herein is avoidance of the system to move to the supersaturation zone. However, the high recirculation prevents regular crystal growth from approaching the crystal size distributions seen in both the Jacobs and multicompartment reaction systems. Figure 5. Fig. (8) - Raytheon Crystallisation Figure (5) Isothermal Crystallisation Weight % CaO B Recirculation - External - 0:1 - Internal - 330:1 Total - 330:1 C 0.5 A Weight % SO4 9

10 Figure (5a) (1) Raytheon Isothermal Reactor System OTHER FACTORS Integral to this point is adequate agitation in the reaction system. If the reactor geometry or low agitation power per unit volume allows for inadequate dissolution of the rock, there will be hypersaturation zones and desaturated zones resulting in losses for competing reasons. This also minimizes the available reaction volume affecting crystal growth which in turn affects filtration rates and overall yields. Each ore and reaction circuit will also have a specific reaction volume for which adequate time is allowed for regular crystal growth. The required volume is often determined in pilot plants prior to design and construction. Volume is diminishing over time as scale forms in the reactor. As the volume decreases, sulfate control will become more difficult. Smaller crystals will pass through the recirculation circuit resulting in less available surface area for crystals to grow which shifts equilibrium to more spontaneous nucleation and a smaller overall particle distribution. Over time, the problem will compound resulting in lower production and higher losses. This phenomenon is a primary reason some designers have moved to larger specific reaction volumes such that at the end of the turnaround cycle the required volume is still available. It is important for the Process Engineer to be apprised and react according during the course of a turnaround cycle. Small changes will result in small results, but over the course of time these small results can have significant economic impacts. Addressed thus 10

11 far are but a few of the variables to be considered, but at point is an understanding of gypsum crystal properties at any given time against yields and production. CONCLUSION So the argument can be made that the higher the recirculation rates the better control is available. So why not increase rates for all plants at every site? It is important is note that a plant is designed for a given rate, a nameplate production. This production rate should take into consideration the required specific reaction volume, heat removal, adequate agitation, and maturation. The designer will provide the required recirculation rates in accordance with a combination of calculations and experience. Given all the interdependencies, any modification of recirculation rates will have consequences. Simply increasing recirculation rates can have unintended consequences if not studied in depth. In some reaction circuits, an increase to power input to unit volume in agitation can increase internal recirculation. The primary concern herein is which agitators are targets such that high shear agitators do not impede in the high efficiency zone. Other considerations are the flow dynamics, which should increase performance but could impact existing recirculation depending on the age of the design or modifications post installation. Many multicompartment reactors will have significant restrictions at overflows and underflows which must be modified requiring expensive concrete and brick work. If an external pump is used, care must be taken that thermal impacts are considered. If the flash coolers are bypassed and the hot slurry is returned to the front end, the supersaturation curve is impacted impacting crystallization kinetics. Often when production rate is increased significantly without increasing recirculation the cooler, the T across the cooler increases to point scaling in the downleg causes frequent shut downs for cleaning. With any increase in recirculation, stresses and flow dynamics must be addressed. In a multicompartment vessel, horizontal stresses on agitator shafts must be accounted so catastrophic failures do not occur. Of particular concern in a mulicompartment reactor are plug flow limitations. Once plug flow is attained, hypersaturation zones will effectively de-rate the entire circuit. As rock or sulfuric acid flows are changed, large unintended swings in sulfate concentration can move around the reactor in plug flow are very difficult to correct. Despite these concerns, the benefits of additional recirculation are such that each facility should consider its merit if any optimizations are attempted. After careful consideration of the consequences, further study is warranted. 11

12 In conclusion, recirculation rates are one of the most powerful tools available to the operator. During plant design, current practice is to have a very high ratio relative to industry practice. This gives the plant superior control relative to older plants and will allow flexibility for future growth. Older plants should consider increasing recirculation ratios, but must take into consideration all elements such that the best intentions do not result in unintended consequences. 12