Title:
Author:
“When Nameplate Is Not Enough”—Expanding Phos Acid Capacity with Defoamers
Dan Partin, Director of International Business ArrMaz Custom Chemicals
Phosphates 2005 International Conference and Exhibition British Sulfur Paris, France April 2005 Abstract: Problematic issues related to surface foaming and entrained gases often adversely effect phosphoric acid production volume and efficiency. efficiency. In newly constructed facilities, facilities, design specialists strive strive to build plants that can operate with minimal reagent usage. Raw materials feed streams occasionally vary from surveyor projections and in such cases, the phosphate rock can contain constituents that cause stable bubble formation, excess reactor fuming and entrained gases. Significant foam presence and/or gas entrainment decreases agitation and slurry circulation by lowering pumping efficiencies. These factors combine to reduce reaction efficiency, capacity ca pacity and chemical control stability. It is very common for phosphoric acid facilities to desire the ability to increase production rate beyond “Nameplate” design capacity. capacity. Nameplate capacity can be defined as the engineering company guaranteed and demonstrated production rate. Once beyond nameplate capacity, pumping, mixing and circulation rates become more critical in maintaining proper cooling and chemical controls. As engineering process parameters are exceeded, the resulting process instability can hasten scale formation in piping and plugging of filter media, necessitating more shutdown periods to wash or clean the effected area. Sulfate excursions due to poor circulation can impact citrate soluble soluble and insoluble digester losses. Poor crystal formations will will reduce filtration filtration rates, thereby reducing operating rate and/or increasing water-soluble P2O5 losses. Defoamers maximize reactor circulation promoting a more stable, controlled environment for formation of crystals suitable for proper filtration and recovery. ArrMaz Custom Chemicals’ produce site specific defoamers, supplied to phosphoric acid plants around the globe and have become an integral solution in achieving production capability often far exceeding nameplate design rating.
Introduction: The chemistry of the commercial production of phosphoric acid has remained relatively unchanged over the last 30 years. Phosphate ore is reacted with sulfuric acid to form calcium sulfate crystals suitable for separation from the phosphoric acid by means of filtration. The principle reaction of digestion is Ca3(PO4)2 + 3H2SO4 + 6H2O
→
3CaSO4 • 2H2O + H3PO4
[KEMWorks]
While there are several side reactions occurring during the digestion based on the type of process employed (dihydrate, hemihydrate, hemidihydrate), it is important to note the reactions are taking place in a strongly acidic environment, are exothermic in nature and produce a viscous liquid slurry. In a side reaction, carbon dioxide gases are released into the slurry. CaCO3 + H2SO4 + H2O
→
Ca SO4 • H2O + CO2
[KEMWorks]
The subsequent bubbles that form have a tendency to stabilize, depending on the contaminants found in the phosphate ore. Organic materials can cause foaming in the reactors and interfere with filtration [Slack 1968]. Two general classifications of foam types are “kugleschaum” and “polyederschaum”. Kugleschaum is widely separated spherical bubbles while polyederschaum is more polyhedral with thin films of liquid between bubbles. It is this polyhedral foam mass that rapidly forms on the surface of phosphoric acid attack systems. The major forces that influence this formation and stability are capillary action (surface tension and interfacial tension), viscosity, temperature, gravity and mechanical actions. [Van Orsdale 1987].
Figure 1 -- ©2000-2004 by SITA measuring technique GmbH
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Foam Description: While the study of bubble structure and the physical forces involved is a science in itself and worthy of doctoral dissertation, the issues that effect the operation of a phosphoric acid plant can be pretty straightforward. Stable bubble accumulation is referred to in the phosphate industry as “foam”. The two types of foam discussed in this presentation are surface foam and entrained gas. •
•
Surface foam can be classified as polyederschaum and is the accumulation of stabilized bubbles that form a layer on top of the reactor slurry liquid phase Entrained Gas is the bubbles of CO2 dispersed through out the slurry beneath the reactor surface that are not detectable by visual observation Va or s ace Surface Foam
Entrained gas bubbles
Figure 2
Common Problems Associated With Foaming: The presence of surface foam and entrained gas present serious issues that hamper the optimization of the reaction to convert phospha te ore to H3PO4 as well as the formation of crystals suitable for optimal filtration. Reactor circulation rates, mixing and cooling are crucial factors in establishing stable chemical controls and thus the platform for optimization. The ability to control excess sulfate level is paramount and yet is highly influenced by mixing and recycle flow in the rock feed and primary reaction zone as well as circulation rate throughout the digester. Defoamers are commonly used to control surface foaming and reduce the entrainment of gas bubbles beneath the reactor surface.
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Surface Foam Surface foam is easily visible and if uncontrolled can reduce the fume evacuation or air spaces, which can result in undesirable fugitive emission of gases from the digester. Fugitive fuming poses unsafe conditions for employees and exposure to environmental regulation violation. During startup or system upsets, excessive surface foam can result in an overflow of the digester or be drawn into the fume evacuation or scrubbing devices. This would contaminate the scrubber liquor and where a packed scrubbing system is used, a fouling of the media can occur. In each case there is the potential loss of P2O5.
Fume Evacuation to Scrubbing system
Vapor Space Surface Foam
Figure 3
Entrained Gases Pumping and mixing are imperative in achievi ng optimal reaction and minimal losses in a phos acid digester. Gases entrained in reactor slurry represent a significant loss of pumping, mixing and cooling efficiency. High gas content reduces impeller flow for both agitators and pumps that can be correlated through amperage measurement and comparison. As was earlier noted with regard to bubble structure, fluid dynamics is a very complex subject. Yet the effects of entrained gases in a phos acid reactor are clearcut and particularly demonstrable in a dihydrate plant. 4
Pumping: One crucial factor to be taken into account when considering flash cooler pumps is the expected influence of the gas contained in the slurry on the head-capacity performances of the pump. In this area, the only guideline is experience. This experience must of course be combined with a knowledge of the characteristics of the phosphate used, especially with regard to its content of organic substances likely to p roduce these gases. It has been demonstrated that within the usual operating range of such pumps, the drop in total head due to air content is far from negligible, reaching up to 30 to 40% with 8 to 10% air. [Plateus 2001]
Influence of gas content on propeller pump performance 4.0 W/o air with 8% d'air 3.0
) m ( d a e h
2.0
1.0 3000
5000
7000
debit (m3/h)
2001 AICHE Clearwater
Figure 4
As illustrated above, gas in liquids dramatically reduces pump performance. In a dihydrate process where the slurry is cooled from 82-85°C to 74-78C°C prior to filtration, entrained gases will reduce cooling capability. There are several differing types of flash coolers used commercially, but all rely on high circulating rates of reactor slurry through a negative pressure vessel where latent heat is dissipated through evaporative cooling. Low pumping rates require reducing the absolute pressure and often result in large delta T across the cooler. It is no secret that increased scaling occurs as phos acid slurry is flashed and cooled. Significant increases in either delta T or pressure drop across the flash cooler can result in accelerated formation of scale and build up that require a process shutdown to clean.
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Mixing: Entrained gases effect phos acid reactor agitators in a similar manner as described with pumps. Gases reduce the flow across the impeller resulting in a reduction of flow in the crucial mixing and reaction zones. Nucleation and crystal formation are greatly influenced by sulfate conditions as the phosphate rock is being reacted with sulfuric acid. Excessively high sulfate conditions can result in the coating or searing of rock particles where the surface of the rock particle is coated with calcium sulfate before the rock is fully digested, thus preventing the reaction from being completed. This is seen as a citrate insoluble loss. Exposing rock to an insufficient sulfate condition will result in cocrystallized P2O5 losses (citrate soluble) and slower filtration rates. The conditions related in the above paragraph are described only to emphasize the importance of proper mixing within the phos acid reactor. It should be recognized that any reduction of circulation within the reactor z one or individual compartment influences sulfate conditions. Entrained gases thereby reduce mixing efficiency and consequently increase P2O5 losses in the reactor.
Gases reduce flow across the impeller
Figure 5
Production increases: Gases to Liberate Although entrained gases may not be as apparent to the casual observer as the surface foaming previously described, its presence is commonly experienced in most phos acid facilities. The severity of gas is directly related to the amount of carbon dioxide released during the reaction process, assuming other design parameters are normal.
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A phos acid plant operating at 1000tpd P2O5 production rate produces a certain volume of CO2 generated in the slurry resulting from the ongoing chemical reactions. If the plant production rate is increased to 1500 tpd with the same rock, there is a substantial increase in the volume of CO2 to liberate with the same reactor surface area available for the degassing process. The bubbles of gas must be moved to the surface of the digester and broken to release the CO2. Any coalescence of bubbles inhibits breaking when the bubble reaches the surface and the small bubbles agglomerate to form surface foam. There must be sufficient surface area to allow for the bubbles to break and allow the gas to disperse. This example of a fifty (50%) percent increase in produ ction rate results in a corresponding increase in the amount of entrained gas to liberate. The volume of gases to be released is increased substantially, yet the surface area av ailable for dispersion remains unchanged. Here lies the danger of increased entrained gases if the increased volume of CO2 is not totally dispersed. Cooling In plants where flashcoolers are used, the heat load is increased as the production rate increases. Insufficient cooling can limit production when higher levels of entrained gases reduce the flow and may require lower levels of absolute pressure on the flash cooler. This combination of high delta temperature and increased pressure drop can produce a significant amount of scaling which serves to reduce operating time for removal and increase the risk of P2O5 loss due to high vapor velocity.
Water/Heat
Flash Cooler
Di ester
Figure 5 7
Rock Impurities and Organic Compounds—Surface Foaming Increasing production rates also increase proportionally the impurity loading into the phosphoric acid digester. Organic elements from phosphate rock stabilize foam and emulsions, thus impeding the effect of agitation, with consequent higher local supersaturations. [Becker] It is common industry knowledge that impurities can also serve to increase slurry viscosity. Increasing slurry viscosity slows bubble movement upward to the surface in phosphoric acid slurry so the gas can be released. Increases in viscosity have an effect of extending the bubble life or stability by reducing the drainage rate of liquid, which would allow the bubble to break. Increases in impurity loading set the stage for a more viscous reactor slurry which impedes bubble movement for degassification. Additional quantities of organic compounds serve to further stabilize surface foam and inhibit dispersion. There are phos acid plants where the natural organic compounds contained in the phosphate rock combined with a large quantity of carbonate produce a very stable agglomeration of surface foam as well as entrained gases that must be liberated through the use of defoamer. These plants typically operate with a controlled layer of surface foam by tightly controlling the reactor level and production rates.
What is “Name Plate” and How Defoamers Help As the world population continues to increase, the need to produce food increases as well. With phosphate fertilizers providing a lion’s share of the nutrient requirements for food production to nations around the globe, the production of phosphoric acid remains an integral building block in feeding the world. When a phosphoric acid facility is constructed, the design and economical modeling consider the chemical composition of the phosphate ore to be processed. The equipment and reaction vessels are sized accordingly to meet the design goals at an expected daily production level that is typically known as the “Name Plate” rate. Defoamer is often a part of the design criteria depending on the engineering firm and design employed on the project. After construction is completed, the plant is started and commissioned at the Name Plate production rate. When the contractual obligations and performance guarantees are satisfied, the engineering firm is normally released from the project and the operation staff assumes control. Given the usual economic opportunities of the fertilizer business and the incremental benefits of scale, the demand for production often increases after the operation has stabilized. Quite often the desired capacity exceeds the excess design factors of the equipment installed. The additional carbonate generated cannot be fully degassed within the 8
confines of the same reactor volume and surface area so the level of entrained gases increase, effecting the operation to the point of increased losses. This is where defoamer consumption costs can be more than offset by the value of incremental production increases. Defoamer can be the viable alternative to additional capital investment in equipment. Defoamer addition to the phosphoric acid digester controls surface foaming and reduces entrained gas content in the slurry. Site specific evaluation is required to fully diagnose the presence and impact of foaming. ArrMaz Custom Chemicals manufactures a wide variety of defoamers, which can withstand a range of harsh process conditions. Defoamer adds after-market horsepower to phosphoric acid plants where nameplate production is just not enough.
REFERENCES th
Pocket Fertilizer Manual- 7 Edition, KEMworks, page 18, 2003 History and Status of Phosphoric Acid, C.C. Legal and O.D. Myrick, Jr., Volume 1, Part 1, Phosphoric Acid (A.V. Slack, Ed.) Marcel Dekker, New York and Basel, page 33, 1968 Defoamer History, James D. Van Orsdale, Westvaco Corporation, AICHE Clearwater Convention, page 3, 1987. How does foam develop? Sita Messtechnik GmbH, http://www.sitamesstechnik.de/schaum/schaumtesten.html, ©2000-2004 by SITA measuring technique GmbH Design Principles of Flash Cooler Pumps Selection of Materials, PLATEUS Pierre, AICHE Clearwater Convention, page 5, 2001 Phosphates and Phosphoric Acid, Pierre Becker Second Edition, Marcel Dekker, New York and Basel, page 132, 1989
Acknowledgements: Dr. Seng Yap, ArrMaz Custom Chemicals Dr. Guoxin Wang, ArrMaz Custom Chemicals James D. Van Orsdale, ArrMaz Custom Chemicals Glen Varnadoe, ArrMaz Custom Chemicals Roger Rixom, ArrMaz Custom Chemicals
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