when nameplate is not enough

8/6/2019 When Nameplate is Not Enough

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Title: When Nameplate Is Not EnoughExpanding Phos Acid

Capacity with Defoamers

Author: Dan Partin,Director of International Business

ArrMaz Custom Chemicals

Phosphates 2005 International Conference and Exhibition

British Sulfur

Paris, FranceApril 2005

Abstract:

Problematic issues related to surface foaming and entrained gases often adversely effect phosphoric acid production volume and efficiency. In newly constructed facilities,

design specialists 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, excessreactor 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 and chemical control stability.

It is very common for phosphoric acid facilities to desire the ability to increase

production rate beyond Nameplate design capacity. Nameplate capacity can be definedas 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 parametersare exceeded, the resulting process instability can hasten scale formation in piping and

plugging of filter media, necessitating more shutdown periods to wash or clean theeffected area. Sulfate excursions due to poor circulation can impact citrate soluble andinsoluble digester losses. Poor crystal formations will reduce filtration rates, thereby

reducing operating rate and/or increasing water-soluble P2O5 losses.

Defoamers maximize reactor circulation promoting a more stable, controlled environmentfor 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 oftenfar exceeding nameplate design rating.


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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 offiltration. 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 thereactions 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 morepolyhedral 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 thatinfluence 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 thephosphate 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 ofstabilized 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 beneaththe reactor surface that are not detectable by visual observation

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 phosphate 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 foroptimization. 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 controlsurface foaming and reduce the entrainment of gas bubbles beneath the reactor surface.

Va or s ace

Surface Foam

Entrained gas

bubbles


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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.

Figure 3

Entrained Gases

Pumping and mixing are imperative in achieving optimal reaction and minimal losses in aphos acid digester. Gases entrained in reactor slurry represent a significant loss of

pumping, mixing and cooling efficiency. High gas content reduces impeller flow for bothagitators 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 clear-cut and particularly demonstrable in a dihydrate plant.

Vapor Space

Surface Foam

Fume

Evacuation toScrubbing

system


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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 produce 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 to10% air. [Plateus 2001]

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 82-85C to 74-78CC prior to

filtration, entrained gases will reduce cooling capability. There are several differingtypes 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 throughevaporative cooling. Low pumping rates require reducing the absolute pressure and oftenresult 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.

Influence of gas content on propeller pump

performance

1.0

2.0

3.0

4.0

3000 5000 7000

debit (m3/h)

head(m)

W/o air

with 8% d'air


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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 citrateinsoluble 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 zone or individual compartment influencessulfate conditions. Entrained gases thereby reduce mixing efficiency and consequently

increase P2O5 losses in the reactor.

Figure 5

Production increases:

Gases to Liberate

Although entrained gases may not be as apparent to the casual observer as the surfacefoaming 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.

Gases reduce

flow across

the impeller


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A phos acid plant operating at 1000tpd P2O5 production rate produces a certain volume ofCO2 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 surfaceand 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 production 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 available for dispersionremains 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 rateincreases. 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 asignificant amount of scaling which serves to reduce operating time for removal and

increase the risk of P2O5 loss due to high vapor velocity.

Figure 5

Di ester

Flash Cooler

Water/Heat


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Rock Impurities and Organic CompoundsSurface Foaming

Increasing production rates also increase proportionally the impurity loading into thephosphoric 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 ofextending 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 whichimpedes bubble movement for degassification. Additional quantities of organic

compounds serve to further stabilize surface foam and inhibit dispersion. There are phosacid plants where the natural organic compounds contained in the phosphate rockcombined 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 lions 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 equipmentand 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 theproject.

After construction is completed, the plant is started and commissioned at the Name Plate

production rate. When the contractual obligations and performance guarantees aresatisfied, the engineering firm is normally released from the project and the operation

staff assumes control. Given the usual economic opportunities of the fertilizer businessand 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


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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 defoamerconsumption 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 widevariety 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

Pocket Fertilizer Manual- 7th

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.sita-messtechnik.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, NewYork and Basel, page 132, 1989

Acknowledgements:

Dr. Seng Yap, ArrMaz Custom ChemicalsDr. Guoxin Wang, ArrMaz Custom Chemicals

James D. Van Orsdale, ArrMaz Custom Chemicals

Glen Varnadoe, ArrMaz Custom Chemicals

Roger Rixom, ArrMaz Custom Chemicals

when nameplate is not enough

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