Inspection Manual for Fertilizers Industry




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Fig (8) Process Flow Diagram for Nitric Acid Manufacturing

Inputs

Operations

Outputs





Air







Backwash wastewater

Filter cake









Spent lube oil

Noise


Liquid Ammonia

Steam







Fugitive ammonia

Heat stress


Pt & rhodium catalyst










Fugitive ammonia, N2O Washing water of filters

Heat stress


Process Water







Fugitive ammonia

Blowdown











Filtered & Compressed air










Cooling Water





Cooling wastewater recycled to cooling towers








To expander (Nox emissions)


Process water

Cooling water





Cooling wastewater to towers

Acid mist, Nox emissions



Air














HNO3 (55 % conc.)



c) Methanol Production


This unit replaced the copper liquor (ammoniacal copper formate or acetate) section which was used for the removal of CO from synthesis. The liquor pollutes water streams as it contains copper, ammonia and carbonic acid. The copper liquor reacts with CO which later released when the solution is regenerated.


Methanol is formed by the reaction of CO and CO2 with H2 according to the reaction:


CO + 2H2 → CH3OH

CO2 + 3H2 → CH3OH + H2O


This reaction takes place at 210o C and 220 atmosphere in presence of Cu O and ZnO catalyst. The product is raw methanol which is purified by distillation to 99 % concentration.


Fig (9) shows the process flow diagram for methanol production.



Fig (9) Process Flow Diagram for Methanol Production

Inputs

Operations

Outputs


Synthetic gas

Water

Catalyst (CuO, ZnO)







NH3, CO, H2 emissions

Heat stress

Steam









Synthesis gas to methanator

CO, H2 emissions


Steam










Wastewater (CH3OH)



















d) Ammonium Nitrate Production



Ammonium Nitrate is in the first place a nitrogenous fertilizer representing. 12.4% of the total nitrogen consumption worldwide. It is more readily available to crops than urea. In the second place, due to its powerful oxidizing properties is used with proper additives as commercial explosive.


The production process comprises three main unit operations: neutralization, evaporation, solidification (prilling and granulation). Individual plants vary widely in process detail.


  1. Neutralization:

Anhydrous liquid ammonia is evaporated in an evaporator using cooling water. The stoichiometic quantities of nitric acid (55% concentration wt/ wt) and gaseous ammonia are introduced by an automatic ratio controller to a neutralizer. The reaction between Ammonia and nitric acid produces ammonium nitrate solution according to the following exothermic reaction.


NH3 + HNO3 NH4 NO3


Neutralization can be performed in a single stage or in two stages. The neutralizer can be carried out at atmospheric (either normal or low emission neutralizers where the temperature does not exceed 105C and pH will be 6 and 3 respectively) or at elevated pressure of almost 4 atmospheres. The normal neutralizers are usually followed by flash evaporation in order to in crease the out let A.N concentration to 70%. In case of pressure neutralizers the temperature will be in the range of 178C and the steam generated from the heat of reaction will be utilized in the subsequent step namely concentration of A.N solution.


During evaporation some ammonia is lost from the solution. The steam which is boiled off is contaminated. The control of the neutralizer is important. The pH and the temperature must both be strictly controlled to limit the losses from the neutralizer. All installations must include pH and temperature controls. At the operating temperature of the neutralizer, impurity control is of great importance because a safety incident will also be a significant environmental incident. The ammonium nitrate solution from neutralizer may be fed to storage without further processing but, if it is used in the manufacture of solid ammonium nitrate, it is concentrated by evaporation.


  1. Evaporation to Concentrate the A.N

The outlet from the neutralizer is received in an intermediate tank. The solution should be made alkaline before being pumped (no need for pumps in case of pressure neutralizers since the pressure will maintain the flow) to the evaporation section (multi-effect) running under vacuum. The solution will be steam heated in the multi effect evaporation section. The solution will be concentrated up to 97.5-99.5% (normally over 99 %) depending on whether ammonium nitrate will be granulated or prilled.


  1. Mixing the Filling Material:

In order to reduce the nitrogen content of A.N from 35% to 33.5%, the proper filling material is added (about 4% by weight of powdered limestone or dolomite or even kaolin)


  1. Prilling or Granulation

The hot concentrated melt is either granulated (fluidize bed granulation, drum granulation … etc) or prilled. Ammonium nitrate is formed into droplets which then fall down a fall tower (prill tower) where they cool and solidify. Granulation requires more complicated plant than prilling and variety of equipment. The main advantage of granulation with respect of environment is that the quantity of air to be treated is much smaller and abatement equipment is cheaper.


  1. Drying, Screening

The ammonium nitrate (prills or granules) is dried (usually in drums) using hot air (steam heated), then screened to separate the correct product size. The oversize and undersize will be recycled either in the mixing tank (in case of prilling) or to the granulator.


  1. Final Cooling

The hot proper size granules, are then cooled (against cooled and humid free air) down to 40C and treated with anti-caking (usually amines) and then coated with an inert material (usually, kaolin, limestone or dolomite) and then conveyed to the storage.


Fig (10) illustrates the block flow diagram for ammonium nitrate production process.


Major Hazards

Ammonia, nitric acid and ammonium nitrate are the hazardous chemicals present in ammonium nitrate plants. A.N is an oxidizing agent and precautions must be taken in manufacturing, transport and storage.

The main chemical hazards associated with ammonium nitrate are fire, decomposition and explosion. Burns caused by hot AN solution should also be considered from a safety point of view.

Ammonium nitrate itself does not burn. Being an oxidizing agent, it can facilitate the initiation of a fire and intensify fires in combustible materials. Hot AN solution can initiate a fire in rags, wooden articles ets., on coming into contact with them. Similarly, fertilizer products or dust contaminated with oil or other combustible materials can also start fires when left on hot surfaces.

Pure solid A.N melts at 169o C. On further heating it decomposes by way of a complex series of reactions. Up to about 250o C it decomposes primarily into N2O and H2O. Above 300o C reactions producing N2, NO, NO2 etc., become significant. These reactions are exothermic and irreversible. They are accompanied by the vapour pressure dependent endothermic dissociation into HNO3 and NH3 vapours which can provide a temperature limiting mechanism, provided the gases can escape freely. If they cannot, the endothermic dissociation is suppressed and a run-away decomposition can develop, leading to explosive behavior. A number of materials have a strong catalytic effect on the thermal decomposition of A.N. These include acids, chlorides, organic materials, chromates, dichromate, salts of manganese, copper and nickel and certain metals such as zinc, copper and lead. The decomposition of AN is suppressed or prevented by an alkaline condition. Thus the addition of ammonia offers a major safeguard against the decomposition hazard. The release of toxic fumes is one of the main hazards associated with the decomposition of AN.

Strongly acidic conditions and the presence of contaminants should be avoided to counter the explosion hazard in AN solutions. Explosions can occur when ammonium nitrate is heated under confinement in pumps. Reasons for pump explosions include:

  1. No (or insufficient) flow through the pump.

  2. incorrect design (design may incorporate low flow and/or high temperature trips).

  3. poor maintenance practices.

  4. contamination.


It is more common for the major storage of these chemicals to be located within their own manufacturing plants. Possible requirements for storage

    • materials of construction used in the building of the store, other buildings in the locality, storage of other product in the same building, absence of drains, fire detection and fire fighting systems, layout and size of stacks

Fig (10) Process Flow Diagram for Ammonium Nitrate Manufacturing

Inputs

Operations

Outputs



Liquid Ammonia












Nitric Acid






Ammonia emissions


Steam condensate (NH3, ammonium nitrate)









Vapours to ammonium nitrate separator


Ammonia injection

Steam











Steam






Vapours to ammonia separator

Condensate (NH3, ammonium nitrate)


Dolomite, Kaolin or Limestone







Particulates of dolomite, kaolin or lime stone









Particulates



Air






Particulates (ammonium nitrate) and NH3


Steam heated air






Heat stress

Water vapour








Particulates

Noise


Cold dry air





Particulates



Polyethylene bags

Clay or diatomaceous earth






Particulates (ammonium nitrates)

e) Ammonium Sulphate


Ammonium sulphate (A.S) is a nitrogenous fertilizer with an additional source of soluble sulphur which is a secondary plant nutrient. The majority of its production is coming from coking of coal as a byproduct. Ammonium sulphate is produced by the direct reaction of concentrated sulphuric acid and gaseous ammonia and proceeds according to the following steps.


  1. Reaction of Ammonia and Sulphuric Acid:

Liquid ammonia is evaporated in an evaporator using 16 bar steam and preheated using low pressure steam.

The stiochiometric quantities of preheated gaseous ammonia and concentrated sulphuric acid (98.5% wt/wt) are introduced to the evaporator – crystalliser (operating under vacuum). These quantities are maintained by a flow recorder controller and properly mixed by a circulating pump (from upper part of the crystalliser to the evaporator)


  1. Crystallization

The reaction takes place in the crystallizer where the generated heat of reaction causes evaporation of water making the solution supersaturated. The supersaturated solution settles down to the bottom of crystalliser where it is pumped to vacuum metallic filter where the A. S crystals are separated, while the mother liquor is recycled to the crystalliser.


  1. Drying of the wet Ammonium Sulphate Crystals

The wet A.S crystals are conveyed (by belt conveyors) to the rotary dryer to be dried against hot air (steam heated) and then conveyed to the storage area where it naturally cooled and bagged.


Fig (12) presents the process block diagram for ammonium sulphate production.

f) Ammonium Phosphate


There are two types of ammonium phosphate, namely: mono-ammonium phosphate and di-ammonium phosphate. Mono-ammonium phosphate is made by reacting ammonia with phosphoric acid, centrifuging and drying in a rotary dryer. Di-ammonium phosphate requires a two-stage reactor system in order to prevent loss of ammonia. A granulation process follows with completion of the reaction in a rotary dryer which is heated by a furnace using fuel.


To produce mono-ammonium phosphate, ammonia to phosphoric acid ratio is 0.6 in the pre-neutralizer and then 1.0 in the granulator. For production of di-ammonium phosphate, the ratios are 1.4 and 1.0 in the pre-neutralizer and granulator respectively.

The resulting ammonium phosphate is then screened. The undersize particles are recycled back to the granulation operation, while the oversized particles are grinded first before recycling to the granulator. After screening the fertilizer granules are coated with specific material in order to regulate its dissolving process in the soil when used.


Fig (13) illustrates the process block diagram for the manufacturing of ammonium phosphate fertilizer.


^ Fig (12) Process Flow Diagram for Ammonium Sulphate Manufacturing

Inputs

Operations

Outputs





Liquid ammonia


Steam








Ammonia emissions


Heat stress

(work place)


Sulphuric acid








Ammonia & acid mist











Wastewater (ammonium sulphate)













Air Steam Heated








Wastewater










Solid waste (crystals)




Polyethylene bags






Solid wastes

(bags and product spills)



^ Fig (13) Process Flow Diagram for Ammonium Phosphate Manufacturing

Inputs

Operations

Outputs


Evaporated Ammonia


Phosphoric acid


Cooling water






Fumes & gases to scrubber

Ammonia leaks



Fuel for heater & air






Ammonia

Furnace flue gases

Particulates of ammonium phosphate to cyclones


Cooling air








Ammonia, particulates





Undersize recycled






Product size


Particulates & NH3

(work place)




Grinded oversize recycled








Particulates

Noise


Coating materials








Particulates to collectors












Polyethylene bags













Ammonium Phosphate



g) Calcium Nitrate


Calcium nitrate is produced by dissolving the calcium carbonate (lime stone) with nitric acid, according to the following reaction:


CaCO3 + 2HNO3 → Ca(NO3)2 + CO2 + H2O


The lime stone is transported to the site as small size stones and lifted to the dissolving tower. The nitric acid is fed to the bottom of the dissolving tower and the formed calcium nitrate is fed to the settling tank. After settling, the excess acid is neutralized with ammonia. The nitrogen content is adjusted with ammonium nitrate. The fertilizer is produced in the liquid state and the nitrogen content of the final product is adjusted to the required specifications using ammonium nitrate.


Fig (14) illustrates the process flow diagram for the production of calcium nitrate fertilizers.


^ Fig (14) Process Flow Diagram for Calcium Nitrate

Inputs

Operations

Outputs


Limestone


Nitric acid







CO2 & acid mist

Solid wastes (flakes of lime stone)











Solid waste CaCO3



Ammonia







Fugitive ammonia and acid mist



Ammonium nitrate








Liquid waste (spills)




Barrels







Spills of liquid fertilizer





h) Urea


Urea (carbamide) is a high-concentration nitrogenous fertilizer, with a 46 % nitrogen content. It is produced from liquid ammonia and gaseous carbon dioxide at about 170- 190º C and 135- 145 bar, according to the following reactions:


CO2 + 2NH3 → NH2COONH4 (1)


NH2COONH4 ↔ NH2CONH2 + H2O (2)


The second reaction is dehydration of the carbamate to produce liquid phase urea.


The urea plant consists of high and low pressure sections. The high pressure section is composed of:

  • Urea synthesis including the high stripper and condenser.

  • Urea rectifying operation.


Whereas, the low pressure section is composed of:

  • Evaporation.

  • Recovery.

  • Prilling.


Carbon dioxide is supplied from the ammonia plant and compressed in the centrifugal CO2 compressor then introduced into the bottom of the high pressure stripper, which is a part of urea synthesis section. Liquid ammonia is pumped from the storage tank to the urea plant and is preheated to a temperature around 10º C. The high pressure NH3 pump raises the pressure to 165 bars and delivers it to the high pressure carbamate condenser. NH3 and CO2 are fed to the synthesis section in the molar ratio of 2 for NH3/ CO2.


In the urea reactor most of the condensate carbamate is converted to urea and water. The reaction mixture, leaving the reactor from the overflow through internal down comer, is distributed over the top of the stripper tubes. CO2 gas is introduced in counter flow. The gases leaving the top of the stripper are led into the high pressure carbamate condenser. Major parts of the stripper off-gases are condensed gases and non-condensed NH3, CO2. They are introduced into the bottom of the reactor where the conversion of carbamate into urea takes place. Non-converted NH3 and CO2 leave the reactor to the high pressure scrubber, where major parts of NH3 and CO2 are recovered and mixed with fresh NH3 feed through the high pressure ejector.


The urea carbamate solution leaving the bottom part of the stripper is sprayed on a bed of ball rings in a rectifying column. The urea solution leaving the bottom part flows to a flash tank and then to the urea solution storage tank (about 70- 80 % concentration). This solution is further concentrated to a melt (98 % urea) by evaporation under vacuum in two stages. The urea melt is pumped to the prilling tower. The prills are received on conveyors and transported to the bagging section.


The condensate containing NH3, CO2 and urea is pumped from the NH3- water tank to the upper part of the first desorber, which is stripped by the overhead vapours of the second desorber. The bottom effluent of the first desorber is pumped to the hydrolyser column. In the hydrolyser the urea is decomposed into NH3 and CO2 and fed to the first desorber after separation in the second desorber. The process condensate is discharged from the bottom of the second desorber to the sewer system of the plant.


The desired temperature of the prills ranges from 60º C to 65º C. If the temperature is 80º C- 85º C then the residual heat in the prills causes some stretching and bursting for the polyethylene bags after the bagging operation.


Fig (15) shows the process flow diagram for the production of urea.

Fig (15) Process Flow Diagram for Urea Production

Inputs

Operations

Outputs











Heat stress

Urea, CO2, NH3



Liquid ammonia











Carbon dioxide


















Fugitive emissions

(NH3, CO2)









Fugitive emissions

(NH3, CO2, H2O)



Steam








Fugitive emissions

(NH3, CO2, H2O)




Cold air







Particulates, NH3, CO2, urea

















Recycled CO2, NH3



Steam stripping agent






Wastewater (urea, NH3, CO2)


Steam






Leaks of NH3, CO2









Particulates

Noise


Urea formaldehyde

Polyethylene bags






Particulates

i) Bagging Section


The produced fertilizers are sent to the bagging section by belt conveyors. Fertilizers are piled in this section and withdrawn by rail scrapers which moves on a rail bars. By using forks the fertilizers slide down to the feed network then to a sliding car with small bunker which ensure constant feeding. The car slides along the section to transport the product to an inclined belt conveyor or then to bucket elevators and hammer mill. By a screw conveyor, the product is discharged to the final product and packed.


In the packaging stage a balance exist between the conveyor and the packaging bunker at which a level meter is fixed. The bunkers feed the balance which consequently feed packaging operation in which polyethylene bags are filled and transported by belt conveyors to the trucks.


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