Phosphoric acid manufacture reaction unit

From the grinding unit the feed is sent to reaction units

Reaction section:

Phosphate rock is subjected to react with sulfuric acid react in this section resulting out the precipitate of calcium sulfate dihydrate CaSO4. 2H2O and liquid phosphate acid.

Various equipments in this section are

  • A reactor
  • Agitators to maintain homogeneity
  • Cooling air fan, connecting ducts, and air nozzles to supply the necessary air for cooling the slurry.
  • Off gas duct leading to fume scrubber followed by exhaust fans and stack.
  • An overflow founder to transport the slurry to filter feed tank.

The reactor is a single tank reactor with central and annulus compartments and the total volume is divided into five processing zones. The annulus compartment is provided with two agitators and the central compartment is provided with one agitator. The central compartment is connected to the outer compartment through an under the port.

The agitator blades are designed such that in addition to ensure a high level of axial mixing, they also provide large circulation of slurry within the annulus compartment. This ensures constant and proper control of free, sulfate content in the slurry which is to be maintained at an optimum level to get gypsum crystals of desired shape and size. In the annulus, the ground is fed at first agitator. A high powered agitator is provided to ensure complete dissolution of rock phosphate into the slurry.

A surface baffle which is nothing but an SS plate dipping form top into the slurry is provided immediately after first agitator. This prevents rock going directly together processing zones. The reaction zone in the annulus is further extended to give sufficient time for complete reaction of rock and growth of the crystals. About 12 hours residence time is provided in the reactor to ensure large sized crystals.

Sulfate level, which is vital to obtain the large sized crystals are almost constant throughout the annulus because of the large back mixing flow produced by the agitators and the “flywheel” effect obtained from there circulating slurry. Reactor temperature is maintained at about 78oC to promote formation of dehydrate crystals and rapid rock digestion.

To maintain this temperature a cooling air fan supplies the necessary air, which then passes through 4 numbers of nozzles provided. The fan is provided with variable speed drive by which fan speed can be increased or decreased to maintain the reactor slurry temperature at the desired level. The slurry form annulus flows to central compartment through under the port.

The central compartment gives about one hour residence time for the slurry. This helps in desupersaturating the slurry and promotes the growth of the gypsum crystal for better filtration. As the level builds up the equivalent slurry outflow through an overflow launder which starts from central compartment and up to filter feed tank.

At the mouth of the overflow launder a gate is provided to control the level of the slurry in the reactor. By opening, closing this gate the slurry level can be dropped or increased so that an optimum gap is maintained between slurry surface and cooling air nozzle. With this arrangement effective cooling of slurry can be achieved with minimum airflow.

The off gases from reactor which contains small amounts of fluorine vapors are routed to fume scrubber for effective scrubbing of the same before the air can be let out to the atmosphere. These off gases consists of those that released during the reaction (mainly CO and some amount of fluorine vapors in the form of  SiF) and the air supplied with cooling air fan, which picks up water from the slurry by evaporative cooling.

The off gas duct is provided with impingement baffles immediately above the reactor for cleaning particulate matter and for capturing fine droplets carried because of the entrainment.

The off gases from reactor which contains small amounts of fluorine vapors are routed to fume scrubber for effective scrubbing of the same before the air can be let out to the atmosphere. These off gases consists of those that released during the reaction (mainly CO2 and some amount of fluorine vapors in the form of  SiF4) and the air supplied with cooling air fan, which picks up water from the slurry by evaporative cooling.

The off gas duct is provided with impingement baffles immediately above the reactor for cleaning particulate matter and for capturing fine droplets carried because of the entrainment.

Partially cleaned gases then go to fume scrubber where the velocity of the gases is reduced and a large number of spray nozzles are provided. These sprays provided large surface area and effectively scrub the all fluorine bearing compounds.

The fume scrubber also cleans the gases coming from various ventilation units of the plant like filter hood, seal tank area and vacuum pump area. After spray section the gases go to packed section where final scrubbing is effected to ensure the last traces of fluorine compounds are removed.

The water required for scrubbing is supplied by a circulating pump. This water is kept in closed circuit with continuous bleed off and make up water facilities. As an exhaust fan provides necessary driving force for the removal of off gases from reactor and negative pressure in reactor filter areas avoids pollution. The clean gases are finally let out through the stack.

Rock phosphate grinding process unit

Dihydrate process phosphoric acid plant contains following units:

  • Rock grinding
  • Reaction
  • Filtration
  • Evaporation

Process flowsheeet developed based on the following information:

Unground rock phosphate section:

Rock phosphate received by trucks is unloaded in rock phosphate godown which can hold 1500 metric – equivalent to approximately 3 days production. A dedicated front- end pay loader will stack the material and also help in feeding the material to rock grinding unit.

The rock phosphate is fed to the rock feeding system- comprising a small hopper with an outlet gate and a vibratory feeder by means of front end loader. Grizzly prevents large lumps entering into the system, thereby preventing damage to downstream equipment. Hopper helps in continuous runs. The silo is also provided with a high level switch to stop the conveyor in case it is full. At the bottom of the silo a slide gate and a chute are provided. The slide gate helps to isolate the silo from the mill system whenever it is required. The chute ends onto a table feeder.

Ball mill section: 

The material from unground silo falls onto a table feeder, from where it drops into ball mill feed inlet chute. To control the feed ate to ball mill; the table feeder is driven by a variable speed drive. Based on the screen analysis of mill outlet this speed can be adjusted to get the required fineness in the ground rock phosphate.

The output of the mill through a chute is connected to the boot of an elevator. The material after grinding to the required fineness falls into the elevator from where it is scooped by the buckets of the elevator. The elevator lifts the ground phosphate to the rock silo and the material discharge into it.

The silo also has a capacity of 600 metric tons and can hold about 8 hours production equivalent of ground rock. Since the material is in fine powder form (about 50% minus 74 microns), retrieval is a problem and the cone portion of the silo is provided with air points and pads to keep the material in fluidized condition. Air required for fluidization is supplied by a separate blower. The silo is provided with a high level alarm to warn the operator in case silo is above a particular level.

For retrieving the material, the bottom of the silo is provided with a motorized plug, gate valve and a variable speed rotary valve. The plug valve helps in isolating the silo from down stream equipment whenever the same is down. Rotary valve discharges the material in a controlled way. Increasing or decreasing of material is done by varying the speed of the rotary valve. This valve is connected to an automatic feedback control loop to rock phosphate feeding to reaction section. Whenever reaction is stopped the plug valve is closed and rotary valve is stopped.

The entire grinding operation is carried out under negative pressure to ensure good working environments by eliminating dust nuisance. For this purpose, the ball mill, elevator boot, both rock silo and material transfer points are connected to a bag filter system which consists of various ducts a bag filter and a vent fan.

The two bag filter is of reverse pulse jet type and is connected to vent fan in the downstream. Air, as it passes through bags, deposits the dust on the outside of the bags. Clean, air passing through bags goes to the suction of the vent fan from where it is passed to the atmosphere. Dust collected on the outside of the bags is discharged by means of air pulses obtained by compressed air operated by a based control. As the bag filter is directly mounted on the top of the ground rock silo the dislodged dust falls into ground rock silo.

Rock phosphate feeding section:

This consists of an inclined belt conveyor, which is fitted with a high sensitive load cell for accurate measurement of rock phosphate being fed to the reactor. Since accurate and constant flow of material, is a prerequisite for efficient operation of the reaction system, measurement facility provided on feed conveyor in addition to giving continuous indication in the control room is provided with a feedback control. Based on plant load rock phosphate rate is set at a particular value. The actual value is compared with this set value and depending on the deviation (or error) the speed of the rotation value located below the ground rock silo is controlled to increase or decrease the material flow. The material is transported by the inclined conveyor and is fed into the reactor near first agitator where it is, because of the high degree of agitational energy provided, immediately dissolute into the circulatory reactor slurry.

Ammonium sulphate manufacturing from coke oven gas

Coke oven gas is generated when coal is heated to higher temperatures without air/oxygen. It is the good source for one of the most consuming base compound, ammonia. Along with the organic compound ammonia is evolved from the coke oven. Any idea of separating it from the organic gases was to scrub the gas with acid so that the solid salt is formed and easily separated. Sulfuric acid is best option tho react with ammonia, that yields ammonium sulfate fertilizer.

Acetone manufacturing from cumene hydroperoxidation process

Acetone produced with one of the best efficient process, it uses cumene as raw material.

 

Hydrogen sulfide conversion to carbon disulfide

Hydrogen sulfide H2S obtained from the desulfurization plants used as off gas and in most cases burned by flare. This operation releases harmful sulfur oxide compounds into the atmosphere. It is better to convert hydrogen sulfide into other useful product if it is available sufficiently for instant it can be reacted with carbon to generate carbon disulphide and hydrogen which is very economical and eliminate the pollution caused by H2S.

The chemical reaction for this process is

  • H2S + C  => CS2 + H2

The process flowsheet drawn for the conversion and separation operations: