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Essay: Hazard and Operability (HAZOP) Study – Production of Diammonium Phosphate (DAP)

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1.1 Background
HAZOP stands for Hazards and Operability Study and it is an important systematic examination of an existing operation to identify and eliminate potential health and safety hazards in the workplace.
In this study, our team will be examining the batch reaction between ammonia and phosphoric acid, which produces Di-Ammonium Phosphate (DAP). The reaction has the following equation:
2NH3(aq) + H3PO4(aq) ‘ (NH4)2HPO4 (aq)
This reaction produces large amount of heat, having a heat of reaction of approximately 1566.9kJ/mol (Indiana University, 2013) Temperature in the reactor increases immensely, converting any aqueous medium present into steam. The presence of steam may give rise to violent boiling of the reaction mixture, leading to sudden surges which could be dangerous. A solution to alleviate this problem would be to have a reactor tank with a larger surface area. However, the presence of steam still might affect the reaction systems, hence, steam must be quickly removed from the reactors so as to ensure proper functioning of the systems. One possible solution would be to install a vent within the vessel. Installing a vent near the upper end of the cooling column would allow for steam removal and prevent pressure build-up. A thermal runaway could also happen due to the immense amount of heat released from the reaction.
Therefore, it is important to ensure that the temperature, pressure, concentration and flow rates in the feed flow and reactors are well regulated by appropriate devices. Unregulated temperature increase in the feed flow would lead to a large amount of energy release. There is a possibility of the reactor vessel rupturing if the increase in temperature and pressure is beyond the specifications of the vessel, leading to the release of hazardous chemicals. Hence, it is important to keep in mind the above-mentioned properties (temperature, pressure, concentration and flow rates) when doing reactor designing.
1.2 General Uses of DAP
DAP is one of a series of ammonium phosphate salts that can dissolve in water and has many uses. It is a common phosphorus-containing fertilizer for industrial usage and it provides an excellent source of Nitrogen and Phosphorus, which are crucial for plant growth. It is highly popular in the industry for its high nutrients source due to its high solubility in soil, allowing for quick release of Phosphorus and Nitrogen. Besides agricultural usage, DAP can also be used as a fire retardant or as a yeast nutrient in winemaking and mead brewing.
Hence in this report, we will be conducting a HAZOP study on this batch production of DAP to identify potential hazards and risks during or after the operation and brainstorm solutions to mitigate such risks.
Hazard and Operability Analysis (HAZOP)
2.1 HAZOP Assumptions
Additional assumptions are made for this HAZOP analysis and they are:
a) The design specifications mentioned above are strictly adhered to.(Cheah, 2005)
b) The design specifications are based on legal requirements, industrial standards and engineering codes.
c) A basic HAZOP form will be used for recording purposes.
d) Analysis will focus on causes and consequences such as plant design equipment error and human error.
e) The operation of process is not higher than operation design previously assumed. (Sutton Technical Books, 2012)
2.2 Deviation 1 (High Level)
Process Section: Ammonia solution storage tank
Design Intention: Safely contain ammonia feed at ambient temperature and pressure
Guide Word: High
Process Parameter: Level
Deviation: High Level
2.2.1 Causes
1) Level indicator fails low
This may cause the display to show a lower number than the actual quantity. This will cause the amount of ammonia that is unloaded to be higher in quantity than it is supposed to be. This might cause higher ammonia level in the storage tank.
2) Human Error
The level set point could be too highly set by the operator. This will lead to more ammonia being unloaded in the tank. Moreover, the operator may set the flow rate of valve A too slow, which will lead to very slow outflow, which results in the accumulation of ammonia in the tank. In addition, the operator may purposely overfill the tank than the intended level. All these will result in a very high level in the tank.
3) Malfunction of the unloading control system
The ammonia is unloaded from the station where the outflow might be high. This can lead to the increase in unloading rate of ammonia which can increase the level of ammonia in the tank.
4) Backflow of content from DAP reactor
A backflow from DAP reactor into the pipe line and storage tank can cause the high level of ammonia in storage tank. This could be due to high pressure in the DAP reactor caused by cooling system dysfunctions.
5) Plugging of the line
The impurities in the solution may cause the obstruction of pipe, which can lead to a lower flow rate of ammonia through the pipe, which causes the ammonia to build up and therefore, increasing the level of ammonia in the tank.
6) Flow indicator fails high
The flow indicator displays a higher value than actual. Together if the outflow rate is too slow and when the operator follows the display, there might be accumulation of ammonia in the tank causing the level to rise.
2.2.2 Consequences
1) Release of Hazardous Ammonia into Surrounding. (how it is relaesed)
a) Backflow of ammonia solution and the product back into the tanks and its unloading pipes
b) The cross contamination of phosphorus with ammonia in the feed lines, there might be a chemical reaction that is exothermic and therefore, will vaporise a portion of the ammonia. This ammonia will increase the pressure in the pipe line and hence, the feed lines may be rupture and the ammonia can leak into the surrounding
c) Ammonia will be released when the pressure valve, at the ammonia tank, is overflowing. The ammonia released will flash and mix with air
2) Health and Environmental Hazard ( hazards of release)
a) There are health risks when one is exposed to high level of ammonia in air. These effects include mild irritation to the respiratory system and mucous membrane to convulsions, coma and possibly death.
b) When the released ammonia gets into the water bodies, it can be thereat to the aquatic life.
3) Fire Hazard
a) The vaporised ammonia in air flammable if its composition is between the lower and upper flammability limit.
b) A fire may cause pressure in the other equipments that results in explosion, which further released combustible chemical that can exacerbate the fire.
4) Mechanical Failure of Equipment
a) High level ammonia in the tank can cause high pressure and high flow rate. High flow rate can damage the valves such as valve A and pressure relieve valve.
b) High pressure build up in the storage tanks may also rupture the tanks.
c) The excess phosphoric acid can corrode the containers
5) Economical Loss
a) There will be a loss of DAP production, as the release of ammonia from pressure valve will lead to a lower concentration of ammonia in the storage tank and therefore, lesser conversion of ammonia into products.
2.2.3 Safeguards
1) Relief valve
2) Level indicator
3) Flow indicator
2.2.4 Actions
1) Periodic inspection and maintenance
a) Ensure regular maintenance of level indicator, valve A and flow indicator.
b) The relieve valve and tank should be replace if there is any rupture or leakage.
c) Level indicators must be calibrated regularly to ensure correct readings are being made.
d) Ensure regular maintenance and inspection on fire fighting equipments.
2) Installation of Additional Hardware
a) Consider installing new alarm systems to alert any abnormality in the system such as high ammonia in the tank.
b) Installation of scrubber to remove ammonia vapour, if it is released.
c) Installing a connector to ensure that the flow is in one direction – preventing reverse flow.
d) Install a overflow line or over flow detector to prevent high flow and to ensure the level respectively.
e) Consider installation of ventilation to dilute ammonia concentration to levels below ERPG-1 (25ppm).
f) Consider installing a fire sprinkler system with dikes to contain and absorb ammonia vapour if it is released.
3) Precautionary measure
a) HAZMAT kit should be placed near the operation site to contain spillage. Inspection of this kit is also necessary.
b) Technical manual kit should be kept near the operating system so that the operator can know how to operate the system safely.
c) Review should be done to be sure of the level of ammonia in the tank before any unloading.
2.3 Deviation 2 (High Flow)
Process Section: Ammonia feed line to the DAP reactor
Design Intention: Deliver ammonia to reactor at y gpm and z psig
Guide Word: High
Process Parameter: Flow
Deviation: High flow
2.3.1 Causes
1′ Mechanical failure of valve A
Prolonged use of valve A may bring about tear and wear to valve A. With tear ad wear, the opening of valve A may be larger than it is supposed to be and higher flow rate is resulted.
2’Flow indicator fails low
Due to accumulation of rust or dust, the sensor of flow indicator may lose its sensitivity and consequently flow controller displays a lower value than actual value. When operator follows the displayed value and open valve A to a larger extent, an actually higher flow rate is resulted.
3) High pressure in ammonia storage tank
High pressure in ammonia storage tank forces ammonia to exit, leading to high flow rate.
4′ Human error
Operator inputs the setting point of flow controller at a higher value.
2.3.2 Consequences
1) Release of Hazardous Ammonia into Surrounding
High flow rate leads to higher pressure in the pipe. It is possible that high pressure causes the pipes to break or disconnect and consequently ammonia is leaked to the environment. Under high pressure, ammonia solution flashes to vapor into the environment.
a) Health hazards
Ammonia vapor, if inhaled in, is toxic to human body. In addition, after being exposed to ammonia for long time, technicians may get less alert to low concentration of ammonia leakage, which increases the risk.
b) Fire hazards
Apart from health threats to on-site technicians, ammonia may cause fire as it becomes combustible if well mixed with appropriate amount of air and ignition source is present. Heat released during fire increase the possibility of explosion. Once any explosion occurs, on-site technicians may get injured and equipment may get damaged, leading to further leakage and even more hazards.
c) Environmental hazards
Leakage of ammonia to water threatens the aquatic life. Drinking water may also be contaminated.
d) Insufficient ammonia for production of DAP
Leakage of ammonia leads to a lower ratio of ammonia to phosphoric acid feed into DAP reactor. The possible consequences are:
i) Product quality is compromised;
ii) Excessive phosphoric acid may corrode the metal material of DAP reactor and DAP storage tank and produce hydrogen gas. Weakened walls of reactor and tank are more vulnerable to high pressure. Apart from ammonia vapor, the accumulation of hydrogen gas poses further hazards to the weakened walls. Furthermore, flammable hydrogen gas at the outlet of DAP reactor may induce fire and explosion with the presence of ignition source and appropriate amount of air. Again, on-site technicians may get injured and equipment may get damaged, leading to more incidents.
2) Excessive ammonia feed into DAP reactor
a) Low product yield and purity
Run away reactions may occur and unwanted side products such as triammonium phosphate (TAP) is produced instead of DAP, leading to low product yield and purity.
b) Damage to equipment
Large amount of ammonia feed into DAP reactor may lead to backflow of both unreacted reactants and products into ammonia storage tank, phosphoric acid storage tank and their respective feed lines. The reaction goes on in the backflow with the presence of unreacted reactants. Since the reaction is exothermic, heat released can cause vaporization of ammonia. Increased pressure inside pipes may lead to rupture of pipes and leakage of chemicals into the environment.
3) Release of unreacted ammonia from the DAP storage tank to the enclosed work area
Unreacted ammonia solution may flash to vapor at the exit to DAP storage tank the moment valve C is open. Flashing of ammonium suddenly increases the pressure inside the pipe; therefore spillage of chemicals may occur.
2.3.3 Safeguards: flow indicator/controller F1 and valve A
2.3.4 Actions
1) Periodic inspection and maintenance
a) Ensure regular maintenance of valve A and flow indicator.
b) Valve A and feed pipes should be replaced if any rupture or damage occurs.
c) Flow indicator must be calibrated regularly to ensure correct readings are being made.
d) Ensure regular maintenance and inspection on fire fighting system.
2) Installation of Additional Hardware
a) Consider installing an alarm and a safety feed cut off valve at ammonia feed line. In the case of high ammonia flow, ammonia flow will be shut down.
b) Consider installing an ammonia detector, which displays the real-time ammonia concentration of ammonia in the surroundings and warns about concentration of ammonia above ERPG-1 (25ppm).
c) Improve ventilation system, such as installing elephant trunks, so that released ammonia vapor can be diluted spontaneously.
d) Installing a non-return valve at feed lines of ammonia and phosphoric acid to prevent backflow.
e) Add a bypass line to existing feed line of ammonia so that if valve A fails, the flow rate of ammonia can be adjusted manually.
f) Consider installing a fire sprinkler system with dikes to contain and absorb ammonia vapor if it is released.
g) Change the current open DAP storage tank to a closed DAP storage tank with vent so that spillage of chemicals can be prevented.
h) Introduce earthing system to reduce static and its associated ignition source.
3) Precautionary measure
a) HAZMAT kit should be placed near the operation site to contain spillage. Inspection of this kit is also necessary.
b) Technical manual kit should be kept near the operating system so that the operator can know how to operate the system safely.
c) Regular training and refresher courses should be provided to on-site operators to avoid human error as much as possible. On-site operators should also be informed of any updates on the operating system.
d) Personal protective equipment (goggles, gloves, etc.) should be made available.
e) Good housekeeping should be applied, such as ventilation after operating hours.
A detailed evacuation plan should be prepared and drills should be conducted regularly.
2.4 Deviation 3
Process Section: Phosphoric Acid Storage Tank
Design Intention: Safely contain acid feed at ambient temperature and pressure
Guide Word: Low
Process Parameter: Concentration
Deviation: Low Concentration
2.4.1 Causes
1) Equipment Error
a) Valve B might have malfunctioned and was fully open during the loading of phosphorus acid with the reaction still on-going in the reaction vessel. This would cause a backflow of mixture from the reactor into the phosphorus acid storage tank and subsequently dilute the acid in the tank.
b) The tank and pipe may not have been sealed properly and tightly after loading and unloading, causing the phosphorus acid in the tank to be diluted and contaminated.
c) An overloaded DAP reactor can cause backflow to happen even when valve B is closed. Overloading of the DAP reactor could possibly be due to high flow in the ammonia feed line. Backflow of mixture is undesirable as dilution of phosphorus acid in the tank will take place
2) Human Error (Upstream Flow)
a) Feed is of poor quality due to improper handling and preparation of phosphorus acid. Phosphorus acid might have already been contaminated or diluted before being fed into the tank.
b) Desired concentration of Phosphorus acid was miscalculated, causing the feed entering the storage tank to be out of specification.
c) The well-prepared phosphoric acid might be diluted or contaminated during loading into the storage tank due to improper or untimely cleaning of storage tank.
3) Unused Phosphoric Acid of lower concentration
a) The previous batch’s unused phosphoric acid of lower concentration might be recycled and reused and mixed with the new batch of phosphoric acid. This will then result in the deviation from the desired concentration of phosphoric acid.
2.4.2 Consequences
1) Poor Product Performance
a) Since the concentration of Phosphorus acid is lower, the rate of production of DAP will be slower. This also means that the reaction mixture in the DAP reactor is not in stoichiometric ratio which leads to excess unreacted ammonia that contaminates the product. TAP may be formed instead of DAP due to the unreacted ammonia.
2) Flashing of ammonia leading to hazardous release at vessel outlet
a) After the reaction has ended, each batch of DAP product is collected through the use of earth’s gravity into the DAP product tank by opening Valve C. However, due to the presence of unreacted ammonia in the reactor vessel, the ammonia would flash the moment it exits the reactor with the rest of the products. This flashing of ammonia would bring about an increase in reactor pressure and consequently, the liquid product DAP would be forced out of the pipe and be released as a high pressurized liquid. This could result in splashing and the products would spill out of the tank.
b) The ammonia vapor released could be inhaled and prolonged exposure and inhalation of the vapor would compromise the on-site workers’ health and safety.
c) Vaporized ammonia could also bring about a fire hazard because it mixes with air to form a combustible mixture. This combustible mixture acts as an ignition source and an explosion could occur which would endanger the lives of many on-site technicians as well as cause structural damage to the plant and equipments. Any dependent cascading operations would be affected and cease as well.
3) Flashing of DAP inside reaction vessel
a) The unreacted ammonia vapor may flash inside the reaction vessel as well and cause a pressure build-up within the vessel. This pressure build-up would cause the vessel to rupture and release the DAP products and unreacted ammonia into the surrounding. As previously mentioned, the release of unreacted ammonia vapour could compromise the health and safety of plant technicians, cause structural damage and possibly lead to an explosion.
4) Health and Environmental Hazard
a) Ammonia is a colorless and toxic gas which is highly soluble in water to form liquid ammonia when it is released into the surroundings. Inhalation of large amounts of ammonia or ingestion of food contaminated with large amounts of ammonia may result in ammonia poisoning. The release of ammonia into water bodies poses as a threat to the aquatic life as well
2.4.3 Safeguard
Pressure release on storage mitigates pressure hazard
2.4.4 Action
1) Personal Protective Equipment (PPE)
a) All on-site technicians and employees should be enforced to put on their PPE such as goggles, face shields and chemical resistant gloves at all times while they are on site.
2) Compulsory Training Courses
a) Compulsory training courses must be provided to all on-site technicians and employees to acquire the proper set of skills to operate the equipments and familiarize themselves with the safety procedures in the case of an emergency.
b) The operators should also familiarize themselves with the safety practices of handling and storing various chemicals.
c) All employees on-site should be familiar with the proper evacuation procedure, route and evacuation zone.
3) Scheduled Maintenance
a) Since Valve B is the valve controlling the flow of Phosphoric acid into the reactor vessel, proper maintenance and inspection should be scheduled regularly to ensure that the valve is working fine and that it is perfectly sealed when closed.
b) Regular checks on the storage tank, feed line and unloading line of the phosphoric acid should be performed to ensure that there is no contamination and impurities present.
4) Sensors and Detection
a) Install a composition analyser in both ammonia and phosphoric acid lines to ensure that the reactants entering the reaction vessel have the exact stoichoimetric ratio required.
b) Sensors that could detect the presence of ammonia vapor due to leakage could be installed together with an alarm system so that the workers on-site could be alerted early that there is a leakage.
c) A sensor that could detect the deviation in phosphoric acid concentration could be installed on the phosphoric acid feed line. On top of this, some device or procedure could be put in place to shut down the entire operation when it detects a deviation in the phosphoric acid concentration from its desired value. This would ensure that phosphoric acid concentration below the desired value would not enter the reaction vessel and dilute the reaction mixture.
d) Use a sealed DAP reactor vessel to prevent the release of ammonia gas into the surrounding.
a) A sprinkler system with dikes could be set-up to absorb as much ammonia vapor in the case when there is flashing of ammonia.
6) Neutralizing Agents
a) Neutralizing agents are effective in treating in spillage of chemicals and should be made available to all employees in the workplace.
7) Fire Sprinkler System
a) Automatic fire sprinkler system should be installed in the workplace, where the detection of a certain amount of heat or smoke would trigger the system to release water at high velocity through the sprinklers.
8) Earthing
a) Earthing or grounding the vessel can remove the static build up on the surface of the vessel to prevent explosion.
9) Ventilation System
a) Methods of ventilation such as elephant trunks and blower fans could be installed in the workplace to allow fast removal or dilution of ammonia vapor concentration in air to a safe level below ERPG-1 (25ppm).
2.5 Deviation 4 (Loss of Agitation)
Process Section: DAP reactor
Design Intention: Contain the reaction at x??C and y psig
Guide Word: Loss
Process Parameter: Agitation
Deviation: Loss of agitation
2.5.1 Causes
1) Electrical Failure
a) A short circuit causes the electrical power outage, which will damage the electric transmission lines.
b) The motor may not work due to the lack of driving force power, which the mixing job of reactants inside the mixer will not be functioning well.
2) Human Error
a) The mixer may not work properly due to operator’s oversight.
3) Mechanic failure
a) Accumulation of undesired materials on the surface of the impeller causes damaged or broken impeller.
b) Agitator mechanical linkage failed to operate or function properly.
c) The impeller may lose its efficiency due to fouled or jammed.
4) Up Stream Error
a) Impurities in the feed could cause damage to the stirrer in the DAP reactor.
2.5.2 Consequences
1) Low quality of the DAP production performance
a) The reactants in the reactor will not be well mixed. Some reactants will be left un-reacted due to non-homogenous reaction mixture.
b) The quality of the product will be poor as DAP output is contaminated with un-reacted reactant.
2) Tank destruction
a) The situation of the hot spot reaction may occur. The high enthalpy of reaction may cause materials to be heated to extremely high temperatures. This may cause the tank wall integrity to be compromised blow expected lifetime from cooling or expansion.
b) With the un-reacted phosphoric acid in the DAP reactor, it will pass through valves C and D then will be released together with the product DAP into the DAP storage tank. The acidity from the phosphoric acid would accelerate corrosion in both valves C and D, as well as the DAP storage tank.
c) The un-reacted ammonia liquid will flash into vapor within the reactor, then end up to the phosphoric acid feed line and react with phosphoric acid in the storage tank. As a result, the damage to the tank will occur due to the high enthalpy of reactions between the reactants.
3) Waste of raw materials
a) Large amount of un-reacted reactant will accumulate inside the DAP storage tank.
4) Hazardous release at vessel outlet
a) At the end of the reaction time, DAP product will be collected and contained in the DAP storage tank through an earth gravity feed. However, due to the un-reacted ammonia in the reactor vessel, ammonia would flash into vapor when it exits the reactor. These flashing will cause a sudden increase in reactor pressure, forcing liquid product DAP out as a high pressured liquid, which could spill out of the tank.
b) The ammonia vapor could also be posed as a fire hazard. It will become as a combustible mixture when vaporized ammonia is mixed with air. An explosion could occur if the ignition source exists.
5) Environmental hazard
a) Ammonia is a highly water soluble, colourless and toxic gas which could be easily dissolved in the water and form liquid ammonia to be released to the surrounding. The ammonia solution will be inhaled by surrounding unsuspecting plant technicians. This would pose as a health risk to workplace.
2.5.3 Safeguards
2.5.4 Actions
1) Maintenance
a) Periodic inspection on the impeller to mak sure it in its optimum working condition and there is no crack and structural damages, as well as no undesired materials on the surface of the impeller.
b) Regular check on the DAP rector vessel, outlet and feed line to ensure there is no cracks.
c) Proper cleaning should be carried out to make sure there are no solid contaminants in the storage tank and feed lines of the reactants.
2) Training
a) Compulsory technical and professional training courses must be offered to the operators so they could acquire new skills and update existing skills to become more familiar with the procedures.
b) Safe practices for handling and storing the chemicals must be strictly followed by the operators.
c) All the individuals in the workplace should be familiarize with the proper evacuation procedure and the location of the evacuation zone.
3) Personal Protective Equipment
a) Personal protective equipment such as goggles, rubber gloves, face shields and etc. should be available in the workplace.
4) Equipment and Detection
a) A sensor which can detect the loss of agitation within the reactor should be installed.
b) Backup power generator may be installed to provide the backup power during power outage.
c) A tachometer could estimate the angular velocity of the motor and detect the sudden impeller failure. Further inspection could then be carried out by technicians.
5) Neutralization
a) Neutralizing agents should be made readily available to all the individuals in the workplace to treat spillage.
6) Ventilation
a) Installation of ventilation systems will allows quick removal or dilution of ammonia concentration to a level well below ERPG-1 (25ppm).
7) Earthing
a) Grounding the tank can be done to remove static, which is responsible for causing the explosion.
3. Suggested Modifications to Current System
In order to improve the safety of working in the given system based on the four deviations given, the common recommended actions that can be taken are summarized as follows:
Preventive Measures a) Trainings must be provided to operators.
b) Personal protective equipment must be made available in the workplace.
c) Maintenance and inspection on hardware equipment must be conducted on a regular basis. Damaged and faulty parts must be notified and replaced immediately.
d) Good housekeeping must be practiced by all people who work in the plant.
e) Equipment must be grounded at all times to prevent fire and explosion.
f) Sensor and alarm system should be installed to detect harmful concentration of chemical in the air.
Mitigation Measures a) Ventilation system should be installed to lower concentration of chemicals.
b) Neutralization kit should be accessible at the site of operation.
c) Install sprinkler system for dilution of chemicals in the event of leakage.
d) Firefighting equipment should be available at the site of operation.
e) Evacuation plan should be drawn up to minimize in the event of an accident.
4. Conclusion
5. Reference

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