Diammonium Phosphate Fertilizer Manufacturing Process
Diammonium phosphate (DAP) is a widely used granular fertilizer, valued for its high phosphorus and nitrogen nutrient content. The manufacturing process involves a series of chemical reactions, physical transformations, and separations, all carefully controlled to produce a high-quality, consistent product. Let's walk through the main steps depicted in this process flow diagram:
1. Pre-Neutralization Process:The journey begins in a pre-neutralizer where concentrated phosphoric acid (H3PO4) reacts with ammonia (NH3) in a controlled environment. The reaction is highly exothermic, releasing heat, and produces a partially neutralized slurry of monoammonium phosphate (MAP) and diammonium phosphate (DAP). The stoichiometry and reactor conditions are carefully controlled to obtain an optimum liquid slurry that can be further processed in the granulator.
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| Preneutralizer Diagram |
Pre-neutralizer equipment: Typically a stirred tank reactor, is designed for efficient mixing and heat removal. It's constructed of corrosion-resistant materials to withstand the harsh chemical environment. It's also equipped with instrumentation to monitor and control temperature, pH, and flow rates.
Reaction Equation:
- NH₃ + H₃PO₄ → (NH₄)₃PO₄ (DAP)
Neutralization Reaction:
- H₃PO₄ + 2NH₃ → (NH₄)₂HPO₄ (Mono-Ammonium Phosphate, MAP)
- (NH₄)₂HPO₄ + NH₃ → (NH₄)₃PO₄ (DAP)
Model Equation:
The model equation for the pre-neutralizer can be represented by a set of ordinary differential equations (ODEs) that describe the changes in concentration of the reactants and products over time.
Let's assume that the pre-neutralizer is a continuous stirred-tank reactor (CSTR) with a constant volume (V) and a feed flow rate (F). The model equation can be written as:
d[MAP]/dt = (F/V) × (0 - [MAP]) + k1 × [H3PO4] × ([NH3])2 / (1 + K1 × [MAP])
d[DAP]/dt = (F/V) × (0 - [DAP]) + k2 × [MAP] × [NH3] / (1 + K2 × [DAP])
d[H3PO4]/dt = (F/V) × ([H3PO4]n - [H3PO4]) - k1 × [H3PO4] × ([NH3])2 / (1 + K1 × [MAP])
d[NH3]/dt = (F/V) × ([NH3]n - [NH3]) - 2 × k1 × [H3PO4] × ([NH3])2 / (1 + K1 × [MAP]) - k2 × [MAP] × [NH3] / (1 + K2 × [DAP])
where:
- [MAP], [DAP], [H₃PO₄], and [NH₃] are the concentrations of mono-ammonium phosphate, diammonium phosphate, phosphoric acid, and ammonia, respectively.
- k1 and k2 are the reaction rate constants for the neutralization reactions.
- K1 and K2 are equilibrium constants of MAP and DAP
- F is the feed flow rate.
- V is the reactor volume.
- [H₃PO₄]ₙ, and [NH₃]ₙ are the inlet concentrations of the reactants and products.
Assuming the production rate of 800 kg/h of DAP, we need to calculate the reactants' required flow rates and the pre-neutralizer's size.
Molecular Weights:
- NH₃: 17 kg/kmol
- H₃PO₄: 98 kg/kmol
- (NH₄)₂HPO₄ (MAP): 117 kg/kmol
- (NH₄)₃PO₄ (DAP): 149 kg/kmol
Material Balance Calculations:
| Component | Molar Flow Rate (kmol/h) | Calculation Formula |
|---|---|---|
| DAP | 5.37 | F_DAP = 800 kg/h / 149 kg/kmol |
| MAP | 5.37 | F_MAP = F_DAP × (1 mol MAP / 1 mol DAP) |
| NH₃ | 10.74 | F_NH3 = F_MAP + F_DAP |
| H₃PO₄ | 5.37 | F_H3PO4 = F_MAP × (1 mol H₃PO₄ / 1 mol MAP) |
Pre-neutralizer Size Calculation:
Assuming a pre-neutralizer with a volume (V) of 10 m³ and a residence time (τ) of 30 minutes:
| Parameter | Value | Calculation Formula |
|---|---|---|
| Volumetric Flow Rate of NH₃ (m³/h) | 240.3 | Q_NH3 = F_NH3 × 22.4 m³/kmol |
| Volumetric Flow Rate of H₃PO₄ (m³/h) | 120.1 | Q_H3PO4 = F_H3PO4 × 22.4 m³/kmol |
| Required Diameter of Pre-neutralizer (m) | 1.43 | d = √(4 × Q_NH3 / (π × τ)) |
| Required Height of Pre-neutralizer (m) | 6.35 | h = V / (π × (d/2)²) |
Therefore, the required size of the pre-neutralizer is approximately 1.43 m in diameter and 6.35 m in height. Note that this calculation assumes a simplified model and does not take into account other factors that may affect the design of the pre-neutralizer, such as mixing, heat transfer, and mass transfer.
The partially neutralized slurry from the pre-neutralizer is fed to a granulator, where it's sprayed onto recycled fine particles and partially granulated products. More ammonia is added to this rotating bed, causing further neutralization and the formation of larger, solid granules. The addition of ammonia also helps to control the pH of the slurry, driving the reaction towards forming DAP. Other additives, such as urea and fillers, may be introduced at this stage.
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| DAP Granulator |
Granulator: Typically a rotating drum granulator, is designed to tumble and agitate the materials. It has internal lifters to enhance the mixing and growth of granules.
3. Drying Process:
The wet granules from the granulator are conveyed to a dryer, where excess moisture is removed using hot flue gas from a combustion chamber. This step is crucial for producing stable, free-flowing granules. The temperature and flow rate of the hot flue gas are carefully controlled to prevent scorching or over-drying.
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| Rotary dryer |
Dryer: Typically a rotary drum dryer is designed for maximum heat transfer and material mixing. It is designed with internal flights which help to improve heat transfer and material flow.
Combustion Chamber: This is where fuel and air are mixed and combusted to generate the hot flue gas, which is then sent to the dryer. It typically has temperature controls to ensure that the gas is heated to the required temperature.
4. Screening and Sizing Process:
After drying, the product is sent to vibration screens where granules are sized according to the desired particle size. Under-sized particles (fines) are recycled back to the granulator, and over-sized particles might be sent for crushing (not shown in this particular diagram). The on-size product moves on to product cooling.
Vibration Screens: These screens are used to separate particles based on size. They consist of one or multiple screens placed at angles, with the particles moved using vibrations.
Feeders: Feeders are used to deliver a consistent and controlled stream of materials to subsequent equipment. These can be belt or screw-type feeders.
5. Product Cooling Process:
The dried-sized granules are conveyed to a product cooler where they are cooled with air to the desired temperature before bagging and storage. This cooling is important to improve product stability and prevent agglomeration.
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| Rotary product cooler |
Product Cooler equipment: Typically a rotating drum cooler which is designed for efficient cooling of solids. Air is passed through the cooler for effective heat removal.
6. Fumes and Scrubber Treatment Process:
The process generates fumes containing ammonia, phosphoric acid, and particulate matter. These fumes are scrubbed and cleaned in a multi-stage scrubber system. This is done to meet environmental regulations and to recover valuable materials.
Fumes Scrubbers Equipment: These scrubbers are used to remove pollutants from gaseous streams. They might use water or other solvents to absorb pollutants before releasing the cleaned gas to the environment through a chimney. These scrubbers may contain spray nozzles, packing material and demisters.
Centrifugal Blower: This blower is used to move the gaseous stream.- Tail gas scrubber: This scrubber is similar to the fume scrubber, and might also be part of the same train or a separate one for further purification of gasses.
- Cooler Scrubber: This cooler scrubber helps in cooling the gas and removing any remaining contaminants.
- Scrubber effluent tank: This is a collection tank for scrubbing liquids for reuse or treatment.
Cyclone Separators: These are used to separate larger particles from gases, based on centrifugal forces.
Additional Equipment:Preparation Tank: A tank for pre-mixing or dissolving materials.
Pulverizers: Used to reduce the size of solid particles when required.
Conveyors: Different types of conveyors (belts, screws, etc.) are used throughout the process for material transport.
Pumps: Used to transport liquids to different sections of the plant.
Instrumentation: Various sensors and controllers are integrated to monitor and control temperatures, pressures, flow rates, levels, and compositions across the plant, ensuring the process runs effectively and safely.
Process Flow:
The overall process starts with the reaction of phosphoric acid and ammonia in a pre-neutralizer, followed by granulation, drying, sizing, cooling and then packaging. Off-gases are scrubbed and treated for reuse or safe disposal. This flow sheet shows the typical process involved in the production of DAP fertilizer.
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| Process flow diagram of DAP fertilizer manufacturing |
By carefully controlling each stage of the process, a consistent high-quality product is manufactured at a large scale. This detailed process description and equipment list are essential to understanding the engineering and science behind DAP fertilizer production.