Continuous USP Fertilizer Granulation via Urea-Sulfuric Acid Reactant
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Solution Overview
Problem
The manufacture of granular urea-superphosphate (USP) fertilizers faces challenges due to the liquefaction of granular mixtures caused by the reaction of urea with calcium phosphate hydrates and sulfate, leading to inefficient energy consumption, equipment issues, and safety concerns from exothermic reactions, particularly when using superphosphates or ammonium nitrate.
Innovation Solution
A method involving the decomposition of phosphate raw materials with a urea-sulfuric acid solution, with controlled water addition and optional phosphoric acid, to produce a nitrogen-phosphate fertilizer, allowing for continuous granulation and reducing the concentration of sulfuric acid, thereby minimizing exothermic reactions and improving product stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If superphosphates and urea are used as intermediates to manufacture NP and NPK fertilizers, then high total nutrients content is achieved, but granular mixtures undergo liquefaction due to water release from urea reaction with calcium phosphate hydrates and sulfate
Solution Approach 1:
The patent changes the physical-chemical parameters of the system by replacing solid superphosphate and solid urea intermediates with a liquid reactant solution containing urea in sulfuric acid. This parameter change prevents the liquefaction problem because the urea is already dissolved and reacts in situ during granulation, eliminating the water release issue that occurs when solid components react after mixing.
Solution Approach 2:
The patent introduces sulfuric acid as an intermediary medium that dissolves urea to form a reactant solution. This intermediary allows urea to be delivered in a controlled, dissolved state during the granulation process, preventing the direct contact and water-release reaction between solid urea and calcium phosphate hydrates that causes liquefaction.
2Stability of the object's composition
If cold recycle is applied to avoid granule disintegration and sieve pores plugging, then granule integrity is improved, but energy consumption increases due to moving increased masses of intermediates
Solution Approach 1:
The patent extracts and eliminates the problematic intermediate stage that requires cold recycle. By using a reactant solution of urea in sulfuric acid that reacts directly during granulation, the process removes the need for separate granulation and drying stages with cold recycle, thereby eliminating the energy-consuming circulation of large intermediate masses.
Solution Approach 2:
The patent achieves continuous granulation where the reactant solution is continuously fed and reacts in situ during the granulation process. This continuous action eliminates the need for interrupting the process for cooling and recycling intermediates, maintaining continuous productive operation without energy-intensive recycle loops.
3Stability of the object's composition
If granulation and drying are conducted at significantly reduced temperatures to prevent liquefaction, then granular stability is improved, but dryer size and drying agent amount increase
Solution Approach 1:
The patent converts the potential harm of water release during reaction into a benefit by using the water already present in the reactant solution as the reaction medium. The controlled water content (16-25%) in the reactant solution prevents excessive liquefaction while enabling complete reaction, eliminating the need for low-temperature processing and large dryer capacity.
4Productivity
If high concentration of sulfuric acid is used in the reactant solution, then reaction efficiency is improved, but exothermic reactions increase causing safety concerns and product instability
Solution Approach 1:
The patent optimizes the concentration parameters of the reactant solution, using a controlled water content of 16-25% rather than highly concentrated sulfuric acid. This parameter change reduces the intensity of exothermic reactions and safety hazards while maintaining effective reaction efficiency, as the moderate concentration allows controlled heat release during granulation.
Data Source
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AI summary
The method according to the invention consists in that the reaction of phosphoric raw material decomposition is carried out using a reactant solution containing from 1.5 to 4.0 mol of urea per 1 mol of sulphuric acid, in a quantity corresponding to 90 to 100% of acid in respect of the stoichiometric requirement for the complete decomposition of the rock phosphate, whereas into the reaction environment water is added in an amount necessary to ensure its content in process pulp in the range of 16-25% in respect of the whole mass of the suspension, and the process pulp obtained is fed directly to a granulation unit. The plant according to the present invention comprises an agitated reactor (1) to manufacture reactant solution, a system of two reactors (3 a i 3b) prepared for continuous operation, a set of granulators (5), a rotary dryer operating on hot air or flue gas (6), a set of sieves (7) and a recycle system comprising a grinding mill (13) and a buffer tank (14). The method and the plant according to the invention are particularly suited for the manufacture of USP fertiliser in a continuous process and allow for avoiding time- and cost-intensive operations related to product aging.