Evaporator and Crystalliser processes
The solubility of a compound is the amount that can be dissolved in a solvent at equilibrium. Each product compound or combination of compounds has its own unique solubility data. When a portion of the solvent, water in most cases, is removed from a saturated solution, i.e. by evaporation, the solubility of the solution will be exceeded. This is an unstable condition known as supersaturation. In order to return to a stable condition, a portion of the solute precipitates, or drops out of solution as a solid phase, until the solution returns to a saturated condition.
Many compounds have solubilities that vary significantly with temperature (without the requirement to evaporate water). Normal solubilities increase with increasing temperature. Compounds such as potassium chloride, potassium nitrate and mono-ammonium phosphate have normal solubilities that increase with increased temperature.
Other compounds, such as calcium sulphate and calcium carbonate, have an inverse solubility, which means that as the temperature increases, the solubility decreases. This can result in a scaling concern on heat transfer surfaces. For many crystallisation applications these low solubility inversely soluble contaminants must either be removed to low levels or controlled to prevent scaling of any heat transfer surfaces.
However, to complicate the process designer’s work, often two or more components with varying solubilities may be present in the same solution. This is the case with sylvinite brines containing potassium chloride and sodium chloride. For sylvinite solutions the solution can first be concentrated at a high temperature to precipitate sodium chloride while the potassium chloride remains soluble. The precipitated sodium chloride is removed and the remaining liquor is cooled to precipitate and recover the potassium chloride. The recovery processes can be further complicated by the presence of magnesium, calcium and sulphate species that impact the solubilities or product purity and often result in more complex processing paths.
There are multiple processes available to achieve evaporation and crystallisation. The processes most frequently found in the production of fertilisers or by-products will be discussed briefly here. These processes include evaporation, evaporative crystallisation, cooling crystallisation and reactive crystallisation. The process that is implemented depends on the raw materials, desired end product quality, physical properties of the materials and any specific project criteria. Sometimes it is necessary to deviate from traditional methods due to project constraints.
Evaporation / Evaporative crystallisation
Evaporation is the removal of solvent, i.e. water vapour, which results in concentration of the solute, which is usually the desired product. As water is evaporated, the concentration of dissolved salts increases. Examples using evaporation in the fertiliser industry include concentrating phosphoric acid or calcium chloride, or pre-concentrating dilute streams prior to crystallisation.
In the case of evaporative crystallisation, the solution becomes supersaturated when water is evaporated resulting in precipitation of crystals. Evaporative crystallisation is often used for compounds whose solubilities do not change significantly with temperature, such as sodium chloride. It can also be used if the feed solution is relatively dilute and requires significant solvent removal. This can be done in one or multiple evaporation stages, as will be discussed in the following section discussing typical equipment.
Cooling crystallisation
Compounds with a relatively steep normal temperature dependent solubility are easily crystallised in cooling crystallisers. A hot saturated solution is cooled, creating supersaturation which results in crystallisation. The cooling is most often achieved by flashing the water vapour under vacuum. The main driving force for the crystallisation is the cooling of the solute, but some concentration also occurs as water vapour is removed. This type of crystallisation is prevalent in potassium chloride production.
Reactive crystallisation
Another type of crystallisation that is used in the fertiliser industry is reactive crystallisation. This is a process where two species are mixed together resulting in a chemical reaction to form a crystalline product. This type of crystallisation is used to make ammonium sulphate from ammonia (either gaseous or liquid) and sulphuric acid. It is also used to make mono-ammonium or di-ammonium phosphate (MAP or DAP) by reacting ammonia with phosphoric acid. In each of these cases the reaction is strongly exothermic, so no additional heat input is required. Typically, relatively pure reactants are used in these types of crystallisers. However, there is increasing interest in using less pure (and less costly) compounds or waste streams for these applications.
Metathesis is a chemical reaction that involves the exchange of chemical bonds between two reacting chemical species. This approach can be used for the production of potassium sulphate.
Process challenges
Often it is imperative to perform specialised laboratory testing, either at the bench or pilot scale to develop or confirm the design parameters. Relevant physical property and solubility data must either be gathered from experience and/or confirmed with laboratory testing.
Chemistry modelling and process simulation software are being used more frequently as available process data improves. Process modelling software has the advantage that it is easy to compare a variety of proposed system configurations in order to optimise the process.
The best system design approach is to confirm system process design in laboratory testing, using the actual feed solution expected for the commercial plant. If the actual solution is not available, a synthetic solution can be used. Testing can be done at multiple points in the designed process to simulate the different conditions throughout the plant.
Production of high quality product salts.
Fertiliser products may have specific crystal habit, size or purity requirements that must be met on a continuous basis. These requirements must be considered during the process design phase. Sometimes a habit modifier may be required to achieve certain requirements. In other cases an impurity may need to be removed or controlled to achieve the product requirements. The impurity can sometimes be removed by a physical or chemical process, i.e. by chemical addition resulting in precipitation followed by separation by filtration or centrifugation. In some cases it is necessary to control an impurity to a certain level by taking a liquid purge stream from the process. In order to meet product size criteria it may be necessary to implement screening or compaction. Sometimes recrystallisation is required to achieve all product quality requirements. Specialised bench scale testing or pilot scale testing with actual material (or synthetic material if actual material not available) is critical to confirm the required crystal quality requirements.
Environmental constraints
Environmental regulations may impose major constraints on some new system designs. These constraints may take the form of a limitation on waste liquid or other discharge to the environment, or it may take the form of a limited energy supply or cooling media supply. In some cases, if no liquid discharge from the plant is allowed, it may be necessary to design a waste concentration step into the process. Companies are often looking to recover additional products from existing tailings piles or ponds for both environmental and/or economic reasons.
Product recovery from waste streams or by-products
Opportunities exist for clients to potentially recover a valuable product from what was once considered a waste stream. These waste streams often have a high impurity content making it difficult to properly recover the desired product.
The phase chemistry of these complex multicomponent waste streams generally cannot be found in literature or predicted via modelling software. As such, laboratory testing is essential to confirm the design. Once a processing path is identified by the designer, laboratory bench or pilot scale testing to confirm the process design is recommended.
Product recovery may require the design of pre-treatment processes or multi-stage processing to achieve the product objectives.
Heat integration and water balance.
Evaporative crystallisation is a very energy intensive process. Some of the processes, such as potash from solution mining, have large recycle flows and very high energy demands on the system. For these systems, it is imperative to consider a system with a highly integrated heat balance. The concentrated brine is cooled to crystallise KCl and the cooled mother liquor is reheated and recycled to increase the product recovery. It is also imperative to limit unnecessary water addition to these systems as any water that is added must be evaporated. A properly designed system will have long run times and will not require significant water addition for cleaning or flushing.
Similarly, in reactive crystallisation applications, there is a fine balance between the water addition and the heat input. Water is often added to dissolve fines or for cleaning, but often any water that is evaporated must be re-evaporated, thus adding to the overall energy requirement of the system.
Materials of construction
Choosing the proper materials of construction can be challenging for evaporation and crystallisation applications. In many cases the feed or mother liquor solution is at a high temperature and contains high concentrations of corrosive compounds. It is important to choose materials that are resistant to the corrosive environment, but are cost competitive. Corrosion testing is often an integral part of the system design to identify the optimum materials of construction.
For many high temperature applications, high alloys such as Hastelloy®, Inconel®, titanium or Monel® are required, especially for heat transfer surfaces. When the metallurgy required becomes exotic it may be possible to fabricate some of the process vessels using high temperature plastics. In other cases, such as phosphoric acid concentration, graphite is used for heat transfer surfaces in lieu of metal. Cost saving methods must be considered to reduce the overall alloy capital costs. These methods include the use of carbon steel plate that can be lined or coated (with epoxy, rubber, or a thin layer of a higher alloy material).
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