Evaporation and crystallisation technology

PRODUCTION TECHNOLOGY

Evaporation and crystallisation technology

Evaporation and crystallisation are widely used throughout the fertilizer industry. Major applications and the types of equipment used are reviewed by Laurent Palierne and Norbert Strieder of GEA Group.

Fertilizer production relies on efficient processes and mature technologies. This article provides an overview of the use of evaporation and crystallisation technologies in fertilizer manufacture. The trend for purification to optimise product quality via precipitation, membrane filtration and ion exchange is also outlined.

There is increasing market demand for water-soluble value-added products for foliar applications, fertigation and NPK blends. For these end-uses, evaporation, stripping, crystallisation and membrane filtration units are a perfect fit for the production of high-purity solids with the right shape and size characteristics. These technologies can be harnessed to manufacture a long list of nitrogen, phosphate and potash products:

• Phosphoric acid
• Monoammonium phosphate (MAP)
• Diammonium phosphate (DAP)
• Urea phosphate
• Ammonium sulphate (AS)
• Ammonium chloride
• Ammonium nitrate
• Calcium nitrate
• Potassium chloride (MOP)
• Potassium sulphate (SOP)
• Magnesium sulphate
• Potassium phosphate
• Potassium nitrate (NOP).

By-products like calcium chloride are also produced in evaporation and granulation plants.

Technology basics

Evaporation and crystallisation are required in a production process whenever:

• Removal of water or another solvent is required
• Concentration has to be increased (product specification, chemistry, etc.)
• Volume has to be reduced (effluent, transportation, etc.)
• By-products or impurities need to be stripped or precipitated
• Valuable, high-purity crystals are being manufactured.

Equipment selection and plant design is influenced by many product- and process-specific factors. Customer requirements, notably site characteristics such as the energy source and its cost, play an equally important role in determining the process engineering design of an industrial plant – and minimising investment and operating costs.

Process scale-up and testing

Laboratory tests and trials in pilot plants are the prerequisite for determining the correct process engineering design. Once physical properties and behaviour are known, evaporation and crystallisation processes can then be modelled easily.

GEA’s research and development laboratories have equipment that accurately represents most types of evaporators, crystallisers and membranes. These can be used to simulate the specific design envisioned for our clients and record relevant data.

These process designs can then be tested in small pilot plants to simulate specific process requirements. The samples produced are also suitable for further tests or market investigations. GEA’s team can even perform the necessary tests or investigations at the client’s own site, if products are too sensitive to be shipped to their facilities, or require special handling due to safety or health concerns.

Evaporator types

Several designs of evaporators (Figure 1) are available to satisfy a wide spectrum of process requirements. They include:

• Plate or tubular type falling film evaporators
• Plate or tubular type forced circulation evaporators
• Flash and multi-flash evaporators.

Different types of evaporator offer their own particular benefits. In each case, the most suitable type is selected by consideration of the main process parameters:

• Scaling tendency
• Product thermal sensitivity
• Required particle size
• Annual operating hours
• Accessibility for maintenance.

To ensure the longest operational lifetime, an evaporation plant is made of the most suitable and durable construction materials. These include carbon steel, stainless and duplex steel, high nickel alloys, nickel and titanium. Graphite, rubber-lined carbon-steel and fiberglass reinforced plastics are also selected for the most highly corrosive applications.

Types of crystallisation process and equipment

Crystallisation plants need to be engineered to meet the customer’s product and process specifications (purity, particle size, operating time, etc.), while minimising investment and operating costs. The crystalliser unit is selected from a wide range of available technologies (forced circulation, draft tube baffled, Oslo or flash cooling) to match individual product requirements. Achievable particle size can ranges from microns to millimetres, depending on the crystalliser type (Figure 2).

Evaporative crystallisation is usually chosen as a process when the solubility of the solute is virtually independent of temperature, thereby allowing supersaturation to be achieved by concentrating the slurry. This often requires a forced circulation system in order to control supersaturation, suspension density or crystal breakage.

Fig. 1: Evaporator types

Fig. 2: Crystalliser types

Vacuum cooling crystallisation, in contrast, is usually chosen when the solubility of the substance to be crystallised is strongly temperature dependent. Supersaturation is generated by adiabatic cooling of the slurry by applying a vacuum, without a cooling surface and avoiding encrustation.

Where high quality standards are demanded, a recrystallisation stage can be added to improve the purity of the final product.

Heating configurations

Evaporation and crystallisation processes require a substantial amount of energy. The operating cost is directly linked to the type of evaporator and heating configuration selected (Figure 3):

• Single-effect
• Multiple-effect
• Thermal vapour recompression
• Mechanical vapour recompression
• A combination of those arrangements.

A new plant must balance energy costs with capital cost. This is generally achieved by finding the right compromise (total cost of ownership) between the customer’s investment budget and operating cost, mainly linked to energy price levels. Existing production units can also be upgraded by reconfiguring and improving the existing heating configuration and/or by introducing more modern technology.

The main types of heating configuration are described below:

• Multiple effect evaporator: Classical multiple effect evaporation uses low-pressure steam for heating of the first effect and vapour for heating in the subsequent stage or stages. The number of stages employed reflects the relative importance of operating costs versus capital investment. These factors are linked to evaporation capacity and the boiling point elevation of the solution.
• Thermal vapour recompression (TVR). TVR is used whenever steam is available at high- or medium-pressure. The flash energy of the steam is used to recompress part of the vapour given off, up to the pressure of the heating steam. This configuration, when available, achieves substantial savings in steam and cooling water at a relatively low cost.

Fig. 3: Mass/energy flow diagrams for an evaporator: four different types of heating configuration*

• Mechanical vapour recompression (MVR). In MVR, all of the process vapours given-off are recompressed to heat the evaporator. Virtually no steam is required, apart from pre-heating when starting the process. The only energy consumption necessary is the power required to drive the compressors. The compressors are usually driven by electrical power, although they can also be driven by a steam turbine. They are usually centrifugal fans, with one or several stages of compression, or positive-displacement type compressors. The current cost of energy makes MVR very attractive for evaporation and crystallisation, and choosing this option also delivers a saving on cooling tower and boiler investment. A scrubber can be installed prior to the fans for corrosive applications (e.g. acidic vapours).

Key applications

Evaporators and crystallisers are widely applied in the production of nitrogen, phosphate and potash fertilizers. They are used in the manufacture of commodity products like merchant-grade phosphoric acid (MGA) and muriate of potash (MOP, KCl) granules. More importantly nowadays, they are also used to manufacture value-added fertilizers, especially water-soluble products such as MAP, DAP, MKP, SOP and KNO3.

GEA Group, with its expertise in evaporation and crystallisation, has developed and successfully installed numerous processes in fertilizer production plants worldwide. As well as operational industrial plants, the company’s know-how includes process design expertise, gained from laboratory- and pilot-scale tests and process simulations carried out at GEA’s development centre.

Potassium chloride (MOP) production

For brine sources, highly pure crystalline potassium chloride is recovered at multiple flash cooling crystallisation plants. Brines typically contains both sodium chloride (NaCl, sylvinite) and potassium chloride (KCl). This process is notable for:

• High efficiency heat recovery
• Temperature drop from 110°C to 45°C
• Heat recovery in four stages is usual with condensation in three re-cooling stages
• K2 O content adjustable between 58-62 percent by remixing condensate
• Crystal size of up to 1.2 mm.

Potassium chloride is also commonly crystallised from conventionally mined carnallite ore (KCl, MgCl2 , NaCl, MgSO4 and CaSO4 ). Additional purification steps are generally necessary to obtain a product of the required commercial quality at an economically-acceptable yield.

Potassium nitrate crystallisation

Potassium nitrate (KNO3 ) can be manufactured via several process routes:

• GEA offers a direct conversion process for KNO3 using crude, natural sodium nitrate (caliche) and fertilizer-grade potassium chloride (MOP). This innovative process design provides a capital cost saving on the sodium nitrate rectification plant.
• The reaction of MOP with nitric acid is another KNO3 production method. This yields ammonium chloride as a by-product.
• Potassium nitrate can also be produced using an ion exchange and crystallisation process.

Ammonium sulphate

The industrial production of caprolactam (CPL), methyl methacrylate (MMA) and acrylonitrile yields large quantities of ammonium sulphate (AS) as a by-product. This route now accounts for 60-70 percent of AS production globally. Other sources of ammonium sulphate include:

• Purge gas washing, e.g. from large urea prilling towers
• Regeneration liquids from continuous ion exchange systems
• Production through reactive crystallisation of ammonia and (spent) sulphuric acid.

The reactive crystallisation of ammonium sulphate produces enough heat of dissolution and reaction that the process can be operated without any external energy source for evaporation. Reactive crystallisation has an energy advantage over evaporative crystallisation but, disadvantageously, produces smaller crystals. Indeed, around 80-90 percent of AS crystallisers are operated in evaporative mode because this produces larger size crystals. The market price of large ‘granular’ crystals (2-3 mm) can be up to three times higher than the price of smaller crystals (<1 mm) – a price premium that strongly favours the production of large crystals.

Fig. 4: Ammonium sulphate reactive crystalliser.

PHOTO: GEA

Monoammonium phosphate (MAP) from green acid

Monoammonium phosphate (MAP) is manufactured from phosphoric and ammonia by a process involving:

• Reactive crystallisation – the reaction of NH3 with H3 PO4 in a reactor
• Decantation
• Membrane filtration
• Forced circulation crystallisers with integrated scrubbers.

This produces MAP in highly concentrated form as white crystalline pellets.

The demand for water-soluble MAP is growing globally. It can be tank-mixed with other fertilizers for fertigation or foliar application to crops. It is notable for the following key attributes:

• Suitable for fertigation and foliar applications, the production of fertilizer blends and nutrient solutions
• Fully water-soluble
• Free of chlorine, sodium and other elements like heavy metals that are harmful to plants
• A safer and less corrosive moderate pH – especially desirable for neutral- and high-pH soils
• Insoluble content below 100 ppm (NTU <10).

High-purity phosphoric acid is generally used as the starting product for MAP manufacture. GEA, however, has developed a ground-breaking process for producing high-quality water-soluble MAP fertilizer from non-purified, merchant-grade phosphoric acid (MGA) (Figure 5). This eliminates the need to purchase or manufacture purified phosphoric acid (PPA).

Fig. 5: Block-flow diagram for the production of MAP with the GEA process

In a major cutting edge ‘lighthouse’ project for the fertilizer industry, GEA has successfully implemented this process for a customer in Eastern Europe. The installed production unit has avoided considerable capital and operating expenditure, thanks to its ability to consume MGA rather than PPA, yet still produces high-purity MAP fertilizer, a premium product with a high market value.

GEA’s process allows the customer to produce pure, soluble MAP with a yield of up to 70 percent. By providing the industry with the ability to manufacture high-quality water-soluble products, GEA is strongly supporting the responsible use of fertilizers.

Innovations and current trends

GEA’s newly-developed MAP production concept, which consumes lower grade MGA, incorporates a special ceramic membrane filtration (CMF) system for removal of impurities (mainly struvite). The filter’s ceramic elements are abrasion resistant and provide the highest level of temperature stability while removing crystalline impurities from the MAP solution. These result from process steps upstream of the CMF filtration system.

Another promising economic innovation is the growing trend for phosphoric acid purification using high-pressure nano-filtration (NF) and ion exchange (IEx). This represents a superior alternative to conventional liquid-liquid extraction. The NF membrane system offers world-class performance and achieves good yields at various concentrations. This new method of phosphoric acid purification, by employing acid-stable nanofiltration membranes, avoid the rapid performance degradation associated with conventional polyamide-based membranes. GEA has developed a high-pressure NF unit that can operate at 120 bar g.

In addition to membrane filtration, a polishing stage with cationic resins is capable of maximising both crystallisation yield from solutions and product purity. This technology can purify any evaporator feed solution. It can be applied to the purification of phosphoric acid, for example, as well as the production of potassium nitrate.

Fertilizer producers, in the current context of over-capacity, generally seek to optimise their energy costs and consumption, both for competitive advantage and to improve their operational sustainability. In a world of relatively cheap fossil fuels – with even negative oil prices being observed in April 2020 – the imperatives for greater energy efficiency should, in principle, still hold true. With this in mind, there are several ways to improve the energy consumption in evaporation and crystallisation:

• Better use of hot condensates
• Use of vapour recompression
• Installation of multiple-effect evaporators
• Heat integration between the evaporator and dryer
• Combine scrubbing or steam reforming with vapour recompression, when dealing with corrosive process vapours.

GEA is able to provide energy use audits and advise on all these options.

Water and product recovery from fertilizer production effluents is also of growing importance. More stringent regulations on wastewater discharge are driving demand for effective waste reduction and treatment technology. In this context, evaporation and crystallisation can bring great benefits to customers by providing a creative solution to the management of the whole water cycle in industrial processes. The technology enables:

• Compliance with environmental regulations
• Recovery of distilled water
• Recovery of valuable chemicals
• Financial profit from waste treatment
• Highly positive improvements to corporate and social responsibility.

The management of phosphogypsum ponds and raffinates remains a great challenge for the phosphates industry. Evaporation and crystallisation technology, together with membrane filtration and ion exchange, offer one potential solution due to their ability to recover both valuable products and water.

Conclusion

The spectrum of available evaporation and crystallisation technologies can be customised to manufacture a wide range individual fertilizers, allowing these to be produced in an optimal and sustainable manner.

GEA – with a team of 150 evaporation and crystallisation specialists and brands like Messo, Wiegand and Kestner – is a long-standing provider of innovative production technology to the fertilizer industry. Founded nearly 140 years ago, the company possesses invaluable experience of full-scale industrial plants, process design and laboratory- and pilot-scale testing

GEA notably combines extensive expertise in evaporation and crystallisation with strong research and development capabilities. GEA plants producing industrial-grade, high-quality straight fertilizers are found around the globe – in locations ranging from the Atacama desert in Chile to Europe’s potash mines, and from the shores of Dead Sea in the Middle East to phosphoric acid plants in Asia and Latin America.

About the authors

Laurent Palierne heads the French Chemical Business Unit at GEA Group, while Norbert Strieder is the company’s head of Chemical Application Marketing.