1. Types of equipment
   1. Falling film evaporation
   2. Forced circulation crystalliser
   3. Draft tube baffle (HPD PIC) crystalliser
   4. Oslo type (HPD Growth) crystalliser
2. Equipment configurations
   1. Steam driven
   2. Multiple effect steam driven
   3. Thermo-compression
   4. Mechanical vapour compression
   5. Multiple stage vacuum flash

Types of equipment

This section will discuss some of the evaporation and crystallisation equipment that is often utilised for fertiliser production.

Falling film evaporation

Falling film evaporation, as shown in Figure 1, is typically used to concentrate a solution in non-scaling applications.

It is often used to pre-concentrate a stream prior to a separate crystallisation process and can be used to concentrate highly soluble salts, such as calcium chloride, to make high concentration solutions. In some cases of falling film evaporation, controlled precipitation of select crystals can be managed to prevent scaling on the heat transfer tubes. This controlled precipitation is known as ‘seeded’ operation, since the crystals act as ‘seeds’ to selectively relieve supersaturation of the fouling species.

A falling film evaporator consists of a vertical shell and tube heater / condenser positioned on top of a vapour body / sump. The feed is added to the recirculating brine in the evaporator vapour body / sump. Brine is recirculated, using a recirculation pump, from the vapour body / sump up through an internal return circuit (external piping is used only to connect the recirculation pump). The major portion of the internal recirculation circuit is a preheated pipe centrally located in the tube bundle. This design offers the advantages of minimised recirculation piping and heat loss, simplifies the installation of the equipment, and reduces the overall height of the evaporator.

The recirculation flow is returned to the top of the evaporator above the top tubesheet and flows radially across a brine distributor located at the top of the heater. The distributor plate evenly distributes the brine to the top tubesheet, delivering brine to every tube and evenly distributes the flow to the inside of each tube as a thin film. Heat is transferred to the thin film by condensing steam on the outside (shell side) of the tube. The brine is at the boiling point (or very close to it) so that all the heat transferred through the tube produces steam with the falling brine. The falling brine and process steam exit the bottom of the tube bundle and separate. The brine falls into the sump to be mixed with feed and the steam is drawn from the vapour space of the sump by a compressor or to a condenser.

Vapour generated in the tubes separates from the brine within the evaporator vapour body. The vapour is cleaned of entrained liquid and solids to various degrees with mist elimination devices. The system employed for mist elimination depends on the quality requirements for the recovered water and/or the need to protect a vapour compressor from foam or water droplet carryover.

The condensed vapour is collected at the bottom of the heater / condenser and typically flows into a tank from which it is pumped to recover heat and for re-use as high quality distilled water within the process plant.

The concentrated brine is drawn off to control the concentration at the desired point, either as the end product or as feed for further processing in a crystalliser.

Forced circulation crystalliser

A forced circulation crystalliser, as shown in Figure 2, is a mixed suspension, mixed product removal (MSMPR) crystalliser since the slurry is well mixed and uniform throughout the system. This approach is used for applications where it is easy to grow large crystals or where particle size distribution of the product is not critical. A simple forced circulation crystalliser consists of a vapour body, a heat exchanger and a recirculation pump. Forced circulation crystallisers are often used for the sodium chloride crystallisation in potash recovery from sylvinite deposits.

The crystalliser heat exchanger is a shell and tube type exchanger. The process liquor is pumped with a recirculation pump (either centrifugal or axial flow) through the tube side of the exchanger with steam applied to and condensing in the exchanger shell. The exchanger is designed with the proper tube side velocity and volumetric flow to assure efficient heat transfer and low temperature rise of the recirculating liquor. The heater liquor is discharged from the heat exchanger to the vapour body where the liquid is flashed.

The vapour body is designed with sufficient volume and surface area for proper liquid retention time and vapour release surface area. The recirculation piping inlet and outlet nozzles are located to provide for proper flow and hydraulics and to minimise short-circuiting of the liquor in the vapour body. The crystalliser design typically utilises a radial recirculation inlet nozzle that directs the entering recirculating liquid across the surface area of the vapour body in order to provide a uniform boiling surface. Because of the active surface area, the vapours are released from the vapour liquid interface at a low rate. The entrainment of liquid droplets is minimal and will not overload the mist eliminator arrangement.

As water is evaporated, supersaturation occurs and crystals precipitate. New feed is added to the crystalliser and solids are removed from the system via a slip stream through a centrifuge or other filtration equipment with the mother liquor (concentrated solution after crystallisation) being returned to the process for further concentration.

This type of crystalliser has limited means of controlling crystal size, but can be designed with additional features to provide control. An elutriation leg can be included for crystal removal. The elutriation leg allows the crystals to settle and thicken prior to removal. Feed brine can be added to the bottom of the leg. This can aid in the removal of fine particles and dilutes and cools the product prior to solids removal.

Additionally the vapour body can be provided with a baffle or a separator can be inserted into the recirculation loop to remove fine particles from the mother liquor.

A variation of the forced circulation crystalliser can also be used for surface cooled applications. A cooling medium is used in lieu of steam and the crystalliser becomes a cooling crystalliser instead of an evaporative crystalliser.

Draft tube baffle (HPD PIC) crystalliser

Another type of crystalliser that is common in the fertiliser industry is the draft tube baffle (DTB) crystalliser or HPD partitioned internal circulation (PIC) crystalliser, as shown in Figure 3.

This type of crystalliser is used in applications that require narrower crystal size distribution and larger crystal size. These crystallisers are common for potassium chloride, ammonium sulphate, MAP and DAP production, among others.

The crystalliser consists of an internal draft tube with an agitator and a baffled section that overflows to an external recirculation loop for heating and/or fines destruction. The agitator, a vertically mounted axial flow impeller, provides the high recirculation flow through the draft tube. Several features of this design contribute to its ability to grow large crystals and deliver long washout cycles:

The high internal recirculation flow results in low levels of super-saturation.
Withdrawal of clear liquid from the baffle area allows for control of the operating slurry density.
Fines destruction can be implemented and controlled in the overflow from the baffle by the addition of solvent or heat to dissolve the fines.

The crystalliser vessel is designed for proper retention time to allow for adequate crystal growth. The crystalliser body has sufficient internal recirculation retention time and operates at a slurry density to allow for deposition of developed supersaturation on crystal surfaces while minimising solid build-up on vessel walls.

The boiling action within the crystalliser is very gentle over the entire surface area without any preferential boiling in any particular part of the unit. Because of the high, active surface area, the vapours are released from the vapour liquid interface at a low rate. The entrainment of liquid droplets is minimal.

The crystalliser agitator uses a modern three or five blade impeller operating at low speed which provides efficient flow with minimal crystal breakage. The agitator is supplied with a large diameter shaft for stability.

Oslo type (HPD Growth) crystalliser

The Oslo type or HPD Growth crystalliser design as shown in Figure 4 is a classified suspension classified product removal (CSCPR) crystalliser with well-developed bed fluidisation and circulation of a magma, or crystal slurry.

The fluidised bed provides classification of the crystalliser inventory by crystal size. The classification advantages of an HPD Growth unit have been demonstrated in applications from high purity to fertiliser grade potassium chloride and from by-product to high purity ammonium sulphate.

The HPD Growth crystalliser unit is composed of a retention chamber, a central downcomer, a recirculation pump and a vapour body. A heater can be included in the recirculation loop if heat input is required for the process.

A classified crystal bed is generated in the retention chamber with the largest sized crystals near the bottom and progressively smaller crystals near the surface of the slurry suspension. This classification of crystals allows selective removal of large particles with a narrow size distribution. The HPD Growth unit has an external axial flow recirculation pump, and the crystal slurry re-enters the vessel through a liquor entry nozzle which eliminates vessel erosion and promotes a homogenous surface boiling action. The design allows for circulation of only fine and mid-size fractions, and provides great flexibility for the selection of fines size and rates for the fines destruction circuit.

The vapour body is sized for gentle release of the vapours. The low density magma circulation results in long cycle times and is beneficial for crystal growth and limits crystal attrition.

Equipment configurations

There are multiple ways to configure equipment for evaporation and crystallisation in order to maximise project economics and process efficiency. Factors that must be considered in the design are:

• Utility (steam, power and cooling water) availability and costs.
• Capital equipment sizing and metallurgy.
• Installation costs.
• Process requirements.
• Environmental constraints.

All of the above can have an impact on the final process equipment configuration.

Steam driven

For evaporative crystallisation, heat input is required to boil the water in order to concentrate the solution. This is done by introducing steam into the evaporator or crystalliser heater in order to transfer the heat from the steam to the solution that is being concentrated in the heater tubes. The vapour that is generated as a result of evaporation must be condensed in order to recover the water. Condensation is typically performed in a water or air cooled condenser.

The energy requirements can be very high, especially in a single effect system, as it basically takes one kilogram of steam to evaporate one kilogram of water. The requirement can be even higher if pre-heating (bringing the feed material up to the boiling temperature) is required.

The cooling demand for vapour condensation can be high also. If cooling water is used, the heat input will result in the same amount of evaporation in the cooling tower, so cooling water makeup requirements will be high. If cooling water is not available, air cooling can be used, but the equipment required for this can be very expensive and has some limitations.

Multiple effect steam driven

In order to improve the efficiency of a steam driven system, it is possible to arrange multiple vessels in series so that the vapour generated from one body becomes the steam required to drive the evaporation for the following body or ‘effect’. The first effect operates at the highest pressure and each of the following effects operates at a lower pressure. By arranging the vessels in this manner, the steam economy (kilograms of water evaporated per kilogram of steam supplied) can be significantly increased. The number of effects may be limited by steam supply pressure, cooling medium conditions (i.e. end pressure limitations), boiling point rise, component solubility, heat transfer area or other project constraints. A schematic of a multiple effect system is shown in Figure 5.

Thermo-compression

If high pressure steam is available, it is possible to reduce the steam consumption by using a thermo-compressor. Thermo-compressors operate under the same principles as the steam jet ejectors that are used to create a vacuum, but the suction pressure may or may not be at a vacuum. In a thermo-compressor driven system, a portion of the vapour that is evaporated is recycled to the suction of the thermo-compressor and the high pressure steam is used as the motive steam to produce steam at an intermediate pressure that can be used as the driving force for evaporation. This can result in significant steam savings from 25 – 50% depending on the steam pressure available and the design compression ratio. One disadvantage of a thermo-compression system is that the steam condensate is contaminated with process vapours so the quality of condensate that is returned may not be as high as is desired. Thermo-compression can also be used in conjunction with multiple effect evaporators to further improve the steam economy. Figure 6 shows a schematic of a thermo-compression system.

Mechanical vapour compression

A mechanical vapour compression (MVC) cycle can be used to achieve evaporation. The water vapour leaving the evaporator or crystalliser is compressed using a centrifugal compressor. Mechanical compression of the water vapour results in an increase in its saturation temperature. The compressed vapour flows to the heating side of the evaporator or crystalliser and is subsequently utilised to drive evaporation in the unit. The MVC process is very efficient (typically equivalent to >10 effects) because the latent heat of vaporisation of the water vapour is recycled and recovered in the evaporator. The energy input to the system is electric power required to drive the compressor motor in lieu of steam. A schematic of an MVC system is shown in Figure 7.

Vapour compressors are limited in capacity and achievable compression ratio. It is very important to correctly identify physical properties such as boiling point rise and heat transfer coefficients when designing a vapour compression evaporation system in order to achieve the desired system capacity.

Multiple stage vacuum flash

This crystalliser configuration is often used for products that have a decreasing solubility with decreasing temperature. The crystallisers use adiabatic cooling, i.e. the evaporation caused by the vacuum in the crystalliser cools the liquor. The product precipitates as a result of the cooling. This is a typical configuration used for KCl production. The system is arranged to achieve the crystallisation using multiple stages of evaporative cooling. Various cooling media can be used to condense the vapours.

Optimisation of the heat integration of these systems into the processing plant is the most critical aspect of the system design.

The degree of system cooling is limited by the available cooling medium (typically cooling water) which is used to condense the vapour and create the vacuum. In order to maximise production yield, chillers can be used to provide a lower temperature cooling medium which can be used in condensers or in surface cooled units.