This overview presents the traditional and emerging types of continuous crystallizers
Crystallization is an important technique for the purification of solids in the chemical process industries (CPI). The operation can be performed batchwise or in continuous operation, and is used for a wide variety of products, ranging from sugar, salts and other bulk chemicals, to pharmaceuticals and other fine chemicals.
This article provides an overview of the most important types of continuous crystallizers, with a brief summary of the pros and cons of each type.
Crystallization from solution is done by creating a supersaturated solution of the product in the solvent. Creating the supersaturation condition can be achieved by any (or a combination) of the following methods:
- Cooling (adiabatic)
- Cooling (surface)
- Cooling (by gas injection)
- Anti-solvent addition
- pH adjustment
Types of crystallizers
For continuous operation, the main crystallizers in operation in the CPI include the following (for more details, see Ref. 1–4):
- Forced circulation (FC)
- Draft tube baffle (DTB)
- Indirect forced circulation (IFC)
Forced circulation (FC). An FC crystallizer (Figure 1) consists of a single heat exchanger, a circulating pump (usually an axial flow pump, which permits large flow and small transport disengagement height (TDH)), and one vessel (the crystallizer-vapor separator). The FC can also be equipped with an elutriating leg, which is used to improve the coefficient of variation (CV) by removing the coarsest solids in the system. The heat exchanger can be vertical or horizontal. The circulation rate in the heat exchanger tubes is usually between 6 and 8 ft/s, and temperature rise (TR) across the heat exchanger is 4 to 6°F (or less than 1°F for surface cooling).
The FC shown in Figure 1 employs a central inlet, but many designers are still using tangential or radial inlets.
FC crystallizers can be used as an evaporator-concentrator when the product to concentrate is incrusting the heat exchange surface (for example, for phosphoric acid and gypsum) and as a crystallizer (for example, for sodium chloride, sodium sulfate and sodium carbonate).
FC is most commonly used as an evaporator and crystallizer, and can also be used as a surface-cooled crystallizer, adiabatic-cooling crystallizer or reactive crystallizer. In addition, it can be used for viscous products (such as citric acid anhydrous or mono hydrate).
The washing cycles are usually performed monthly, if the tubes are correctly submerged.
Crystals produced in an FC are usually small (150–400 microns) with a large CV (50–60%), but such crystals are sufficiently large to be separated by a pusher-type centrifuge (salt with 3% moisture is usual).
The crystal size distribution (d50) in an FC crystallizer can be improved by achieving the right balance between primary nucleation and secondary nucleation . Increasing the retention time will also increase the the secondary nucleation .
Draft tube baffle (DTB). A DTB crystallizer (Figure 2) has an internal circulation that is performed by an agitator instead of a pump.The low speed agitator permits a reduction of the secondary nucleation and increases the size of crystals, a feature that gives the DTB a reputation for producing large crystals.
The DTB is equipped with baffles, which permit the system to operate as an evaporative crystallizer. The baffles are often refered to as fines killers because they destroy fine crystals, enabling the production of larger crystals.
DTB is largely used as an adiabatic cooling crystallizer, for producing salts, such as potassium chloride, boric acid and other borates, or as a reactive crystallizer, for producing ammonium sulfate.
The TR in a DTB is very small (0.5–2°F) and again it is necessary to find the good compromise between primary and secondary nucleation to achieve the best size distribution.
Usually DTBs are not installed with a leg.
Oslo or crystal growth. The Oslo crystallizer (Figure 3) is the oldest type of industrial crystallizer, and was first developed specifically to produce large sodium chloride crystals. As with the FC crystallizer, the Oslo has an external recirculation loop, one heat exchanger and one fluidized-bed vessel. However, the Oslo crystallizer recirculates a clear liquid instead of a slurry, as in the FC. There is no secondary nucleation, and normally there is no limit for the size of crystals.
Usually Oslo crystallizers are installed without a leg and without fine killers.
The Oslo crystallizer has one technical disadvantage: short washing cycle (due to encrustation of the downcomer). The price can also be high compared to a DTB. Despite these shortcomings, new Oslo crystallizers are still being used today.
Indirect forced circulation (IFC). The IFC is a relatively new type of continuous crystallizer that uses a different hydraulic concept (Figure 4). Only medium-sized crystals come into contact with the agitation device; the larger crystals are indirectly circulated without making contact. To increase the crystal size, the system can be equipped with a leg and, if necessary, a fines killer. The d50 in an IFC is 2–3 times higher than achieved in an FC, and the CV is around 20%, compared to 40–50% for the FC and DTB.
Design of a new crystallizer
For a new product, or a new application, the question to be asked is, what is the best design? To answer this question, the designer needs to know the following:
- Physical data
- Solubility curves
- Specific gravity and heat capacity of liquid and solid
- Heat of crystallization
- Viscosity and boiling-point elevation of the liquid
- Market requirements for the product
- Crystal size distribution (CSD)
- Temperature limitations
- Quality of condensates
- Utilities available
- Cooling water
- Chilled water
- What are the habits of the market for the application (type of crystallizer and arrangment generally used for this application)?
- Multiple effect? mechanical vapor recompression? thermo compression?
- Evaporative? or adiabatic cooling? or reactive? or surface cooling
- Other parameters
- Operability and operators training
For new applications, it is often necessary to perform laboratory tests to confirm the physical data (first bullet above), followed by pilot tests to verify the crystallizability and the characteristics of the crystallized product.
One of the benefits of the IFC crystallizer is that laboratory and pilot tests are fully representative of the product produced in an industrial plant. This is not necessarily the case for a DTB crystallizer, because the crystal size is affected by the tip speed of the agitator that generates secondary nucleation.
Choice of a flowsheet
More and more, several types of crystallizers are associated with additional equipment, such as the following:
- Preconcentration (falling film, plate and other types of concentrators)
- Vapor condensation (surface or mixing)
- Vacuum equipment (ejectors, vacuum pumps or combined systems)
- Vapor compression
- Chilled water
- Meshes, catchalls and treatment of vapors
- Solid-liquid separation (hydrocyclones, settlers, legs, pusher centrifuges, solid bowl centrifuges, filters and so on)
Over time, the flowsheets used for industrial applications are evolving to take into consideration the needs of the market and the new material of construction available, so that for each application, there has emerged something like a standard. Some examples are given below.
Salt in caustic soda. A first example is the concentration of caustic soda from a diaphragm electrolyzer, with salt crystallization in a triple-effect FC crystallizers (in pure nickel). It took more than ten years to arrive at the final flowsheet. Initial static decanters are now replaced by elutriated legs.
Potassium chloride. For a long time, KCl crystallization has been done in adiabatic cooling DTB crystallizers (up to seven stages).
Ammonium sulfate. (NH4)2SO4 is a byproduct from many sources, primarily from the production of caprolactam and methyl methacrylate, but also from waste ammonia or sulfuric acid solutions. Over the last 10–15 years, DTBs are replacing Oslo crystallizers for this application, but some end users continue to make large investments with Oslo crystallizers today.
Sodium chlorate. NaClO3 is produced by electrolysis of NaCl. Typically, electrolyte leaving the cells at 85°C is sent to a DTB evaporative crystallizer. For many decades, the crystallization was done at 30°C, which required steam for evaporation and chilled water or steam for boosters.
In the mid 1980s, the author and J.C. Studler developed a new flowsheet in which no steam and no chilled water are required, by operating between 30–45°C, and recovering the heat of the cells. This flow sheet is now a standard.
Sodium sulfate. Na2SO4 is produced from mines in Spain, Turkey and other countries by water injection. It is also a byproduct of many different processes in the CPI.
The main use for Na2SO4 is for laundry detergent, and therefore it is necessary that the product be very white. Normally, a first crystallization as glauber salt is required
Today, most crystallizers for this application are FC crystallizers, which produce crystals with a d50 of 180–200µm and a large CV. One plant in Spain has installed a DTB crystallizer, giving crystals that seem larger than those produced by the FC crystallizer, but in fact, agglomerates are being produced.
One plant in France has installed an IFC crystallizer giving 300 µm monocrystals with a CV of 20–25% and less than 4% fines at the outlet of the dryer. For this application IFC is the new standard.
1. Genck, W., A clear view of crystallizers, Chem. Eng. July 2011, pp. 28–32.
2. Mullin, J.W., “Crystallization,” 4th ed., Butterworth-Heinemann, Burlington, Mass., 2001.
3. Gallot, J.C. and Schur, A., Crystallizers and Agitators, Chem. Eng., December 2006, pp. 38–41.
4. Randolph, D. and Larson, M.A., “Theory of Particulates Processes,” Academic Press, N.Y., 1968.
Michel Malfand is president of Crystal Evap Consult (42 Bd Risso, 06300 Nice, France; Telephone:+33-4-9331-0075; Email: email@example.com), and a consultant on evaporation and crystallization. Malfand is the inventor of the indirect forced circulation (IFC) crystallizer, which is patented around the world. He has been with the company since 2004. Prior to this, he has worked at Applexion (for 3 years), Agrochem (12 yrs), and Swenson (18 yrs). He studied chemical engineering at HEI (École des Hautes Études d’Ingénieu) in Lille, France.
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