A Guide to Evaporative Crystallizers

Evaporative crystallizers are by far the most common form of crystallizer. In the industrial setting, they are used to separate liquid from solid. These are crucial pieces of machinery for chemical processing since they can produce high-purity products with only a small amount of energy input.

In this complete guide about Evaporative Crystallizers, we will talk about:

• What is Crystallization
• Phases of Crystallization
• Types of Crystallization
• Difference Between Evaporation and Cooling Crystallization
• Advantages of Evaporative Crystallizers Over Cooling Crystallizers
• Where is Crystallization Used in the Industry, and
• Who to Contact If You Need Evaporative Crystallizers

Crystallization

Crystallization is a process in which dissolved solids in a liquid solution are separated out and form a solid crystal structure. This process can be used for precipitation and purification of crystalline solids and as a cost-effective method for reducing the volume of waste streams in a variety of industries such as chemical, pharmaceutical, food, utility and pollution control.

There are several factors that influence the process of crystallization. These include but are not limited to:

• the concentration of the solution
• temperature and pressure
• composition of the solvent combination
• composition of the dissolved solids in solution
• ionic potency

When the right physical circumstances are present, the liquid's molecules organize themselves into these highly structured and tightly bound structures named crystals.

There are three primary types of crystallization processes:

1. Evaporative crystallization
2. Cooling crystallization
3. Reactive crystallization

In evaporative crystallization, solvents are removed by heating the solutions until the solvent is evaporated to reach saturation level. This is the process we use at Dedert in our evaporative crystallizers to produce solids from a wide range of processes and waste solutions.

On the other hand, cooling crystallization involves chilling the solution until the solubility of the compound is lowered enough to reach saturation level, allowing the dissolved solids to separate from the solvent through crystallization.

In reactive crystallization, a reactant is added to the solution to generate a chemical reaction which creates supersaturation and therefore crystals. Usually, the chemical reactions are fast compared to the crystals growth rate.

Hybrid systems also exist, where evaporation is used to cool the solution, resulting in a concentration of the solution.

It's worth noting that each crystallization system should be individually developed for maximum efficiency as different equipment types are better suited for different purposes, depending on the mixture's components, the amount of energy used, and the desired crystal size distribution.

At Dedert, we specialize in providing innovative solutions for handling products with high boiling point elevations. Our crystallizers are trusted by repeat customers and we offer a wide range of options, from basic skid setups to full turn-key solutions.

Our pre-assembled skid-mounted projects include heat exchangers, pumps, equipped motors, instrumentation, control valves, piping, electrical junction box or MCC/PLC, wiring, insulation, and structural steel.

We also offer 3D modeling for equipment, piping, steel, as well as optional code stamps for pressure vessels. We handle a variety of crystallizer products, including amino acids, ammonium sulfate, citric acid, cyclodextrin, dextrose, gluconic acid, glucosamine, itaconic acid, monosodium glutamate, potassium carbonate, potassium citrate, sodium chloride, sodium citrate, sodium sulfate, threonine, and Vitamin C.

Phases of Crystallization

This process involves two distinct phases:

1.     Primary Nucleation

This is the initial phase of crystallization where new crystals form. In this phase, small clusters of molecules start to come together and form the initial structure of the crystals. This phase typically requires a high degree of supersaturation and is relatively rare.

2.     Secondary Nucleation

This phase is the primary process that results in the mass formation of crystals and maintains growth. In this phase, the newly formed crystals act as nucleation sites for the growth of additional crystals. This phase is much more common than primary nucleation, and it results in the formation of large, visible crystals. Secondary nucleation typically requires a lower degree of supersaturation, and it can continue for an extended period.

It's important to note that the efficiency of the crystallization process depends on both the rate and the number of nuclei that form during primary nucleation, as well as the rate of crystal growth during secondary nucleation.

Types of Crystallizers

1.     Evaporative Crystallizers

By evaporating the solvent, evaporative crystallization raises the concentration of the solution. The solution becomes supersaturated as the concentration rises, which triggers the onset of nucleation. The nuclei start to expand, and unique crystals begin to form.

This is the most popular method of crystallization, especially when working with simple substances such as inorganic salts and sucrose. These crystallization units typically use steam as the heat source and incorporate forced circulation. The transition occurs under nearly isothermal conditions because evaporation is the primary process involved.

Commercial evaporative crystallizers come in 3 primary varieties:

1. Forced Circulation (FC) Crystallizers
2. Draft Tube Baffled (DTB) Crystallizers
3. OSLO Crystallizers

A.    Forced Circulation (FC) Crystallizers

Forced Circulation (FC) crystallizers circulate a suspension while adding heat to drive evaporation. The circulating stream is flashed in the separator, and the stirring is caused by the impulse of the circulated suspension.

There is a high secondary nucleation rate in this type of crystallizer which represents the best solution when large crystals are not required.

B.     Draft Tube Crystallizers

Draft Tube Baffled (DTB) Crystallizers are continuous vacuum evaporating crystallizers that apply heat to a crystal lean stream of liquid that is drawn from the crystallizer's settling zone in order to cause evaporation. A stirrer in the shape of a low-speed propeller and a draft tube work together to keep the slurry suspended.

So, the mixture goes upward through a draft tube toward the boiling surface to evaporate.

A small degree of supercooling minimizes the buildup of crystals on the walls while the remaining liquid flows back downward through the draft tube. This crystallizer is useful for small-capacity applications with moderate control of crystal size.

Crystal size and uniformity of size depend on:

1. External separation
2. Fines destruction, and
3. Crystal population control.

C. OSLO Crystallizers

OSLO crystallizers, also known as Fluidized Bed Crystallizers, apply heat to the nearly crystal-free top stream leaving the fluid bed in order to cause evaporation. The supersaturated liquid that results from flashing the overheated stream in the evaporator is then circulated over the fluidized bed, where the supersaturation is deposited on the bed's crystals.

2.     Cooling Crystallizers

Cooling Crystallizers are a type of crystallization process that involves decreasing the solution temperature until the solubility of the compound is lowered, allowing it to separate from the solvent through crystallization. There are several systems that can be used to carry out the cooling crystallization process.

For our purposes today, we’re not going to dive deep into Cooling Crystallizers here. Just know that one of the following three systems is used to carry out the cooling crystallization process:

A.    Vacuum Cooling Crystallizers

This model uses either a batch or a continuous crystallization method, which is useful for keeping tighter control over crystal size. As each crystal goes through the procedure for the same length of time, resulting in consistent dimensions, the batch operation is ideal for controlled crystal sizing.

B.     Continuous Cooling Crystallizers

This model centrifugally separates a slurry and sends the concentrated warm solution from the evaporator to the cooling crystallizer after it has been centrifugally separated into its component parts.

C.    Scraped Surface Crystallizers

This device uses rotating blades against the wall surfaces after its chilled vessel walls cause the solution to crystallize on the wall surface to remove the generated crystals. After being collected, the crystals are put back into the main body of the solution, where they can develop and eventually be extracted as a slurry for centrifugal separation.

The liquid in the heat exchanger is not overheated but rather undercooled while cooling crystallization applications. The undercooled solution must stay in the metastable zone in order for the fluidized bed to be maintained and for spontaneous nucleation to be avoided.

In crystallizers with non-scraped heat exchangers, the typical temperature difference between the coolant and the crystallizing solution is less than 2°C. The Double Tube Configuration is the most typical kind of crystallizer with a scraped heat exchanger. While the coolant circulates in the outer tube, crystallization occurs in the inner tube. Rotating scrapers keep the inner tube's surface clear of solids.

Comparative Analysis of the Most Common Types of Evaporative Crystallizers

Now, let’s give you a comparative analysis of the Evaporative Crystallizers that we have described above in terms of their complexity and reliability.

Type of Crystallizer	Intricacy	Reliability
Forced Circulation Type	Most straightforward	Most reliable
Draft Tube Type	Average	Average
OSLO Type	Most intricate	Least reliable

As you can see, of the three, the Forced Circulation crystallizer is the most straightforward and reliable, while the OSLO crystallizer is the most intricate and least reliable.

So, would there be a reason to use OSLO Crystallizers in place of Forced Circulation (FC) Crystallizers?

Yes. The OSLO crystallizer often produces crystals of a higher grade with a greater average size and a narrower size distribution.

Evaporative vs Cooling Crystallization

The table below shows the comparison between evaporative and cooling crystallization:

	Evaporative Crystallization	Cooling Crystallization
Method of Solvent Removal	Heating	Chilling
Suitability for High Boiling Point Elevations	Yes	No
Equipment Types	Evaporative Crystallizers	Cooling Crystallizers
Energy Used	High	Low
Desired Crystal Size	Can vary	Can vary
Types of Crystallizers	Batch and Continuous	Vacuum Cooling, Continuous Cooling, Scraped Surface
Phase of Crystallization	Primary and Secondary Nucleation	Primary and Secondary Nucleation
Efficiency of the Process	Depends on nucleation rate and crystal growth rate	Depends on nucleation rate and crystal growth rate
Examples of Crystallizer products	Amino acids, ammonium sulfate, citric acid, cyclodextrin, dextrose, gluconic acid, glucosamine, itaconic acid, monosodium glutamate, potassium carbonate, potassium citrate, sodium chloride, sodium citrate, sodium sulfate, threonine, and Vitamin C	Same as evaporative crystallization

Note: The table above is a general comparison of Evaporative and Cooling Crystallization, specific crystallization methods and equipment may have different features and parameters. It's worth noting that each crystallization system should be individually developed for maximum efficiency as different equipment types are better suited for different purposes, depending on the mixture's components, the amount of energy used, and the desired crystal size.

Advantages of Evaporative Crystallizers Over Cooling Crystallizers

Evaporative crystallizers have a number of advantages over cooling crystallizers. Some of these advantages include:

1. Lower operating costs: Evaporative crystallizers require less energy to operate than cooling crystallizers, which can result in significant cost savings over time.
2. Higher crystal quality: Evaporative crystallizers tend to produce crystals with a higher degree of purity and fewer defects than cooling crystallizers.
3. Greater flexibility: Evaporative crystallizers can be used to produce a wide range of crystal sizes and shapes, whereas cooling crystallizers are typically limited to producing a single crystal size and shape.
4. Better scalability: Evaporative crystallizers can be scaled up or down more easily than cooling crystallizers, making them more versatile for different production needs.
5. Reduced waste: Evaporative crystallizers produce less waste than cooling crystallizers, which can help to reduce environmental impact.
6. Better product recovery: Evaporative crystallizers can recover a higher percentage of product than cooling crystallizers, resulting in greater efficiency and cost savings.

Where is Crystallization Used in the Industry?

Crystallization is a widely used process in various industries for the purification, separation and recovery of solid compounds from solution. In the chemical and pharmaceutical industries, crystallization is used to purify active ingredients and produce high-purity products.

In the food industry, it is used to produce sugar, salt, and other food ingredients. In the mining industry, it is used to separate and purify valuable minerals from ore. In the petrochemical industry, it is used to separate and purify products such as gasoline, diesel, and jet fuel.

Crystallization is also used in the production of many consumer goods, including personal care products, household cleaning products, and electronic materials.

Evaporative Crystallizers from One of the Leading Industry Experts: Dedert

Since this article particularly focuses on the topic of evaporative crystallizers, let’s briefly talk about who you can contact in the industry for your evaporative crystallizer needs.

In its 50+ years of operation, Dedert Corporation has provided custom-designed evaporators and dryers to customers all over the world. Our skilled engineering team, who has an average tenure of over 20 years, works tirelessly to position Dedert as our customers' top choice for evaporation and drying technology.

In fact, we’re known for:

• Offering creative solutions for handling products with elevated boiling points.
• Offering crystallizers for clients who opt for our service repeatedly over others in the industry.

Evaporative crystallizers from Dedert create solids from a variety of industrial and waste solutions. Applications include cost-effective volume reduction of waste streams in chemical, pharmaceutical, food, utility, and pollution control operations, as well as the precipitation and/or purification of crystalline solids.

Our Forced Circulation crystallizers feature a feed and a circulation slurry of precipitated solids suspended in liquid that constantly mix. Condensation takes place outside the heat-exchange tubes of a chest as this slurry moves through them, generating the heat needed for evaporation. The same energy sources that are used for evaporators can also be used for our crystallizers.

If you're in need of a dependable and efficient evaporative crystallizer, Dedert Corporation should be your first choice. Contact us to learn more about how we can meet your specific needs and requirements.

Products that Our Crystallizers Handle

Our crystallizers are one of the leading solutions in the industry to handle products like:

• Amino acids
• Ammonium sulfate
• Citric acid
• Cyclodextrin
• Dextrose
• Gluconic acid
• Glucosamine
• Itaconic acid
• Monosodium glutamate
• Potassium carbonate
• Potassium citrate
• Sodium chloride
• Sodium citrate
• Sodium sulfate
• Threonine
• Vitamin C

Solutions That We Offer

Dedert can offer everything from a simple skid to a complete turnkey solution.

Features of pre-assembled skid-mounted projects include:

• Heat exchangers
• Structural steel
• Pumps and motors
• Instrumentation and control valves
• Piping
• Electrical junction box or MCC / PLC
• Assembly, insulation, and wiring
• 3D Modeling for equipment, piping, and steel
• Optional code stamp for pressure vessels

Conclusion

In conclusion, evaporative crystallizers are a cost-effective solution for volume reduction and purification of industrial and waste streams. They work by mixing feed and a circulation slurry of precipitated solids suspended in liquid, and condensation taking place outside the heat-exchange tubes, generating the heat needed for evaporation. These types of crystallizers are commonly used in chemical, pharmaceutical, food, utility, and pollution control operations. Companies like Dedert Corporation, with 50+ years of experience and a skilled engineering team, provide custom-designed evaporative crystallizers and are known for their creative solutions and repeat clients. If you're in need of evaporative crystallizer technology, Dedert is a top choice in the industry.

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