Comparison of functions and structures of various crystallization equipment

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I’ve gathered some information on my own and would like to share it with everyone; I hope it will be useful. Comparison of the functional structures of various crystallization equipment: There are many types of crystallizers, which can be classified into evaporation crystallizers and cooling crystallizers based on the method used to achieve supersaturation in the solution ； Based on the flow pattern, they can be divided into mother liquor circulation crystallizers and slurry circulation crystallizers ； Based on the operation method, they can be divided into continuous crystallizers and batch crystallizers. 1. Cooling crystallizers (1) Air-cooled crystallizers: Air-cooled crystallizers are the simplest type of open crystallizer. They rely on a large open liquid surface at the top, as well as heat exchange between the wall of the container and the air, to reduce their own temperature and thereby facilitate the cooling and precipitation of crystals. No seed crystals are used, nor is any stirring employed; there is no method to control the cooling rate or the formation of crystal nuclei and the growth of crystals. This type of crystallizer has the simplest structure and lowest cost, and it enables the production of high-quality crystals with large particle sizes; it is particularly suitable for the crystallization of substances containing multiple crystal waters. The disadvantages are the slow heat transfer rate, batch operation, low production capacity, and large floor space required. It is still used when the quantity of the product is not very large and the requirements regarding purity and particle size are not strict. (2) Stirred crystallization tank: A heat transfer jacket is installed outside the air-cooled crystallizer, or a coiled tube heat exchanger is placed inside to facilitate heat transfer; by adding a power circulation device, it becomes a forced-circulation cooling crystallization tank or a stirred crystallization tank. The slurry is forced to circulate between the external cooler and the crystallization tank, which enables better mixing of the slurry within the tank and increases the heat exchange rate at the cooling surface. Such crystallization tanks can operate in batch or continuous mode. For natural cooling, an internal cooler can be installed if necessary. The mixer can be driven from below or from above. The slurry can flow upward or downward in the guide cylinder. In such crystallizers, the temperature is relatively uniform; fewer crystals are produced, but their size is more consistent. This also reduces the cooling time and increases production capacity. For the crystallization of substances that are prone to oxidation in air, a closed tank can be used, with an inert gas introduced into the tank. (3) Long-tank stirring-type continuous crystallizer: The long-tank stirring-type continuous crystallizer is a widely used type of continuous crystallizer with high production capacity. Its structure is an open or closed long groove, with a semi-circular bottom; a water jacket is welded around the outside of the groove, and a low-speed screw agitator with a long pitch is installed inside the groove. During operation, the concentrated hot solution is added from one end of the tank, while cooling water (or chilled brine) typically flows counter-currently to the solution in the jacket. The spiral stirrer can mix and transport crystals, prevent them from accumulating on the cooling surface, and lift the already formed crystals to disperse them in the solution, allowing the crystals to remain suspended in the solution and grow thereby producing uniform crystals. Long-groove stirring continuous crystallizer: 1. Groove; 2. Water tank; 3. Stirrer; 4, 5. Connectors; 6, 7. Cooling water inlet and outlet. 2. Evaporation crystallizer: An evaporation crystallizer is a type of crystallization equipment that concentrates solutions by evaporating the solvent, thereby causing crystals to form. Taking the Oslo evaporation crystallizer as an example, its structure mainly consists of a crystallization chamber, an evaporation chamber, and a heating chamber. During operation, the raw material liquid is added through the feed inlet and sent to the heater via a circulation pump for heating. The heated liquid then enters the evaporation chamber, where part of the solvent evaporates; the resulting secondary steam is discharged from the top of the evaporation chamber. The concentrated liquid flows downward through the central tube to the bottom of the crystallization chamber, from where it moves upward and crystals are formed. The crystallization chamber is conical in shape, with its cross-sectional area increasing gradually from bottom to top; as a result, as the solid-liquid mixture flows upward within the chamber, its flow velocity decreases gradually. Crystals with a larger particle size will accumulate at the bottom of the crystallization chamber, where they can come into contact with the supersaturated solution; as a result, their particle size will continue to increase. The crystals with a smaller particle size are located in the upper layer of the crystallization chamber and can only come into contact with solutions having a lower degree of supersaturation; as a result, their particle size can only increase slowly. Therefore, the crystals in the crystallization chamber are automatically graded, which is a notable advantage of the Oslo crystallizer. The Oslo crystallizer boasts excellent operational performance, but its disadvantages are its complex structure and high investment cost. 3. Vacuum crystallization: In vacuum crystallization, an unsaturated solution at atmospheric pressure is subjected to reduced pressure under adiabatic conditions, resulting in the vaporization of part of the solvent. This causes the solution to become concentrated and cooled, eventually reaching a supersaturated state and thus crystals to precipitate. During operation, the hot concentrated solution is fed into a sealed and insulated container, where a high vacuum level is maintained. This results in the boiling point of the solution inside the container being lower than the temperature of the feed material; as a result, the hot solution rapidly evaporates and cools adiabatically to the equilibrium temperature corresponding to the pressure inside the container. The vacuum crystallizer has both a cooling effect and a slight concentration effect. The sensible heat released by the cooling of the solution, along with the crystallization heat of the solute, provides the latent heat of vaporization required for the evaporation of the solvent. The solution can be cooled without needing to come into contact with a cooling surface, and the solvent can be evaporated without the need for the solution to be in contact with a heating surface; therefore, there is no need to include any heat exchange surfaces inside the vessel. (1) Batch vacuum crystallizer: The body of a batch vacuum crystallizer is a container with a conical bottom. The liquid material is placed in a container, where its flash evaporation causes intense boiling, allowing the solvent vapor to be discharged from the top of the container and enter a ejector or other vacuum device. Stirring is intensified to make the solution temperature fairly uniform and to keep the crystals suspended until they have grown sufficiently and then sink to the bottom of the cone. After each batch of operations is completed, the mixture of crystals and mother liquor is discharged to the slurry tank through a discharge valve, and then filtration is carried out to separate the crystals from the mother liquor. The main advantage of this crystallizer is its simple structure; the solution undergoes adiabatic evaporation cooling, eliminating the need for heat transfer surfaces and thus preventing crystal agglomeration on those surfaces. As a result, it has a low cost and high production capacity. Intermittent vacuum crystallizer (2): Multi-stage vacuum crystallizer. The body of a multi-stage vacuum crystallizer is a horizontal cylindrical vessel, which is divided into several interconnected chambers by vertical partitions; this allows the slurry to flow between these chambers. However, the vapor spaces at the top of each chamber are isolated from one another, with each vapor space being connected to a vacuum system. Air distribution tubes are installed at all levels at the bottom of the vessel and are connected to the atmosphere; thus, during operation, a small amount of air can be drawn in from outside the vessel. This air passes through the liquid layer via the distribution tubes in the form of bubbles, thereby creating stirring action. When the temperature of the solution drops below its saturation point, crystals begin to precipitate. Thanks to the stirring effect of these air bubbles, the crystal particles remain suspended and can grow, while also flowing along with the solution at each level. Multi-stage vacuum crystallizer 4, general-purpose crystallizer (1) FC-type crystallizer. The FC-type crystallizer is also known as a forced external circulation crystallizer; it belongs to the category of slurry circulation crystallizers. Its structure mainly consists of a crystallization chamber, circulation pipes, a circulation pump, and a heat exchanger. The crystallization chamber has a conical bottom; after the slurry is discharged from this conical bottom, it is pumped to the heat exchanger via circulation pipes using an axial-flow circulation pump. After being heated or cooled, it returns to the crystallization chamber in a tangential direction, and this cycle repeats itself, thereby enabling the discharge of the slurry. The slurry outlet is located near the conical bottom of the crystallization chamber, while the feed inlet is on the inlet pipeline of the circulation pump. Forced external circulation crystallizer: 1. Atmospheric condenser; 2. Circulation pipe; 3. Heat exchanger; 4. Circulation pump. (2) Oslo-type crystallizer: The Oslo-type crystallizer is also known as a Krvstal crystallizer or a particle size classification crystallizer, and it is mainly divided into vacuum cooling crystallizers, cooling crystallizers, evaporation crystallizers, etc. The crystallizer is mainly composed of a vaporization chamber and a crystallization chamber. The body of the crystallization chamber has a certain degree of taper, with a larger cross-sectional area at the upper part than at the lower part. After being mixed with the hot concentrated solution, the mother liquor is pumped by a circulation pump to a vaporization chamber located at a higher level. In this chamber, the solution vaporizes and cools, resulting in supersaturation; thereafter, it flows to the bottom of the crystallization chamber through a central downcomer before flowing upward again. In the crystallization chamber, the upward flow rate of the liquid gradually decreases; the particle size of the suspended crystals gets smaller as one moves upward. When the solution reaches the top of the crystallization chamber, it flows out at the top as a clear mother liquor and enters the circulation circuit. The advantage of the Oslo crystallizer is that the circulating liquid contains virtually no crystals, which prevents nucleation due to contact between the impeller and crystals. Coupled with the particle size sorting effect in the crystallization chamber, this type of crystallizer produces large and uniform crystals, making it particularly suitable for producing crystals with a high settling velocity in saturated solutions. The drawback is that the production capacity is limited, as it is necessary to restrict the liquid circulation rate and suspension density in order to keep the clear interface of the suspension in the crystallization chamber below the overflow level. 0slo-type vacuum cooling crystallizer (3) DTB-type crystallizer: The DTB-type crystallizer has a guide tube in its middle, with cylindrical baffles surrounding it. These cylindrical baffles divide the crystallizer into a crystal growth zone and a clarification zone. Near the lower end of the guide tube, there is a propeller (an internal circulation axial flow pump) that rotates at a low speed; this propeller drives the suspension to rise toward the surface of the liquid inside the tube, after which it moves downward along the annular channel between the guide tube and the baffles, reaching the bottom of the crystallizer, where it is then drawn back up to the lower end of the guide tube. This cycle repeats continuously, thereby creating favorable mixing conditions. The DTB type crystallizer has high efficiency; it is capable of producing larger crystals, features high production capacity, and is less prone to the formation of crystalline scale inside. Applicable to various crystallization methods; one of the main forms of continuous crystallization. DTB-type crystallizer: 1. Guide cylinder; 2. Ring baffle; 3. Spiral impeller; 4. Bent leg; 5. Heater; 6. Circulation pipe; 7. Crystallizer body; 8. Jet vacuum pump; 9. Atmospheric condenser. (4) DP-type crystallizer: The structure of the DP-type crystallizer can be regarded as an improvement over the DTB type. The DP type not only installs propellers inside the guide cylinder to push the circulating fluid upward, but also has a set of propeller blades on the outside of the guide cylinder in the annular gap, with blades arranged in the opposite direction to those inside the guide cylinder, so as to push the circulating fluid in the annular gap downward. The inner and outer sets of blades together form a large-diameter propeller, resulting in a very small gap between its outer diameter and the inner diameter of the circular baffle; this allows the intermediate guide cylinder to rotate in sync with the large propeller. The DP-type crystallizer is suitable for various crystallization methods; it can reduce the rate of secondary nucleation, increase the average particle size of the product, and shorten the average residence time of the crystals within the crystallizer. This leads to an improved production capacity, low circulation resistance, uniform flow, and easier suspension of solid particles with higher density.