Short-path distillation

Short-path distillation

Basic Principles of Short Path Distillation/Molecular Distillation
Short-path distillation is a distillation process that achieves liquid-liquid separation by evaporating molecules on a heating surface and condensing them on a condensing surface over the shortest possible distance under high vacuum conditions (<10 Pa). When the distance between the heating surface and the condensing surface is less than or equal to the mean free path of the evaporated molecules at the operating vacuum and temperature, this distillation process is also referred to as molecular distillation. It is characterized by low distillation temperatures, high distillation vacuums, short heating times, and high separation efficiency.
Material enters the short-path distillation column through the top flange, and is continuously and evenly distributed across the vertical cylinder heating surface by the distribution disk on the rotor. As the material descends under gravity, it is forced to form a thin turbulent liquid film on the heating surface by the rotating scraper device.

The distillation process is divided into the following steps:
One: As the liquid film forms on the heating surface and reaches the internal condenser, it condenses and is discharged through the outlet at the bottom of the evaporator. The reconstituted components then enter the residue collection tank of the short-path distiller and are discharged through the side outlet.
Two, the evaporated molecules condense on the condensation surface.
Section 3: Collection and Discharge of Distillates and Residues.

Performance Features
High vacuum degree, low distillation temperature; working vacuum: 0.1 Pa to 10 Pa.
High thermal conductivity, high evaporation efficiency: The rotating scraping device forms a uniform film of material on the inner surface of the evaporating cylinder, reducing thermal resistance. Under high vacuum conditions, it facilitates the easy distillation and separation of volatile substances from the material. Due to the intense turbulent state of the liquid film between the inner surface of the cylinder and the scraping device, the transfer of different components within the material and the heat transfer with the heating medium inside the jacket are enhanced. Additionally, the high smoothness of the cylinder inner surface and the continuous movement of the scraper prevent coking and scaling on the heating surface, which also contributes to increasing the overall thermal conductivity and improving the evaporation efficiency.
Short heating time: The material entering the short-path distiller is evenly distributed over the vertical heating surface by the distribution plate. Under the combined action of gravity and the scraper, it spirals down along the limited heating surface, with a very short residence time inside the evaporator, typically a few to several seconds (average residence time is related to the length of the heating surface, the type of scraper, the characteristics of the material, and parameters such as film thickness). Additionally, the "dead angle-free" design of the DZ series short-path distiller prevents material accumulation in areas such as the distribution plate, heating surface, and scraper system, reducing thermal polymerization and thermal decomposition of heat-sensitive substances.

Structural Features
One: Transmission and mechanical seal design for long-term stable operation
The drive unit and mechanical seal of the DZ series short-range distiller are designed and manufactured to ensure long-term stable operation of the equipment. The transmission ratio of the drive unit, the lubrication method of the mechanical seal (forced circulation lubrication, static lubrication), and the cooling method of the mechanical seal are determined based on the operating conditions and material characteristics of the short-range distiller.
The entire evaporator cylinder is machined into shape, without any intermediate flange connections.
The evaporator body of the short-path distiller consists of an inner cylinder and a heating jacket. The bodies of the DZ series short-path distillers are all integrally machined, with no intermediate flange connections. The integrally machined bodies eliminate leakage and material accumulation caused by flange connections, and prevent the formation of low-temperature zones on both sides of the flange, ensuring effective utilization of the evaporation surface. Additionally, the integrally machined bodies contribute to the equipment's precision and excellent film-forming effect.
Three, the inner surface of the cylinder has a high degree of smoothness and is not prone to sticking materials.
The inner surface of the cylinder serves as the heating and evaporation surface for the short-path distiller, and it has strict requirements for surface smoothness. After the rough machining of the cylinder inner surface of the DZ series short-path distiller meets the dimensional requirements, it is finely ground to achieve a surface smoothness of Ra0.8μm. This allows the inner surface to be less prone to material adhesion and scaling, reducing material retention, and enhancing heat and mass transfer efficiency. In accordance with the requirements of the food industry and others, polishing techniques can also be employed to achieve a surface smoothness of Ra0.2μm.
Four: Design of liquid distributors and splash prevention devices, effectively ensuring product quality
To enhance evaporation efficiency, the material in the short-path distiller is continuously and evenly distributed over the vertical cylinder heating surface. The liquid distributor of the DZ series short-path distiller not only distributes the material evenly over the heating surface but also prevents a small amount of material from remaining in the distributor for an extended period, even for high-viscosity materials.
The DZ series short-path distillers are designed and manufactured to ensure long-term stable operation of the scraping device. Their excellent film-forming properties make the evaporation process of the material gentle, without splashing or carryover. In most cases, even without splash prevention devices, they can meet product requirements. When handling certain materials and when product quality is a concern, the splash prevention devices of the DZ series short-path distillers can guarantee product quality.

Film Removal Method
A variety of film removers available, with wide application range.
Oblong slot scraper, drum-type;
Utilizes special PTFE material with a working temperature up to 280℃.
Metal hinge-type fixed scraper.

Application Range
Can replace conventional film evaporators, used for distillation, purification, concentration, and decolorization of petrochemical and chemical products.
Distillation purification suitable for thermally sensitive and easily oxidizable chemical industrial products.
Also suitable for distillation separation of high-boiling point products.

Typical Applications
Distillation and refining of polyunsaturated fatty acids (oleic acid, linolenic acid, fish oil fatty acids (ethyl esters)) and their derivatives.
Refining of monoacylglycerols
Purification of adipic acid;
VE Refining; Plant Sterol Refining
Distillation of liquid crystal
Deaggregate high molecular polymer;
Refinement of other thermosensitive products.

Product Selection
Based on theoretical calculations and years of experience in the application and promotion of short-path distillation, the processing capacity of short-path distillation is directly related to the molecular weight of the raw material (product), material viscosity, operating conditions, and the effective heat exchange area of the evaporator.

Common Feed Rate: 100 kg / (m² h), (80 - 150 kg / (m² h))
Small Size: DZ-5 Effective Evaporation Area 0.05 Square Meters
Large Size: DZ-3600 with an effective evaporation area of 36 square.
Material of Equipment: SUS304, SUS316L, SAF2205, and Hastelloy.
Operating Temperature: 350°C.

Ascending Film Evaporator
The ascending film evaporator, also known as the climbing film evaporator, operates based on the siphon pump principle. It utilizes the upward force of steam bubbles generated during the boiling process, causing the liquid and steam to flow upwards together. Consequently, as the steam volume increases, a flowing film is formed on the tube wall, with the liquid "climbing" upwards. The upward concurrent motion aids in creating a high degree of turbulence within the liquid.
This evaporator features long heating tubes (about 5~7M in length), but the temperature difference between the heating chamber and boiling chamber must also be significant. Otherwise, the energy of the steam flow is insufficient to transport the liquid and generate an ascending film.
Material Import
Condensate Water Outlet
Steam Import
D: Concentrate Export
E: Second Steam Outlet

Film Evaporator
In a falling film evaporator, the liquid and steam flow downward concurrently. The liquid to be concentrated is preheated to boiling temperature, and a uniform liquid film enters the heating tubes through the liquid distribution device at the top of the evaporator. As it flows downward at the boiling temperature, it partially evaporates, and the downward movement is continuously enhanced by the gravitational force from the concurrent steam flow.
In a film evaporator, the trouble-free operation of the device for fully wetting the heating surface is crucial. If the heating surface is not adequately wetted, localized dry walls and scaling may occur, with the worst-case scenario being complete blockage of the heating tubes. In critical conditions, the wetting rate increases with the number of stages or divisions in the evaporator, hence, it is advisable to maintain the advantages of single-pass operation. In this mode of operation, the contact time between the liquid and the heating surface is almost constant.
In addition to pipe sizes, selecting the appropriate liquid distribution device is crucial for achieving proper wetting. With two types of liquid distribution devices available, due to the low liquid volume and fast flow rate within the heating tube, the product's dwell time is quite brief. Therefore, it is possible to gently vaporize heat-sensitive materials.