Molecular distillation, short-path distillation

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Molecular distillation, short-path distillation I. Overview Many substances such as petroleum residue, chemicals, pharmaceuticals, natural foods, health supplements, and fatty acids are often heat-sensitive, viscous, and/or have high boiling points. To separate these materials from their other components while maintaining the quality of the product, distillation must be carried out at low boiling temperatures, or even below the boiling point, and over a very short period of time, in order to minimize thermal decomposition or polymerization and thus avoid damaging the product. When the operating vacuum is around 500 Pa, it can be carried out in a wiped-film evaporator or a falling-film evaporator. However, problems arise if distillation must be carried out under high vacuum (e.g., at pressures between 0.001 and 100 Pa). An evaporator whose product vapor pressure does not exceed the pressure difference between the heating surface and the condensing surface must be used; therefore, evaporators with external condensers are excluded from distillation processes in high-vacuum conditions. Compared to those that only have a lower section, the SPE-type short-path evaporator is highly suitable for such applications, as the condenser is built inside the evaporator, directly opposite where heating and distillation take place. Therefore, the short-distance distillation method for fish oil is a relatively new technique that has not yet been widely used in industrial production; it represents a novel separation method capable of solving problems that conventional distillation techniques cannot address. II. Working Principle The pressure difference between the boiling film and the condensation surface is the driving force for the flow of steam; even a slight pressure drop can cause steam to flow. Operating at 1 mbar requires a very short distance between the boiling surface and the condensing surface; distillers built on this principle are called short-path distillers (Fish oil short distance distills). In a short-path distiller (for fish oil), short-path distillation – also known as molecular distillation – features a built-in condenser on the opposite side of the heating surface, which reduces the operating pressure to below 0.001 mbar. Its low boiling point makes it very suitable for heat-sensitive substances with high boiling points. Basic composition: cylindrical tank with an external heating jacket, rotor, and built-in condenser ； A film-scraping blade and a splash prevention device are precisely installed on the rotor’s mounting frame. The built-in condenser is located at the center of the evaporator, with the rotor rotating between the cylindrical tank and the condenser. The distillation process involves feeding the material from the top of the evaporator; it is then distributed evenly and continuously over the heating surface by a liquid distributor on the rotor. Subsequently, a wiper blade spreads the liquid into a very thin, turbulent film, which is pushed downward in a spiral pattern. During this process, the light molecules that escape from the heating surface travel along a short path and, with almost no collisions, condense into a liquid on the built-in condenser. This liquid then flows down through the condensation tube and is discharged via the outlet pipe located at the bottom of the evaporator ； The residual liquid, namely the heavy molecules, is collected in the circular channels in the heating area and then flows out through the outlet pipes on the side. Molecular distillation is a special distillation process that takes place under a high vacuum of 1 Pa to 0.01 Pa, with molecular flow moving directly from the heating surface to the surface of the condenser. As shown in the figure below, the molecular distillation process can be divided into the following four steps: 1. Molecules diffuse from the bulk of the liquid phase toward the evaporation surface. Typically, the diffusion rate in the liquid phase is the main factor determining the speed of molecular distillation; therefore, it is necessary to minimize the thickness of the liquid layer and enhance its flow. 2. Free evaporation of molecules at the surface of the liquid layer: The evaporation rate increases as the temperature rises, but the separation factor sometimes decreases with rising temperature. Therefore, an economically reasonable distillation temperature should be selected based on the thermal stability of the substance to be processed. 3. Molecules travel from the evaporation surface to the condensation surface: As steam molecules move from the evaporation surface toward the condensation surface, they may collide with each other, as well as with air molecules remaining between the two surfaces. Since the evaporating molecules are much heavier than air molecules and mostly move in the same direction, their collisions with each other have little effect on the direction of emission and the evaporation rate. The residual gas molecules are in a chaotic state of thermal motion between the two surfaces; therefore, the number of residual gas molecules is the main factor affecting the direction of emission and the evaporation rate. 4. Condensation of molecules on the condensation surface: As long as there is a sufficient temperature difference between the hot and cold sides (usually 70–100°C), and the condensation surface is properly shaped and smooth, the condensation process can be completed in an instant; therefore, it is very important to choose the right type of condenser. It follows that the conditions for molecular distillation are: 1. The partial pressure of the residual gas must be very low, so that the mean free path length of the residual gas is a multiple of the distance between the surfaces of the distiller and the condenser. 2. At saturation pressure, the average free path length of steam molecules must be on the same order of magnitude as the distance between the surfaces of the evaporator and the condenser. Under these ideal conditions, evaporation occurs from the residual gas molecules without any obstacles. All steam molecules reach the surface of the condenser before encountering other molecules and returning to the liquid. The evaporation rate reaches its possible maximum under the given conditions. Since the distillation rate is proportional to the vacuum level, the amount of distillate obtained through molecular distillation is relatively small. In SPE short-path distillation, the distance between the condenser and the heating surface is approximately 20–60 mm. When the pressure of the residual gas is 10–3 mbar, the average free path length of the residual gas molecules is about twice this value. Therefore, the short-path distiller is fully capable of meeting all the necessary conditions for molecular distillation. III. Equipment Features ▲High-boiling-point materials: Special metal components are used to handle materials with a boiling point of up to 450°C under a vacuum pressure of 0.001 mbar. ▲Viscosity and presence of solid materials: The scraper immersed in the liquid film generates strong shear forces, which can significantly reduce the viscosity of the mixture; thus, it is possible to handle materials with a viscosity of up to 50 Pas. ▲Excess solid-containing material: The scraper immersed in the liquid film generates a strong stirring force, which also prevents solids from scaling on the heating surface. ▲Color-sensitive materials: In SPE-type short-path distillers for fish oil, HEC has specially designed a device to prevent metal wear. This device is very important, in case the processing of color-sensitive materials is affected due to metal wear resulting from the flow of non-volatile substances (heavy components) through the distiller. ▲High distillation ratio: The use of precisely manufactured film-scraping blades ensures that the liquid film is evenly applied to the heating surface, allowing the entire area to be heated and moistened. This meets the requirements for a high distillation ratio, enabling over 90% of the product to be distilled while maintaining a constant level of residue. ▲Low maintenance costs: Thanks to a rational structural design and a unique rotor, wear on the film-scraping elements and metal surfaces is avoided. ▲It has a self-cleaning effect; therefore, there is no need for disassembly. It is sufficient to use a suitable solvent or cleaning solution in place of the product feed, and to pass it through the device at a reduced jacket heating temperature. IV. Structural features: 1. Precise rotor – The rotor has a squirrel-cage cross-section. HEC uses two different types of rotors to generate a liquid film; the strong turbulence created by the scraping blades that are deeply immersed in this liquid film results in better heat transfer on the heating surface compared to other rotor systems (for example, using PTFE tubes around metal rods). An example is the purification of isocyanate from prepolymers, with the final concentration being less than 0.1% (Table 2). It can be seen from these examples that, for the same heating area, different types of scraping blades yield different results, and SPE can increase the distillation capacity by 20%. 2. For precise short-path distillation using condensers, bolted condensers generally employ coiled tubes, U-tubes, or tube bundles, which are fixed at the bottom of the distiller. The shape of the condenser and the appropriateness of the gap with the heating surface directly affect the separation purity and yield of the product, as well as the vacuum level inside the distiller. Therefore, based on its long-standing experience in evaporation/distillation, HEC Company designs different built-in condensers according to the specific materials used in various processes. The built-in condenser in the right diagram is of the tube bundle type, located precisely at the center of the distiller. It features equal distances between its condensing and heating surfaces, a smooth surface, uniform temperature, and a large space for gas flow. This allows for an optimal adjustment of the distance between the heating and condensing surfaces, thereby ensuring that the short-path distillation column of the SPE fish oil short-distance distiller operates under high-vacuum conditions. 3. Mist separator: The mist separator is essential for some products; it can be placed between the rotor blade channels or behind the blade. These splash guards remove any droplets carried by the steam flow. As the droplets pass through them, they are deflected and flung back toward the heating surface where they separate. The splash guard is a complete component that is easy to extend. Depending on the process properties, they can be louvered, herringbone, grooved, or open. V. Device Type: Short-path Distiller (For fish oil short-distance distillation). The short-path distillation unit operates under high vacuum conditions, and it has specific requirements regarding the scraper blade and the internal condenser. In order to reduce the thickness of the liquid film and enhance the movement of the liquid layer, HEC Company has classified this type of device into the following 2 categories based on the design of the scraper blade: ▲SPE-A type short-path distiller (For fish oil short-distance distillation): The scraper blade in this model is of the radial sliding type, and it evolved from the WFE-VA scraper evaporator ； The scraper is embedded in the U-shaped guide groove of the rotor; as the rotor rotates, the scraper is pushed by centrifugal force to slide radially from within the U-shaped guide groove toward the heating wall surface. The guide grooves are evenly distributed around the rotor’s circumference (see figure above). The rotor is of the squirrel-cage type, and dynamic vapor-liquid separation devices are installed between the guide grooves to prevent droplets from splashing or being carried along. Scraper materials are generally composed of polytetrafluoroethylene, graphite, etc. ▲SPE-B type short-path distiller (for fish oil short-distance distillation): The scraping blade of the short-path distillation unit in SPE-B is similar to that of WFE-VC (see figure on the right); this scraping blade adheres tightly to the heating wall surface due to centrifugal force. Handle color-sensitive materials, as the non-volatile components of these materials flow through this distiller, and their color is not affected by metal wear. This type is widely used; by changing the frequency and quality of the scraper, the centrifugal force can be adjusted to suit various viscosities. The dimensions of the standard equipment are listed in the third column of the table on the right: For short-distance distillers used for fish oil distillation, the part of the distillation column that comes into contact with the product is usually made of stainless steel; however, depending on the customer’s requirements, it can also be made of carbon steel, composite materials, or other special materials such as Hastelloy, nickel, titanium, Monel alloy, etc. VI. Typical Applications Table 4 below lists some typical applications of the SPE short-path distiller for various materials: a) In the single-stage distillation unit of the SPE short-path distiller, the material that has been preheated and degassed enters the distiller, where the light components are vaporized and condensed on an internal condenser. It and the reconstituted components are discharged through different channels separately (Flowchart 1). b) Film-evaporator and short-path distiller (The fish oil short distance distills): A combination distillation unit using a short-path distiller; a large amount of low-boiling substances are removed in the film-evaporator, while the distillation of the product takes place in the subsequent short-path distiller (The fish oil short distance distills) (Flowchart 2). VII. Test first, then produce – we hope you can achieve the best possible results. To this end, HEC Company has short-path distillation equipment for operational testing, in order to determine the required liquid volume and the efficiency of the short-path distiller (the fish oil short distance distill) and its distillation columns. Similarly, we build complete short-path distillation units for the distillation processes in laboratories and testing stations. This can help accelerate product development and optimize the production process. We offer complete, ready-to-use laboratory and pilot plant systems, including short-path distillers for fish oil, short-path distillation columns, as well as the necessary auxiliary equipment such as pipes, fittings, cooling wells, pumps, and valves. VIII. Distillation plant systems: HEC Company not only designs and manufactures individual equipment but also designs, develops, and manufactures complete plant systems. The design is based on the company’s years of experience and the technical data you provide. Completing product distillation and evaporation requires a complete set of equipment; if multiple fractions need to be separated from a mixture, several sets of equipment are generally needed as well. As required, the short-path distiller (The fish oil short distance distills) can be supplied as a complete unit together with auxiliary equipment (cold wells, material pumps, concentration pumps, vacuum pumps), or it can be supplied as a standalone unit. To implement a short-path distillation process, in addition to the short-path distiller itself, it is also necessary to choose other appropriate equipment, such as degassing systems, product processing units, heating devices, and vacuum systems. HEC has many years of experience in selecting the most effective combinations and types of equipment units; we are always striving to achieve the best balance between your benefits and our expertise. ▲The material to be processed in the degassing unit usually contains dissolved gases and small amounts of low-boiling components; under medium to high vacuum conditions, these substances do not condense with cooling water at normal temperatures, thus not affecting the vacuum level. To ensure that the load on the vacuum pump is as low as possible before it enters the short-distance distiller for fish oil, it is recommended to install a degassing unit in front of the distiller to remove low-boiling components in advance. Practice has shown that the thin-film evaporator is a quite good degassing device, as it can preheat the product while simultaneously performing degassing. ▲The product treatment degassing unit and the short-path distiller (the fish oil short distance distills) are connected via a pressure difference; under the action of gravity, the material flows from the degassing unit into the short-path distilling chamber of the short-path distiller (the fish oil short distance distills) ； If they are required to be at the same height, a pump must be used to transfer the material from the degassing unit to the short-distance distiller (the fish oil short distance distill). The feed must be continuous, while the discharge can be continuous or intermittent. ▲In the short-path distillation column of fish oil short-distance distillers, a certain level of vacuum must be maintained; therefore, it is very important to select an appropriate vacuum system. Mechanical pumps, or combinations of mechanical pumps and steam jet pumps, are usually used for this purpose. When the absolute pressure is in the range of 0.1–1 Pa, a diffusion pump must also be used. Furthermore, to keep the outward partial pressure as low as possible, a cold well must be installed between the distiller and the vacuum pump, with low-temperature water or coolant used as the cooling medium.