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Scraping film evaporator

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Scraping film evaporator

I. Introduction

Separation technology is one of three major chemical production technologies. The separation process has a great impact on product quality, efficiency, consumption and benefit. The TFE Mechanically-agitated Thin Film Evaporator is a device used to perform separation through the volatility of the materials. This device has a high heat transfer coefficient, low evaporation temperature, short material residence time, high thermal efficiency and high evaporation intensity. It is widely used in the industries of petrochemicals, fine chemicals, agricultural chemicals, food, medicine and biochemical engineering, to conduct the processes of evaporation, concentration, solvent removal, purification, steam stripping, degassing, deodorization, etc.

The Thin Film Evaporator is a new and efficient evaporator that can carry out falling film evaporation under vacuum conditions, in which the film is forcibly made by the rotating film applicator and has a high flow speed, high heat transfer efficiency and short residence time (about 5-15 seconds). It also possesses a high heat transfer coefficient, high evaporation strength, short flow time and large operating flexibility, which is especially suitable for the concentration by evaporation, degassing, solvent removal, distillation and purification of heat-sensitive materials, high viscosity materials and easy crystal and particle-containing materials. It consists of one or more cylinders with jackets for heating and a film applicator rotating in the cylinder. The film applicator continuously scrapes the feed materials into a uniform liquid film on the heating surface and pushes them downward, during which components with low boiling points evaporate and their residues are discharged from the bottom of the evaporator.

II. Performance Characteristics

Low vacuum pressure drop:

When the vaporized gas of the materials transfers from the heating surface to the external condenser, there exists a certain differential pressure. In a typical evaporator, such a pressure drop (Δp) is usually relatively high, sometimes to an unacceptable degree. In contrast, the Thin Film Evaporator has a larger gas space, the pressure of which is almost equal to that in the condenser; therefore, there is a small pressure drop and the vacuum degree can be ≤1Pa.

• Low operating temperature:

Thanks to the above property, the evaporation process can be conducted at a high vacuum degree. Since the vacuum degree increases, the corresponding boiling point of materials decreases rapidly. Therefore, the operation can be conducted at a lower temperature and the thermal decomposition of the product is thus reduced.

Short heating time:

Due to the unique structure of the Thin Film Evaporator and the pumping action of the film applicator, the residence time of the materials in the evaporator is short; in addition, the rapid turbulence of the film in the heating evaporator makes the product unable to stay on the evaporator surface. Therefore, it is especially suitable for the evaporation of heat-sensitive materials.

High evaporation intensity:

The reduction of the boiling point of materials increases the temperature difference of heated media; the function of the film applicator decreases the thickness of the liquid film in a turbulent state and reduces the thermal resistance. Meanwhile, the process suppresses the caking and fouling of materials on the heating surface and is accompanied by good heat exchange, thus increasing the overall heat transfer coefficient of the evaporator.

Large operating flexibility:

Due to its unique properties, the scraper film evaporator is suitable for treating heat-sensitive materials which require smooth and steady evaporation and high-viscosity materials whose viscosity increases dramatically with the increase of concentration, as its evaporation process is smooth and steady.

It is also suitable for the evaporation and distillation of materials containing particles or in cases of crystallization, polymerization and fouling.

III. Application Areas

he scraper film evaporator has been widely used in heat exchange projects. It helps the heat exchange of heat-sensitive materials (short time) especially, and can distil complex products with its various functions.

The scraper film evaporator has been used for concentration by evaporation, solvent removal, steam-stripping, reaction, degassing, deodorization (de-aeration), etc. in the following areas, and has achieved good results:

Traditional Chinese medicine and Western medicine: antibiotics, sugar liquor, thunder godvine, astragalus and other herbs, methylimidazole, single nitrile amine and other intermediates;

Light industrial foods: juice, gravy, pigments, essences, fragrances, zymin, lactic acid, xylose, starch sugar, potassium sorbate, etc;

Oils and daily chemicals: lecithin, VE, cod liver oil, oleic acid, glycerol, fatty acids, waste lubricating oil, alkyl polyglycosides, alcohol ether sulfates, etc;

Synthetic resins: polyamide resins, epoxy resins, paraformaldehyde, PPS (polypropylene sebacate esters), PBT, formic acid allyl esters, etc;

Synthetic fibers: PTA, DMT, carbon fiber, polytetrahydrofuran, polyether polyols, etc;

Petrochemistry: TDI, MDI, trimethyl hydroquinone, trimethylolpropane, sodium hydroxide, etc;

Biological pesticides: acetochlor, metolachlor, chlorpyrifos, furan phenol, clomazone, insecticides, herbicides, miticides, etc;

Waste water: Inorganic salt wastewater;

IV. Working Principle

Molecular distillation achieves separation by using the difference of molecular free paths when the light and heavy component molecules in the liquid phase escape from the liquid phase body. The basic conditions for achieving molecular distillation are as follows: the molecular free path of the escaped light component molecules is equal to the distance between the heating surface and the condensing surface, so the light component molecules can accurately reach the internal condenser to achieve trapping; while the molecular free path of the escaped heavy component molecules is less than the distance between the heating surface and the condensing surface, so the heavy component molecules cannot reach the internal condenser. That is to say, the internal condenser only traps light component molecules, thereby achieving the separation of light and heavy component molecules.

Designed on the basis of the molecular distillation principle, the short-path distiller(short path distillation system) is a model which simulates molecular distillation. Its heating surface is very close to the condensing surface and it has very little resistance, hence 'short-path distiller(short path distillation system)'. As the internal condenser can liquefy the vapor phase instantly to contract the volume, a high vacuum can be maintained inside the device. The short-path distiller(short path distillation system) has an operating vacuum of up to 1Pa, which is difficult to achieve for other evaporation or distillation equipment. Therefore, the short-path distiller(short path distillation system) is particularly suitable for materials which have a high boiling point at atmospheric pressure and are difficult to separate by conventional separation methods. It has obtained a great deal of successful experience in many industries as a new type of liquid-liquid separation equipment. Depending on the properties of the material, various forms of short-path distiller(short path distillation system) can be selected, the principle of which is similar to that of the scraper evaporator.

Processing capabilities of short-path distiller(short path distillation system):

As a short-path distiller(short path distillation system) is generally used to treat materials with a high boiling point and high viscosity that are usually heated by conduction oil, its separation capacity is relatively small: in the range of 30-70kg/m2/hr depending on the materials.

Materials are entered into the evaporator from the top of the heating zone in a radial direction, and distributed on the heating face of the evaporator by the distributor. The rotating film applicator then continuously and uniformly scrapes the materials into a uniform liquid film on the heating surface, pushing them downward in a spiral way. In this process, the rotating film applicator ensures the rapid turbulence of the continuous and uniform liquid film and prevents the liquid film from caking and fouling, thereby improving the overall heat transfer coefficient. After being evaporated, the light components form into a vapor stream and rise through a vapor-liquid separator to reach the external condenser directly connected to the evaporator, and the heavy components are removed from the cone at the bottom of the evaporator.

The unique distributor can not only evenly pour the materials onto the inner face of the evaporator to prevent them from spilling to the inside and being injected into the steam flow, but also prevent the new feed from flash-evaporating while distributing. It also helps with the elimination of foam and ensures that the materials can only be evaporated along the heating surface.

The top of the scraper film evaporator is equipped with a centrifugal separator designed on the basis of the properties of the material, which can separate the droplets from the rising steam-flow and return them to the distributor.

V. Operating Instructions

Molecular distillation achieves separation by using the difference of molecular free paths when the light and heavy component molecules in the liquid phase escape from the liquid phase body. The basic conditions for achieving molecular distillation are as follows: the molecular free path of the escaped light component molecules is equal to the distance between the heating surface and the condensing surface, so the light component molecules can accurately reach the internal condenser to achieve trapping; while the molecular free path of the escaped heavy component molecules is less than the distance between the heating surface and the condensing surface, so the heavy component molecules cannot reach the internal condenser. That is to say, the internal condenser only traps light component molecules, thereby achieving the separation of light and heavy component molecules.

Role Description of Instrument Parts:

1. Motor and reducer

This is the actuating device for the rotation of the rotor. The rotational speed of the rotor will depend on the form of the scraper, the viscosity of the materials and the inner diameter of the evaporation cylinder; properly-selected linear velocity is an important parameter that can ensure the stable and reliable operation and satisfying effect of the evaporator.

2. Separation tube

The materials are entered into the evaporator from the inlet on the lower end of the separation tube in a tangential direction, and are continuously and uniformly distributed on the inner face of the evaporation cylinder by the distributor inside the separation tube. The secondary steam evaporating from the evaporation cylinder rises into the separation tube, and any droplets or foam that may exist in the secondary steam are separated by the vapor-liquid separator installed inside the separation tube. The secondary steam then escapes out of the evaporator from the outlet on the top.

The rational design of the separation tube based on the calculation of resistance inside the evaporator is one of the key factors for preventing the 'short circuit' of materials. (This so-called 'short circuit' means a circumstance in which materials which have just entered the evaporator but have not yet completed the evaporation process leave the evaporator prematurely from the secondary steam outlet.)

3. Distributor

The distributor is mounted on the rotor. Its rational design enables materials entering the evaporator tangentially to pass through the rotating distributor and be distributed on the evaporation surface in a continuous and uniform film.

4. Vapor-liquid separator

Installed on the top of the separation tube, the rotary-vane vapor-liquid separator captures droplets or foam entrained in the secondary steam and makes them fall onto the evaporation surface.

5. Evaporation cylinder body

Also known as the heating cylinder body, this is the evaporation surface for heat exchange between materials forcibly made into films by the rotating scraper and the heat media within the jacket. The inside diameter and length of the evaporation cylinder is determined by the evaporation area and proper length-diameter ratio.

The inner face of the heating cylinder is machined and polished by a special purpose machine, and its machining is completed simultaneously with that of the joint faces of the flanges on both ends to ensure the circular center degree of the whole device. The inner face of the cylinder is bright, smooth and resistant to materials sticking to it and forming a scale after polishing, effectively ensuring the high heat transfer coefficient of the device.

6. Rotor

Installed inside the evaporator cylinder, the rotor is composed of a rotor shaft and turret. The rotor is driven by the motor and reducer, and it drives the scraper in a circular motion. The turret is made of precision-cast processed stainless steel, thus effectively guaranteeing the strength, geometry and stability of the rotor.

7. Scraper

Due to the movement of the scraper, the material is constantly scraped into thin films on the evaporation surface in order to achieve the results of film evaporation.

We have focused on top quality lab instruments, pilot apparatus and biochemical instruments field for over 10 years in China.

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Our main products include the Short Path Distillation Apparatus, Thin Film Evaporator, Rotary Evaporator, Glass Reactor, Vacuum Filter, Liquid-Liquid Extractors, Heating and Cooling Circulator, Water Bath, Heating Mantle,Lab Vacuum Pump, Lab Drying Oven, Spray Drier and so on。


 

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