SEMI-SOLID EXTRACTS PRODUCTION.
Extracts are preparations of liquid (liquid extracts and tinctures), semi-solid (soft extracts) or solid (dry extracts) consistency, obtained from herbal drugs or animal matter, which are usually in a dry state.
Different types of extract may be distinguished. Standardised extracts are adjusted within an acceptable tolerance to a given content of constituents with known therapeutic activity ; standardisation is achieved by adjustment of the extract with inert material or by blending batches of extracts. Quantified extracts are adjusted to a defined range of constituents ; adjustments are made by blending batches of extracts. Other extracts are essentially defined by their production process (state of the herbal drug or animal matter to be extracted, solvent, extraction conditions) and their specifications.
Extracts are prepared by suitable methods using ethanol or other suitable solvents. Different batches of the herbal drug or animal matter may be blended prior to extraction. The herbal drug or animal matter to be extracted may undergo a preliminary treatment, for example, inactivation of enzymes, grinding or defatting. In addition, unwanted matter may be removed after extraction.
Herbal drugs, animal matter and organic solvents used for the preparation of extracts comply with any relevant monograph of the Pharmacopoeia. For soft and dry extracts where the organic solvent is removed by evaporation, recovered or recycled solvent may be used, provided that the recovery procedures are controlled and monitored to ensure that solvents meet appropriate standards before re-use or admixture with other approved materials. Water used for the preparation of extracts is of a suitable quality. Except for the test for bacterial endotoxins, water complying with the section on Purified water in bulk in the monograph on Purified water (0008) is suitable. Potable water may be suitable if it complies with a defined specification that allows the consistent production of a suitable extract.
Where applicable, concentration to the intended consistency is carried out using suitable methods, usually under reduced pressure and at a temperature at which deterioration of the constituents is reduced to a minimum. Essential oils that have been separated during processing may be restored to the extracts at an appropriate stage in the manufacturing process. Suitable excipients may be added at various stages of the manufacturing process, for example to improve technological qualities such as homogeneity or consistency.
Suitable stabilisers and antimicrobial preservatives may also be added.
Extraction with a given solvent leads to typical proportions of characterised constituents in the extractable matter ; during production of standardised and quantified extracts, purification procedures may be applied that increase these proportions with respect to the expected values ; such extracts are referred to as ‘refined’.
Soft extracts — extracta spissa
Soft extracts are semi-solid preparations obtained by evaporation or partial evaporation of the solvent used for extraction.
Herbal drug preparations are obtained by subjecting herbal drugs to treatments such as extraction, distillation, expression, fractionation, purification, concentration or fermentation. These include comminuted or powdered herbal drugs, tinctures, extracts, essential oils, expressed juices and processed exudates.
Instant herbal teas consist of powder or granules of one or more herbal drug preparation(s) intended for the preparation of an oral solution immediately before use.
Herbal drugs are mainly whole, fragmented, or cut plants, parts of plants, algae, fungi or lichen, in an unprocessed state, usually in dried form but sometimes fresh. Certain exudates that have not been subjected to a specific treatment are also considered to be herbal drugs. Herbal drugs are precisely defined by the botanical scientific name according to the binominal system (genus, species, variety and author).
Herbal drugs are obtained from cultivated or wild plants.
Suitable collection, cultivation, harvesting, drying, fragmentation and storage conditions are essential to guarantee the quality of herbal drugs.
Herbal drugs are, as far as possible, free from impurities such as soil, dust, dirt and other contaminants such as fungal, insect and other animal contaminations. They are not rotten. If a decontaminating treatment has been used, it is necessary to demonstrate that the constituents of the plant are not affected and that no harmful residues remain. The use of ethylene oxide is prohibited for the decontamination of herbal drugs.
Extractions
Extraction, as the term is used pharmaceutically, involves the separation of medicinally active portions of plant or animal tissues from the inactive or inert components by using selective solvents in standard extraction procedures. The products so obtained from plants are relatively impure liquids, semisolids or powders intended only for oral or external use.
These include classes of preparations known as decoctions, infusions, fl uid extracts, tinctures, pilular (semisolid) extracts and powdered extracts. Such preparations popularly have been called galenicals, named after Galen, the second century Greek physician.
The purposes of standardized extraction procedures for crude drugs are to attain the therapeutically desired portion and to eliminate the inert material by treatment with a selective solvent known as menstruum.
The extract thus obtained may be ready for use as a medicinal agent in the form of tinctures and fl uid extracts, it may be further processed to be incorporated in any dosage form such as tablets or capsules, or it may be fractionated to isolate individual chemical entities such as ajmalicine, hyoscine and vincristine, which are modem drugs. Thus, standardization of extraction procedures contributes signifi cantly to the fi nal quality of the herbal drug.
Extraction is the withdrawing of a active agent or a waste substance from a solid or liquid mixture with a liquid solvent. The solvent is not or only partial miscible with the solid or the liquid. By intensive contact the active agent transfers from the solid or liquid mixture (raffinate) into the solvent (extract). After mixing the two phases are separated which happens either by gravity or centrifugal forces. For recovery of the solvent and to get the active agent in pure form a further separation process is necessary (rectification or re-extraction)
Depending on the phases following types of extraction exist:
• Solid – liquid extraction
• Liquid – liquid extraction
The gas – liquid extraction is called absorption.
The main area of extraction is for hydrometallic processes, for pharmaceutical industry (producing active agents), for petroleum industry (production of monomers and aromates) and for cleaning of waste water to separate solved compounds.
The solvent for extraction has to withdraw the active agent from a mixture.
• selectivity: Only the active agent has to be extracted and no further substances which means that a high selectivity is required.
• capacity: To reduce the amount of necessary solvent the capacity of the solvent has to be high.
• miscibility: To achieve simple regeneration of the solvent the miscibility of solvent and primary solvent has to be low.
• difference in density: After extraction the two phases have to be separated in a separator and for this a high difference in density is positive.
• optimal surface tension: a low -> low amount of energy for dispersing required; if surface tension < 1 mN/m stable emulsions are produced, a > 50 mN/m -> high amount of energy for dispersing and high tendency to coalesce
• recovery: The solvent has to be separated from the extract phase easily to produce solvent free active agents.
• corrosion: If the solvent is corrosive prices for construction increase
• low price
• no or low toxicity
• flame temperature:
• vapour pressure: To prevent loss of solvent by evaporation a low vapour pressure at operating temperature is required.
• viscosity: A low viscosity of the solvent leads to low pressure drop and good heat and mass transfer.
• chemical and thermal stability
Methods of Extraction
Percolation
This is the procedure used most frequently to extract active ingredients in the preparation of tinctures and fl uid extracts. A percolator (a narrow, cone-shaped vessel open at both ends) is generally used (Figure 1). The solid ingredients are moistened with an appropriate amount of the specifi ed menstruum and allowed to stand for approximately 4 h in a wellclosed container, after which the mass is packed and the top of the percolator is closed. Additional menstruum is added to form a shallow layer above the mass, and the mixture is allowed to macerate in the closed percolator for 24 h. The outlet of the percolator then is opened and the liquid contained therein is allowed to drip slowly. Additional menstruum is added as required, until the percolate measures about three-quarters of the required volume of the fi nished product. The marc is then pressed and the expressed liquid is added to the percolate. Suffi cient menstruum is added to produce the required volume, and the mixed liquid is clarifi ed by fi ltration or by standing followed by decanting.
Hot Continuous Extraction (Soxhlet)
In this method, the fi nely ground crude drug is placed in a porous bag or “thimble” made of strong fi lter paper, which is placed in chamber E of the Soxhlet apparatus (Figure 2). The extracting solvent in fl ask A is heated, and its vapors condense in condenser D. The condensed extractant drips into the thimble containing the crude drug, and extracts it by contact. When the level of liquid in chamber E rises to the top of siphon tube C, the liquid contents of chamber E siphon into fl ask A. This process is continuous and is carried out until a drop of solvent from the siphon tube does not leave residue when evaporated. The advantage of this method, compared to previously described methods, is that large amounts of drug can be extracted with a much smaller quantity of solvent. This effects tremendous economy in terms of time, energy and consequently fi nancial inputs. At small scale, it is employed as a batch process only, but it becomes much more economical and viable when converted into a continuous extraction procedure on medium or large scale.
Aqueous Alcoholic Extraction by Fermentation
Some medicinal preparations of Ayurveda (like asava and arista) adopt the technique of fermentation for extracting the active principles. The extraction procedure involves soaking the crude drug, in the form of either a powder or a decoction (kasaya), for a specifi ed period of time, during which it undergoes fermentation and generates alcohol in situ; this facilitates the extraction of the active constituents contained in the plant material. The alcohol thus generated also serves as a preservative. If the fermentation is to be carried out in an earthen vessel, it should not be new: water should fi rst be boiled in the vessel. In large-scale manufacture, wooden vats, porcelain jars or metal vessels are used in place of earthen vessels. Some examples of such preparations are karpurasava, kanakasava, dasmularista. In Ayurveda, this method is not yet standardized but, with the extraordinarily high degree of advancement in fermentation technology, it should not be diffi cult to standardize this technique of extraction for the production of herbal drug extracts.
Counter-current Extraction
In counter-current extraction (CCE), wet raw material is pulverized using toothed disc disintegrators to produce a fi ne slurry. In this process, the material to be extracted is moved in one direction (generally in the form of a fi ne slurry) within a cylindrical extractor where it comes in contact with extraction solvent. The further the starting material moves, the more concentrated the extract becomes. Complete extraction is thus possible when the quantities of solvent and material and their fl ow rates are optimized. The process is highly effi cient, requiring little time and posing no risk from high temperature. Finally, suffi ciently concentrated extract comes out at one end of the extractor while the marc (practically free of visible solvent) falls out from the other end.
This extraction process has signifi cant advantages:
i) A unit quantity of the plant material can be extracted with much smaller volume of solvent as compared to other methods like maceration, decoction, percolation.
ii) CCE is commonly done at room temperature, which spares the thermolabile constituents from exposure to heat which is employed in most other techniques.
iii) As the pulverization of the drug is done under wet conditions, he heat generated during comminution is neutralized by water. This again spares the thermolabile constituents from exposure to heat.
iv) The extraction procedure has been rated to be more effi – cient and effective than continuous hot extraction.
Ultrasound Extraction (Sonication)
The procedure involves the use of ultrasound with frequencies ranging from 20 kHz to 2000 kHz; this increases the permeability of cell walls and produces cavitation. Although the process is useful in some cases, like extraction of rauwolfi a root, its large-scale application is limited due to the higher costs. One disadvantage of the procedure is the occasional but known deleterious effect of ultrasound energy (more than 20 kHz) on the active constituents of medicinal plants through formation of free radicals and consequently undesirable changes in the drug molecules.
Supercritical Fluid Extraction
Supercritical fl uid extraction (SFE) is an alternative sample preparation method with general goals of reduced use of organic solvents and increased sample throughput. The factors to consider include temperature, pressure, sample volume, analyte collection, modifi er (cosolvent) addition, fl ow and pressure control, and restrictors. Generally, cylindrical extraction vessels are used for SFE and their performance is good beyond any doubt.
The collection of the extracted analyte following SFE is another important step: signifi cant analyte loss can occur during this step, leading the analyst to believe that the actual effi ciency was poor.
There are many advantages to the use of CO2 as the extracting fl uid. In addition to its favorable physical properties, carbon dioxide is inexpensive, safe and abundant. But while carbon dioxide is the preferred fl uid for SFE, it possesses several polarity limitations. Solvent polarity is important when extracting polar solutes and when strong analyte-matrix interactions are present. Organic solvents are frequently added to the carbon dioxide extracting fl uid to alleviate the polarity limitations. Of late, instead of carbon dioxide, argon is being used because it is inexpensive and more inert. The component recovery rates generally increase with increasing pressure or temperature: the highest recovery rates in case of argon are obtained at 500 atm and 150° C.
The extraction procedure possesses distinct advantages:
i) The extraction of constituents at low temperature, which strictly avoids damage from heat and some organic solvents
ii) No solvent residues.
iii) Environmentally friendly extraction procedure. The largest area of growth in the development of SFE has been the rapid expansion of its applications. SFE fi nds extensive application in the extraction of pesticides, environmental samples, foods and fragrances, essential oils, polymers and natural products. The major deterrent in the commercial application of the extraction process is its prohibitive capital investment.
Phytonics Process
A new solvent based on hydrofl uorocarbon-134a and a new technology to optimize its remarkable properties in the extraction of plant materials offer signifi cant environmental advantages and health and safety benefi ts over traditional processes for the production of high quality natural fragrant oils, fl avors and biological extracts.
Advanced Phytonics Limited (Manchester, UK) has developed this patented technology termed “phytonics process”. The products mostly extracted by this process are fragrant components of essential oils and biological or phytopharmacological extracts which can be used directly without further physical or chemical treatment.
The properties of the new generation of fl uorocarbon solvents have been applied to the extraction of plant materials. The core of the solvent is 1,1,2,2-tetrafl uoroethane, better known as hydrofl uorocarbon-134a (HFC-134a). This product was developed as a replacement for chlorofl uorocarbons.
The boiling point of this solvent is -25° C. It is not fl ammable or toxic. Unlike chlorofl uorocarbons, it does not deplete the ozone layer. It has a vapor pressure of 5.6 bar at ambient temperature. By most standards this is a poor solvent.
For example, it does not mix with mineral oils or triglycerides and it does not dissolve plant wastes.
The process is advantageous in that the solvents can be customized: by using modifi ed solvents with HFC-134a, the process can be made highly selective in extracting a specifi c class of phytoconstituents.
Similarly, other modifi ed solvents can be used to extract a broader spectrum of components. The biological products made by this process have extremely low residual solvent. The residuals are invariably less than 20 parts per billion and are frequently below levels of detection. These solvents are neither acidic nor alkaline and, therefore, have only minimal potential reaction effects on the botanical materials. The processing plant is totally sealed so that the solvents are continually recycled and fully recovered at the end of each production cycle. The only utility needed to operate these systems is electricity and, even then, they do no consume much energy. There is no scope for the escape of the solvents. Even if some solvents do escape, they contaio chlorine and therefore pose no threat to the ozone layer. The waste biomass from these plants is dry and “ecofriendly” to handle.
Advantages of the Process
• Unlike other processes that employ high temperatures, the phytonics process is cool and gentle and its products are never damaged by exposure to temperatures in excess of ambient.
• No vacuum stripping is needed which, in other processes, leads to the loss of precious volatiles.
• The process is carried out entirely at neutral pH and, in the absence of oxygen, the products never suffer acid hydrolysis damage or oxidation.
• The technique is highly selective, offering a choice of operating conditions and hence a choice of end products.
• It is less threatening to the environment.
• It requires a minimum amount of electrical energy.
• It releases no harmful emissions into the atmosphere and the resultant waste products (spent biomass) are innocuous and pose no effl uent disposal problems.
• The solvents used in the technique are not fl ammable, toxic or ozone depleting.
• The solvents are completely recycled within the system.
Applications
The phytonics process can be used for extraction in biotechnology (e.g for the production of antibiotics), in the herbal drug industry, in the food, essential oil and fl avor industries, and in the production of other pharmacologically active products. In particular, it is used in the production of topquality pharmaceutical-grade extracts, pharmacologically active intermediates, antibiotic extracts and phytopharmaceuticals.
However, the fact that it is used in all these areas io way prevents its use in other areas. The technique is being used in the extraction of high-quality essential oils, oleoresins, natural food colors, fl avors and aromatic oils from all manner of plant materials. The technique is also used in refi ning crude products obtained from other extraction processes. It provides extraction without waxes or other contaminants. It helps remove many biocides from contaminated biomass.
Parameters for Selecting an Appropriate Extraction Method
i) Authentication of plant material should be done before performing extraction. Any foreign matter should be completely eliminated.
ii) Use the right plant part and, for quality control purposes, record the age of plant and the time, season and place of collection.
iv) Conditions used for drying the plant material largely depend on the nature of its chemical constituents. Hot or cold blowing air fl ow for drying is generally preferred. If a crude drug with high moisture content is to be used for extraction, suitable weight corrections should be incorporated.
v) Grinding methods should be specifi ed and techniques that generate heat should be avoided as much as possible.
vi) Powdered plant material should be passed through suitable sieves to get the required particles of uniform size.
vii) Nature of constituents:
a) If the therapeutic value lies ion-polar constituents, a non-polar solvent may be used. For example, lupeol is the active constituent of Crataeva nurvala and, for its extraction, hexane is generally used. Likewise, for plants like Bacopa monnieri and Centella asiatica, the active constituents are glycosides and hence a polar solvent like aqueous methanol may be used.
b) If the constituents are thermolabile, extraction methods like cold maceration, percolation and CCE are preferred.
For thermostable constituents, Soxhlet extraction (if nonaqueous solvents are used) and decoction (if water is the menstruum) are useful.
c) Suitable precautions should be taken when dealing with constituents that degrade while being kept in organic solvents, e.g. fl avonoids and phenyl propanoids.
d) In case of hot extraction, higher than required temperature should be avoided. Some glycosides are likely to break upon continuous exposure to higher temperature.
e) Standardization of time of extraction is important, as:
• Insuffi cient time means incomplete extraction.
• If the extraction time is longer, unwanted constituents may also be extracted. For example, if tea is boiled for too long, tannins are extracted which impart astringency to the fi nal preparation.
f) The number of extractions required for complete extraction is as important as the duration of each extraction.
vii) The quality of water or menstruum used should be specifi ed and controlled.
viii) Concentration and drying procedures should ensure the safety and stability of the active constituents. Drying under reduced pressure (e.g. using a Rotavapor) is widely used. Lyophilization, although expensive, is increasingly employed.
ix) The design and material of fabrication of the extractor are also to be taken into consideration.
x) Analytical parameters of the fi nal extract, such as TLC and HPLC fi ngerprints, should be documented to monitor the quality of different batches of the extracts.
Steps Involved in the Extraction of Medicinal Plants
In order to extract medicinal ingredients from plant material, the following sequential steps are involved:
1. Size reduction
2. Extraction
3. Filtration
4. Concentration
5. Drying
Filtration
The extract so obtained is separated out from the marc (exhausted plant material) by allowing it to trickle into a holding tank through the built-in false bottom of the extractor, which is covered with a fi lter cloth.
The marc is retained at the false bottom, and the extract is received in the holding tank. From the holding tank, the extract is pumped into a sparkler fi lter to remove fi ne or colloidal particles from the extract.
Condensing by evaporation
In the general sense, evaporation refers to any change in phase of a component from liquid to gas. Vaporization, sometimes used interchangeably with evaporation, is at times specifically used to designate the total change of a liquid phase to gas (vapor).
In this article only the term evaporation will be used. Evaporation will be defined as processes carried out in process equipment conventionally classified as evaporators. This, in turn, implies that nonequipment- contained classes of evaporation, such as solar ponds and oil tanker spills, will be ignored.
Evaporators are used to increase the concentration of relatively non-volatile dissolved or suspended components in a solution or slurry (the liquor) by evaporating portions of the liquid phase using energy supplied by a medium, often steam. The dissolved or suspended components do not appear in the vapor phase to a substantial extent. (If they do, the process is referred to as distillation.)
Other methods that will not be discussed here, but also can be used to increase concentration (some with and some without concomitant evaporation) are reverse osmosis, ion exchange, dialysis, electrodialysis, osmotic distillation, and applications that involve fluidized beds, cooling towers, or evaporation of aerosols.
In most evaporators, the solvent or suspending phase is primarily or totally one constituent, most frequently water. The important product in evaporation can be either the more concentrated mixture left behind or the overhead vapor (which is often, but not necessarily, subsequently condensed).
The overhead solvent vapor in solvent recovery processes or boiler water vapor in power plant applications typifies vapor products. Blowdown refers to the periodic or continuous purging of the bottoms used to control buildup of undesirable material in the liquid phase when producing a vapor product.
Some processes of evaporation can be accompanied by crystallization, as the residual liquor grows more and more concentrated. Carried yet further, evaporation evolves to drying (or dehydration, if the constituent removed is water), as the bottoms product obtained becomes primarily solid rather than liquid.
Typical Applications For Evaporators
Historically, a classic example of an evaporation process is the production of table salt. Maple syrup has traditionally been produced by evaporation of sap. Concentration of black liquor from pulp and paper processing constitutes a large-volume present application. Evaporators are also employed in such disparate uses as: desalination of seawater, nuclear fuel reprocessing, radioactive waste treatment, preparation of boiler feed waters, and production of sodium hydroxide. They are used to concentrate stillage waste in fermentation processes, waste brines, inorganic salts in fertilizer production, and rinse liquids used in metal finishing, as well as in the production of sugar, vitamin C, caustic soda, dyes, and juice concentrates, and for solvent recovery in pharmaceutical processes.
Types Of Evaporators
Extended discussion of types (including photographs and schematic diagrams), design, and operation of evaporators can be found in the literature.
Because evaporation of a liquid phase usually requires addition of large amounts of thermal energy, the method of transferring this heat to the liquor tends to dominate evaporator capital cost. The source of heat for evaporators is usually a medium such as hot combustion gases or a condensing vapor, typically steam. Molten salts and electrical resistance heaters are less commonly used sources of thermal energy.
Flash evaporators operate by an adiabatic decrease of the pressure on a liquid that has been previously heated. These were first used for production of potable water on ships; now they are used for more general brackish waters and seawater as well as for processed liquids.
Disk or cascade evaporators use the partial immersion of either disks mounted perpendicular to, or bars mounted parallel to a rotating shaft to carry films of liquid into a hot gas stream.
The most efficient method of transferring the energy of a heating medium to the liquor is direct injection of the heating medium. Because of the consequent contamination of the liquor with the heating medium, this method of heat transfer is of relatively less importance in the pharmaceutical industry and will not be discussed here.
The more useful methods for pharmaceutical products maintain purity at the expense of additional resistance to heat transfer by interposing a solid wall of some thermally conductive material between the heating medium and the liquor. The solid wall is usually metallic, but can be coated with materials such as glass, porcelain enamel, or polymers. Glass or ceramic themselves can be used for walls.
The solid wall can be the wall of the evaporator itself, as in jacketed evaporators. The area available for heat transfer in jacketed vessels, however, is quite limited. Jacketed vessels frequently incorporate some sort of internal agitator.
Heat transfer can be supplied from within a vessel by a heating coil, but again, the available heat transfer area is not large; however, such coils can be designed in ways that make their removal for cleaning relatively easy. The alternative is to have the heat exchange external to the main chamber of the evaporator.
Some applications use plate-type exchangers. In plate exchangers, the bounding surface may be in the plateand-frame form (parallel plates with the heating medium and the liquor flowing in alternate interstitial spaces), or in a spiral-plate configuration that contains a concentric pair of spiral passages. Such exchangers can be cleaned easily. They do, however, require a large gasketed area. Fig. 1 shows a typical plate-type evaporator.
The most common geometry for the separating surface between the heating medium and the liquor is probably that of tube bundles, which can be oriented either horizontally or vertically, with the liquor flowing on either the outside or the inside of the tubes. Depending on the application, the tube bundle can be either inside or outside the vessel in which the evaporation takes place.
The heating element of an evaporator is sometimes referred to as a calandria. Usually this term is applied to a heating system in which the liquor rises through a vertical tube bundle surrounded by the heating medium and then descends through a central well. The short-tube vertical evaporator is an early type that still sees considerable industrial use.
The heating element, a vertical bundle of tubes around a center well, is sometimes colloquially referred to as the basket. Circulation is upward through the tubes, the rising film mode, and then downward through the central well or downtake. Liquid boils in the tubes, which decreases the overall density therein and thus creates the driving force for circulation, since the density of the (non-boiling) fluid in the downtake is greater than that in the tubes.
Mechanical cleaning is fairly easy with such units, and the capital investment is relatively low. Circulation stops, however, if the heat input is interrupted, creating the danger of the settling of any solids suspended in the liquor. This type of unit is not well suited to viscous liquids because of the low heat transfer coefficients associated with the low velocities of natural convection. Short-tube vertical evaporators have largely been surpassed by other types, particularly for applications involving liquors that foam, deposit excessive scale, are excessively viscous, or are heat sensitive.
Long-tube vertical evaporators are normally the cheapest per unit of capacity. When operated in the rising film mode, temperature variation along the inside of the tubes is both substantial and difficult to predict. Thevariation in pressure from high at the bottom to low at the top normally means that the liquid enters the bottom of the tubes below its boiling temperature. The liquid is subsequently heated to boiling as it rises, and the boiling temperature simultaneously decreases as the pressure decreases toward the top of the tube (assuming any boiling point rise from increasing concentration is overshadowed by the effect of the reduced pressure on the boiling point).
By operating a long-tube evaporator in the falling film mode, the problem of temperature variation induced by pressure differences is mitigated. Here, a film of liquid surrounding a gas core flows down the walls of the tube, so pressure drop is very much less than in the rising film mode. The low residence time of the falling film units makes them useful for heat-sensitive materials, but the necessity of maintaining a film on the walls of the tubes makes feed distribution a problem. They are readily adapted to sanitary processing. Evaporators that combine rising film sections and falling film sections in the same unit are also available.
Forced circulation evaporators have relatively higher heat transfer coefficients, and are somewhat less subject to fouling, salting, and scaling. This advantage is offset by both the cost of external power required for the circulating machinery and a relatively high holdup.[8] At times they more frequently experience plugging from deposits detached from the walls of the unit by the force of the circulating fluid. The introduction of a pump may lead to mechanical problems, particularly with liquors that are slurries.
Liquor velocities required to prevent surface deposits are often greater than can be obtained with natural circulation at reasonably low temperature differences. In addition to mitigating scale formation, forced circulation also improves the heat transfer coefficient.
For viscous liquids, one way to increase the heat transferred is to improve the heat transfer coefficient by scraping or stirring the fluid adjacent to the wall, as in agitated film or wiped film evaporators. Accommodation of the mechanical devices used to mix the fluid close to the wall requires a fairly large diameter tube, so these devices tend to consist of only a single tube; thus, heat transfer area is relatively small. The introduction of moving mechanical parts may lead to maintenance problems.
In horizontal tube evaporators, the liquor is usually on the outside of the tubes and the heating medium on the inside. Rather than submerging the tubes, the boiling liquid is sometimes sprayed on the outside of the tubes. This gives a performance approaching that of falling film evaporators.
Evaporators can be operated at a variety of pressures. bReduced pressure, with its concomitant reduction in boiling temperature, offers advantages for heat-sensitive materials and materials that are sensitive to exposure to air.
Evaporation operations are often staged in multiple effect systems to achieve better efficiency. Such systems can have a variety of relative directions for flow of liquor and vapor. A typical example of such staging is illustrated in Figs. 2 and 3.
Detailed discussion of the advantages and disadvantages of various types of evaporators is available. A table summarizing the advantages and disadvantages of common types of evaporators also is available.
Since most evaporators are purchased from outside suppliers either prefabricated or on-site-fabricated, such suppliers can be an excellent source of information on selection of evaporator type. Suppliers and addresses can be found in the literature.
