The main processes in the distillation column. Industrial application of rectification. Theoretical foundations of the rectification process
Rectification is used to separate liquid mixtures into components or fractions that differ in volatility (fugacity), and is carried out by multiple two-sided mass and heat exchange between countercurrently moving steam and liquid flows - reflux.
The interaction of phases during rectification is the diffusion of a highly volatile component (l.c.) from liquid into vapor and a non-volatile component (l.c.) from vapor into liquid. The method of contacting the streams can be staged (in tray columns) or continuous (in packed columns).
The purpose of the contact devices (plates, packing) is to create conditions conducive to the maximum approximation of vapor and liquid flows. In order for these flows to exchange matter and energy, they must be nonequilibrium to each other. When vapor and liquid streams come into contact as a result of mass and heat exchange, the value of nonequilibrium decreases, then the streams are separated from one another, and the process continues by contacting these phases again at another adjacent stage, with other liquid and vapor streams. As a result of the contacting of liquid and vapor moving in counterflow along the height of the column, repeated many times on successive trays (stages), the composition of the interacting phases changes significantly: the vapor flow enriches the LLC while moving upward, and the liquid flow, flowing down, depletes it, i.e. e. it is enriched tl. With a sufficiently long contacting path of oppositely moving streams, it is possible to obtain steam leaving the upper part of the column, which is a more or less pure LLC, the condensation of which gives a distillate, and from the lower part of the column - a relatively pure HLC. , the so-called VAT residue.
Reflux is formed as a result of partial condensation of vapors coming out of the upper part of the column in special heat exchangers: dephlegmators - or is introduced into the column as food. To create a steam flow in the column, a certain amount of heat is introduced into its lower part by direct inlet of heating steam (in the case of open heating of the column) or by supplying it to a special heat exchanger, through the heat transfer surface of which heat is transferred to the boiling bottoms (in the case of closed heating).
More often the mixture to be separated (feed) in liquid, vapor or mixed form is fed into the middle of the column (Fig. 2) between the concentration, or strengthening and stripping, or exhaustive part of the column. The top tray of the stripping part of the column is called a feed tray. A column with a concentration and a distillation part is called a complete distillation column (Fig. 2a). In such a column, the most favorable conditions are created for obtaining both components of a binary mixture in an almost pure form, however, an independent action of the stripping and concentration columns is also possible. Such columns are called incomplete.
Figure: 2 Diagrams of rectification columns
1 - reflux condenser; 2 - column; A - the mixture to be separated; B - water; D - distillate; P - steam; O is the remainder.
From the lower part of the incomplete stripping column (Fig. 2b), almost pure HLC is removed in liquid form; above the upper tray, steam is obtained, which is somewhat enriched with HLC. Into an incomplete concentration column (Fig. 2c), the mixture to be separated is introduced in vapor form under its lower tray. From the upper part of the concentration column, it is withdrawn in a vaporous form, in the form of an almost pure LC, and from the lower tray, reflux is obtained, somewhat enriched in HLC. In contrast to the complete distillation column in incomplete columns for further enrichment of the distillate of the stripping column of the LLC. or the remainder of the concentration column t.l. their additional rectification is needed.
Refluxing, which is required for the rectification process, in the stripping columns is achieved by feeding liquid feed to the upper tray. In full and reinforcing columns, reflux is carried out due to part of the steam condensate leaving the top of the column. The rest of the steam forms the distillate - the top product of the column, so reflux and distillate withdrawal are quantitatively related.
The ratio of the amount of hot (at dew point) reflux or reflux (L)to the amount of distillate (D)called the reflux ratio (R):
R \u003d L / D \u003d (G - D) / D, (1)
where G is the amount of steam leaving the column.
The reflux ratio can vary from 0 to ∞. At R \u003d 0, there will be no mass transfer and no enrichment of the LLC steam. At R \u003d ∞, all the vapor condensate leaving the column is completely fed to reflux; in this case, the selection of distillate is zero, the column operates "for itself" (in a steady process, the bottom product of the column will have the same composition as the original feed). In practice, the column should operate at 0 The distillate can be removed after partial or complete condensation of steam (Fig. 3). Option 1 provides additional enrichment of the L.L.K. distillate. due to partial condensation of steam and mass transfer between reflux and steam during their counter-current movement in a reflux condenser. In the 2nd variant, the steam leaving the column, the distillate and the reflux have the same composition, and the reflux condenser does not give any strengthening effect. In the alcohol industry, the first option is usually used. Figure: 3. Methods for irrigation of columns: 1 - reflux condenser; 2 - column; 3 - capacitor. The heat of condensation of steam is usually removed by water, products to be heated, or air in special air reflux condensers. Open heating of columns is applicable when the heating steam does not adversely affect the quality of end products, does not interact with rectification products and does not form new, difficult to separate systems in the column. When heating is open, the heating steam condensate is mixed with the final separation product (residue). Indoor heating requires higher steam parameters. The process of mass transfer between vapor and liquid flows on contact devices is determined by the size of the phase contact surface (F m 2), the average concentration difference, or the average driving force of the process (∆С kg / kg), and the mass transfer coefficient referred to 1 m 2 of the phase contact surface [K kg / (m 2 * h)]. The mass transfer coefficient depends on the nature of the substance and the hydrodynamic mode of phase contact. The amount of a substance that has passed from one phase to another (in kg / h) is determined by the equality M \u003d K * F * ∆C (2) The design of the contact device should provide the greatest possible mass transfer on it. This is achieved primarily by creating a developed phase contact surface. Distillation column trays can be cap, lattice, flake, valve, etc. (Fig. 4). A packed column is a cylinder filled with a packing - bodies with a developed surface (rings, balls, saddles, nets, blocks, packages, slats, etc.). Steam and liquid come into contact on the surface of the packing in countercurrent movement. The operation of contact devices is evaluated by the steam and liquid throughput, the ability to separate the working mixture, the range of stable operation, hydraulic resistance, etc. The steam and liquid throughput determines the productivity of the columns, or the specific removal of the final product from a unit of column cross-section. The ability to separate the distilled mixture is called the efficiency of the contact device or the column as a whole and is usually estimated by the number of theoretical plates (concentration steps), or the number of transfer units. The efficiency of tray columns is usually evaluated by the number of theoretical trays (tb). Suppose the liquid entering the plate (Fig. 5, a)contains X i + 1l.l.k., and leaving it - X * i; steam passing through the tray contains respectively Y i and y * i +1 of the same component. If the tray provides vapor / liquid contact, resulting in vapor leaving the tray Y *. i +1 and liquid X * i will be in equilibrium, then such a tray has an efficiency equal to one theoretical tray. Figure: 4. Types of cymbals: a - sieve (scaly): 1 - drain glasses; 2 - without glasses (lattice); b - caps: 1 - single caps,;. 2 - multi-cap and valve disc valves; c: 1 - round; 2 - rectangular .
Figure: 5. Theoretical dish in the X-Y diagram In practice, this balance is almost never achieved. A theoretical tray is an ideal tray and serves as a benchmark for evaluating the performance of real trays. The measure of the efficiency of the real, or real, plate is its efficiency (efficiency). In practice, the efficiency of not an individual tray is determined, but the average efficiency of the plates of the entire column or its significant section, which is equal to the ratio of the number of trays (P)required to carry out a given separation of the mixture to the number of real ones (N) required for the same purpose: The value of the efficiency of the trays depends on their design, the diameter of the column, the inter-tray distance, the steam velocity, the loading of the column, the physical properties of the mixture to be separated and many other factors, therefore, the efficiency is usually determined empirically. The efficiency of the packed columns is assessed by the number of transfer units, representing the change in concentration in the column per unit of driving force. The most commonly used nozzle height is equivalent to one transfer unit (TUU). It varies widely depending on the design and size of the packing, as well as the hydrodynamic mode of operation; columns. For a shallow nozzle, VEEP can be several millimeters, for a large one (with a high throughput for steam and liquid) - 1-1.5 m. In alcohol production, the most widespread are cap (capsule) plates. Multi-cap trays are used in columns for separating liquids that do not contain suspended particles, single-cap trays are used for separating liquids with suspended particles. Less commonly, sieve trays are used, which have holes of 2.5-3.5 mm (for distilling the first of the mentioned liquids) and 8-12 mm (for the second). In recent years, new types of trays have begun to be used in the alcohol industry: lattice failures (without drain devices), flake and valve. They have higher steam and liquid carrying capacity. When choosing the type of tray, one takes into account its specific productivity, efficiency, design economy, as well as the ability to provide optimal operating conditions for the column for a given technological regime. The stable operation of the trays must correspond to such loads on steam and liquid, at which their most intense contact and high efficiency are achieved. At high loads on steam, a large carryover of liquid from the tray to the tray can occur, and liquid in excess of the permissible amount can accumulate on the tray. The upper steam load limit is characterized by "flooding" of the trays. An external sign of flooding is a sharp increase in pressure in the lower part of the column and a decrease in pressure in the upper part. With steam loads approaching the minimum allowable, part of the liquid (reflux) passes from the tray to the tray without coming into contact with the steam. High liquid loading can also lead to flooding of the column. The maximum permissible liquid load is determined by the amount required to create an active contact zone of the exchanging media. The work of the plates is greatly influenced by the inter-plate distance. It is defined in. first of all, the need to create conditions for the contact of vapor and liquid occurring in the zones of bubbling, foam and splashes. These zones are located sequentially above the tray and must fit between adjacent trays. The height of each zone is determined by the physical properties of the liquid to be separated, the design of the tray, the steam load, and is usually found empirically. When working with fluids that give loose foam, the entrainment of fluid mainly occurs due to foam flocks with high windage. For columns processing fluids that do not foam and do not contain suspended particles, the inter-plate distance is usually 178-230 mm; for columns processing liquids with suspended particles - 280-500 mm. The efficiency of the trays, due to different operating conditions of the columns, can vary in the range of 0.35-0.65. Rectification allows to obtain alcohol of high strength and purity. Both qualities depend on how well the person managing the process understands its essence. Therefore, everyone who wants to make pure and strong alcoholic beverages on a moonshine still needs to know the theory of rectification. Let's start with the distillation process, because it is he who is the predecessor of rectification. There is no exact information about who first invented distillation. V. Schneider, compiler of a dictionary of alchemical and pharmaceutical terms, believes that this merit belongs primarily to the Persians, who used distillation to obtain rose water (rose ether). It can be concluded that the history of distillation is more than 3500 years old. Originally, all processes of separation of mixtures into components were called distillation. As they were studied, the processes were classified and given a name. Thus, in the present time distillation is called the separation of substances based on the evaporation of a liquid and the subsequent condensation of vapors. Alambiks were the first distillation apparatus and have practically not changed structurally for several thousand years. They were originally used to make aromatic oils. Science did not stand still, the distillation process was carefully studied and improved. Since the beginning of the 16th century, there has been a large number of works on the selection of evaporation cubes and heating systems for apparatus. To ensure the continuous operation of the column, water and sand baths were used, wax candles were used. Only by 1415 was it first proposed to use thermal insulation, namely animal hair. At the end of the 16th century, the advantage of water cooling of the condenser was revealed, until that time the cooling was air. In the period from the 16th to the 19th century, the modernization of equipment was rapidly taking place. Based on the inertness of materials with respect to sublimated liquids, glass and ceramics, later stainless steel, were used as optimal in the distillation stills. In 1709, theories about reflux (the return of part of the condensed vapors to the column) first appeared. The result of all research and development was the invention of the first continuous distillation column by the French engineers Adam, Berard and Perrier, who received a patent for it in 1813. It still matches modern rectification columns. From this period begins the history of rectification in science and industry. There are various definitions of rectification. Rectification is a process of separating binary (two-component mixtures, for example, alcohol-water) or multicomponent mixtures due to countercurrent mass and heat exchange between vapor and liquid. Rectification - separation of liquid mixtures into practically pure components, differing in boiling point, by repeated evaporation of liquid and vapor condensation. Despite such complex formulations, there is nothing difficult in the rectification process. Having the necessary equipment and basic knowledge, it can be easily done in your kitchen. E. Krel in his works "Guide to laboratory distillation" outlined the basic principle of rectification: Metabolism (mass transfer and heat exchange) occurs by passing the vapor mixture through the column filler. The following factors influence the speed and quality of this process: It can be concluded that the process of alcohol rectification will proceed better under the following conditions: good diffusion, high concentration of the separated component, developed contact area. Krel paid special attention to the importance of the state of the interface and listed the factors that determine the rectification process: Depending on the applied contact devices, the columns are divided into poppet and packed ones. Mostly common in the oil refining industry and in large industries. Tray columns are a vertical pipe in which trays of different configurations are installed at a certain distance, where there is contact between the vapor and liquid phases. Lack of columns: high cost and large dimensions. Benefits: disc-type distillation column separates fractions finer. To date, packed columns are widely used. These are the same vertical pipes, only another contact device is installed in them - a nozzle. The attachments are divided into two types: Irregular - a disordered layer of bulk or filled inert material (for example, a spirally prismatic SPN packing). Benefits: light weight, large contact area. disadvantages: high resistance, difficulty in correct distribution of vapors and reflux. Regular - represents perforated nets and sheets arranged in cassettes (these include Panchenkov's regular wire nozzle (RPN). Benefits: high efficiency, low pressure drop. disadvantages: the packed distillation column showed no obvious shortcomings. Let's consider what happens in the column itself using the example of the equipment of the "Doctor Guber" Factory. There is no magic or secret technologies here, everything is very simple. Distillation columns for private use are vertical tubes with a diameter of 40 to 50 mm, a height of no more than 180 cm, filled with on-load tap-changers or SPN nozzles. These columns are equipped with a refrigerator or reflux condenser, as well as an alcohol sampling unit. Let us consider periodic rectification on a packed column with a regular on-load tap-changer packing, which everyone can repeat at home. When heating a cube with home brew, which is a multicomponent mixture, which, in addition to water and alcohol, includes fermentation by-products (aldehydes, acids, ethers, etc.), the process of boiling and evaporation of these components begins. The temperature of the beginning of the process can be different, it all depends on the qualitative and quantitative composition of the brew or raw alcohol. During the process, steam rises along the column, begins to warm it up and partially condense, thus forming a "wild phlegm". The formation of wild reflux occurs due to the cooling of the column body, due to heat losses to the environment. There are qualitative and quantitative losses in alcohol (up to 10%). In standard rectifiers, the problem of the formation of wild reflux is solved by thermal insulation of the column. The highly qualified specialists of the Dr. Guber Factory found another way to solve this problem by creating a Tornado Column. The structure of the column allows the rising steam to pass first along the outer contour of the column, while creating active heating. As a result, heat loss to the environment from the working section of the column becomes minimal. At the exit, the finished product is obtained with improved organoleptic and physico-chemical characteristics. After warming up the column, the vapors reach the cooler or reflux condenser, in which they condense and return to the column in the form of reflux. The reflux stream is directed towards the vapor rising along the column. Mass and heat exchange takes place. The temperature during the rectification of alcohol is of key importance: the reflux on its way from a zone with a low temperature to a zone of higher temperatures absorbs high-boiling components (fusel oils) from the vapor stream and releases low-boiling components (alcohol). Since these processes take place at the interface, it is very important to create the largest possible contact surface. For this purpose, Dr. Huber's distillation columns are equipped with an on-load tap-changer, which creates the maximum contact surface along its entire length. The quality of the obtained alcohol depends on the rate of selection. Namely, the more reflux is taken from the column, the worse is the process of mass transfer, therefore, the strength of alcohol at the exit from the column decreases. And vice versa, the less phlegm is taken, the better the process of mass transfer and the increase in the strength of the final product. To control the rate of alcohol withdrawal, needle valves for fine adjustment and sight glasses are installed on the columns. It is not enough to create a developed contact surface; it is necessary to irrigate it correctly. A wall effect takes place in packed columns. Reflux does not pass through the packing, but flows down the walls of the column, as a result of which its efficiency decreases. With the correct filling of the column, this effect is minimal; it is practically absent in the Tornado column, where a bubble cap tray with a central spout is installed. As a result, the reflux is directed exactly to the packing and the maximum efficiency of this column is achieved. As for the diameter and height of the column, according to Steadman and McMahon, the diameter of the packed columns has little effect on the quality of the separated mixtures. Column height. We are talking about its working part (the part of the column that is filled with packing) should be no more than (6-8) xD. If the height is greater than this expression, then the columns are filled in sections to avoid the wall effect. When choosing a column, pay attention to the following points: P.S. Alcohol rectification is not a complicated process, and if you have the necessary equipment, it can be easily done at home. By 2016, the range of rectifying equipment is growing infinitely. Despite the small design differences of all devices, the rectification process remains unchanged and its quality will primarily depend on the knowledge and experience of the person controlling the process. Distillation (distillation and rectification) is one of the most common methods for separating liquid homogeneous mixtures consisting of two or more components. In a broad sense, distillation is a process involving the partial evaporation of the mixture to be separated and the subsequent condensation of the resulting vapors, carried out once or repeatedly. As a result of condensation, a liquid is obtained, the composition of which differs from the composition of the initial mixture. Rectification is a process of multiple partial evaporation of liquid and vapor condensation. The process is carried out by contacting streams of vapor and liquid having different temperatures, and is usually carried out in columns. At each contact, a predominantly low-boiling component (LCC) evaporates from the liquid, with which the vapors are enriched, and from the vapors, a predominantly high-boiling component (VCC) condenses, passing into the liquid. Such a two-way exchange of components, repeated many times, makes it possible, in the end, to obtain pairs that are almost pure CQ. After condensation in a separate apparatus, these vapors give distillate (rectified) and reflux - liquid returned to reflux the column and interact with rising vapors. Vapors are obtained by partial evaporation from the bottom of the column of the residue, which is almost pure VCC. The rectification processes are carried out in apparatuses, the technological scheme of which depends on the purpose of the apparatus and the pressure in it, and the design depends on the way of organizing the phase contact. With a stepwise implementation of the rectification process in column apparatus, the contact of vapor and liquid can occur in countercurrent (on discontinuous-type trays), in cross-flow (on bubble cap trays), in co-current (jet trays). If the rectification process is carried out continuously throughout the entire volume of the column apparatus, then the contact of vapor and liquid during the movement of both phases can occur only in a counterflow. Modern rectifiers can be classified according to the process application, pressure and internal device that provides contact between vapor and liquid. According to their technological designation, rectifiers are subdivided into columns of atmospheric vacuum units, thermal and catalytic cracking, secondary distillation of petroleum products, as well as for gas rectification, stabilization of light petroleum fractions, etc. The following requirements are imposed on modern rectifiers: high separation capacity and productive capacity, sufficient reliability and flexibility in operation, low operating costs, low weight and simplicity, technical design. The latter requirements are no less important than the first ones, since they not only determine capital costs, but also significantly affect the value of operating costs, ensure the ease and convenience of manufacturing the apparatus, installation and disassembly, repair, control, testing, as well as operational safety and etc. In addition to the requirements listed above, rectifying apparatus must also meet the requirements of state standards, departmental norms and inspections of Gostekhnadzor. The technological scheme of the apparatus depends on the composition of the mixture to be separated, the requirements for the quality of the products obtained, on the possibilities of reducing energy costs, the purpose of the apparatus, its place in the technological chain of the entire installation and on many other factors. The process of rectification of liquid mixtures is carried out in rectification units consisting of several apparatuses. Let us consider the principle of separation of a two-component mixture by rectification using the example of the operation of such an installation (Fig. 10.1). The mixture to be separated is continuously fed into the distillation column through an inlet located slightly above the middle of the column body. The introduced liquid mixture is lowered through contact devices (trays) to the bottom of the column, called a cube. Steam rises towards the liquid flow, which is formed as a result of liquid boiling in the column bottom. The resulting vapors contain mainly NCC and a number of VKKs. When steam interacts with liquid on the trays of the column, the VCC condenses and is carried down the column by the liquid flow. Due to this, the number of NCC... Thus, as the vapors rise, they are enriched NCC, while the liquid flowing down is enriched VKK. The initial mixture from the intermediate tank 1 by the centrifugal pump 2 is fed to the heat exchanger 3, where it is heated to the boiling point. The heated mixture enters the separation into the distillation column 5 on the feed plate, where the composition of the liquid is equal to the composition of the initial mixture. Flowing down the column, the liquid interacts with the rising vapor formed during the boiling of the bottom liquid in the boiler 4. The initial composition of the vapor is approximately equal to the composition of the bottom residue, that is, it is depleted in a highly volatile component. As a result of mass transfer with liquid, the vapor is enriched with a highly volatile component Figure: 10.1. Schematic diagram of the rectification plant: 1 - container for the original mixture; 2, 9 - pumps; 3- heat exchanger-heater of raw materials; 4 - a boiler; 5 - rectification column; 6 - reflux condenser; 7 - distillate refrigerator; 8 - container for collecting distillate; 10 - cooler of vat liquid; 11 - container for vat liquid. For a more complete enrichment, the upper part of the column is refluxed in accordance with a given reflux ratio with liquid (reflux), which is obtained in reflux condenser 6 by condensation of vapor leaving the column. Part of the condensate is removed from the reflux condenser in the form of a finished product of separation - a distillate, which is cooled in a heat exchanger 7, and sent to an intermediate tank 8. From the bottom part of the column pump 9 continuously withdraws bottoms liquid - a product enriched with a non-volatile component, which is cooled in heat exchanger 10 and sent to tank 11. Thus, in the distillation column, a continuous non-equilibrium process of separating the initial binary mixture into a distillate with a high content of a volatile component and a bottom residue enriched with a non-volatile component is carried out. The rectification of multicomponent mixtures, which are more common in practice than two-component mixtures, proceeds according to the scheme discussed above, although the number of equipment used in this case increases. In rectification plants, mainly two types of apparatus are used: columns with stepped phase contact (disc-shaped) and continuous contact (film and packed). The rectification of multicomponent mixtures can be carried out in a different sequence, using many simple columns (one less than the number of components in the initial mixture) and using one complex column. Tray columns are mainly used for the rectification process. They are equipped with horizontal trays with devices that ensure good contact between liquid and vapor. The column diameter is determined depending on the productivity of the installation and the vapor velocity in the column, which is selected in the range of 0.6 - 1.0 m / s... Distillation columns of various sizes are used: from small columns with a diameter of 300 - 400 mm up to high-performance plants with columns with a diameter of 6, 8, 10, 12 m and more. The height of the column depends on the number of trays and the distance between them. The smaller the distance between the trays, the lower the column. However, with a decrease in the distance between the trays, the entrainment of splashes increases and there is a danger of transferring liquid from the lower trays to the upper ones, which significantly reduces the efficiency. installation. The distance between the trays is usually taken depending on the diameter of the column, taking into account the possibility of repair and cleaning the column. The recommended distances between the distillation columns' trays, depending on their diameter, are given below: Column diameter, mm up to 800, 800 - 1600, 1600 - 2000 Distance between plates, mm 200 -350, 350 - 400, 400 - 500 Column diameter, mm from 2000 - 2400 and over 2400 Distance between plates, mm 500 - 600, over 600. The number of plates in the distillation column or the height of the packing are determined by technological calculation; it depends on the physicochemical properties of the separated components, the required separation purity and efficiency. plates. Usually rectification columns have 10 - 30 trays, but columns for separating mixtures with close boiling points have hundreds of trays and, accordingly, have a height of up to 30 - 90 m. The distillation columns are usually operated at atmospheric or light overpressure. Vacuum columns and columns operating at elevated pressure are of limited use. Vacuum rectification is used when one wants to lower the temperature in the column, which is necessary when separating components with a high boiling point or substances that are unstable at high temperatures. High pressure rectification is used to separate liquefied gases and highly volatile liquids. Rectification -the process of separation of mixtures of mutually soluble components differing in boiling points by countercurrent multiple contacting of nonequilibrium liquid and vapor. Contacting is carried out, as a rule, in column devices on disc-shaped or packed contact devices in countercurrent flow - steam from bottom to top, liquid from top to bottom. Column apparatus is a vertical steel pipe with contact devices located inside. In tray columns, contact occurs stepwise in separate stages called trays (perforated, bubble cap, valve, etc.), usually by bubbling steam through the liquid layer or by spray mixing, or in another way that provides the most efficient heat and mass transfer. In packed columns, contact is carried out continuously between steam and liquid film in the packed bed with a developed surface, which is filled with the column (crushed stone, rings, springs, nets, etc.). The liquid, which is relatively rich in low-boiling components and has a relatively lower temperature, enters the contact device from the top. The steam, rich in high-boiling components and having a higher temperature, enters the contact device from below. On the contact device, liquid and vapor strive for equilibrium through heat and mass transfer. If equilibrium is reached between the vapor and liquid leaving the contact device, then such a contact device is called a theoretical stage or theoretical plate. Simple distillation ("Moonshine still") provides a one-time good contact of liquid and vapor and is equivalent to one theoretical stage. Real trays of industrial columns have an efficiency of 0.3 ... 0.8 theoretical stage. For packed columns, there is a quantity called the height of the equivalent theoretical tray - this is the height of the packed bed, the mass transfer efficiency of which is equivalent to one theoretical stage. This height can be 100 ... 600mm. On contact devices, the vapor is enriched with a low-boiling component, and the liquid with a high-boiling one. Passing successively a number of stages, liquid and vapor reach the specified concentrations of the components. Low-boiling components are concentrated at the top of the column, high-boiling components at the bottom. By increasing the number of steps, you can get any given clarity of separation of components. Along the height of the column, the concentrations of the components change sometimes very nonlinearly. In devices continuous rectification the raw material is introduced at about the middle of the column height, i.e. on the plate where the concentrations of the components are approximately equal to those of the raw materials. Distillate rich in low-boiling components is taken from the top of the column. A residue rich in high-boiling components is taken from the bottom. Vapors from the upper tray of the column are cooled in a condenser, part in the form of vapors or liquid is taken as a distillate, the rest is returned to the column in the form of liquid. The liquid from the lower tray is heated in a boiler, part of the liquid is taken as a bottom product (residue), the rest is returned to the column in the form of vapor. The ratio of the mass flow rate of the liquid coming from the condenser to the column to the mass flow rate of the distillate is called reflux... The ratio of the mass flow rate of vapor from the boiler to the mass flow rate of the residue is called steam number... These numbers characterize the operating mode of the upper (above supply) and lower (below supply) sections of the column. The higher the reflux (and vapor) number, the easier (with a smaller number of stages) the specified clarity of separation of the mixture by rectification is achieved, but the specific energy consumption also increases and the productivity of the column decreases. The reflux (and vapor) number cannot be less than a certain minimum, at which the specified clarity of rectification is not achieved with an arbitrarily large number of stages. When periodic rectification a portion of the raw material is loaded into a boiler of the corresponding volume (called the column cube), no raw material is added during the rectification process, and the composition of the bottoms residue is continuously changing from the composition of the raw material to a given high-boiling residue. Accordingly, a distillate of a time-varying composition is taken from the top of the column. If the number of components of the mixture is small (2 ... 5), and the number of stages and reflux ratio are sufficient for a relatively clear separation, then the composition of the distillate and the temperature on the upper plate change stepwise, at first the distillate consists of the concentrated lowest-boiling component (we will call it the first component) , then a short transition period follows, when the distillate is a mixture of variable composition, in which the concentration of the first component decreases, and the concentration of the second component increases, then the distillate consists of a concentrated second component, etc. for all components. Transition distillates are traditionally called bad cuts and are mixed with the next batch of raw materials. If the clarity of separation is low and / or the number of components is large (oil mixtures), then the graded composition of the distillate becomes imperceptible, the composition of the distillate and the temperature on the upper tray change continuously. Multicomponent mixtures can be separated into individual components by repeated rectification of narrow distillate fractions containing already a small number of components. Features of rectification of oil mixtures are due to the requirements for the quality of separation into fractions and the fact that oil mixtures consist of thousands of components. The multicomponent composition of oil mixtures determines the continuous composition of the distillate during periodic rectification for any practically achievable number of stages and reflux ratio. The quality of separation into fractions is determined by the results of simple distillation (ASTM D86 standard) of samples of this fraction, at temperatures of 5% and 95% of the distillate. The standards for the corresponding petroleum products determine that the overlap of temperatures of 95% and 5% of the distillate between adjacent fractions should be no more than 10 ... 15C. For example, if 95% of the gasoline fraction obtained on a given column is distilled off according to D86 at no more than 180C, then 5% of the diesel fraction obtained on the same column must be distilled off according to D86 at least at 170C. In the rectification column (Figure 2.2), vapors move from below, and a liquid is fed from above towards the vapors, which is an almost pure low-boiling component (reflux). On each tray in the column, vapor and liquid come into contact. Moreover, from steam predominantly the high-boiling component is condensed, and the predominantly low-boiling component evaporates from the reflux (Figure 2.3). As a result, the compositions of vapor and liquid change, approaching equilibrium. The vapor becomes richer in a low-boiling component, and the liquid is saturated with a high-boiling component. The steam is condensed in a condenser. Part of this condensate goes in the form of reflux to the reflux of the column, and the other part - the distillate is taken as a finished product. The liquid leaving the bottom of the column is called the bottom of the column. Figure 2.2 - Scheme of the rectification column Figure 2.3 - Scheme of interaction of liquid and vapor Usually the rectifier consists of two parts - upper and lower, each of which is an organized surface of phase contact between vapor and liquid. Initial mixture F is fed to the middle part of the column and as a result of the process is divided into two parts: the part enriched with a low-boiling component (LB) - distillate D, and the part depleted by the NK - the VAT residue W. When carrying out continuous rectification, the initial mixture is introduced onto a feed tray, which divides the column into two parts (Figure 2.4). In the upper part of the column, the greatest possible strengthening of the vapors must be ensured, i.e. enrichment of their NR so that vapors close in composition to pure NR are sent to the reflux condenser. Therefore, this part of the column is called strengthening. At the bottom of the column (from the feed to the bottom tray) it is necessary to remove the low-boiling component from the liquid to a minimum, i.e. to exhaust the liquid so that the liquid that is close in composition to the pure high-boiling component flows into the boiler. Accordingly, this part of the column is called complete. 1 - rectification column (1a - reinforcing part, 1b - exhaustive part); 2 - a boiler; 3 - reflux condenser; 4 - phlegm divider; 5 - heater of the initial mixture; 6 - distillate refrigerator (or condenser refrigerator); 7 - refrigerator of the remainder (or bottom product); 8.9 - collections; 10 - pumps Figure 2.4 - Scheme of a continuous rectification plant Steam for feeding the distillation column is formed in the still by evaporation of part of the liquid entering the still; liquid for irrigation of the apparatus W (reflux) is obtained in a reflux condenser by condensation of steam having a composition similar to that of the distillate. The heat required to vaporize the mixture is transferred to it in the cube. In the dephlegmator, heat is removed, as a result of which the vapors entering it are completely or partially condensed. Material balance equations for a continuous distillation column: where F,
D,
W
- mass or molar consumption of food, distillate and vat residue; Working line equations: a) the upper (reinforcing) part of the distillation column b) the bottom (exhaustive) part of the column where y and x - non-equilibrium concentrations variable along the column height, mole fractions of a volatile component in steam and in liquid in a given column section; R \u003d F/
D - reflux number; In a rectification column, in contrast to an absorption column, based on theoretical assumptions, the total molar flow rates of vapor and liquid are taken to be constant along the column height, in accordance with this, molar rates and concentrations are used in equations (2.3) and (2.4). In the upper part of the column, above the inlet of the initial liquid mixture, the molar liquid flow rate constant along the column height is Rectification history
Rectification concept
Rectification process
Types of rectification columns
Tray distillation columns
Packed distillation columns
Distillation column processes
How to choose a distillation column
2.3.3 Continuous rectification
,
(2.1)
,
(2.2)
- the content of the low-boiling component in the feed, distillate and distillation residue, mass or mole fractions.
,
(2.3)
,
(2.4)
–Relative (per 1 kmol of distillate) molar feed consumption.
, at the bottom of the column it is equal to. The molar steam flow rate constant along the height is the same in the upper and lower parts of the column.