Distillation is a separation process in which each component in the mixture is separated by taking advantage of the different volatility of each component. The common equipment includes plate distillation tower and packed distillation tower.The principle and equipment process of precision distillation are the same as those of ordinary distillation, except that the relative volatility between components in the system to be separated is small (<1.05~1.10), so efficient precision packing is used to realize the separation and purification of components to be separated.In the natural essential oil, which is the raw material for the production of isolated perfumes, isomers often coexist. For example, citronellol and rosanol, which exist in natural essential oils such as geranium oil, rose oil and rose grass oil, are optical isomers.The boiling point difference of these isomers is relatively small, and it is difficult to realize the effective separation of this isolated flavor with ordinary distillation process. Therefore, precision distillation is widely used in the production of isolated flavor.High efficiency packing used in precision distillation tower includes bulk packing such as Dixon packing, McMahon packing and wire mesh packingCorrugated packing(Sulyer filler) is the representative regular filler.These packings have large specific surface, good wettability, small liquid holding capacity and fluid resistance, but the diameter of bulk packed tower should not be too large, otherwise the mass transfer and separation efficiency will drop sharply, which is the "amplification effect" of packed tower.The amplification effect of structured packing is small, so the application of corrugated structured packing is more and more extensive.[1]
Distillation is usually carried out in a distillation tower. The gas-liquid two phases are contacted by countercurrent to conduct interphase heat and mass transfer.The volatile components in the liquid phase enter the gas phase, and the nonvolatile components in the gas phase enter the liquid phase, so nearly pure volatile components can be obtained at the tower top, and nearly pure nonvolatile components can be obtained at the tower bottom.The feed liquid is added from the middle of the tower, and the tower section above the feed inlet further thickens the volatile components in the rising steam, which is called the distillation section;The tower section below the feed inlet extracts volatile components from the descending liquid, which is called the stripping section.The steam from the tower top is condensed, and part of the condensate returns to the distillation tower from the tower top as the reflux liquid, and the rest of the distillate is the product from the tower top.The liquid drawn out from the tower bottom is partially gasified through the reboiler, and the vapor rises along the tower. The remaining liquid is used as the product at the tower bottom.The ratio of the amount of liquid reflux into the tower top to the amount of product overhead is called reflux ratio, which will affect the separation effect and energy consumption of distillation operation.
classification
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According to the operation mode, distillation can be divided into continuous distillation and intermittent distillation;According to the composition of the mixture, it can be divided into binary distillation and multi-component distillation;It can be divided into ordinary distillation and special distillation (including extractive distillation, azeotropic distillation and salt distillation) according to whether or not additives affecting gas-liquid balance are added to the mixture.If there is chemical reaction, it is called reactive distillation.In nonferrous metallurgy, distillation has been successfully used for refining crude zinc.In industry, metals are often converted into chlorides and then rectified
Equipment and process
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doubleComponentsThe separation of mixed liquid is the simplest distillation operation.typicalDistillation equipmentyesContinuous distillationDevice (Figure 1), consisting ofDistillation tower、Reboiler、condenserEtc.Two phase contact of steam and liquid supply for distillation towerInterphase mass transferThe condenser on the top of the tower partially condenses the steam, part of the condensate returns to the top of the tower as the reflux, and the rest of the distillate is the overhead product.
The reboiler at the bottom of the tower partially vaporizes the liquid, the vapor rises along the tower, and the remaining liquid is used as the product at the bottom of the tower.The feed is added in the middle of the tower, the liquid in the feed and the liquid from the upper tower section descend along the tower, and the vapor in the feed and the vapor from the lower tower section rise along the tower.In the whole distillation column, the vapor and liquid two phase countercurrent contact for interphase mass transfer.liquid phaseThe volatile components in the vapor phase enter the vapor phase, and the nonvolatile components in the vapor phase enter the liquid phase.To not formAzeotropeOfSystemAs long as the design and operation are proper, the distillate will be a highly pure volatile component,Bottom productIt will be a high purity volatile component.The tower section above the feed inlet makes the rising steam volatileComponentsFurther concentration, called distillation section;The tower section below the feed inlet extracts volatile components from the descending liquid, which is called the stripping section.The combination of two stages of operation can completely separate the two components in the liquid mixture to produce the requiredpurityTwo products of.When makingnObtained by relatively complete separation of component mixturenFor high purity single component productsn-1 tower.
The key to complete separation of liquid mixture by distillation is the application of reflux.Reflux includes the return of high concentration volatile component liquid at the tower top and high concentration volatile component vapor at the tower bottom to the tower.Vapor liquid reflux forms vapor liquid two phases in countercurrent contact, so relatively pure single component products can be obtained at both ends of the tower.The ratio of the amount of liquid flowing back into the tower from the tower top to the amount of product from the tower top is calledReflux ratioIt is an important control parameter of distillation operation, and its change affects the separation effect andenergy consumption。
Operation evaluation
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The main indicators for evaluating distillation operation are: ①purity。Plate towerNumber of trays in orPacked towerThe height of the middle filling layer, and the position andReflux ratioAnd has a certain impact on the purity of the product.Adjust the reflux ratioDistillation towerMain means used to control product purity during operation. ②Componentsrate of recovery.This is the ratio of the component content in the product to the component content in the feed liquid. ③Total operating expenses.Mainly includingReboilerHeating costscondenserCooling costs andDistillation equipmentThe first two expenses are directly affected by the change of reflux ratio during operation.In addition, even with the same heating and cooling capacity, the heating cost and cooling cost also depend on the boiling temperature andCondensation temperatureAnd change, especially when not usedsteamAs a heating agent or when air or cooling water cannot be used as a coolant, these two costs will be greatly increased.Select appropriate operating pressure, sometimes it can avoid using high temperature heating agent or low temperature coolant (or refrigerant), but increase pressurization or pumpingvacuumOperating expenses of.
Distillation calculation
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MainlyDistillation towerCalculation of.WhetherPlate towerorPacked towerThe number of theoretical plates is usually calculated according to the concept of graded contact mass transfer.For doubleComponentsFor the design calculation of distillation tower, usually the given design conditions are: the amount of liquid mixture (feed liquid)FAnd concentrationxF (with volatile componentsmoleFraction), as well as the concentration of products at the top and bottom of the towerxD andxw。Calculate the number of theoretical plates requiredNT and actual number of platesNP 。ReasonableReflux ratio。Number of theoretical traysThe calculation methods of are:
graphic method
The most commonly used method is the McCabe Tilly graphic method (the double diagram designed by W.L. McCabe and E.W. Tilly in 1925ComponentsGraphical method of distillation theoretical plate calculation) is used for calculation of two-component distillation.This method assumes that the vapor phase flowing through the distillation sectionmoleflowVandliquid phaseMolar flowLAnd vapor liquid two-phase flow in stripping sectionV'andL'remain constant.This assumption is often referred to asConstant molar flow assumption, which is applicable to the mole of two components in the feed liquidLatent heat of vaporizationA system that is roughly equal, has little thermal effect when mixing, and has similar boiling points of the two components.The graphical method is based on compositionMaterial balanceandVapor-liquid equilibriumRelationship.
Strict calculation method
With the maturing of distillation technology and the expansion of production scale, it has special functions such as multi strand feeding and side line extraction, as well as side tower and middle towerReboilerAnd so onDistillation towerAppear one after another.Nowadays, it is more and more necessary to make strict calculation for distillation to understand the changes of temperature, flow and concentration in the tower, so as to achieve more reasonable design and operation.The application of electronic computers provides conditions for strict calculation.All strict calculation methods are based on four typesbasic equation : i.eComponentsMaterial balanceType, steamliquid phaseEquilibrium relation, normalization equation (components in vapor phase and liquid phasemoleThe sum of fractions is 1) andHeat balanceEquation.These equations can be established for each theoretical plate to form a high-dimensional equation set, and then solved by electronic computer.According to different specified conditions, in principle, this equation group can be used for the design of new towers or the calculation of the operating performance of existing towers.
