1、Unit 1 Unit Operations,Arp 26th，2011 Ying, Xiaoguang,lesson 1 Classification of Unit Operations,Fluid flow This concerns the principles that determine the flow or transportation of any fluid from one point to another. Heat transfer This unit operation deals with the principles that govern accumulati
2、on and transfer of heat and energy from one place to another. Evaporation This is a special case of heat transfer, which deals with the evaporation of a volatile solvent such as water from a nonvolatile solute such as salt or any other material in solution. Drying In this operation volatile liquids,
3、 usually water, are removed from solid materials,Distillation This is an operation whereby components of a liquid mixture are separated by boiling because of their differences in vapor pressure. Absorption In this process a component is removed from a gas stream by treatment with a liquid. Membrane
4、separation This process involves the diffusion of a solute from a liquid or gas through a semipermeable membrane barrier to another fluid. Liquid-liquid extraction In this case a solute in a liquid solution is removed by contacting with another liquid solvent which is relatively immiscible(不混溶的) wit
5、h the solution.,Liquid-solid leaching(浸提) This involves treating a finely divided solid with a liquid that dissolves out and removes a solute contained in the solid. Crystallization This concerns the removal of a solute such as a salt from a solution by precipitating(沉淀) the solute from the solution
6、. Mechanical physical separation These involves separation of solids, liquids, or gases by mechanical means, such as filtration, settling, and size reduction, which are often classified as separate unit operations.,Lesson 2 Fractional distillation,Fractional distillation is the separation of a mixtu
7、re of compounds by their boiling point, by heating to high enough temperatures. Fractional Distillation in a Laboratory Apparatus,conical flask,1. The apparatus is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few anti bumping granules(沸石), and the fracti
8、onating column is fitted into the top. 2. As the mixture boils, vapor rises up the column. The vapor condenses on the glass platforms inside the column, and runs back down into the liquid below, refluxing distillate. 3. Only the most volatile of the vapors stays in gaseous(气态) form all the way to th
9、e top. The vapor at the top of the column will be almost pure ethanol. 4. This then passes into the condenser, which cools it down until it liquefies(液化). The process continues until all the ethanol boils out of the mixture. 5. This point can be recognized by the sharp rise in temperature shown on t
10、he thermometer, from the boiling point of ethanol to the boiling point of water.,Example Consider the distillation of a mixture of ethanol and water as an example. Ethanol boils at 78.5oC whilst water boils at 100oC. So by gently heating the mixture, the ethanol will boil off first. Some mixtures fo
11、rm azeotropes(共沸), where the mixture boils at a lower temperature than either component. In the ethanol example, a mixture of 95% ethanol and 5% water boils at 78.2C, so the ethanol cannot be completely purified by distillation.,Industrial uses of Fractional DistillationOil refinery,The most importa
12、nt industrial application of fractional distillation is the distillation of crude oil. The process is similar in principle to the laboratory method described above except for scale, continuous feed and operation, and the fact that crude oil has many different compounds mixed together. The fractionat
13、ing column has outlets at regular intervals up the column which allow the different fractions to run out at different temperatures, with the highly volatile gases coming out the topmost outlet graduating to the less volatile road tar, (bitumen沥青) coming out at the bottom.,Lesson 3 Crystallization,Cr
14、ystallization is a technique which chemists use to purify solid compounds. It is one of the fundamental procedures each chemist must master to become proficient in the laboratory. Crystallization is based on the principles of solubility: compounds (solutes) tend to be more soluble in hot liquids (so
15、lvents) than they are in cold liquids. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound. Impurities are excluded from the growing crystals and the pure solid crystals can be separated from the dissolved impurities by f
16、iltration.,Lesson 4 Membrane Separation,1 What is a Membrane? The membrane can be defined essentially as a barrier, which separates two phases and restricts transport of various chemicals in a selective manner. A membrane can be homogenous or heterogeneous, symmetric or asymmetric in structure, soli
17、d or liquid, can carry a positive or negative charge or be neutral or bipolar. Transport through a membrane can be effected by convection or by diffusion of individual molecules, induced by an electric field or concentration, pressure or temperature gradient. The membrane thickness may vary from as
18、small as 100 micron to several mms.,2 Membrane Separation Technology A membrane separation system separates an influent stream into two effluent streams known as the permeate and the concentrate. The permeate is the portion of the fluid that has passed through the semi-permeable membrane. Whereas th
19、e concentrate stream contains the constituents that have been rejected by the membrane.,Membrane separation process enjoys numerous industrial applications with the following advantages: Appreciable(可观的) energy savings Environmentally benign(温和) Clean technology with operational ease Replaces the co
20、nventional processes like filtration, distillation, ion-exchange and chemical treatment systems Produces high, quality products Greater flexibility in designing systems.,Membrane Separation Processes Various types of membrane separation have been developed for specific industrial applications. Some
21、of the widely used processes are discussed hereunder:,Reverse Osmosis (RO) Unlike water filtration, that can only remove some suspended materials larger than 1 micron, the process of reverse osmosis (RO) will eliminate the dissolved solids, bacteria, viruses and other germs contained in the water. R
22、O is essentially a pressure driven membrane diffusion process for separating dissolved solutes. The RO is generally used for desalination seawater for its conversion into potable(可饮用) water. The salient(显著) features of the process are that it involves no phase change and it is relatively a low energ
23、y process.,RO membranes have the smallest pore structure, with pore diameter ranging from approximately 5-15 A (0.5 nm - 1.5 nm). The extremely small size of RO pores allows only the smallest organic molecules and unchanged solutes to pass through the semi-permeable membrane along with the water. Gr
24、eater than 95-99% of inorganic salts and charged organics will also be rejected by the membrane due to charge repulsion (斥力) established at the membrane surface.,Nanofiltration (NF)Nanofiltration is a form of filtration that uses membranes to separate different fluids or ions. NF is typically referr
25、ed to as “loose” RO due to its larger membrane pore structure as compared to the membranes used in RO, and allows more salt passage through the membrane. Because it can operate at much lower pressures, and passes some of the inorganic salts, NF is used in applications where high organic removal and
26、moderate inorganic removals are desired.,NF is capable of concentrating sugars, divalent (二价) salts, bacteria, proteins, particles, dyes and other constituents that have a molecular weight greater than 1000 Daltons(道尔顿). An advantage of NF over RO is that NF can typically operate at higher recoverie
27、s, thereby conserving total water usage due to a lower concentrate stream flow rate. NF is not effective on small molecular weight organics, such as methanol.,Ultrafiltration (UF)Ultrafiltration is most commonly used to separate a solution that has a mixture of some desirable components and some tha
28、t are not desirable. UF is somewhat dependent on charge of the particle, and is much more concerned with the size of the particle. Typical rejected species include sugars, bio-molecules, polymers and colloidal particles.,The driving force for transport across the membrane is a pressure differential.
29、 UF processes operate at 2-10 bars though in some cases up to 25-30 bars has been used. UF processes perform feed (锅炉给水) clarification, and is typically not effective at separating organic streams.,Microfiltration (MF) This is by far the most widely used membrane process with total sales greater tha
30、n the combined sales of all other membrane processes. Microfiltration has numerous small applications. It is essentially a sterile filtration with pores (0.1-10.0 microns) so small that micro-organisms cannot pass through them.,Microfiltration is a process of separating material of colloidal size an
31、d larger than true solutions. A MF membrane is generally porous enough to pass molecules of true solutions, even if they are large. Microfilters can also he used to sterilize solutions, as they are prepared with pores smaller than 0.3 microns, the diameter of the smallest bacterium, pseudomonas(假单孢菌
32、).,Lesson 5 Supercritical Fluid Extraction,1 Introduction of the physico-chemical properties of the supercritical fluids A pure supercritical fluid (SCF) is any compound at a temperature and pressure above the critical values (above critical point). Above the critical temperature of a compound the p
33、ure, gaseous component cannot be liquefied regardless of the pressure applied. The critical pressure is the vapor pressure of the gas at the critical temperature.,In the supercritical environment only one phase exists. The fluid, as it is termed, is neither a gas nor a liquid and is best described a
34、s intermediate to the two extremes. This phase retains solvent power approximating liquids as well as the transport properties common to gases.,A comparison of typical values for density, viscosity and diffusivity of gases, liquids, and SCFs is presented in Table 1.,Table 1. Comparison of physical a
35、nd transport properties of gases, liquids, and SCFs,The critical point (C) is marked at the end of the gas-liquid equilibrium curve, and the shaded area indicates the supercritical fluid region. It can be shown that by using a combination of isobaric (恒压) changes in temperature with isothermal(等温) c
36、hanges in pressure, it is possible to convert a pure component from a liquid to a gas (and vice versa) via the supercritical region withoutincurring a phase transition.,The behavior of a fluid in the supercritical state can be described as that of a very mobile liquid. The solubility behavior approa
37、ches that of the liquid phase while penetration into a solid matrix is facilitated by the gas-like transport properties. As a consequence, the rates of extraction and phase separation can be significantly faster than for conventional extraction processes. Furthermore, the extraction conditions can b
38、e controlled to effect a selected separation.,Supercritical fluid extraction is known to be dependent on the density of the fluid that in turn can be manipulated(操控) through control of the system pressure and temperature. The dissolving power of a SCF increases with isothermal increase in density or
39、 an isopycnic (恒密度) increase in temperature. In practical terms this means a SCF can be used to extract a solute from a feed matrix as in conventional liquid extraction. However, unlike conventional extraction, once the conditions are returned to ambient the quantity of residual solvent in the extra
40、cted material is negligible(可以忽略，微不足道).,The basic principle of SCF extraction is that the solubility of a given compound (solute) in a solvent varies with both temperature and pressure. At ambient conditions (25oC and 1 bar(1工程大气压 ) the solubility of a solute in a gas is usually related directly to
41、the vapor pressure of the solute and is generally negligible. The dissolution of solutes in supercritical fluids results from a combination of vapor pressure and solute-solvent interaction effects. The impact of this is that the solubility of a solid solute in a supercritical fluid is not a simple f
42、unction of pressure.,Although the solubility of volatile solids in SCFs is higher than in an ideal gas, it is often desirable to increase the solubility further in order to reduce the solvent requirement for processing. The solubility of components in SCFs can be enhanced by the addition of a substa
43、nce referred to as an entrainer(夹带剂), or cosolvent. The volatility of this additional component is usually intermediate to that of the SCF and the solute. The addition of a cosolvent provides a further dimension to the range of solvent properties in a given system by influencing the chemical nature
44、of the fluid.,Cosolvents also provide a mechanism by which the extraction selectivity can be manipulated. The commercial potential of a particular application of SCF technology can be significantly improved through the use of cosolvents. A factor that must be taken into consideration when using cosolvents, however, is that even the presence of small amounts of an additional component to a primary SCF can change the critical properties of the resulting mixture considerably.,
