1、 附录 1 Distilling Equipment Distillation is a separation process based on the relative volatility of the materials to be separated and on a change in phase of the original mixture. In the simplest example, a volatile component as a liquid residue. With slight exception, distillation differs from evap
2、oration and drying in the means provided for saving the volatile component, in the degree of difficulty of separation, and in the complexity of the operation when more than one volatile component is to be separated, each from the other. Generally, distillation applies to liquid mixtures, the notable
3、 exceptions being the destructive distillation of wood and coal where liquid fractions are separated from a solid. The modern science of chemical engineering treats distillation as a unit operation to which several principles and design methods can be applied, regardless of the materials to be handl
4、ed or the industry involved. The trend in equipment is away from special or peculiar designs for different industries and toward designs fitted to the needs of the process. Distillation equipment is built in many typed, arrangements, and sizes to meet the conditions of the particular mixture to be h
5、andled and the products to be made. Selection of the type to be used is based on the physical properties of the material to be distilled, the degree of the separation to be effected, and the magnitude of the operation. The size of the component parts of the distillation system that is chosen is a ma
6、tter of engineering design based on well-developed methods as given in handbooks and textbooks, but tempered by the extensive experience of manufactures pf distillation equipment. This chapter is intended to guide the choice of equipment by type and arrangement but purposely omits the lengthy treati
7、se that would be required for instruction in detailed design. Distillation calculations for determining the degree of separation and the size of equipment are based on vapor-liquid equilibrium data , heat and material balances , allowable vapor velocities, disengaging rates. The calculations are sim
8、plest for batch distillation of one volatile constituent from a nonvolatile residue. They become increasingly complicated as the number of constituents becomes greater whether a batch or a continuous process is used. Although it is assumed that the reader is not seeking detailed instruction in desig
9、n of equipment , it is well that he be aware of the information required to make a complete over the range of temperature and pressure of the operation as follows: 1.Specific gravity of liquid. 2.Specific volume of vapors. 3.Solubility of each component in the other and in water if open steam is use
10、d . 4.Specific heat of liquid and vapor. 5.Latent heat of liquid. 6.Viscosity of liquid and vapor. 7.Surface tension (at least approximate values for estimating entrainment). 8.Thermal conductivity of liquid and vapor(for heat transfer calculations ). 9.Foaming characteristics. 10.Corrosion rate on
11、probable materials of construction. In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are l
12、arge diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as sieve or valve trays. Depth of packing without intermediate supports is limit
13、ed by its deformability； metal construction is limited to depths of 2025ft, and plastic to 1015ft. Intermediate supports and liquid redistributors are supplied for deeper beds and at sidestream withdrawal or feed points. Liquid redistributors usually are needed every 2.53 tower diameters for Raschig
14、 rings and every 510 diameters for Pall rings, but at least every 20ft. The various kinds of internals of packed towers are represented whose individual parts may be described one-by-one: 1.Is an example column showing the inlet and outlet connections and some of the kinds of internals in place. 2.I
15、s a combination packing support and redistributors that can also serve as a sump for withdrawal of the liquid from the tower. 3.Is a trough-type distributor that is suitable for liquid rates in excess of 2 gpm/sqft in towers feet and more in diameter. They can be made in ceramics or plastics. 4.Is a
16、n example of a perforated pipe distributor which is available in a variety of shapes, and is the most efficient critical, they are fitted with nozzles instead of perforations. 5.Is a redistribution device， the rosette, that provides adequate redistribution in small diameter towers； it diverts the wa
17、ll towards which it tend to go. 6.Is a hold-down plate to keep low density packings in place and to prevent fragile packings such as those made of carbon, from disintegrating because of mechanical disturbances at the top of the bed. The broad classes of packings for vapor-liquid contacting are eithe
18、r random or structured. The former are small, hollow structures with large surface per unit volume that are loaded at random into the vessel. Structured packings may be layers of large rings or grids, but are most commonly made of expanded metal or woven wire screen that are stacked in layers or as
19、spiral windings. There are several kinds of packings. The first of the widely used random packings were Raschig rings which are hollow cylinders of ceramics, plastics, or metal. They were an economical replacement for the crushed rock often used then. Because of their simplicity and their early intr
20、oduction, Raschig rings have been investigated thoroughly and many data of their early introduction, Raschig rings have been investigated thoroughly and many data of their performance have been obtained which are still useful, for example, in defining the lower limits of mass transfer efficiency tha
21、t can be realize with improved packings. Structured packings are employed particularly in vacuum service where pressure drops must be kept low. Because of their open structure and large specific surface, their mass transfer efficiency is high when proper distribution of liquid over the cross section can be maintained.
