过程装备与控制工程专业英语翻译18.doc

Reading Material 18Packed TowersIn 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 large 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 much as sieve or valve trays. Depth of packing without intermediate supports is limited by its deformability; metal construction is limited to depths of 2025 ft, and plastic to 1015 ft. 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 rings and every 510diameters for Pall rings. But at least every 20 ft. The various kinds of internals of packed towers are represented in Fig. 4. 2 whose individual parts may be described one-by-one:(a) is an example column showing the inlet and outlet connections and some of the kinds of internals in place.(b) Is a combination packing support and redistributor that can also serve as a sump for withdrawal of the liquid from the tower.(c) Is a trough-type distributor that is suitable for liquid rates in excess of 2 gpm / sqft in towers 2 feet and more in diameter. They can be made in ceramics or plastics. (d) Is an example of a perforated pipe distributor which is available in a variety of shapes, and is the most efficient type over a wide range of liquid rates; in large towers and where distribution is especially critical, they are fitted with nozzles instead of perforations.(e) Is a redistribution device, the rosette, that provides adequate redistribution in small diameter towers; it diverts the liquid away from the wall towards which it tends to go.(f) Is a hold-down plate to keep low density packings in place and to prevent fragile packings such as those made of carbon, for instance, from disintegrating because of mechanical disturbances at the top of the bed. The broad classes of packings for vapor-liquid contacting are either random or structured. The former are small, hollow structures with large surface per unit volume that are loaded at random into the vessel. Structured pakings may be layers of large rings or grids, brt are most commonly made of expanded metal or woven wire screen that are stacked in layers or as 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 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 that can be realized 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. 阅读材料18填料塔与板式塔相比，填料塔适用于小直径、低压力、小储量及塑料或陶瓷材料。不利于用于那些需要大直径塔、尤其是液体量少而蒸发率高的液体。因为液体分布难以控制及不能随时调节。在大型塔设备中，分散的填充物可能会导致两倍多于筛式或阀式塔的成本。不需中部支撑的填充物的深度受其可变形能力的限制；金属结构的深度大概在20-25英尺 ,塑料结构大概是10-15英尺。中部的支撑和液体再分配器应用在深床、测流回收点或进料口。对于拉西环，每2.5-3倍塔径就需要液体再分配器，而对于鲍尔环，每5-10倍塔径就需要，每个最少要20英尺。如图4.2中所示填料塔的内部结构可以一一描述如下：(a) 是一个典型的筒体，展示了众多的入口和出口、连接部分和一些内部结构。(b) 是一个连接填料支撑和再分配器的部分，也可以作为从塔中流出液体的回收盘。(c) 是一个沟型分配器，适用于流速超过2m/s塔径超过2英寸的塔；可以用陶瓷或塑料制成。(d) 是一个典型的穿孔管型分配器，可适用于各种形状，在流速大范围变化时效率最高；在大型塔设备和对分配要求严格的情况下，改用喷嘴取代穿孔。(e) 是一个再分配装置，玫瑰花形物能为小径塔提供合理的分配；它将塔壁上的液体转移到该去的地方。(f) 是一个滞留盘，用来保持填料的低密度，并防止一些易碎的诸如碳制的填料在床层顶部的机械干扰下发生破碎。用于气液接触的大面积填充物一般分为散装的，或者有规则的。前者是较小的的带孔结构，单位体积面积较大，在容器中受分散力。有规则