Distillation systems

Distillation systems are used in a wide variety of applications, including  

Products made by distillation include natural gas, propane, butane, gasoline, kerosene. Jet fuel, light oil, and heavy oil. These products can be used to make plastic, synthetic rubber, medicine, chemicals, and many other useful compounds and components. 

Chemical engineering emerged as a discipline in the 1890s, as a science that employed empirical methods. However, the new discipline had a long history. The first distillation system (apparatus) was used by Babylonian alchemists in Iraq (Mesopotamia) in 2000 BCE. Large-scale (spirits) distillation was practiced by Greek alchemists in 100 AD. Detailed instructions for a distillation process were written by an Alexandrian named Zosimos in the fourth century CE. Other significant contributions included those of  

As the heated charge leaves the furnace and enters the distillation column, a fraction of the feed vaporizes and rises up the column, while the heavier components (still in liquid state) drop down the column. This initiates the process of separation by boiling point. Because the energy in the process stream begins to dissipate immediately, a reboiler or heating source is attached to the column. This allows the separation process to continue. Some distillation columns are steam traced to ensure even temperature control. 

The distillation process is represented by four distinct systems and one super system  

---

Consider first the design of distillation systems comprising only simple columns. These simple columns employ ... 

Kakhu, A. L, and Flower, J. R., Synthesi ng Heat-Integrated Distillation Systems Using Mixed Integer Programming, Trans. IChemE ChERD, 66 241, 1988. 

Two new processes usiag 2eohte-based catalyst systems were developed ia the late 1980s. Unocal s technology is based on a conventional fixed-bed system. CR L has developed a catalytic distillation system based on an extension of the CR L MTBE technology (48—51). 

The distillation system is designed to recover a high purity cumene product. The unconverted benzene and polyisopropylbenzenes are separated and recycled to the reaction system. Propane ia the propyleae feed is recovered as fiquid petroleum gas (LPG). 

The design of these distillation systems and the operating conditions used depend on the physical properties of the alkylphenols involved and on the product requirements. Essentially all alkylphenol distillation systems operate under vacuum, but the actual pressures maintained vary considerably. Vacuum operation allows reasonable reboder temperatures (200—350°C) so that thermal dealkylation reactions of the alkylphenols are slow. 

Use in distillation systems are rare. The reason is the recognition that almost the same benefits can be achieved by integrating the rehoiling—condensing via either steam system (above ambient) or refrigeration system (below ambient). 

The sulfate ester hydrolysate is stripped to give a mixture of isopropyl alcohol, isopropyl ether, and water overhead, and dilute sulfuric acid bottoms. The overhead is neutralized using sodium hydroxide and refined in a two-column distillation system. Diisopropyl ether is taken overhead in the first, ie, ether, column. This stream is generally recycled to the reactors to produce additional isopropyl alcohol by the following equiUbrium reaction ... 

The fatty acids that emerge from the top of the column contain entrained water, partially hydroly2ed fat, and the Zn—soap catalyst. This product stream is passed into a vacuum dryer stage where the water is removed through vapori2ation and the fatty acid cooled as a result of this vapori2ation process. The dried product stream is then passed to a distillation system. 

Distillation System. The cmde condensate consists of the desired product, some low boiling constituents, and a smaller quantity of high boiling tar. Distillation separates the low boiling components, which are invariably incinerated, followed by the product fraction. Tar accumulates in the stiU ketdes, from which it is periodically removed, again to incineration. Stills work at atmospheric pressure and are vented to the incinerator. 

The dried malted barley is ground and mashed in a tub, after which the Hquid portion is drained off, cooled, and placed in the fermentor. After fermentation, a batch distillation system is usually used to separate the whisky from the fermented wort. The stiU consists of a copper ketde with a spiral tube or "worm" leading from the top. The dimensions and shape of the stills have a critical effect on the character of the whisky. The product taken off in the first part of the distillation is called foreshots (heads). The middle portion is the high wines and the last portion is the feints (tails). The middle portion is redistilled at the 140—160° proof (70—80%) range and matured in used oak cooperage. 

Absorptive distillation, involving the addition of water to the upper section of a column in the whiskey distillation system, is a method of controlling the level of heavier components in a product. 

Fatty acids are corrosive at high temperatures and selection of materials of constmction for distillation systems is critical. Stainless steels with various contents of molybdenum have proved satisfactory. For example, 316 L has 2% Mo and is satisfactory for service up to 260°C 317 L has 3% Mo and can be used satisfactorily up to 285°C, whereas 904 L can be used up to 310°C (31). 

The HCl gas is absorbed in water to produce 30—40% HCl solution. If the HCl must meet a very low organic content specification, a charcoal bed is used ahead of the HCl absorber, or the aqueous HCl solution product is treated with charcoal. Alternatively, the reactor gas can be compressed and passed to a distillation column with anhydrous 100% Hquid HCl as the distillate the organic materials are the bottoms and are recirculated to the process. Any noncondensible gas present in the HCl feed stream is vented from the distillation system and scmbbed with water. 

Multiple Products. If each component of a multicomponent distillation is to be essentially pure when recovered, the number of columns required for the distillation system is N — 1, where AJ is the number of components. Thus, ia a five-component system, recovery of all five components as essentially pure products requires four separate columns. However, those four columns can be arranged ia 14 different ways (43). 

Most distillation systems ia commercial columns have Murphree plate efficiencies of 70% or higher. Lower efficiencies are found under system conditions of a high slope of the equiHbrium curve (Fig. lb), of high Hquid viscosity, and of large molecules having characteristically low diffusion coefficients. FiaaHy, most experimental efficiencies have been for biaary systems where by definition the efficiency of one component is equal to that of the other component. For multicomponent systems it is possible for each component to have a different efficiency. Practice has been to use a pseudo-biaary approach involving the two key components. However, a theory for multicomponent efficiency prediction has been developed (66,67) and is amenable to computational analysis. 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

The hquid product streams are fed to a distillation system to remove the light impurities and to recover the ethanol as a 95% volume ethanol—water a2eotrope. To produce anhydrous ethanol, the ethanol—water a2eotrope is fed to a dehydration system. 

Experimental values of Hqg -nd Hql for a number of distillation systems of commercial interest are also readily available. Extrapolation of the data or the correlations to conditions that differ significantly from those used for the original experiments is risky. For example, pressure has a major effect on vapor density and thus can affect the hydrodynamics significantly. Changes in flow patterns affeci both mass-transfer coefficients and interfacial area. 

Total instrumentation cost does not vary a great deal with size and hence is not readily calculated as a percentage of basic equipment. This is particularly true for distillation systems. If in doubt, detailed estimates should be made. 

The problem presented to the designer of a gas-absorption unit usually specifies the following quantities (1) gas flow rate (2) gas composition, at least with respect to the component or components to be sorbed (3) operating pressure and allowable pressure drop across the absorber (4) minimum degree of recoverv of one or more solutes and, possibly, (5) the solvent to be employed. Items 3, 4, and 5 may be subject to economic considerations and therefore are sometimes left up to the designer. For determining the number of variables that must be specified in order to fix a unique solution for the design of an absorber one can use the same phase-rule approach described in Sec. 13 for distillation systems. 

The reader may refer to the data in the preceding edition. For the current work, emphasis will be given to one absorption system, carbon dioxide-air-caustic, and to several distillation systems. 

Bums and Hazzan demonstrated tlie use of event tree and fault tree analysis in tlie study of a potential accident sequence leading to a toxic vapor release at an industrial chemical process plant. The initiator of tlie accident sequence studied is event P, the failure of a plant programmable automatic controller. Tliis event, in conjunction willi the success or failure of a process water system (a glycol cooling system) mid an operator-manual shutdown of tlie distillation system produced minor, moderate, or major release of toxic material as indicated in Fig. 21.4.1. The symbols W, G, O represent tlie events listed ... 

O failure of operator-manual shutdown of the distillation system... 

A distillation system is to operate with a horizontal overhead condenser. Figure 6-19, and pressures are as marked. The estimated air leakage into the system is 7 Ibs/hr. The molecular weight of the product vapor going out the condenser into the ejector (at 80°F) is 53. The vapor pressure of the condensing vapors is 3 mm Hg abs at 80°F. 

To determine the proper operating pressure for a distillation system, whether trays or packed column, exam-... 

Liebert [218] studied feed preheat conditions and the effects on the energy requirements of a column. This topic is essential to the efficient design of a distillation system. 

Kister [94, 95] examines binary distillation systems with multiple feeds, one or more side products, one or more points of heat removal or addition, and various combinations. 

Open or direct injection of steam into a distillation system at the bottom may be used to heat the mixture as well as to reduce the effective partial pressure of the other materials. In general, if steam is used to replace a reboiler, one tray is added to replace the reboiler stage, and from one-third to one or more trays may be needed to offset the... 

This type of evaluation of a distillation system involves a material and heat balance around each tray. It is extremely tedious to do by conventional means, and is now handled with computers. But even with this untiring worker, the volume of calculations is large and requires a relatively long time. Only those special systems that defy a reasonable and apparently economical solution by other approaches are even considered for this type of solution. 

---