Heat exchangers and separators

The distillation tower is the most important equipment used in rectification units. The different types of rectification towers and schemes have been described already in the preceding sections. In addition to towers, there are also other major items of equipment used in the rectification unit. These are essentially units used for heat transfer. Heat exchangers play a very important role not only in rectification units, but also in petroleum processing as a whole. All the heat exchangers used in the petroleum industry can be classified as follows  

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The schemes with natural reflux are operated based on the principle of natural convection (i.e. circulation based on density difference between cold and heated fluid streams) to create a thermo-siphon. The hotter stream has the lower density. In natural reflux, the heated feed passes to the top of the heat exchanger and then to the bottom of tower by itself while the colder stream drains from the tower bottom to the heater. Forced reflux is based on using pumps for the circulation. 

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Figure 5-5. The ICI low-pressure process for producing methanol " (1) desulfurization, (2) saturator (for producing process steam), (3) synthesis loop circulator, (4) reactor, (5) heat exchanger and separator, (6) column for light ends recovery, (7) column for water removal.

The hydrogenated gaseous stream enters the cold section, where it is passed through a number of heat exchangers and separators. A portion of the liquid stream from unit 10 is used as the recycle stream. Hydrogen is separated as a gaseous stream in unit 12. 

Consider a simplified chemical process shown in Figure 4.2, which converts raw material in the feed stream into a desired product through reaction, heat exchange and separation operations. A process model for each of these operations will be in the form of y = g(u) where the output variables (y) are related to the input variables (u) through a set of alge-braic/differential equations (g). Input variables, here, include inlet streams conditions and... 

Figure 4.2 A simple chemical process consisting of a reactor, heat exchanger and separator.

Heat exchanger-reactors are now in the company s inventory, as well as heat exchangers and separators and whole process plant design and build. 

If high wellhead pressures are available over long periods, cooling can be achieved by expanding gas through a valve, a process known as Joule Thomson (JT) throttling. The valve is normally used in combination with a liquid gas separator and a heat exchanger, and inhibition measures must be taken to avoid hydrate formation. The whole process is often termed low temperature separation (LTS). 

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

Fig. 5. Hoechst/Rhc ne-Poulenc oxo flow scheme A, stirred tank reactor B, separator C, phase separator D, stripping column E, heat exchanger and F,...

After compression and removal of impurities, the air is cooled ia heat exchangers and expanded to low pressure through a turbiae, to recover energy, or through a valve. Liquid air, which forms at about 80 K, is separated via a distillation column. The column as well as the heat exchangers and the associated piping are placed within a cold box, which is packed with iasulation to minimise heat transfer (qv) between streams and to protect the system from the ambient air external to the cold box. 

The carbon monoxide-rich, Hquid condensate from the primary separator is expanded and exchanged against the incoming feed and is then sent to a distillation column where the carbon monoxide is purified. The bottoms Hquor from the methane wash column is expanded, heat-exchanged, and sent to the bottom section of the distillation column for methane rectification and carbon monoxide recovery. The methane bottom stream is recompressed and recycled to the top of the wash column after subcooling. A sidestream of methane is withdrawn to avoid a buildup of impurities in the system. 

Even the best modern low-temperature air separation plant has an efficiency only a small fraction of the theoretical optimum, that is, about 15 to 20 percent. The principal sources of inefficiency are threefold (1) the nonideality of the refrigerating process, (2) the imperfection of the heat exchangers, and (3) losses of refrigeration through heat leak. 

The extraction process at BP-Amoco Empress begins with natural gas arriving at the plant at about 15°C and 600 psi pressure. The gas is dehydrated to a -90°C dewpoint by means of molecular sieves. Still at 600 psi, the gas is introduced into heat exchangers and cooled to -70°C, at which point it begins to liquify in a separator. 

Atmospheric Distillation - The desalted crude oil is then heated in a heat exchanger and furnace to about 750°F and fed to a vertical, distillation column at atmospheric pressure where most of the feed is vaporized and separated into its... 

Oil Scale Sludge hoaming Fouling of pipe work and heat exchangers Baffle separators Strainers Coagulation Diatomaceous earth... 

A typical ethane cracker has several identical pyrolysis furnaces in which fresh ethane feed and recycled ethane are cracked with steam as a diluent. Figure 3-12 is a block diagram for ethylene from ethane. The outlet temperature is usually in the 800°C range. The furnace effluent is quenched in a heat exchanger and further cooled by direct contact in a water quench tower where steam is condensed and recycled to the pyrolysis furnace. After the cracked gas is treated to remove acid gases, hydrogen and methane are separated from the pyrolysis products in the demethanizer. The effluent is then treated to remove acetylene, and ethylene is separated from ethane and heavier in the ethylene fractionator. The bottom fraction is separated in the deethanizer into ethane and fraction. Ethane is then recycled to the pyrolysis furnace. 

In the UOP process (Figure 10-5), fresh propylene feed is combined with fresh and recycled benzene, then passed through heat exchangers and a steam preheater before being charged to the reactor.The effluent is separated, and excess benzene recycled. Cumene is finally clay treated and fractionated. The bottom product is mainly diisopropyl benzene, which is reacted with benzene in a transalkylation section ... 

Reaction occurs in the loop as well as in the stirred tank, and it is possible to eliminate the stirred tank so that the reactor volume consists of the heat exchanger and piping. This approach is used for very large reactors. In the limiting case where the loop becomes the CSTR without a separate agitated vessel, Equation (5.35) becomes q/Q > 10. This is similar to the rule-of-thumb discussed in Section 4.5.3 that a recycle loop reactor approximates a CSTR. The reader may wonder why the rule-of-thumb proposed a minimum recycle ratio of 8 in Chapter 4 but 10 here. Thumbs vary in size. More conservative designers have... 

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