Distillation units

The distillation unit, with standard arrangement, serves for separation of C3-C7 hydrocarbons. The reflux ratio equals 1.3. The thermodynamic analysis is rather tedious we shall give only the results, summarized in Tab.6.2. 

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In the distillation units for producing petroleum cuts, the curves are determined in the laboratory from samples taken at regular intervals. 

The graph gives the yields that the refiner would obtain at the outlet of the atmospheric distillation unit allowing him to set the unit s operating conditions in accordance with the desired production objectives. 

Reflux Distillation Unit. The apparatus shown in Fig. 38 is a specially designed distillation-unit that can be used for boiling liquids under reflux, followed by distillation. The unit consists of a vertical water-condenser A, the top of which is fused to the side-arm condenser B. The flask C is attached by a cork to A. This apparatus is particularly suitable for the hydrolysis of esters (p. 99) and anilides (p. 109), on a small scale. For example an ester is heated under reflux with sodium hydroxide solution while water is passed through the vertical condenser water is then run out of the vertical condenser and passed through the inclined condenser. The rate of heating is increased and any volatile product will then distil over. 

Fig. 77,13, 1 illustrates a distillation unit when it is desired to protect the distillate from moisture in the atmosphere. The drying tube may be filled with anhydrous calcium chloride held in position by loose plugs of glass wool or with a loose plug of cotton wool. Fig. 77,13, 2 depicts the use of an air condenser for liquids of boiling point above 140-150°. 

Fig. II, 17, 2 illustrates a fractional distillation unit f for use with glass helices. The column is provided with an electrically-heated jacket the resistance shown in the Figure may be replaced by a variable transformer. The still head is of the total-condensation variable take-off type aU the vapour at the top of the column is condensed, a portion of the condensate is returned to the column by means of the special stopcock (permitting of...

Cyclopentadiene (2.5) was prepared from its dimer (Merck-Schuchardt) immediately before use. Dimineralised water was distilled twice in a quartz distillation unit. Ethanol (Merck) was of the highest purity available. Acetonitrile (Janssen) was mn over basic aluminium oxide prior to use. 2,2,2-Trifluoroethanol (Acros) was purified by distillation (bp 79 - C). Co(N03)2 6H20,... 

The eadiest use of heat-exchange network synthesis was in the analysis of cmde distillation (qv) units (1). The cmde stream entering a distillation unit is a convenient single stream to heat while the various side draws from the column are candidate streams to be cooled in a network. So-called pumparounds present additional opportunities for heating the cmde. The successful synthesis of cmde distillation units was accompHshed long before the development of modem network-synthesis techniques. However, the techniques now available ensure rapid and accurate development of good cmde unit heat-exchange networks. 

Azeotropic and Extractive Distillations. Effective as they are for producing various Hquid fractions, distillation units generally do not produce specific fractions. In order to accommodate the demand for such products, refineries have incorporated azeotropic distillation and extractive distillation in their operations (see Distillation, azeotropic and extractive). 

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

Obtaining maximum performance from a seawater distillation unit requires minimising the detrimental effects of scale formation. The term scale describes deposits of calcium carbonate, magnesium hydroxide, or calcium sulfate that can form ia the brine heater and the heat-recovery condensers. The carbonates and the hydroxide are conventionally called alkaline scales, and the sulfate, nonalkaline scale. The presence of bicarbonate, carbonate, and hydroxide ions, the total concentration of which is referred to as the alkalinity of the seawater, leads to the alkaline scale formation. In seawater, the bicarbonate ions decompose to carbonate and hydroxide ions, giving most of the alkalinity. 

Shinskey, F. G., Energy-Conserving Control Systems for Distillation Units, Chemical Engineering Progress, May 1976. 

Where is naphthenic acid corrosion found Naphthenic acid corrosion occurs primarily in crude and vacuum distillation units, and less frequently in thermal and catalytic cracking operations. It usually occurs in furnace coils, transfer lines, vacuum columns and their overhead condensers, sidestream coolers, and pumps. 

While working in a plant, a troubleshooter read a pressure gauge daily for several weeks and only realized it was inaccurate when one day the blower was down. The gauge still read about normal operating pressure. Had this have been a distillation unit, it could have been more serious. In distillation service, pressure is a more important variable than in many other unit operations. Relative volatility is a function of pressure. Pressure, or more accurately delta-P, is the best indication of the tower hydraulics. 

An electrostatic precipitator is used to remove more tar from coke oven gas. The tar is then sent to storage. Ammonia liquor is also separated from the tar decanter and sent to wastewater treatment after ammonia recovery. Coke oven gas is further cooled in a final cooler. Naphthalene is removed in a separator on the final cooler. Light oil is then removed from the coke oven gas and is fractionated to recover benzene, toluene, and xylene. Some facilities may include an onsite tar distillation unit. The Claus process is normally used to recover sulfur from coke oven gas. During the coke quenching, handling, and screening operation, coke breeze is produced. The breeze is either reused on site (e.g., in the sinter plant) or sold offsite as a by-product. 

Vacuum Distillation - Heavier fractions from the atmospheric distillation unit that cannot be distilled without cracking under its pressure and temperature conditions are vacuum distilled. Vacuum distillation is simply the distillation of petroleum fractions at a very low pressure (0.2 to 0.7 psia) to increase volatilization and separation. In most systems, the vacuum inside the fractionator is maintained with steam ejectors and vacuum pumps, barometric condensers, or surface condensers. 

The column internals are housed within a vertical shell, and together with the condenser and reboiler, constitute a distillation column. A schematic of a typical distillation unit with a single feed and two product streams is shown in Figure 1. 

In light ends fractionation it is usually just as important to remove light material from the heavier cut as it is to keep heavy material out of the lighter cut sidestreams are seldom withdrawn. The desired purity (expressed as per cent of impurity) of the overhead and bottoms is determined by product specifications or by the requirements of subsequent processing units. To meet these purity requirements, higher reflux ratios and greater numbers of plates between cuts are required than in crude distillation units. 

A combination unit is a special type of unit that was developed to reduce the investment for a small refinery. In effect, one main distillation unit serves as a crude fi-actionator as well as the cat unit primary fractionator. This same tower also serves the naphtha reformer and visbreaker. A schematic diagram of a combination unit is shown in Figure 2. Crude oil is topped (material boiling below 650°F is removed) in the atmospheric tower, and the topped crude is sent to the combination tower along with cat products and naphtha reformer products. These latter streams provide heat to distill the topped crude and also, being more volatile than topped crude, provide a lifting effect which assists in vaporizing more of the crude. 

Usually, product specifications for a crude distillation unit are expressed in terms of the products 15/5 or ASTM distillation curves. The prediction of a product 15/5 distillation is accomplished simply by blending the quantities of the pseudo components in the stream so as to form a true boiling point, 15/5 equivalent, distillation curve. This curve can then be converted to an ASTM type distillation using an empirical method. Figure 5 illustrates how a typical ASTM curve compares to the 15/5 curve for the same material. 

Pumps, instruments and alloys for corrosion protection are also vital parts of a distillation unit and must be carefully selected for maximum efficiency. 

Acetonitrile serves to greatly enlarge the spread of relative volatilities so that reasonably sized distillation equipment can be used to separate butadiene from the other components in the C4 fraction. The polar ACN acts as a very heavy component and is separated from the product without much difficulty.The feed stream is carefully hydrogenated to reduce the acetylene level rerun, and then fed to the single stage extractive distillation unit. Feed enters near the middle of the extractive distillation tower, while (lean) aqueous ACN is added near but not at the top. Butenes and butanes go overhead as distillate, with some being refluxed to the tower and the rest water washed for removal of entrained ACN. 

Modem crude oil distillation units are larger than those in the chemical process industry, producing up to 200,000 barrels per day of product. 

FIGURE 7.18. Overview Display of the Four Furnaces of the Distillation Unit. 

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