Naphtha vapor, condensation

Naphtha desulfurization is conducted in the vapor phase as described for natural gas. Raw naphtha is preheated and vaporized in a separate furnace. If the sulfur content of the naphtha is very high, after Co—Mo hydrotreating, the naphtha is condensed, H2S is stripped out, and the residual H2S is adsorbed on ZnO. The primary reformer operates at conditions similar to those used with natural gas feed. The nickel catalyst, however, requires a promoter such as potassium in order to avoid carbon deposition at the practical levels of steam-to-carbon ratios of 3.5—5.0. Deposition of carbon from hydrocarbons cracking on the particles of the catalyst reduces the activity of the catalyst for the reforming and results in local uneven heating of the reformer tubes because the firing heat is not removed by the reforming reaction. 

The steam and naphtha vapors would pretty much totally condense in the precondenser, shown in Fig. 16.7. 

As an example, a heat exchanger in which naphtha vapors in the shell are being condensed by cooling water in the tubes may experience corrosion on both shell and tube side. Corrosion by cooling water and the attendant scaling and fouling problems are of great importance. 

Absorption recovers valuable light components such as propane/propylene and butane/ butylene as vapors from fractionating columns. These vapors are bubbled through an absorption fluid, such as kerosene or heavy naphtha, in a fractionating-like column to dissolve in the oil while gases, such as hydrogen, methane, ethane, and ethylene, pass through. Absorption is effectively performed at 100 to 150 psi with absorber heated and distilled. The gas fraction is condensed as liquefied petroleum gas (LPG). The liquid fraction is reused in the absorption tower. 

The hydrocarbon vapors flow to the wet gas compressor. This gas stream contains not only ethane and lighter gases, but about 95% of the C3 and and about 10% of the naphtha. The phrase wet gas refers to condensable components of the gas stream. 

The HPS liquid consists mostly of C3 s and heavier hydrocarbons however, it also contains small fractions of Cj s, H2S, and entrained water. The stripper removes these light ends. The liquid enters the stripper on the top tray. The heat for stripping is provided by an external reboiler, using steam or debutanizer bottoms as the heat medium. The vapor from the reboiler rises through the tower and strips the lighter fractions from the descending liquid. The rich overhead vapor flows to the HPS via the condenser and is fed to the primary absorber. The stripped naphtha leaves the tower bottoms and goes to the debutanizer. Usually, at least one draw is installed in the tower to remove the entrained water. 

For the primary step in the overall pyrolysis plant, the feedstock must be vaporized, if in liquid form, then mixed with steam, and finally preheated to the reactor temperature. When the feedstock (e.g., ethane or propane) is in gaseous form, vaporization is usually achieved by simple heat exchange with other product components such as condensing propylene. To vaporize liquid stocks such as naphthas, higher temperatures must be used. These feeds may be partially preheated before entering the furnace itself and then fully vaporized as they flow through the convective zone of the furnace. Typical furnace and tube geometries are shown in Fig. 4. 

A process involving catalytic dehydrogenation in the presence of hydrogen is known as hydroforming. Toluene, benzene, and other aromatic materials can be economically produced from naphtha feed stocks in this way. After the toluene is separated from the other components, it is condensed and cooled in a process such as the one shown in Fig. P4.87. For every 100 kg of stock charged into the system, 27.5 kg of a toluene and water mixture (9.1% water) are produced as overhead vapor and condensed by the charge stream. Calculate ... 

The vapor leaving the top of the column is condensed in a water- or air-cooled condenser. The liquid distillate is a heavy naphtha stream, which is used for the production of gasoline. It has ASTM 5% and 95% boiling points of 195 and 375 °F, respectively. In some refineries, it is sent to a reforming unit to produce aromatics (benzene, toluene, and xylenes) and hydrogen. The condensed water is decanted off the reflux drum. Note that this water stream is quite large (17,180 Ib/h) because of all the open stripping steam that is used in the column base and sidestream strippers. 

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