Water-hydrocarbon separations

Why do we usually want to separate water from the reflux stream, as shown in Fig. 39.3 Some of the bad things that happen to the distillation tower if water persistently entrains into the reflux are 

The water acts as a highway for the electrons (the term electrons comes from the word electricity) to move between these potentially corrosive molecules and the vessel wall. 

Water in reflux tends to get trapped in the tower if the tower-bottom temperature is above the boiling point of water at the tower s operating pressure. The water trickles down the tower and revaporizes off of the hot reboiler tubes. As the water may be saturated with corrosive salts and gases, reboiler tube corrosion can be rapid. In almost every petroleum refinery service, refluxing water is a quick route to reboiler tube leaks. 

Water droplets settle out of a lighter liquid hydrocarbon phase because of gravity. The water is denser than the liquid hydrocarbons. Liquid droplets settle out of a lighter gas phase faster because of gravity. What is the difference The difference is viscosity. We neglected the effect on the gas viscosity when calculating the K value in KO drums. Gas viscosities are almost always very low. 

But liquid viscosities are extremely variable. At a constant viscosity, the settling rate of water in another liquid due to gravity is proportional to 

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If the recycle soot in the feedstock is too viscous to be pumped at low temperature, naphtha is used to extract the carbon from the water slurry. Naphtha is used rather than oil feedstock to facilitate the water-hydrocarbon separation. A slip stream of oil feedstock is then used to extract the soot from the carbon-naphtha slurry and a naphtha stripper is used to recover the naphtha for recycle. The oil-carbon slip stream is recombined with the feed to the reactor and the unreacted carbon is recycled to extinction. 

The neutralized cleavage product, consisting of acetone, phenol, water, hydrocarbons, and trace organic impurities, is separated in a series of distillation columns. Also in this section alpha-methylstyrene is either recovered as a product or hydrogenated to cumene. 

The first vessel in the blowdown system is therefore an acid-hydrocarbon separator. This drum is provided with a pump to transfer disengaged acid to the spent acid tank. Disengaged liquid hydrocarbon is preferably pumped back to the process, or to slop storage or a regular non-condensible lowdown drum. The vented vapor stream from the acid-hydrocarbon separator is bubbled through a layer of caustic soda solution in a neutralizing drum and is then routed to the flare header. To avoid corrosion in the special acid blowdown system, no releases which may contain water or alkaline solutions are routed into it. 

Many processes require the separation of immiscible liquid/liquid streams that is, water/hydrocarbon. The setding unit must be of sufficient height (diameter) and length to prev ent entrainment of the aqueous phase into the hydrocarbon and vice versa. Horizontal units are usually best for setding and possibly vented units for decanta-don (but not always). 

Unstabilized gasoline and light gases pass up through the main column and leave as vapor. The overhead vapor is cooled and partially condensed in the fractionator overhead condensers. The stream flows to an overhead receiver, typically operating at <15 psig (<1 bar). Hydrocarbon vapor, hydrocarbon liquid, and water are separated in the drum. 

The basic refining tool is the common distillation unit. It is usually the first process in refining crude oils. Crude oil normally begins to vaporize at a temperature somewhat less than what is required to boil water. Hydrocarbons with the lowest molecular weight vaporize at the lowest temperatures, whereas successively higher temperatures are applied to separate or distill the larger molecules. 

The major source of wastewater in thermal cracking is the overhead accumulator on the fractionator, where water is separated from the hydrocarbon vapor and sent to the sewer system. This water usually contains various oils and fractions and may be high in BOD, COD, ammonia, phenol, sulfides, and alkalinity. 

The Ferret process actively draws in water, as well as free and dispersed hydrocarbons. The inward pull causes water and product to move toward the Ferret inlet, enhancing recovery. Once the product/water mixture is taken inside the Ferret inlet, separation takes place specific gravity is used to split the hydrocarbons and water into separate pumping paths. 

Molecular sieves are used in a variety of fuel processing applications. Uses include drying and water removal from fuel, product purification, hydrocarbon separation and catalysis. Molecular sieves are composed of sodium and calcium aluminosilicate crystals which have been produced from natural or synthetic zeolite compounds. The crystals are dehydrated through heating and are processed to ensure that pore sizes are tightly controlled. 

Point Q2 is a quadruple point. At Q2, four phases are in equilibrium liquid water, hydrocarbon liquid, hydrocarbon gas, and solid hydrate. The almost vertical line extending from point Q2 separates the area of liquid water and hydrocarbon liquid from the area of liquid water and hydrate. 

Some of the many solvents that have been examined for certain hydrocarbon separations are listed in Table 13.8 part (c) for n-butane and butene-2 separations includes data showing that addition of some water to the solvent enhances the selectivity. The diolefins butadiene and isoprene are available commercially as byproducts of cracking operations and are mixed with other close-boiling saturated, olefinic and acetylenic hydrocarbons, often as many as 10-20 different ones. The most widely used extractive... 

Figure 183. Drums with coalescers for assisting in the separation of small amounts of entrained liquid, (a) A liquid-liquid separating drum equipped with a coalescer for the removal of small amounts of dispersed phase. In water-hydrocarbon systems, the pot may be designed for 0.5 ft/sec (Facet Enterprises, Industrial Division), (b) An oil-water separator with corrugated plate coalescers (General Electric Co.).

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