Middle distillate streams

Calcium chloride is the most common nonregenerative reagent used to dry low molecular weight refinery streams to moderately low dew points. Anhydrous potassium or sodium hydroxide have also been used at times to dry liquefied petroleum gas. Sodium chloride is used most commonly to remove entrained and some soluble water from middle distillate streams. 

The DCC has two reactor operating modes DCC-I (Riser-plus fluidized dense-bed reactor, maximum propylene mode) and DCC-II (Riser reactor, maximum iso-olefins mode). The DCC can process different heavy feeds— VGO, DAO, coker gasoil, atmospheric residue, VR, etc. Paraffinic feedstocks are the best feeds for DCC. In DCC maximum propylene operation mode, over 20 wt% propylene yield can be obtained from paraffinic feedstocks. The naphtha and middle distillates streams from the DCC unit can be used as blending components for high-octane, commercial gasoline and fuel oil, respectively. 

Refiners use sweetening processes to remove mcr-captans that give a vei y unpleasant odor to gasolines and middle distillates (the skunk uses mercaptans to protect itself). This is done by washing the hydrocarbon stream with a caustic solution followed by a wash with water to remove die caustic. 

Liberated gasses are drawn off at the top of the tower with the naptha. The gas is recovered to manufacture refrigerated liquefied petroleum gas (LPG). The naptha is condensed at a temperature of about 52 °C (125 °F). Part of the condensed naptha is normally returned to the top of the tower. The naptha product stream is split into light naptha for gasoline blending and heavy naptha for further reforming. Inside the tower, kerosene is withdrawn at a temperature of about 149 °C (300 °F). Diesel is withdrawn at a temperature of 260 °C (500 °F). These middle distillates are usually brought up to specification with respect to sulfur content with hydrodesulfurization. The heavy oil... 

Consecutively, the heavy paraffins are cracked into lighter hydrocarbon fractions by hydro-cracking. For example, for the Shell Middle Distillate Synthesis (SMDS) process, the liquid product stream is composed of 60% gasoil (diesel), 25% kerosene and 15% naphtha. The gaseous product mainly consists of LPG (a mixture of propane and butane) (Eilers et al., 1990). Figure 7.3 shows a simplified diagram comprising all process steps to produce synthetic hydrocarbons from biomass, natural gas and coal. 

Table 7 shows the yield distribution of the C4 isomers from different feedstocks with specific processing schemes. The largest yield of butylenes comes from the refineries processing middle distillates and from olefins plants cracking naphtha. The refinery product contains 35 to 65% butanes olefins plants, 3 to 5%. Catalyst type and operating severity determine the selectivity of the C4 isomer distribution (41) in the refinery process stream. Processes that parallel fluid catalytic cracking to produce butylenes and propylene from heavy cmde oil fractions are under development (42). 

The saturates remain the major component in the mid-distillate fractions of petroleum but aromatics, which now include simple compounds with up to three aromatic rings, and heterocyclic compounds are present and represent a larger portion of the total. Kerosene, jet fuel and diesel fuel are all derived from middle distillate fractions and can also be obtained from cracked and hydropro-cessed refinery streams. 

The vapor stream from the hot high-pressure separator is cooled stepwise to produce middle distillate and naphtha that are sent to fractionation. High-pressure purge of low-boiling hydrocarbon gases is minimized by a sponge oil circulation system. 

The next processing stage, hydrodesulfurization, is where most of the sulfur, some of the nitrogen, and the residual metals are removed. A limited amount of conversion also takes place. From the final reactor, the gas phase is separated, hydrogen is recirculated to the reaction section, and the liquid products are sent to a conventional fractionation section for separation into naphtha, middle distillates, and heavier streams. 

The main column is the first step in the separation and recovery of cracked hydrocarbon vapors from the reactor. Heavy naphtha and middle distillates are withdrawn as side cuts, the bottom product is used as fuel oil or recycle stream. 

No long-term studies of the chronic health effects, including cancer, of JP-8 exposure have been conducted. With regard to epidemiologic studies of related jet fuels, one additional study published after the release of the 1996 NRC report was identified (Parent et al. 2000). Numerous studies of the carcinogenic potential of gasoline streams and related middle distillates have appeared in the open literature. 

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