Feed vacuum distillation unit

While originally designed for cracking the overhead stream from vacuum distillation units, known as vacuum gas oil (4), most FCC units currently operate with some higher boiling vacuum distillation bottoms (Resid) in the feed. Table 5.1 illustrates the difficult challenges faced by refiners, process licensors and FCC catalysts producers the resid feeds are heavier (lower API gravity), contain many more metals like Ni and V as well as more polyaromatic hydrocarbons prone to form coke on the catalysts (Conradson Carbon Residue, or CCR). 

Figure 6.5 Modern refinery vacuum distillation unit. One of the product streams is the feed to the plant prodticiny lubricating oils... 

Cmde is distilled and the bottoms, atmospheric resid, is sent to a vacuum distillation unit (VDU) sometimes called a vacuum pipestill (VPS) for further fractionation. Vacuum fractionation is used to separate the atmospheric resid into several feed streams or distillates. Conventional solvent processing uses selected solvents in physical processes to remove undesirable molecules... 

As shown in Figure 1, hydrocracking often is an in-between process. The required hydrogen comes from catalytic reformers, steam/methane reformers or both. Liquid feeds can come from atmospheric and/or vacuum distillation units delayed cokers fluid cokers visbreakers or FCC units. Middle distillates from a hydrocracker usually meet or exceed finished product specifications, but the heavy naphtha from a hydrocracker usually is sent to a catalytic reformer for octane improvement. The fractionator bottoms can be recycled or sent to an FCC unit, an olefins plant, or a lube plant. 

Heavy Coker Gasoil (HCGO) HCGO may be used as heavy fuel oil, as FCC feed after hydrotreating, or sent to the vacuum distillation unit. 

Other components in the feed gas may react with and degrade the amine solution. Many of these latter reactions can be reversed by appHcation of heat, as in a reclaimer. Some reaction products cannot be reclaimed, however. Thus to keep the concentration of these materials at an acceptable level, the solution must be purged and fresh amine added periodically. The principal sources of degradation products are the reactions with carbon dioxide, carbonyl sulfide, and carbon disulfide. In refineries, sour gas streams from vacuum distillation or from fluidized catalytic cracking (FCC) units can contain oxygen or sulfur dioxide which form heat-stable salts with the amine solution (see Fluidization Petroleum). 

Vacuum distillation increases the amount of the middle distillates and produces luhricating oil base stocks and asphalt. The feed to the unit is the residue from atmospheric distillation. In vacuum distillation, reduced pressures are applied to avoid cracking long-chain hydrocarbons present in the feed. 

The catalytic cracking unit is often referred to as the gasoline workhorse of a refining unit. As shown in Fig. 18.9, feeds to the catalytic cracking unit are gas oils from the atmospheric and vacuum distillation columns and delayed coker. These heavier fractions also carry metals such as nickel, vanadium, and iron. More important, sulfur compounds concentrate in the heavier product fractions. Table 18.8 lists a typical mass balance for sulfur.25 FCC blend-stocks comprise 36 percent of the volume of the gasoline pool. However, this stream also contributes 98 percent of the sulfur concentration to blended procucts.25 As specifications on sulfur concentrations in diesel and gasoline tighten, more efforts are focused on how feeds and product streams from the FCC are pre- and posttreated for sulfur concentrations. 

With the advent of better catalysts and effective additives, FCC feed preparation by vacuum distillation has become redundant for certain low-metal/CCR crude oils, so that the atmospheric residue can be processed in an FCC unit directly. The range of crude oils from which the atmospheric residue can be processed directly can be extended by revamping existing FCC units or building special new FCC units, RFCC units, and/or heavy-oil crackers. 

In 1966, a plant with a capacity of 380 Mg D20/yr was commissioned at Port Hawkesbury, Nova Scotia, and production started in 1970. The plant at Glace Bay, Nova Scotia, built in 1963, faced numerous problems caused by seawater feed. The plant was rehabilitated in 1976. The Bruce HWP (BHW-A) at the Bruce site on Lake Huron was built in 1971. Two identical enriching units in parallel were used to double the plant capacity. The final enrichment was achieved by vacuum distillation. Three additional identical plants (BHWP-B, BHWP-C, and BHWP-D) were constructed at Bruce, Ontario, and at Le Prade in Quebec. BHWP-B was commissioned in 1979. Owing to lack of demand, the remaining plants were cancelled. ... 

An existing lube hydrocracker can be operated at higher severity to make this special product, but the sharp reduction in yield may not be attractive for the base oil plant economics. However, an alternative source of hydrocracked base oil is available from some of the many existing fuel crackers. These hydrocrackers are important refinery conversion units and are used to make a range of fuel products from vacuum distillate feedstocks. Some plants do not fully convert the feed in one pass to low-boiling products and the limited amount of residue which remains, 5-10%, can be recycled within the plant, used as a fuel oil blending component or upgraded to make the special base oils. 

Liquid feeds can be atmospheric or vacuum gas oils from crude distillation units gas oils (light and heavy) from delayed cokers, fluid cokers, or visbreakers and cycle oils (light and heavy) from FCC units. 

Figure 17.28 shows a schematic of the SRC-I process. The feed coal is crushed and mixed with a recycle solvent and hydrogen prior to entering the preheater. The preheater effluent, at 700 to 750°F (370-400°C), then is fed to the dissolver unit, or thermal liquefaction unit (TLU), which operates at 840 to 870°F (450-465°C). There is no upgrading step, as the desired product is a solid at room temperature and not a distillate. The solids removal from the liquid slurry is accomplished by critical solvent de-ashing (CSD). The solids-free resid from the CSD was separated by vacuum distillation into a recycle solvent (the light fraction) and a solvent refined coal product (the bottoms). 

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