Visbreakers

The gas oils from visbreaking and coking have better cetane numbers than LCO but they are unstable and need hydrotreatment before they can be used. 

In the 1970 s, heavy fuel came mainly from atmospheric distillation residue. Nowadays a very large proportion of this product is vacuum distilled and the distillate obtained is fed to conversion units such as catalytic cracking, visbreaking and cokers. These produce lighter products —gas and gasoline— but also very heavy components, that are viscous and have high contaminant levels, that are subsequently incorporated in the fuels. 

The main feedstock for catalytic reforming is heavy gasoline (80 to 180°C) available from primary distillation. If necessary, reforming also converts byproduct gasoline from processes such as visbreaking, coking, hydroconversion and heart cuts from catalytic cracking. 

The visbreaking process thermally cracks atmospheric or vacuum residues. Conversion is limited by specifications for marine or Industrial fuel-oil stability and by the formation of coke deposits in equipment such as heaters and exchangers. 

Visbreaking conversion products are unstable, olefinic, and very high in sulfur and nitrogen. They must be upgraded by processing before they can be incorporated into finished products. 

Table 10.11 provides some general performance data on a visbreaking process applied on a typical VR. 

Figure 10.4 shows the position of visbreaking units in a refining flowsheet. 

Typical composition of a visbreaking feedstock. Performance and product properties. 

Feedstocks for this very flexible process are usually vacuum distillates, deasphalted oils, residues (hydrotreated or not), as well as by-products from other processes such as extracts, paraffinic slack waxes, distillates from visbreaking and coking, residues from hydrocracking, converted in mixtures with the main feedstock. 

Feedstocks are light vacuum distillates and/or heavy ends from crude distillation or heavy vacuum distillates from other conversion processes visbreaking, coking, hydroconversion of atmospheric and vacuum residues, as well as deasphalted oils. 

The feedstocks in question are primary distillation streams and some conversion products from catalytic cracking, coking, visbreaking, and residue conversion units. 

Fractions treated by this process are light products from the primary distillation LPG to Kerosene, or light products from thermal and catalytic cracking (visbreaking, coking, FCC). 

Figure 10.16 presents the refining flowsheet of the two decades between 1950 and 1970. An optional unit, visbreaking, appears there. 

Visbreaking. Viscosity breaking (reduction) is a mild cracking operation used to reduce the viscosity of residual fuel oils and residua (8). The process, evolved from the older and now obsolete thermal cracking processes, is classed as mild because the thermal reactions are not allowed to proceed to completion. 

Cracking (visbreaking residual oils) TU LG None 470-495 10-30 450 s, 8 LHSV [11]... 

Example 3 Thermal Cracking of Heavy Oils (Visbreaking)... 

Thermal Cracking/Visbreaking Heater stack gas (CO, SO, NO, HCs and PM), vents and fugitive emissions (HCs). 

Residuum Conversion This includes fluid coking, delayed coking, visbreaking, and residuum hydroprocessing. 

Product separation for main fractionators is also often called black oil separation. Main fractionators are typically used for such operations as preflash separation, atmospheric crude, gas oil crude, vacuum preflash crude, vacuum crude, visbreaking, coking, and fluid catalytic cracking. In all these services the object is to recover clean, boiling range components from a black multicomponent mixture. But main fractionators are also used in hydrocracker downstream processing. This operation has a clean feed. Nevertheless, whenever you hear the term black oil, understand that what is really meant is main fractionator processing. 

Gas plants are integrated tower systems intended to recover LPG range material and separate it from naphtha products. This stabilizes the naphtha and reduces its vapor pressure. The LPG material may either be saturate gases going to LPG or unsaturates going to further processing. Gas plants on preflash and atmospheric crude processing units are saturate gas plants. Gas plants on FCC units are unsaturate gas plants. Coker and visbreaker gas plants are somewhere between the two. 

Visbreaking is the least expensive of the cracking processes but is limited to the lowest conversion of perhaps 20 to 25% of the feed to 680 °F material. 

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