Visbreaking unit

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

The process applied in a conventional visbreaking unit in which a proprietary additive and water are added to the feedstock/recycled bottoms prior to the soaker reduce the inventory of heavier feedstocks and products. The presence of the oil-soluble dual catalyst (Carrazza et al., 1997a, 1997b) and water prevents the buildup of coke precursors and deposition of sediment that often occurs during visbreaking. The catalyst may be supported on support material or mixed directly with the feedstock. The first metal is chosen from the nonnoble Group VIII metals and the second is an alkali metal such as potassium or sodium. 

A visbreaking unit consists of a reaction furnace, followed by quenching with a recycled oil, and fractionation of the product mixture. All of the reaction in this process occurs as the oil flows through the tubes of the reaction furnace. The severity is controlled by the flow rate through the furnace and the temperature typical conditions are 475-500°C (885-930" F) at the furnace exit with a residence time of 1-3 minutes operation for 3-6 months on stream (continuous use) is possible before the furnace tubes must be... 

Visbreaking units typically convert about 15% of the feedstock to naphtha and diesel oils and produce a lower-viscosity residual fuel. 

ADU = Atmospheric distillation unit LOU = Lube oil unit HOT = Hydrotreating VDU = Vacuum distillation unit lieu = Hydrocracker unit VBU = Visbreaker unit FCC = Fluid catalytic cracking H1)S= Hydrode-sulfiirization ... 

The feedstock for the SGP unit is a heavy, vacuum-flashed cracked residue from an existing visbreaker unit. This material is typically characterized by high viscosity and high metal content. 

At most North American refineries, including Suncor-Samia, most of the sulfur in the gasoline pool comes from a catalytic cracking imit. To make low-sulfur gasoline, an Axens Prime-G unit was installed to desulfurize gasoline from the HCC and visbreaker units. Another option would have been to pretreat all of the HCC feed. (Already, hydrocracker bottoms comprise some of the HCC feed, but only a minor fraction. )... 

FIGURE 3.1 Typical soaker visbreaking unit. (Adapted from Stratiev, D. and Nikolaev, N., Petrol. Coal, 51(2), 140, 2009.)... 

The number, length, and diameter of furnace tubes as well as diameter and height of the soaker reactor vary, of course, depending on the design and type of visbreaking unit. For instance, Reza et al. (2011) reported the simulation of a commercial visbreaker unit with the following characteristics of coil and soaker reactors ... 

Properties of the products were obtained from a commercial visbreaking unit... 

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