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Visbreaking is a non-catalytic thermal process that Figure 1

converts atmospheric or vacuum residues via thermal Typical Visbreaking Process

cracking to gas, naphtha, distillates, and visbroken Gas + Gasoline

residue. Atmospheric and vacuum residues are typically Fractionator

charged to a visbreaker to reduce fuel oil viscosity and Distillate

increase distillate yield in the refinery. Stripper


The process will typically achieve a conversion to gas, Steam

gasoline, and distillates of 10% to 50%, depending on Feed Light Gas Oil


the severity and feedstock characteristics. Visbreaking Resid

reduces the quantity of cutter stock required to meet

fuel oil specifications and, depending upon sulfur specs, mechanism will result in precipitation of unconverted

can decrease fuel oil production by 20%. materials if conversion is taken too far. The “crackable”

oils dissolve the precipitates. As the “solvent” is con-

In 1993, UOP and Foster Wheeler USA Corporation verted away, the heavy materials fall out of the solution.

(FWUSA) collaborated to combine their respective vis-

breaking technologies into a single, optimized offering Conversion is limited by asphaltene and Conradson car-

supported by both companies. bon content. A feedstock with a higher asphaltene con-

P D tent will limit the overall conversion possible while

ROCESS ESCRIPTION maintaining production of a stable fuel oil product. The

The thermal conversion of the residue chargestock is presence of sodium, as well as higher levels of feed

accomplished by heating at high temperatures in a spe- Conradson carbon, can increase the rate of coking in the

cially designed furnace. The residence time, tempera- heater tubes. Minimizing the sodium content will result

ture, and pressure of the furnace’s soaking zone is in significantly longer operating cycles.

controlled to optimize the thermal free radical cracking

to produce the desired products. The heater effluent is Residues with low softening points and low n-pentane

quenched to stop the reaction and the quenched prod- insolubles contain a greater portion of the heavy distil-

ucts flow to the fractionator for separation of the visbro- late, non-asphaltenic oil. It is this heavy oil that cracks

ken naphtha, distillate, and residue. After steam strip- into lower boiling and less viscous oils, which results in

ping, the distillate is recombined with the visbroken an overall viscosity reduction. The asphaltenes (n-hep-

residue for heavy fuel oil production (Figure 1). tane insolubles) go through the furnace relatively unaf-

fected at moderate severities.

When considering a visbreaking unit for residual

streams upgrading, specific objectives must be defined. Data indicate that for a given feedstock, as the severity

is increased, the viscosity of the 400 F+ (204 C+) vis-

o o


EEDSTOCK ONVERSION MPACT broken tar initially decreases and then increases dramat-

Feedstocks with low n-pentane insolubles and softening ically. This viscosity reversal indicates the point beyond

points show good susceptibility to visbreaking, while which fuel oil instability may occur (Figure 2).

those with high values respond poorly. T V


Stability of fuel oil product is a limit to the conversion The commercially available visbreaking processes are

attainable in the visbreaking process. The thermal either “coil” or “soaker” designs. Product qualities and




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+1 anno fa

Corso di laurea: Corso di laurea in chimica industriale e tecnologie del packaging
Università: Parma - Unipr
A.A.: 2007-2008

I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher Atreyu di informazioni apprese con la frequenza delle lezioni di Chimica industriale II e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Parma - Unipr o del prof Moggi Pietro.

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Parex Process
Steam Cracking
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