Visbreaker Plant | Energy Efficiency Guidelines in Refinery

Visbreaker Plant – The Visbreaker process reduces the viscosity of residual oils by thermal cracking at a relatively low temperature and long residence time.

Visbreaker Plant | Energy Efficiency Guidelines in Refinery

Visbreaker Plant | Energy Efficiency Guidelines in Refinery

Visbreaker residue and gas oil, when mixed, require much smaller quantities of expensive cutter stock than would be necessary to blend with vacuum residuum to produce the same specification fuel oil.

The basic design of the Visbreaker Unit is to process about 50 to 60 MBOD of Arabian Light vacuum residue.

The plant normally consists of two parallel arranged feed preheater/visbreaker residue cooling systems, two parallel arranged heater/soaker systems, and a fractionator. There is a gas treatment and recovery section consisting of a DGA contactor and a Naphtha wash column to recover C4+ from the off gas. The off gas is routed to the Refinery fuel gas system.

The Products from the Visbreaker Unit are: Visbreaker Residue Product fractionator bottom residue and side-stripper gas oil are cooled, mixed and routed to fuel oil blending, Un-stabilized Naphtha routed to the NHT\CCR Plant and Off-Gases.

Vis-Breaking Process Targeting

Typical Visbreaking HEN Retrofit

The Visbreaker process reduces the viscosity of residual oils by thermal cracking at a relatively low temperature and long residence time. Visbreaker residue and gas oil, when mixed, require much smaller quantities of expensive cutter stock than would be necessary to blend with vacuum residuum to produce the same specification fuel oil.
The basic design of the Visbreaker Unit was done by ABB Lummus Crest B.V. using the Shell Soaker Visbreaker technology. The unit is designed to process 60 MBOD of Arabian Light vacuum residue.

The plant consists of two parallel arranged feed preheater/visbreaker residue cooling systems, two parallel arranged heater/soaker systems, and a fractionator. There is a gas treatment and recovery section consisting of a DGA contactor and a Naphtha wash column to recover C4+ from the off gas. The offgas is routed to the Refinery fuel gas system.

The products from a typical Visbreaker unit are: Visbreaker Residue Product (55.72 MBOD) fractionator bottom residue and side-stripper gas oil are cooled, mixed and routed to fuel oil blending; unstabilized Naphtha (4.175 MBOD) routed to the NHT\CCR Plant and off-Gases (4.3 MMSCFD) processed and routed to the refinery fuel gas system.

Visbreaking Plant HEN Retrofit

The following are the cold streams to be heated and hot streams to be cooled considered in this example:

The global ΔTmin used for the system is found by drawing the hot and cold composite curves and moving it until the existing hot utility (100 MMbtu/hr) and cold utility (110 MMBtu/hr) are obtained/ match with the current used. The test shows that the current design uses global ΔTmin equal to 142°F. Form the best practices this global ΔTmin is very high.

Composite Curves (Existing)

Composite Curves (Existing)

Grand Composite Curve (Existing)

Grand Composite Curve (Existing)

Using ΔTmin equal to 30°F, the flowing table and graph shows the visbraking plant targeting results:

Composite Curves at ΔTmin =30°F

Composite Curves at ΔTmin =30°F

Grand Composite Curve at ΔTmin =30°F

Grand Composite Curve at ΔTmin =30°F

Pinch analysis shows that a total of 50 MMbtu/hr can be saved by modifying the existing HXN. The grand composite curve also shows that there is no possibility to change the hot utility but there is a possibility to change cold utility used /steam generation capability.
The Bottom Product from fractionator (C-101) is coming out from the column at 662°F. It exchanges heat in E-103 with Feed to surge drum D-101. It gets cooled via steam generation in E-102 to 440°F before part of it goes to C-101 as a quench. After flashing the feed in D-101, the feed is heated and cooked in F-101& F-102 to reach 850°F. The minimum temperature different around the E-103 is 142°F.

Vis-breaker Base case

Vis-breaker Base case

Vis-breaker Base Case Data

Vis-breaker Base Case Data

There is a possibility recover more heat from the bottom product using the feed stream entering the furnace (the feed after D-101). This can be done by adding new heat exchanger after D-101 and increasing the surface area of the E-103. This energy saving initiative is evaluated for two cases: phase (I) at Minimum Approached Temperature-ΔTmin of 50°F and on Phase (II) at ΔTmin of 20°F.

The following graph illustrates this energy saving initiative:

Waste Heat Recovery Improvements (in red)

Waste Heat Recovery Improvements (in red)

There is another possibility to save more energy via increasing the waste heat recovery from visbreaking bottom products (integration with other refinery units).


 

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