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Jan-2010

Achieving optimal visbreaker severity

A refinery has managed to run a visbreaker unit continuously for more than two years without any loss of conversion

Gaetano Petralito, ISAB
Marco Respini, Baker Hughes Europe
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Article Summary
This article discusses the case of extending a complex visbreaker unit cycle to more than two years without requiring a shutdown and without comprom-ising conversion. A significant factor in achieving these results was the use of the Baker Petrolite VisTec Program, which comprises chemical treatments to reduce unit fouling and a variety of monitoring tools used specifically on those areas that affect fouling.

Armed with this information, refinery personnel were able to optimise their visbreaker unit, running at optimal severity without fouling limitations. Monitoring of visbreaker residue stability and coke particle concentration and size was applied to continuously optimise unit severity, control visbreaker residue fouling tendency, minimise the impact of residual feed on heater fouling and visbreaker residue stability, and optimise the critical vacuum unit wash grid. As a result, there was no detrimental effect on average conversion, which showed a slight increase while unit run length exceeded two years.

Visbreaking units suffer many limitations, most of them attribu-table to fouling. Fouling occurs as a result of thermal cracking of critical and highly unstable asphaltenes.  As asphaltene molecules are thermally cracked they become less soluble, ultimately leading to their phase separation as micron-sized particles. This results in the generation of coke particles at thermal cracking temperatures above 400–410°C (in the furnace), and further precipitation when the temperature is then reduced below thermal cracking by quenching after the furnace and soaker. The solubility/dispersion of asphaltenes is typically known as stability. Stability is greatly reduced by visbreaking, well below typical values of vacuum feed residua. Feed residua sometimes contain a high level of insoluble solids, either from crude or from corrosion in upstream units, and these solids tend to co-precipitate with coke in the heater. 

Many visbreaker units that were originally designed to reduce residua feed viscosity are now being used to maximise gas oil yield. There are significant financial incentives for operating the unit in this way.1 High conversion typically requires higher severities, resulting in lower stability and higher coke generation. Coke generation and stability also depend on feed 
quality. Fouling tendencies typically increase exponentially with increas-ing visbreaker operating severity.2 If not properly controlled, this higher fouling tendency can lead to shorter heater and downstream section run lengths. This can affect the unit’s throughput and lead to more frequent shutdowns for cleaning and decoking, offsetting any potential increases in refinery margin from higher distillates yields. Since fouling is a problem for targeting long unit runs between cleaning and decoking, particularly when typical unit conversions need to be maintained or even increased, an effective fouling control management programme becomes essential.

Baker Hughes has developed two proprietary methods for the rapid and reliable assessment of residual stability for fouling and heavy fuel stability control. The first method is the VisTec Stability Index (VSI), which uses near infrared analysis 
to calculate visbreaker residue stability. The second, the VisTec Coke Index (VCI), uses a patented system to measure the physical characteristics of the particulate coke in the visbreaker residue downstream of the furnace and soaker to calculate the furnace fouling/coking tendency.

VSI and VCI are complementary elements of the Baker Petrolite VisTec Program, which has been used to predict fouling tendencies for a wide range of feedstocks over a range of processing conditions. Determination of visbreaker residue stability and furnace fouling/coking tendency can help refiners boost visbreaker economics by maximising conversion, while also maintaining run lengths.3

Refineries operating visbreakers are keen to optimise cycle run lengths and increase conversion, while maintaining production of a stable heavy fuel oil.4 In trying to achieve these objectives, some refiners have experienced problems with heater coking, which has reduced run lengths; others want to optimise their production of heavy fuel oil from visbreaker residue, and some are simply searching for alternatives to traditional methods for controlling visbreaker severity such as the p-value.

This article discusses how a refinery was able to run a visbreaker unit for more than two years with-out any shutdowns and loss of conversion by applying VisTec monitoring information and using VisTec Program antifoulants.

Visbreaker fouling control
The VisTec Program combines monitoring and process optimisation with chemical additives that can affect the behaviour of the hydro-carbon stream during the visbreaking process. VisTec additives increase the inherent stability of visbreaker process streams, enabling refiners to further increase visbreaker severity (and therefore conversion) without causing fouling and fuel instability. VisTec additives also enable 
refiners to extend run lengths 
by mitigating fouling in critical 
parts of the visbreaker process, such as heat exchangers, furnaces and columns.

The monitoring technologies that are included in the VisTec Program enable refiners to maximise visbreaker conversion and run lengths within the natural con-straints imposed by the feed that is charged to the visbreaker unit. VisTec additives are an integral 
part of the VisTec Program because they alter the chemical behaviour 
of the feed to the visbreaker as it is processed and relax the natural constraints imposed by the feed. In effect, the additives expand the possible operating envelope. The VisTec monitoring and software tools enable refiners to operate the visbreaker at the optimum economic position within this new, expanded envelope. Conversion and run lengths are therefore effectively extended by the combined use of a customised chemical treatment programme, which adds to the benefits that can be obtained by VisTec monitoring.

Each VisTec application is customised to provide the most appropriate treatment and monitor-ing protocol to meet each vis-breaker’s design and operating objectives. The range of additives includes:
• Asphaltene dispersants that reduce separation of asphaltenes from the fluid phase during processing
• Asphaltene stabilisers that hinder the aggregation of precipitated asphaltenes and promote the transport of asphaltene through the visbreaking process
• Coke suppressants that inhibit 
the catalytic sites on the internal surfaces of furnace tubes that would otherwise promote the rapid formation of coke during the first few weeks of operation.

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