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Increasing conversion and run length 
in a visbreaker

Combining anticoke/antifoulant treatments with monitoring technology enabled a refinery to keep its visbreaker unit at the best process severity for any feed

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Article Summary
Visbreaker economics are mainly based on achievement of maximum conversion. The main barrier to this goal is in the loss of stability reserve of asphaltenes, causing their precipitation to give fouling in the form of coke. These foulants can severely shorten unit run lengths by deposition at the heater, pre-heat exchangers and columns.

Maximum conversion can be obtained by setting the proper process severity for any processed feed (typically by controlling heater outlet operating temperature) while the use of antifoulants/anticoke chemicals mitigates the fouling rate, particularly when the unit is set at optimal severity as a result of monitoring. This approach results in the best trade off between fouling control and conversion.

This article presents several advances that were put in place for the ISAB Priolo refinery visbreaker, resulting in improved performance after successful results were obtained in the past1 and met all of the desired targets. This was possible due to Baker Hughes VisTec technology and the capability of the ISAB refinery to use this technology in a very effective way.

Visbreaking (thermal cracking) limitations due to fouling
The main impact as a result of thermal cracking reactions is a progressive destabilisation of asphaltenes contained in the unit feed residuum. Visbreaker bottom resid (vistar) has a higher fouling tendency than the unit feed due to the stability reserve of the asphaltenes and their potential to precipitate and result in coke and deposits.

This loss of stability is related to the thermal cracking of asphaltenes and their associated stabilising resins.

Asphaltenes de-alkylate to give lower molecular weight but less soluble, aromatic carbon rich, free radicals, sometimes called ‘cores.’ This is shown schematically in Figure 1.

Apart from this key driver to fouling, the generation of lighter paraffins by thermal cracking of heavier molecules contributes to the decrease in stability. Thermal cracking reactions that occur in visbreaking can be schematically simplified (see Figure 2).

The asphaltene cores (low molecular weight asphaltenes) tend to oligomerise (terminate free radicals by recombination). This generates coke precursors, which are barely soluble asphaltenes that are rich in aromatic carbon.

As long as these are dispersed in the oil, their further polymerisation, resulting in coke particles, is controlled as their contact is limited and is due to some saturation of free radicals by the naphthenoaromatics present in the residuum and vistar (hydrogen donors).

Once their concentration exceeds the solubility limit, they separate into a second liquid phase, called mesophase, and rapidly polymerise, resulting in coke particles.

From a kinetic point of view, the generation of coke precursors has high activation energy and is favoured at higher reaction temperatures. As a result of the process temperatures used, visbreaking tends to produce high levels of coke particles.

A very interesting insight and confirmation on the impact of thermal cracking comes from the direct measurement of changes in the solubility blending number (SBn) and insolubility number (In) from visbreaking units.2 A typical trend is presented in Figure 3.

Figure 3 shows that there is no major change in the SBn from the visbreaker feed resid to the visbreaker bottom atmospheric distillate (vistar). However, the In of the asphaltenes increases due to thermal cracking of asphaltenes into more insoluble ones along with the loss of stabilising resins.

Stability reserve of asphaltenes is proportional to the ratio of SBn to In. The wider the gap, the lower is the tendency of asphaltenes to phase separate and precipitate to give either coke (above 400°C) or deposits.

Asphaltene classification captures a broad range of molecular structures. The In is an average value of the asphaltenes; when the stability reserve is reduced by visbreaking to low values (small difference between SBn and In) a portion of the less stable asphaltenes (the ones with higher In) begin to phase separate to give fouling.

Therefore, although the visbreaker resid has a stability reserve, asphaltenes are on average still soluble/dispersible in the matrix without precipitation, and a relatively small portion of them are at the solubility limit and give fouling. The lower the stability reserve, the higher is the fraction of asphaltenes likely to result in fouling.

It is also clear how feeds with the highest stability reserve can be processed at higher severities (conversions).

Limitations in visbreaker conversion
For any blend of processed crudes there is an optimal conversion for the downstream visbreaker.

Fouling rates have an exponentially increasing trend with severity and conversion. At low severities, the impact of the conversion increase is very limited. As conversion limits are reached, any further minor increase in severity has drastic consequences on fouling rates, resulting in severe reduction in unit run length. This is shown schematically in Figure 4.

Fouling can severely impact the heater run length due to coke deposition. Visbreakers typically show a clear exponential increase of coke generation with heater temperature (see Figure 5).

Heat exchanger fouling is another area of severe problems for most visbreaker units due to the high fouling tendency of vistar. This problem is related to the low stability reserve of asphaltenes in vistar, which can be measured using the VisTec Stability Index (VSI). Normally, fouling tendency increases exponentially with a decrease of VSI and this trend limits, together with coke particle generation, maximum achievable conversion and severity (see 
Figure 6).

Anticoke and antifoulants are able to extend unit run lengths in our experience by 50% to 75% without a reduction in severity or conversion; however, they give the best results in high fouling situations below the exponential fouling range. The extent of fouling in the exponential range is clearly too high to be reasonably controlled, even by anticoke and antifoulants at reasonable dose rates and treatment cost.
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