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Jan-2011

Simulation model increases 
visbreaker conversion

Introduction of advanced analytical monitoring, chemical treatment and process optimisation has raised the profitability of a visbreaker unit

Ernesto Agorreta, Carlos Angulo and Aleixandre Soriano, Repsol Tarragona
Cristina Font and Marco Respini, Baker Hughes
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Article Summary
In today’s refining climate, the economics of a visbreaking unit are mainly related to the conversion levels achieved. Conversion is limited by the impact of cracking reactions on the progressive destabilisation of the asphaltenes present in the unit feed. This destabilisation causes fouling phenomena, due to precipitation of the asphaltenes 
and conversion of some of the asphaltenes to generate coke particles at cracking temperatures above 400°C (752°F), and these phenomena limit unit run lengths. On the other hand, destabilised asphaltenes can easily aggregate over time and, since produced visbreaker resid is often a base stock for the production of heavy fuel oils, this can lead to problems with hot filtration test (HFT) results.

Visbreaker severity is monitored to maximise conversion, taking into account excessive coke generation and the tendency of asphaltenes to precipitate, either inside process equipment or as sediments in 
the heavy fuel oil produced. As fouling increases exponentially at higher conversions, process control becomes more and more important to prevent drastic negative outcomes.

Severity is a function of operating temperatures and velocity steam. This article presents a case of optimisation of these two variables at the Repsol Tarragona visbreaker, resulting in major improvements to visbreaker conversion.

Repsol Tarragona
The Repsol Tarragona refinery is a state-of-the-art 200 000 b/d refinery that has broad experience processing opportunity crudes and feeding different residues to the visbreaker unit. The first step of this project was the implementation of Baker Hughes’ advanced analytical VisTec technologies for the rapid and reliable assessment of visbreaker tar (vistar) and heavy fuel oil stability, and to evaluate fouling tendencies in the unit. This step enabled the refinery to run the unit close to its operating limits and maximise conversion, while rapidly adapting process conditions to changes in processed feed quality or flow rate.

A second step for increasing unit profitability was based on a new development using a proprietary simulation model of flow regimes and liquid-phase velocities across the visbreaker heater coils as a function of flow rates, operating temperatures and velocity steam.

The results of the model were validated in the unit by Repsol process and operation engineers with Baker Hughes service engineers and its technical service group. Operational changes were implemented based on the results from the simulation work, while the impact on fouling and sediments formation, which could potentially affect produced fuel oil quality, was continuously monitored. At the same time, furnace skin temperature trends were continuously tracked and normalised to achieve targeted unit run length.

The implementation of all the steps described in this article resulted in further valuable increases in conversion without negative effects on unit run length and fuel oil specifications.

Visbreaker monitoring
The main purpose of a visbreaker unit is to reduce feed viscosity up to the vistar stability limit, as well as to achieve high distillate yields; in other words, to achieve maximum conversion value, which, in turn, has a strong dependence on feed quality.

Increasing conversion means operating the unit at higher severities, either by increasing the heater outlet temperature or by decreasing the velocity steam and thus increasing residence time. Thermal cracking results in a decrease in the asphaltene fraction’s solubility and dispersibility. When extreme cracking occurs, asphaltene molecules can no longer stay in a dispersed phase and start to aggregate as a separate phase.

Aggregation of asphaltenes at thermal cracking temperatures above 400°C (752°F) leads to cross- linking and dehydration, and yields coke particles with a radius of typically 1–5 microns. Aggregation of asphaltenes at lower temperatures results in fouling of heat exchangers and columns downstream of the process furnace. High generation of coke particles causes rapid heater coking and shortens unit run length.

In addition, aggregated asphaltenes tend to precipitate with time, leading to sediment problems. The heavy fuel oil fails the HFT when vistar 
is blended with cutter stocks. 
Both heater coking and vistar stability limit maximum conversion. Therefore, severity must be optimised regularly to achieve the best compromise between maximum conversion and acceptable unit run length.

The tendency for asphaltenes to aggregate is measured as stability reserve, or simply stability; the higher the stability, the lower the tendency to produce fouling problems and/or development of sediments. Stability is normally defined as the maximum amount of aliphatic solvent (usually heptane or hexadecane) that can be added per gram of resid/vistar or heavy fuel oil before asphaltene precipitation occurs. The higher this ratio, the higher the stability reserve.

Controlling visbreaking severity to maintain visbreaker tar stability minimises serious fouling problems and the formation of sediments. Whenever severity is pushed excessively and vistar stability drops below the minimum control limit, fouling and sediments tend to increase exponentially compared to the increase in conversion. Even when using antifoulants and/or asphaltene stabilisers, the extent of fouling and asphaltene precipitation to produce heavy fuel sediments tends to be so extensive that it is difficult to control.

Visbreaker severity is usually controlled in refineries with p-value and HFT sediments tests. Both techniques are not fully reliable and accurate. Details of the VisTec methods and improvements over traditional techniques are given in reference 1.

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