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Apr-2009

Visbreaker optimisation: 
a step change

Optimising visbreakers is not just a matter of monitoring and adding chemicals. Opportunity crudes demand a more integrated approach

Marc Rijkaart, Mario Vanacore and Christopher Russell
Nalco Europe
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Article Summary
The role of visbreaking in recent years has changed in many refineries, from the simple viscosity reduction of residue feeds to the value-driven goals of maximising gas oil yields and extending process run length. Optimisation programs, such as Nalco’s proprietary Conversion Plus 2, have helped refiners to safely increase and control cracking severity in pursuit of these goals. A successful optimisation program usually requires continuous, detailed process and residue stability monitoring combined with the strategic application of selected fouling control additives.

However, the introduction of opportunity crudes to refinery slates poses additional challenges for vis-breaker optimisation. Experience in dealing with frequently changing complex blends has identified unappreciated problematic feed characteristics that can severely limit visbreaker performance. Advanced feed fouling assessment techniques developed by Nalco have further enhanced the Conversion Plus 2 program by helping refiners to anticipate and overcome these limitations.

A question of balance
In simple terms, the visbreaker operation is a balancing act. Increasing the process cracking severity improves distillate yield, but this is balanced by the negative impact on Vistar stability and an increase in the rate of fouling.

The change in stability of the visbroken residue with an increase in severity usually follows a more predictable path than the increase in fouling, and is often used as the main guide in monitoring cracking severity. Of critical importance then is a rapid and accurate means of monitoring residue stability.

Additional monitoring of sediment or insolubles is often prescribed by finished product specification and generally considered a better guide to the fouling rate within the unit. At a certain point, maximum allowable conversion is reached, limited by either the prescribed stability or insolubles level.

Accurate responsive monitoring facilitates operation at or near the optimum severity condition, maximising distillate yield or attaining run length targets. More importantly, relaxing the fouling constraints with an application of antifoulant treatment additives, as shown in Figure 1, can significantly extend this operating envelope.

Impact of fouling
Fouling can occur in many locations within the visbreaker circuit and the indications can be very diverse. The principal effect of fouling is usually seen on heat transfer efficiency, resulting in either high tube metal temperatures (TMT) in the fired heater or a reduction in the Vistar exchanger’s cooling capacity.1 Fouling in these areas can also cause hydraulic effects and throughput restrictions. Flow rate imbalance in the furnace creates conditions for rapid fouling in the lower flow rate coils.

Fouling in the soaker can impair pressure control system performance, and tray fouling can result in back mixing, a broadening of the residence time distribution profile and reduced cracking efficiency. Similarly, main column fouling affects distillate fractionation efficiency. Problems with level control, foaming and vapour velocity control can result in entrainment of residue and black gas oil production.

Such fouling occurrences are the main reasons for premature visbreaker shutdown, thus limiting run length, unit conversion or both. Alleviating these major problems allows the unit to run for longer at a higher conversion, and allows the visbreaker turnaround to be aligned with other process equipment maintenance schedules.

The potential savings associated with visbreaker fouling reduction can be calculated in terms of energy efficiency, reduced maintenance and potentially improved conversion. On a typical size visbreaker with a capacity of 180 tonnes per hour (tph) or 30 000 bpd, a reduction of each 1°C in the normalised furnace inlet temperature (NFIT) for a fuel oil-fired heater leads to a reduction of 433 tonnes of CO2 per year and can generate a saving that is a function of energy cost.2 If the energy value is 30 $/Gcal, savings are $34 000 for each single degree (°C) of NFIT reduction and can be 25–30% higher once carbon credits are applied (assumed carbon credit of 30 $/tonne CO2).

Measurable savings are also associated with a unit run length extension. Typically, the cost of a visbreaker turnaround ranges from $1–2M and a furnace decoking takes seven to ten days. This adds up to $500 000 per decoke and $8–10/tonne ($1.30–1.60/bbls) in lost production revenue. Therefore, increasing unit run length can generate important savings and, additionally, this will reduce emissions to the atmosphere, soil contamination, waste disposal and risk of accidents during maintenance activities, all of which contribute to meeting refinery sustainability targets.

Optimisation program
Nalco’s Conversion Plus 2 visbreaker optimisation program is an integrated approach to improving the performance and profitability of the visbreaker operation. The start point involves a full unit survey, carried out in cooperation with the refiner and, in many cases, a specialist engineering company such as KBC. Utilising KBC’s engineering experience and Petro-SIM and VIS-SIM advanced process simulations capability a model of either the furnace alone or the entire unit can be developed. This model, along with historical performance data, forms the basis for identifying key constraints, potential improvements and a comprehensive process monitoring and treatment program. Development of a suitable chemical treatment program often requires pyrolysis studies on specific feeds using the research visbreaker simulation unit (VSU), as described in the following discussion.

In implementing the program, a suite of tools has been developed to be used by the Nalco service engineer or the customer in the daily monitoring and control of the unit. The main improvements when compared to traditional tests (P-value and hot filtration test) are a significant reduction in time per analysis, as well as operator independency through automated analysis. This allows for the possibility of an increased interval of monitoring the visbreaker.
 
Residual stability analyser (RSA) Residue stability is determined using the Nalco residual stability analyser (RSA). This is a rapid and accurate system, producing stability results equivalent to ASTM D7157 within 15–20 minutes from receipt of sample.
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