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Existing Process adopted by Refineries

Sludge formation, which occurs when a Fuel is unstable, affects both the Refiner and the ultimate Engine Operator. For the Refiner, it is a direct loss when sludge separates and precipitates out besides problems posed in terms of blockages and pumping. For the end user, it is not only a direct loss, but can lead to problems of filter choking and inoperability.
Such sludge formation can occur when the fuel is composed of two different cycle streams or when it is a mixture of different fuels.
On the other hand, with severe Visbreaking, some coke formation in the fuel is inevitable. Generally however, the particulate size of such suspended coke is so small that the suspension is actually very stable. Also such small particles of coke 
(<5 microns) in no way affect the engine or components such as fuel filters.

To overcome stability problems, an earlier approach used by the refineries was to operate the visbreaker at less than optimum severity and then dilute the visbroken fuel with aromatic diluent streams. Such aromatic diluent streams not only lower viscosity to the desired level but also help in homogenizing and stabilizing the fuel.
It has been the common practice at backward refineries to depend on the "œdrop test" to gauge stability. The drop test is basically to put a drop of fuel on a porous paper where the asphaltenes tend to form a small ring while the oil spreads out to a wider ring.
A visbroken fuel may appear to be unstable when actually it may be quite stable. As a result unnecessary extra cost is incurred "“ either, by using up valuable material for further blending or by not operating the visbreaker at optimum severity.
The Result of a Drop test suffers from a lack of uniformity from person to person, from paper to paper and can even show variance between two tests with the same sample. Consequently, the development of a scientific alternative became imperative.

This led to the development of the S-Value Analyser by ROFA
ROFA France specialise in the Research, Manufacturing and Marketing of Analysers and Certified Reference Material for Petroleum industry. It also manufactures S "“ Value Analysers for Fuel stability Measurements - for usage in Refineries.
The Principle - Measurement of The point of flocculation. (An interim improvement consisted of the P-value Analysis, which lacked a proper Regression Analysis.)

Features of S - Value Analyser
In the S-Value Analyser, three titrations are conducted, determining with great accuracy the actual points of precipitation. It thus gives accurate readings of true stability. A Regression analysis is conducted to safeguard for errors.
Three different mixtures from the same sample of fuel are prepared with toluene (aromatic solvent) and then minutely titrated within-heptane (paraffinic solvent). Thus, three readings are obtained of the precipitation of asphaltenes - through three optical probes.
The S - Value Analyser is computerised - employing a number of inbuilt safeguards and reminders for the technicians conducted the test. The kit comes complete with weighing scales, beakers, heaters, optical probes, titration pumps, et al.
The system is quick and can handle a large work-load. It allows analysis of the gamut of refinery thruput and products, from crudes to visbroken residues.
It allows for the determination of three Values
S : The overall stability of the sample
Sa : The aromaticity of the asphaltenes - the ability of the asphaltenes to stay in colloidal dispersion.
So : The aromaticity of the resins - their capability to maintain asphaltenes in colloidal dispersion.

To operate the Visbreaker optimally, stability of the output needs to be monitored constantly. The ROFA analyser is unique in its ability to handle a large number of samples in any given time frame, quickly and with precision. Being an S-value analyser, it not only states the current stability of a fuel but also gives indications for potential instability.
Refineries using the S-value system, pertaining to the ASTM Standard : D 7157 - 05, huge benefits are derived from optimal refinery operation.

Marine Engine & Stationary Engines on land-based DG Sets:
For such engines the Viscosity stipulation is entirely different at 180 CST max at 50 C. To meet this viscosity specification the visbreaker needs to be operated at optimum severity, often at over 470 C. However, such visbroken fuel is generally found to be very unstable.
Further, employment of aromatic diluents have two significant drawbacks
The combustion quality of the fuel deteriorates sharply on account of the aromaticitiy of the diluent.
In addition, there is considerable value down gradation of a higher value diluent stream to FO. 
(The preventable wastage aspect).