Visbreaker

A visbreaker is a processing unit of an oil refinery whose purpose is to reduce the quantity of residual fuel oil produced in the processing of crude oil and to increase the yield of more valuable middle distillates (heating oil, diesel) by the refinery. A visbreaker thermally cracks large hydrocarbon molecules in the oil by heating in a furnace to reduce its viscosity and to produce small quantities of light hydrocarbons (LPG, gasoline). The process name - visbreaking - refers to the reduction in viscosity ("viscosity breaking") resulting. The process is non-catalytic.

Process objectives

The objectives of visbreaking are:

1. To reduce the viscosity of the feed stream. Typically this is the residue from vacuum distillation but can also be the residue from hydroskimming operations, natural bitumen from seeps or tar sands, and even certain high viscosity crude oils.

2. To reduce the amount of residual fuel oil produced by a refinery. Residual fueloil is generally regared as a low value product and demand for it continues to decrease as it is replaced in its traditional markets such as electricity generation by cleaner burning alternative fuels, for example natural gas. The heavy residual product coming from a visbreaker is referred to as "tar".

3. To increase the proportion of middle distillates in the refinery output. Middle distillate is used as a diluent with residual oils to bring their viscosity down to a marketable level. By reducing the viscosity of the residual stream in a visbreaker, a fueloil can be made using less diluent and the middle distillate saved can be diverted to higher value diesel or heating oil manufacture.

Technology

"Coil" Visbreaking

The term coil (or "furnace") visbreaking is applied to units where the cracking process occurs in the furnace tubes (or "coils"). Material exiting the furnace is quenched to halt the cracking reactions: frequently this is achieved by heat exchange with the virgin material being fed to the furnace, which in turn is a good energy efficiency step, but sometimes a stream of cold oil (usually gasoil) is used to the same effect. The gasoil is recovered and re-used. Providers of coil visbreaking technology include the Foster Wheeler coporation. The extent of the cracking reaction is controlled by regulation of the speed of flow of the oil through the furnace tubes. The quenched oil then passes to a fractionator where the products of the cracking (gas, LPG, gasoline, gasoil and tar) are seperated and recovered.

Soaker Visbreaking

In soaker visbreaking, the bulk of the cracking reaction occurs not in the furnace but in a drum located after the furnace called the soaker. Here the oil is held at an elevated temperature for a pre-determined period of time to allow cracking to occur before being quenched. The oil then passes to a fractionator. In soaker visbreaking, lower temperatures are used than in coil visbreaking, the comparitively long duration of the craking reaction being utilised instead.

Process Options

Visbreaker tar can be further refined by feeding it to a vacuum fractionator. Here additional heavy gasoil may be recovered and routed either to catalytic cracking, hydrocraking or thermal cracking units on the refinery. The vacuum-flashed tar (sometimes refered to as "pitch") is then routed to fueloil blending. In a few refinery locations, visbreaker tar is routed to a delayed coker for the production of certion specialist cokes such as anode coke or needle coke.

Soaker -vs- Coil

From the standpoint of yeild, there is little or nothing to chose between the two approaches. However, each offer significant advantages in particular situations:

De-coking: The cracking reaction forms petroleum coke as a byproduct. In coil visbreaking, this lays down in the tubes of the furnace and will eventually lead to fouling or blocking of the tubes. The same will occur in the drum of a soaker visbreaker, though the lower temperatures used lead to fouling at a much slower rate. Coil visbreakers therefore require frequent de-coking. This is quite labour intensive, but can be developed into a routine where tubes are de-coked sequentially without the need to shutdown the visbreaking operation. Soaker drums require far less frequent attention but their being taken out of service normally requires a complete halt to the operation. Which is the more disruptive activity will vary from refinery to refinery.

Fuel Economy: The lower temperatures used in the soaker approach mean that these units use less fuel. Where a refinery is buying in fuel at the margin to support process operations any savings on consumption could be extremely valuable. In such cases, soaker visbreaking may be advantagous.

Quality & yields

Feed quality and product quality

The quality of the feed going into a visbreaker will vary considerably with the type of crude oil that the refinery is processing. The following is a typical quality for the vacuum distillation residue of Arabian light (a crude oil from Saudi Arabia and widely refined around the world:

Density	Viscosity at 100oC	Sulphur Content
1.020	930	4.0
kg/l	centistokes	wt %

Once this material has been run through a visbreaker (and, again, there will be considerable variation from visbreaker to visbreaker as no two will operate under exactly the same conditions) the reduction in viscosity is dramatic:

Density	Viscosity at 100oC	Sulphur Content
1.048	115	4.7
kg/l	centistokes	wt %

Yields

The yields of the various hydrocarbon product will depend on the "severity" of the cracking operation as determined by the temperature the oil is heated to in the visbreaker furnace. At the low end of the scale, a furnace heating to 425^oC would crack only mildly, while operations at 500^oC would be considered as very severe. Arabian light residue visbroken at 450^oC would yield around 76% (by weight) of tar, 15% middle distillates, 6% gasolines and 3% gas and LPG.

Fueloil stability

The severity of visbreaker operation is normally limited by the need to produce a visbreaker tar that can be blended to make a stable fueloil.

Stability in this case is taken to mean the tendency of a fueloil to produce sediments when stored. These sediments are undesirable as they can quickly foul the filters of pumps used to move the oil necessitating time-consuming maintenance.

Vacuum residue fed to a visbreaker can be considered to be composed of the following:

- Asphaltenes: large polycyclic molecules that are suspended in the oil in a coloidal form;

- Resins: also polycycyclic but of a lower molecular weight than asphaltenes;

- Aromatic hydrocarbons;

- Parafinic hydrocarbons.

Visbreaking preferentially cracks aliphatic compounds: these have relatively low sulphur contents, low density and high viscosity and the effect of their removal can be clearly seen in the change in quality between feed and product. A too severe cracking in a visbreaker will lead to the asphaltene colloid becoming metastable. Subsequent addition of a diluent to manufacture a finished fuel oil can cause the colloid to break down, preciptating asphaltenes as a sludge. It has been observed that a parafinic diluent is more likely to cause precipitation than an aromatic one. Stability of fuel oil is assessed using a number of propriatory tests (for example "P" value and SHF tests).

Economics

Blending viscosity

Blending of viscosity in making fueloils is non-linear. The exercise can be made linear by the use of viscosity blending numbers (VBN - also referred to as viscosity blending indices). VBN is calculated using formulas such as the following:

1 - $VBN=14.534ln(ln(v+0.8))+10.975$

where "v" is the viscosity measured in centistokes.

In calculating the viscosity of a blend of two or more components it is necessary that all viscosities are determined at the same temperature, for example, 100^oC. The blending formula can then be written as:

2 - $VBN(Blend)=\%A(VBN(A)+\%B(VBN(B))+...+\%x(VBN(x))$ - note that percentages are weight, not volume.

Example economics for a two-component blend

A marketable fueloil, such as for electricity generation, might be required to have a viscosity of 40 centistokes at 100^oC. It might be prepared using either the virgin or visbroken residue described above combined with a distillate diluent ("cutterstock"). Such a cutterstock could typically have a viscosity at 100^oC of 1.3 centistokes. Rearranging formula "2" above for a simple two component blend shows that the percentage of cutterstock required in the blend is found by:

3 - $\%cutter=(VBN(40)-VBN(Residue))/(VBN(Cutterstock)-VBN(Residue))$

Using the viscosities quoted in the tables above for Arab Light residues and calculating VBNs according to "1" gives:

For virgin (ie, unconverted - the feed to the visbreaker) residue, 27.5% cutterstock in the blend;

For visbroken residue, 13.3% cutterstock in the blend.

As middle distillates have a far higher value in the market place than fueloils, it can be seen that the use of a visbreaker will considerably improve the economics of fueloil manufacture. For example, if the cutterstock is taken to have a value of $300 per tonne and fueloil $150 (oil prices naturally change quickly, but these prices - and more importantly the differences between them - are not unrealistic), it is a simple matter to calculate the value of the different residues in this example as being:

Virgin residue - $93.1 per tonne

Visbroken residue - $127.0 per tonne

References

1.Foster Wheeler Visbreaking Overview

2.Shell Soaker Visbreaking

3. Shell Deep Thermal Conversion

4.Fuel Oil Stability Testing

5.Bunkerworld: "Sediment Stability and Compatibility - The Structure of Fuel Oil"