Tank Bottom Sludge Cleaning and removal

Whatever the composition of the sludge be it paraffinic or naphtenic, there are problems galore with non human intervention methods. First and foremost is the viscosity of the sludge. Most of the pumps available commercially can not pump high viscous sludge material from the tanks. Therefore solutions such as mixing with light hydrocarbons or crude oil are the common method of dissolving sludge and reducing the viscosity. However this method has its own pitfalls

Firstly most of the modern methods utilize high pressure jetting of crude oil into the tanks. Due to the dissipation of energy at the nozzle orifice, severe agitation is created and the sludge dissolves forming a lower viscosity suspension which can be pumped by the existing pumps . This method requires on an average four times the volume of sludge accumulated in the tank. Further, an extra tank is required for providing the crude oil for mixing and the same tank is generally used as a recirculation tank. The refineries are already facing a shortage of tankage and vacant area to build new tankagaes and dedicating one tank for the purpose of tank cleaning throws the refinery schedules haywire.

Further, this is an economically unviable proposition as good crude is used to create bad crude. Thus the entire volume of crude oil gets contaminated with the sludge and this volume of contaminated crude oil has to be pumped into the lagoons for further processing. Simplistically put, this is an economic waste. If we examine the economics of this exercise, the following scenario evolves.

Economics

If we take one simple scenario as below:

Estimated sludge in a storage tank : 3000 tons
Volume of crude oil required for dilution : 12,000 tons
Cost per ton of crude oil : US$ 360.00
= Rs. 15,500.00 (app.)
Total value of diluent crude : Rs. 18,60,00,000.00 (app)

This is the value of crude oil which is now contaminated with sludge and will now be a dead stock till the time the same is re-processed. In an already high priced crude scenario as exists to-day and is expected to be continued for a longer period to come, the amount of dead stock locked up using fresh crude is economically unjustifiable. Once, we take into account the other expenses such as high price paid for the above system on hire and also for the reprocessing charges of the entire 15,000 tons, then the figures will be phenomenally high.

This is one of the reasons why many refiners prefer the manual cleaning method which is a few lakhs Rupees, compared to the above automation. However, the advent of ISO standards will not allow the refineries the luxury of low cost manual method of sludge removal from the tanks. The health and safety of the workers and the equipment coupled with the environmental issues are now forcing the refiners to look at using the modern method of tank cleaning without man entry.

Time Scale

Normally, if a tank takes 45 days to 60 days for manual cleaning, in the jetting systems, the same time is consumed also. There is no significant saving in time due to the increased piping, utilities, erection etc. in case of automated systems. Further, the amount of sludge and the crude oil handled is five times the normal and thus additional time is required for blending, pumping and separation phases.

However, when the aqueous phase solvents are used, the time for blending remaining the same, the volume handled is 1/5th the volume otherwise handled by an automated system and thus there is significant time reduction. The saving in time taken for cleaning the tank is reduced by at least 15 days in case of aqueous phase solvent usage, which is direct savings to the refinery on account of gain in tankage, reduced wages, rentals etc.

One more advantage of the aqueous phase solvents is the cleaning action of the surfactants. The tanks bottoms are thoroughly cleaned and the inspection of the bottom plates will be much easier and safer. The absence of residual oil makes it safer for man entry and subsequent cleaning with water is not required as this is automatically accomplished at one go.

Application

Introducing the chemical into the tanks is not a problem. The chemical can be entered into the tank using the same jet nozzles used for introducing the crude oil. However, areas in which jet cleaning facilities and equipment are not available, then the chemicals can be introduced through man holes if the man holes can be opened. If the opening of man holes is not possible due to above the man hole sludge accumulation, then the chemical can be introduced through the roof top vents. This adds tremendous flexibility for the sludge cleaning operations which is not possible otherwise.

Further time reduction is possible by using vapour suppressants inside the tank wherein the venting time is reduced from 7 days to under one day.

Some considerations

This method is very apt for paraffinic sludges as the paraffins are naturally hydrophobic and thus aid in demulsification faster than naphtenics. However, napthenic based sludges can also be handled with this system by adjusting the dosages. Napthenic sludges are naturally hydrophilic and they tend to create strong emulsions with water. This phenomenon can be taken advantage of during the creation of micro-emulsions and by adjusting the dosage, the demulsification effect of the solvent can be increased significantly. Thus, by finer adjustments in the blends and dosages, all kinds of sludges can be handled by the refineries with ease. Another, unwritten advantage is in the pipe lines which carry significant volumes of paraffinic crudes. These pipe lines choke during winters, when the temperatures fall below the pour point of the crude oil. Introduction of the Aqueous phase solvents into the pipe line along with some water will prevent the accumulation of wax and choking of the pipe lines.

Conclusion

Aqueous phase solvents is a very new concept which had just crossed the experimental stage and it is now commercial. Significant advances will be made by the introduction of this method for tank cleaning as further refinement of the method from experience gained over a few jobs will add to the strength of the new system.

The economic advantage coupled with the ease of use, will make the Aqueous phase Solvents a major contributor to tank cleaning operations globally in a very short time.

It may be noted that Aqueous Phase Solvents are in essence emulsifiers and demulsifiers which act as solvents for all practical purposes and thus the name solvent may seem to be a misnomer but the pseudo solvent action of these new materials is going to revolutionize the oil industry due the versatility and multiple applications possible with the new exciting science of these solvents.