IMPROVED TANK CLEANING REDUCES HAZARDOUS WASTE AND BOOSTS RETURN

ON INVESTMENT

INTRODUCTION

Dalmeiclean is agent for Innovative Technologies Worldwide (ITW) which is a company operating since several years in the field of applied research, developing new technologies for industry. For all the technologies, ITW possesses the licenses of the patented applications that MEG (the research branch of ITW) has filed all over the World.

Environmental legislation is becoming increasingly restrictive over waste disposal. This requirement becomes even more important in the cleaning of tanks, as the removal of tank sludge is an expensive and time consuming step before achieving gas free certification. In most cases sludge in the bottom of the tanks is removed manually to carry out maintenance. Manual removal of the sludge is a very expensive and time consuming operation, complicated by the fact that tanks are difficult to operate in. Manual cleaning always implies a special attention to safety of operations. ITW has patented a novel technology for asphaltene stabilization. Such technology makes use of chemical additives to be added to hydrocarbons and has proved very effective in many industrial applications.

The technology has been suitably modified for tank cleaning and successfully tested in many cases of aboveground storage tank cleaning (e.g. fuel oil and crude oil tanks), as well as cargo tanks. Asphaltene stabilization achieves an improvement in sludge reuse, in that asphaltene association is reduced, compatibility with the receiving hydrocarbon is enhanced, precipitation does not occur and cracking of asphaltene is facilitated.

According to the ITW approach, sludge is removed by the addition of a chemical additive, which contains asphaltene stabilizers, patented by ITW. The formulation also includes paraffin solvents and fluidizing agents. The additive is utilized to help sludge penetration, thus favouring its solubilisation into a carrier.

Sludge dissolution occurs due to the chemical action of the additive during recycling of the oil phase.

After a brief description of the existing tank cleaning technologies, we will report some results achieved in the application of the novel technology.

TANK CLEANING EXISTING TECHNOLOGIES

Manual cleaning is the most widespread today’s method for cleaning tanks.

This method has many disadvantages in that it:

• is unsafe

• generates a huge amount of wastes

• is time-consuming

• is costly

Other methods have been developed to improve manual tank cleaning and, among the others, the most interesting of them are:

• Crude Oil Washing (COW) catalyst

• Chemical cleaning

• Robot machines

Although the above methods improve manual cleaning they do still have pitfalls.

COW simply moves the sludge from one tank to another (it is a mechanical dispersion method). In some cases, reprocessed crude oil sludge leads to unscheduled shutdown of the Topping units.

For chemical cleaning, chemicals used until now are basically dispersants: again, they transfer the problem from one point to another.

Robot machines improve the safety and sometime the time of the operations, but they do not have any impact on sludge reduction, therefore generating the same amount of sludge.

ITW TECHNOLOGY

ITW uses patented asphaltene stabilizers to make sludge a reusable product.

The used chemical additive by itself is capable of stabilizing and solubilising the sludge. The chemical is not a dispersant, so it doesn’t

create any problem in downstream equipments.

Moreover, ITW stabilizers/solubilizers:

• do not contain any metallic compound

• do not contain any catalyst poison for petroleum processes

• do not contain any halogen compound

• do not contain any carcinogenic compound

• do not contain any compound which, at operating dosages, can be a poison for waste water treatment plants

• do not contain any compound which can be harmful to plant metallurgies

Therefore, the core of ITW processes is highly effective chemicals which are able to solubilise per se the sludge. This means, the sludge will be solubilised chemically, i.e. stabilized permanently, with no danger of subsequent precipitation. To improve the performance of the chemical (especially in large tanks), a modified Crude Oil Washing is also used together with chemical stabilization of the sludge.

CASE STORY 1

A 5.000 m³ fuel oil tank needed to be cleaned after almost 20 years operations. The tank was emptied with an external pump up to 50 cm, hence under the suction limit of tank pump. A certain amount of fuel oil was left above the sludge. Before starting the addition of the chemical, fuel oil was analyzed for SHF (Sediments by Hot Filtration - IP 375), which resulted as Not Filterable (i.e. no oil filtered through a Whatman GF/A filter of nominal porosity of 2µm).

This accounted for the asphaltene being a precipitate, hence strongly associated. Moreover, sludge contained a huge amount of catalyst fines (deriving from the blending during fuel oil formulation with decant oil from Fluid Catalytic Cracking Unit). Therefore a strong aggregation between sediments (substantially cat fines) and asphaltene occurred. In such a case, precipitated asphaltene incorporated cat fines create not filterable macrostructures. This phenomenon was operatively well known, as during fuel oil combustion frequent problems of burners and filters plugging occurred. Once suction of pumpable hydrocarbons was ended (therefore leaving about 80 m³ of sludge), the chemical was added directly in the tank, by connecting the suction of the recycle pump with the container of the chemical. No carrier was utilized as diluent, so the chemical was injected directly in the not filterable fuel oil above the sludge. Tank recycle started, and during recycle the quality of the oil phase (fuel oil + sludge) was analyzed. After one day recycle, SHF of the sludge was 3 %. Such a result is extremely important, as it highlights the reactivity of the chemical. The fact that the oil phase went from not filterable to filterable is a clear demonstration of the stabilizing effect of the chemical. The chemical has stabilized asphaltene to such an extent that they were no longer aggregated, hence filterable. The sludge was freed from cat fines, which contributed to the formation of not filterable macrostructures. If the asphaltene structure had not been disjointed, than a release of sediments from asphaltene would be not have been possible. The concept can be better illustrated as follows:

By continuing recycling, sediments in the sludge increased with time during recycle: 7% the second day and 18 % the sixth day. The chemical effect is then evident: the additive has stabilized asphaltene in the sludge, thus allowing release of catalyst fines which were bound to them. It is important to note that the oil phase achieved from the sludge after ITW treatment was filterable, and SHF consisted almost entirely of cat fines. The oil phase of the sludge after ITW treatment has always been filterable and sediments increased gradually, once asphaltene were attacked from the chemical. In practical terms, the word filterable means, the oil did not contain sediments having dimension greater than 2 µm (nominal porosity of the filter utilized to measure SHF). At the end of recirculation, additivized and stabilized sludge was transferred into another fuel oil tank. The transfer has been performed gradually, by completing the operation in about 6 hours (rate of about 15 T/h). In order to reduce potential operating problems, the sludge was transferred directly in the bottom of the receiving tank and hence fed the boiler immediately (the charge pump sucked from the bottom of the tank). The sludge was entirely transferred into the receiving tank, and the level of the cleaned tank deepened down to zero. This was confirmed by visual inspection from manways. The chemical additive solubilised and stabilized all the sludge (80 m³) present in the tank after only 6 days recycle. To confirm the success of the treatment, the sludge fed in the boiler gave no rise to any operating problem in that free cat fines were easily stopped by hot filters, which did not suffer any fouling problem. The filters were able to dispose of stopped sediments in their normal cleaning time (20 min).Pre-heaters did not suffer any fouling problem too. Still more noticeable is that no combustion and/or no burners plugging problems arose, which is a great success even compared to the normal combustion problems encountered during fuel oil combustion (burners, filters and pre-heaters plugging). Any of the abovementioned operating problems would have been, in a way, justifiable as the boiler was fed with tank sludge, but asphaltene stabilization achieved with the chemical had been so effective to give rise to no pitfall in the combustion of the sludge. To further confirm the above, pressure of the burners did not increase during sludge combustion: on the contrary, it decreased due to the stabilizing action of the chemical. Flame characteristics have been always at best during combustion of additivized sludge: flame has been clear and flame pattern has been regular, without sputtering. Particulate matter emissions during combustion of additivized sludge have not increased; on the contrary they have slightly decreased, once blowing and load change are excluded. NO_x emissions have been significantly decreased during combustion of additivized sludge.

CASE STORY 2

A power station had the need of cleaning the service tank, as it had not been cleaned since boiler construction (roughly 30 years).Manual cleaning was not the solution however, as boiler turnaround was scheduled for only 20 days and an important revamping had to be implemented. As manual tank cleaning is a dirty, time consuming and almost unsafe operation, the management decided to test ITW technology. The purpose of the test was to have indications of cleaning during additivation of an ITW chemical. To give more added value to the application, ITW formulated a tailor made chemical, containing both asphaltene stabilizers and combustion catalysts. ITW fuel oil stabilizer and catalyst (hereinafter referred to as “ITW additive”), was injected upstream the service storage tank; the additivized fuel oil entered the tank from the bottom. After about one and a half month of treatment, a digging was performed in the tank with the following results: a layer of 20 cm solids, 30 cm of sludge with not measurable viscosity at 100 °C and 150 cm of fuel oil more viscous than the one above. These results were interpreted in the sense that ITW additive, by entering the tank from the bottom, was solubilizing the sludge, so this solubilized sludge rendered the lower portion of the oil more viscous. After still another one month of treatment, a further digging was performed in the tank and the results were surprising: the solid layer had disappeared, the same for the very viscous sludge. At their place a single not viscous sludge layer (viscosity 132 CST at 100 °C) was found. The amazing results in tank cleaning were also confirmed by those in pre-flame and post-flame zones of the boiler. As ITW additive contains both asphaltene stabilizers and combustion catalysts it performs its action also downstream the tank. Actually both pre-flame and post-flame zones were cleaner than in the previous situation. In particular, hot filters ∆P was nearly nil after cleaning; on a normalized basis, the filter didn’t increase its ∆P. Fouling factor monitoring in the pr-eaters revealed no increase in fouling. The most significant improvements in boiler operating parameters can be summarized as follows:

• combustion chamber pressure decreased from about 260 mmH₂O to about 230 mmH₂O

• flue gas pressure drop in the Ljungstroem decreased from about 100 mmH₂O to an average of about 70 mmH₂O

• air temperature at Ljungstroem outlet increased from about 395 °C to about 403 °C

• normalized burners pressure was almost constant

The improved cleanliness of the boiler translated into improved combustion efficiency: specific steam production for Kcal of incoming fuel improved from about 0.0566 tons steam/fuel Kcal to about 0.0574 tons steam/fuel Kcal with an improvement of about 1%. Apart from being tremendously effective in improving both pre- and post-flame cleanliness, ITW additive is also effective in reducing flue gas emissions.

CASE STORY 3

An oil tanker (120.000 m³ capacity) needed to be cleaned before dry-docking. Standard oil tanker cleaning procedure involves the use of Crude Oil Washing (COW) followed by water cleaning. However, these operations are not completely effective as tank washing machines have some shadow areas, where the flow has no direct impact on the sludge. Therefore after COW some sludge is left in the bottom of the tanks, and has to be removed manually.(See drawing of manual cleaning). ITW additives have been added both in the COW phase and in the water-washing phase. In the water-washing phase a patented hydrocarbon solubilizer has been added. This product is capable of temporarily solubilizing hydrocarbons in water, when the two phases (additivized water and hydrocarbons) are in direct contact with each other (e.g. under agitation).The solubilization is temporary and, after some minutes, the hydrocarbilic and the water phases separate out. The water phase is very clear and no emulsion is formed. The additive also facilitates the separation of oil from water. COW was performed with Bouri crude oil, which is a very severe crude, like the others transported by the ship. Before performing the additivized COW, some standard COW were performed: a) one upper cycle 120°/30° and b) two bottom cycles 0°/30°/0°.

At the end of those cycles the slop tank contained 1300 m³ of Bouri crude and sediments. This is what the standard COW procedure would have achieved in terms of recovered sludge. One further bottom cycle with the additive (injected in the suction of the COW pump) was performed, and stopped after only 12 hours washing (total time for all the cargo tanks).At the end of this additivized cycle, the slop tank contained 1500 m³ of Bouri crude and sediments.

Therefore, the use of ITW stabilizer allowed for a recovery of 200 m³ of sludge in the COW phase. The additivized water-washing phase was performed in the same way as the additivized COW, with water heated at 60 °C. At the end of this phase, further recovery of 130 m³ of oil was achieved. Therefore, following injection of ITW additives, the total amount of recovered sludge was 330 m³. All the 1630 m³ of crude oil + recovered sludge were sent to the refinery for reprocessing. No operational problems were reported at all. The results achieved are extremely positive in terms of recovered sludge.

Above results are even more valuable when taking into account the type of crude oils carried by the ship and hence the kind of sludge generated by them. The carried crudes are mainly Belayim, Bouri, Bu Attifel and Es Sider. Belayim and Bouri are particularly well-known in the petroleum industry as crudes which bear fouling problems. Bouri, in particular, is a very unstable crude, which causes severe fouling problems in refinery plants. Its visbroken TAR instability has been well recognized; some FCCU overhead section restrictions (following coke deposition) have also been reported when processing Bouri gas-oils. Despite Bouri’s characteristics, the additivized COW procedure was extremely successful, which confirms the stabilizing properties of ITW additives.

The payout of the application has been in the range of 16:1, due to the dramatic reduction in oil tankers’ cleaning time, and hence recovered freight costs.

ECONOMICS

Apart from being environmental friendly, the application of ITW additives makes tank cleaning an economic viable operation.

We will evaluate an approximate pay-out for the operation by taking into account the following items:

• sludge recovery

• waste minimization

• cost of alternative cleaning

• storage capacity recovery

We will make a cost recovery estimation for 100 m³ recovered sludge.

The value of recovered oil (sludge, e.g., valorised to fuel oil) accounts for: 100 m³ * 200 Euro/m³ = 20,000 Euro

The costs of waste disposal (tank sludge is considered as hazardous waste) are accounted for by considering a disposal cost of about 520 Euro/m³. The corresponding saving for a total sludge recovery of 100 m³ is: 100 m³ * 520 Euro/m³ = 52,000 USD.

When not recovered, the sludge would have been manually removed, with the connected costs of manual cleaning to be added to the evaluation. Another item to be added to the evaluation is the time of the tank out of operation and hence storage capacity recovery. This depends on tank capacity and on time for manual cleaning (hence on sludge amount) and can not be normalized for 100 m³.A figure could be 0.026 Euro/day/m³ tank capacity.

For a 50,000 m³ tank and for a 45 days cleaning time reduction with ITW technology, the saving is: 0.026 Euro/day/m³ * 50,000 m³ * 45 days = 58,500 Euro

Finally, although not valuable in terms of money savings, the application of ITW technology results in a much safer operation for sludge removal.

These returns are even bigger in the case of oil tanker cleaning, where you have to add a freight cost of 50,000 USD/day (e.g., for a 120,000 dwt tanker) for each day needed to manually remove the tank. We can compute a minimum saving of about 80,000÷100,000 Euro/100 m³ sludge.

Normal figures of return on investment range from 3:1 to 11:1.For example, total savings for a 50,000 m³ tank, with 600 m³ sludge will be about 620,000 Euro.

CONCLUSIONS

By utilizing ITW technology for tank cleaning is possible to effectively recover and reutilize the sludge, thus achieving the following results:

• overall costs reduction

• hydrocarbons recovery

• reduction of sludge disposal costs

• safe and environmental friendly operations

• reduction of cleaning time

The recovered sludge gives no rise to operating problems in processing plants.

It is also possible to perform on stream tank cleaning without any detrimental effect for downstream equipment.

For tank cleaning, those advantages account for return on investment ranging from 3:1 to 11:1.

TODAYS METHOD FOR CLEANING OIL TANKERS

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