The present invention concerns a high-yield and low-pollutio process to re-refine used lubricating oils. In this particular branch technique, the term "re-refining" means transforming used oils and fats derived from engines or from other industrial uses - into base oils. Su base oils are then used, after mixing with suitable additives, as n lubricating oils having qualitative features which can be fully compar to those of the oils directly obtained by petroleum refinery.

The first industrial plants for re-refining used oils appeared i the nineteensixties, mainly in response to the considerable ecologic problems resulting from the ground or water disposal of used oils. In th years that followed - due, on one hand, to the consumption increases an on the other hand, to the costs of virgin oils of first refinery - th re-refined oils have been conquering an increasingly important positi even as raw material.. At present they are competitive, both as far availability and as far as cost, with the oils directly obtained fr petroleum.

The increasing economic interest in re-refined oils has led th governmental authorities to be more sensitive to the problem of the collection and concentration, by creating associations for collecti used oils, to which all the users are compelled to deliver their oil This trend is up-to-date still in full expansion; in fact, in the E countries, against an annual consumption of lubricating oils amounting about 4 millions of tons, and a corresponding theoretical quantity recoverable used oils amounting to about 2.5 millions of tons, only 1. millions of tons of said used oils are actually collected and deliver to the re-refining centres, the remaining amount being disposed of in uncontrolled manner, thus certainly harmful for the environment.

There are substantially two processes, mostly known and adopted technique, to carry out the re-refining of used oils: the sulphuric ac process and the propane process. In both these processes, the used oi

are first of all freed from the water contained therein, and are then purified - with sulphuric acid or, respectively, with propane - to elimi¬ nate the other impurities present therein. These last ones substantiall consist of compounds with high molecular weight contained in the oil additives, and of metals derived either directly by wear of the metallic parts, or indirectly by the metals present in the fuels or in the oil additives, as for example lead in petrols.

The sulphuric acid process was the first one to be conceived an adopted, but is now being dropped due to the considerable pollution pro¬ blems which it involves. Said process essentially comprises the followin steps, as shown on the block diagram of fig. 1:

A - preflash, under moderate vacuum, of the used oils Al in order to separate a head product, formed by water A2 and light hydrocarbons A3 (gas oil), from a tail product formed by the dehydrated oils A4; B - treatment of the dehydrated oils A4 with concentrated sulphuric aci Bl, in an amount of 20-25 by weight, in order to obtain the extrac¬ tion and then the decantation of the compounds with high molecula weight and of the heavy metals; and

- separation of the acid sludges B2 with high content of oily fraction from the purified and acidized oils B3;

C - neutralization of the oils B3 and decolorization of the same with mixture Cl of lime and decolorant earths; and

- filtration of said oils, allowing to obtain decolorized neutral oil C3 and to recover exhausted earths C2 soaked in high concentration of oily fractions;

D - hot vacuum distillation of the oils C3, with separation of the pro cess water and of the gas oil Dl from the column head, and of th re-refined oils D2 into fractions of different viscosity from latera outlets of the column; injection of superheated steam from the colum bottom;

E - final decolorization of the oils with decolorant earths El, to obtai a lubricating base E3, with recovery of exhausted earths E2 soaked i

high concentrations of oily fractions.

Fig. 1 also indicates the fluid masses flowing into and out of eac single block, with reference to an inlet flow-rate of used oils Al equa to 100. This shows that the final yield of the sulphuric acid proces does not even reach 60 .

A considerable amount of oils is in fact retained in the acid slud ges B2 and cannot be easily recovered therefrom. The composition of sai sludges is in fact quite varied and includes - besides sulphuric acid an the hydrocarbon compounds - polymeric compounds with high molecula weight, organic and inorganic compounds of sulphur, chlorine, nitroge and phosphorus, and heavy metals as chromium, copper, zinc, iron, nicke and lead.

Seen the technological problems which an ecological elimination o this type of waste would involve, the solution still adopted at presen in the existing plants is that of the disposal in dumps or quarries. A times, disposal is preceded by an operation to neutralize the acid slud ges with lime, which however considerably increases their volume and th costs for their elimination, considering that 0.4 to 0.5 kg of lime ar required for each kg of acid sludges. Also the incinerating technique i unadvisable, as it requires the installation and operation of complicate and costly equipment to depurate the smokes.

It is above all due to the pollution damages caused by the elimina tion of the acid sludges (less in volume, but certainly not less harmfu than those produced by the direct elimination of used oils) that th installation of new re-refining plants, making use of sulphuric acid, ha rapidly ceased as soon as the second of the known processes, making us of propane, has appeared on the market.

The block diagram of this second process is shown in fig. 2, in way similar to that done for the above described sulphuric acid process also in this case, the different flows into and out of the single block are indicated with reference to a feeding flow-rate of Al equal to 100 In the propane process, the acid treatment step is totally replaced by a

operation B of clarification with liquid propane.

Said hydrocarbon can in fact be easily liquefied and has, in this liquid phase, a very low density. When mixed with the used oils, propane thus acts thereon as a thinning and fluidizing agent, allowing to easily obtain the separation of a first liquid fraction Bl with high viscosity, containing the high polymers and heavy metals - generally indicated by "asphalts" - from a second liquid fraction B2 consisting exclusively of the clarified and dehydrated oils. The plant of course provides for a depropanizing treatment of the liquid fraction B2, and for the recycle of the propane thus recovered.

The successive filtration step C with decolorant earths Cl is car¬ ried out with a hot treatment so that, as well as recovering the exhaust¬ ed earths C2, one also recovers a gaseous fraction C3 consisting of a minor portion of the gas oil contained in B2. The remaining part of gas oil abandons this block together with the decolorized oils C4 and is recovered in Dl in the successive distillation step D. The distillation step D and the final decolorization step E are fully identical to those previously described with reference to the sulphuric acid process.

The propane process represents a considerable improvement in res¬ pect of the sulphuric acid process, both as far as yield - which has risen to about 68 - and as far as pollution, which is notably reduced. Nevertheless, also this process has some inconveniences. To start with, the handling of propane creates problems of safety for the workers on account of its high explosiveness; furthermore, in spite of the above mentioned depropanizing treatments, the heavy fraction of asphalts Bl still shows traces of propane. This even small quantity of propane is sufficient to prevent the use of such asphalts in potentially interesting fields as the roadway or the building field. The presence of propane is in fact apt to notably increase the inflammability of asphalts, making it on one hand difficult to handle them when bituminous roadbeds have to be formed, and preventing on the other hand one of the most interesting uses thereof in the building field, namely the forming of sealing coatings. In

fact, sealing coatings realized with such asphalts could not - as usuall done - be flame-welded, without risking fires.

Consequently, in the propane process, the asphalts should be consi¬ dered in every respect as a waste product, which can be disposed of - b previous dilution with gas oil so as to make it sufficiently fluid - onl as fuel having a good heat value but a high pollution rate. A typical us thereof is in the casehardening furnaces, already provided with specia equipment to depurate the smokes and apt to eliminate also the pollutant present in such asphalts.

Finally, from the energetic point of view, the propane process i far more demanding than the sulphuric acid process, both in terms o electric energy consumption and in terms of fuel consumption to produc the process steam and to heat the oils.

In recent years, a third type of process has been introduced where in the used oils, after the conventional dehydration step, are subjecte to a thin-film distillation process. In said continuous process, the use oils are sent to an axial rotor of a distillation column and, from sai rotor, they are sprayed onto the inner walls of the column, which ar kept at high temperature by a diathermic fluid flowing into a lining o said column.

In contact with the column wall, the light vaporizable fraction o the oils instantly turns into a gas phase, while the heavy fraction stay liquid and collects onto the bottom of the column. The vaporized fractio is then collected and fractionated into the desired cuts in a successiv fractionating column.

This continuous process allows interesting yields, but involves series of inconveniences which limit its diffusion. To start with, it re quires a pretreatment to purify the used oils so that it can be continue for a fairly long time without having to provide for extra cleaning ope rations; in particular, the presence of the rotor makes this process ver delicate as far as fouling is concerned, and makes periodic maintenanc very costly as far as cleaning is concerned. In the second place, sai

process involves high capital and running costs which do not make it eco nomically competitive in respect of the propane process. Finally - sinc the height of the thin-film distillation column has to be limited due t the need to fully draw out the rotor for its cleaning - when the proces has to be applied to high feeding flow-rates, it requires the use o several parallel columns, without being able to achieve any scale saving.

The inventors of the present patent have thus proposed to themsel ves the object of preparing a process to re-refine used oils: allowing t reduce or fully abandon the use of reagents which are difficult to elimi nate or anyhow harmful for the environment or dangerous for human safety; apt to be carried out with a simple and compact plant, not weighing o the production costs and also allowing to reduce energy consumption; ap to be carried out in a completely static plant, the single components o which should not include any moving parts, highly subject to great foul ing problems typical of the used oil treatments; and, finally, apt t increase the still not satisfactory yields of the known processes, bot for what concerns the quantity and quality of the recovered oils, and fo what concerns the possibility to utilize the by-products in economicall profitable uses, apt to make said process globally more advantageous.

According to the present invention, said objects are reached with process to re-refine used oils - of the type in which said oils are sub jected to a first preflash treatment to eliminate the water and the mos volatile fractions contained therein, to a purification treatment to eli minate the high polymers and heavy metals, to a fractional distillatio treatment to obtain one or more oil fractions for use as lubricatin bases, and finally to a decolorization treatment - characterized in that:

- said fractional distillation treatment is carried out into single filled-type column;

- said purification treatment is carried out in three successiv steps: a first step of pretreatment wherein, before said preflash treat ment, a highly basic reagent is added and mixed with the oils; a secon step wherein, before the fractional distillation treatment, the oil

resulting from the preflash treatment are decanted; a third step where a heavy liquid fraction containing said high polymers and heavy metals discharged from the base of said fractionating column.

Further characteristics and advantages of the re-refining proce according to the present invention will anyhow be more evident from t following detailed description of a preferred plant, into which sa process is carried out, illustrated on the accompanying drawings, which:

Fig. 1 is a block diagram, illustrating the known-type sulphur acid re-refining process;

Fig. 2 is a block diagram, illustrating the known-type propane r refining process;

Fig. 3 is a block diagram, illustrating the process to re-refi used oils according to the present invention;

Fig. 4 is a diagram of the plant to carry out the process of t invention, wherein the basic reagent addition step and the preflash st are illustrated in further detail; and

Fig. 5 is a diagram of the plant to carry out the process of t invention, wherein the fractional distillation step and the step separate the heavy fraction containing the high polymers and heavy meta are illustrated in further detail.

The process to re-refine used oils, according to the present inve tion, differs from the known processes for the fact of having notab simplified the purification treatment and of having combined into a si gle column all the distillation treatments (which, in the known proces es, are carried out into several cascade columns) as well as the oper tion to separate a heavy fraction containing the high polymers and hea metals. Moreover, this last separation is carried out by means of cyclone device positioned in the lower part of the distillation colum which does not provide for the use of any movable parts and is therefo not subject to problems of fouling or blocking, caused by the high visc sity and the special composition of the materials being treated.

Fig. 3 reports a block diagram illustrating the main steps of th process according to the invention. Like the similar diagrams of figs. and 2 - illustrating the known processes and already discussed heretofor - this diagram indicates the fluid masses flowing into and out of th single blocks, with reference to a feeding flow-rate of used oils A equal to 100.

As said above, the treatment to purify the used oils from the hi polymers and heavy metals includes a pretreatment A with a basic reagent A reagent preferred for this purpose is NaOH, which is added in A2 an mixed with the oils Al before the preflash treatment. Said pretreatmen does not act directly on the above pollutants, but helps on one hand t saponify the fatty acids present therein, favouring their precipitation and on the other hand to neutralize the chlorine present both in boun and in free form. The saponification of the fatty acids, besides facili tating their elimination by decantation - hence improving the flowabilit of the system - destroys their volatility and thus prevents said acid from turning into vapour in the distillation column, thereby pollutin the oils both directly and with the polluting parts eventually boun thereto. The neutralization of chlorine also preserves the plant fro dangerous phenomena of corrosion, lengthening its working life.

The successive preflash treatment B involves - as in the processe of known technique - the separation of an oily water flow Bl and a ga oil flow B2. Before being sent to the successive step, the flow of deh drated oils B3 resulting from the preflash treatment is left to rest fo a length of time sufficient to obtain the decantation of the saponifie or flocculate parts.

The flow of oils, thus treated, is sent to a treatment of therma deasphalting and of fractional distillation C which simultaneously invol ves, both the separation of a light phase Cl containing gas oil and of heavy phase C2 containing the high polymers and heavy metals, and th fractionation of the oils into three cuts C3 of different density. Th three cuts thus obtained, suitably purified with a stripping treatment

are sent to the final decolorization treatment D - known per se - int which is fed a flow of decolorant earths Dl and from which the flow o exhausted earths D2 is removed, thereby obtaining a main flow of re-refi ned oils D3, with a very high yield equal to about 7 . With an additio nal process to recover the oils into which the exhausted earths D2 ar soaked, the yield can even rise to 75%- Alternatively, the decolorizatio of the re-refined oils D3 can be carried out with a hydrogen treatment also perfectly known in technique.

The process steps of blocks A, B and C are now described in detai with reference to the plant illustrated in figs. 4 and 5-

Fig. 4 reports the blocks A and B. The used oils Al are fed throug the pump 1 into the preflash column 2. Into the outlet of said pump i also fed the basic reagent A2, with a flow-rate controlled by the meter ing pump 3- As specified above, said reagent preferably consists of NaO and is added, as a 30% solution, in an amount such as to settle the pH o the used oils - at the outlet of column 2 - at a value between 10 and 13 and preferably between 11.8 and 12.2. The flow-rate of the pump 3 is thu preferably regulated automatically by a pH control device 4 positioned a the outlet of column 2.

The used oils Al and the reagent A2 are then thoroughly mixed int a line mixer 5, and are subsequently preheated to a temperature of abou 120°-140°C into the exchanger 6 fed by the steam line VAP.

The preflash column 2 is kept under a moderate vacuum degree, equa to about 200 mmHg, by means of an ejector 7 fed by the same steam li VAP. The flow A3 fed into column 2 is then separated into a vapour phas containing water and gas oil, and into a liquid phase, containing t dehydrated oils and the pollutants. The vapour phase is condensed by t water-cooled condenser 8a and sent to the gravity separator 9> The sa separator also receives the steam let out from the ejector 7 , after has been condensed in the condenser 8b.

The separator 9 provides to separate an incondensable gaseous pha GAS - which is sent to a high-temperature burner, so as to destroy a

gaseous pollutants present therein - from the two already described li quid flows Bl and B2. The water flow Bl, slightly oil-polluted, is sen to a biological depuration plant together with the process waters let o of the other parts of the plant. This is actually the only waste produc of the process according to the present invention, which does not excee 4% by weight of the feeding and which creates no particular problems fo its proper elimination. The gas oil B2 - of excellent quality, seen tha the oils have not been subjected to treatments with polluting reagents can be introduced in the normal trade channel of fuels.

The bottom product B3 of the column, eventually condensed by cooling coil 10, is sent by a pump 11 into a collection tank 12. The di mensions of the tank 12 and the positioning of the inlet and outlet pipe are calculated so that the stay time therein of the flow B3 is at leas 48 hours. In this way, the saponified fatty acids and the other floccula te parts - thanks to the treatment with NaOH - are able to decant on th bottom of the tank 12, from which they are periodically removed throu the drain 13, entraining therewith a certain amount of impurities.

Fig. 5 illustrates the thermal deasphalting and fractional distil lation treatments carried out on the flow B3 of dehydrated oils. Sai oils are first of all heated to a temperature of about 36θ°C in the me thane gas furnace 20. The oils are then fed to the bottom of the fracti nating column 21, into which is kept a vacuum degree of at least 10-2 mmHg (at the top of the column), thanks to a main ejector 22a, seri connected to two secondary ejectors 22b and 22c, all fed by the VAP lin More precisely, the heated and partially vaporized oils are sent to t top of a cyclone device 23, positioned in the lower part of the column and consisting of an open-spiral channel leading down around a lar central tube which connects the bottom part of the column to its pa immediately above.

At the inlet of column 21, the flow B3 almost totally vaporizes a the cyclone device 23 allows a perfect and complete separation of th gaseous phase from the liquid phase, which contains the very heavy oi

fractions and all the pollutants consisting of the high polymers heavy metals. This liquid phase C2, commonly called "asphalts", is s to the collection tanks through a pump 24.

The vapour phase, released from the cyclone 23, moves up along column 21, meeting four successive filling packs 25, positioned o respective collection plates 26, which allow - in known manner - to through the vapour phase which rises towards the top, and to retain liquid which has condensed. The fractionating column 21 of the pres invention has been conceived so as to allow cutting the inlet flow int head fraction, substantially containing the process vapour and the oil, and into three intermediate cuts (indicated by C3/I, C3/II C3/IH) of re-refined oils of different density, ready for the differ commercial uses, as well as into the already described tail fraction C

To obtain this result, the filling should have a very low f resistance (5-10 mm Hg at most, for each pack) and a high gas/liq contact surface. An extremely positive result has been obtained by us a metal filling consisting of side-by-side, superposed, accordion-fol metal sheets, forming zig-zag channels allowing the vapours to rise, average size from 1 to a few centimeters. A filling of this type, m from stainless steel, is produced by Glitsch Italiana - Campoverde Aprilia (Latina), Italy - and is known on the market under the trade n GEMPACK.

The liquid cut collected on the top plate 26a, namely the pl positioned under the first filling pack 25, is reflowed by the pump into a column zone just below said plate and, after having been coo into the water exchanger 28, it is sent to the head of the column 21. portion exceeding the requirement of the reflux is collected in C1, gas oil, to be put on the market together with the flow B2 separated the previous preflash step.

The liquid cuts collected on the two underlying plates 26b and are reflowed into the column zones just below said plates, through circulation pumps 27b and, respectively, 27c. The portion of these

liquid cuts exceeding the reflux requirements are sent to separate part of a stripping column 29 formed of three different sections, into one o which also flows directly the cut collected on the bottom plate 26d. I the three sections of the stripping column 29 the three cuts are purifie even further, in a countercurrent of steam fed from the VAP line an superheated in the furnace 20.

The cuts collected in the stripping column 29 are finally sent t the collection tanks by the pumps 30b, 30c and 30d, after cooling int air coolers 31-

The flows coming from the main ejector 22a and from the secondar ejectors 22b and 22c, containing process vapour, incondensable gases an gas oil vapours, are condensed into the water condensers 32 and then col lected into a gravity phase separator 33- In the separator 33 , "the ga oil phase escaped from the head of the column 21 is separated from th process water phase and from the incondensable gases. The gas oil is fe in Cl; the water flow 34 is recycled to the steam production boiler after having possibly undergone a depollution treatment; while the incon densable gases are sent into GAS and then into a combustion furnace t eliminate any possible polluting gases.

The cuts of re-refined oils thus obtained - heavy oils C3/l, middl oils C3/II, light oils C3/HI - are sent to a final decolorization plant wherein said oils are subjected - in known manner - to a decolorizatio treatment with decolorant earths or with hydrogen, so as to make the suitable for the market.

The asphalt flow C2 - thanks to the total absence of solvents an for the very fact of containing, as vehicle of the pollutants, a fractio of very heavy and viscous oils (bright stock) - can be marketed, in a economically interesting way, to form bituminous roadbeds, to which i confers special elastomeric and deadening characteristics.

The process to re-refine used oils according to the present inven tion is thus particularly advantageous compared to the processes of know technique. In fact, apart from being carried out into a considerabl

simple plant, which thus involves less installation costs, there is al a reduced need for periodic maintenance, thanks to the basic purificati pretreatment as well as to the complete absence of movable parts.

Also as far as running costs, the process of the present inventi is highly competitive, as results from the following comparison Tab reporting the costs of the expendable products, as well as the costs f the disposal of the waste products, which - as can be seen - are altog ther 25% in respect of the costs met with the sulphuric acid process, a 6θ% in respect of those met with the propane process.

Finally, the waste products of the process merely consist of t water separated from the oils in the preflash step. Said water forms very small fraction - about 4% - of the initial feed Al, and anyh creates, as seen above, no ecological problems as far as disposal concerned.

T A B L E

PRODUCTS TYPE OF PROCESS

(per ton of used oil)

Sulph. Acid Propane Invention

160.2 215.8 75-6 102 3-44 4-9

2.5 119-4 1

190.67 131

TOTAL COSTS

(per ton of used oil)

YIELDS

TOTAL COSTS (per ton of re- refined oil produced)

Reference values:

Natural gas

Electric current

Reagents

Propane

Acid

Nitrogen

Decolorant earths

Disposal of exhausted earths

Disposal of acid sludges