The invention is concerned with a process of oxidation of bitumen or the like.
Bitumen is a hydrocarbon product which is received for example as the distilla¬ tion bottom product of an oil refining process and has a high boiling point and viscosity and is soluble in trichloroethylene. Also the soluble binder product separated from natural asphalts by extraction is bitumen. To improve the properties of bitumen 150/5 it can be oxidated to be suitable for intended applications. This oxidation can be carried out by leading air to the bitumen which is oxidated by the effect of the oxygen diffused into the oil phase from the air.
In modern preparation methods of oxidated (blown or semi-blown) bitumen qualities the air needed for the oxidation and the bitumen oil is brought into contact in a large volumed reactor (so-called bubble column technique). The drawbacks of the method are the safety risks because of the large oil and gas volume, the high oxygen content of the rest gas, a bad adjustability of the reaction and the reduced quality properties of the product because of the long retention time.
In patent publication WO 87/03896 the contact between the bitumen and the oxygen has been improved by using a turbine mixer in the oxidation of the bitumen. This turbine mixer comprises a mixer foreseen with an impeller that consists of a cogged face plate. The use of such a mixer does not, however, essentially improve the transfer of the material.
The published publication FI 78495 of the applicant is also mentioned as prior art which is concerned with a method of oxidation of bitumen by air in a large volumed reactor. The aim of this invention is to reduce the retention time of the oxidation reaction and thus avoid reducing of the quality properties. The cold properties are even improved. The aim is e.g. to achieve a bitumen product suitable to be used as road coating expressly in cold conditions, as in the Northern countries, which for instance means that no fractures are allowed to be formed into the coating.
The method of the invention is mainly characterized in that the oxidation is carried out by leading a supply stream of bitumen or the like and an air stream or another pressurized gaseous oxidant to a reactor which is a so-called rotating apparatus and in which reactor the contact between the air or other gaseous oxidant and the bitumen or the like is improved by generating turbulence by mixing devices of rotor-stator type that generate shear forces to achieve a good mixing. ”
The contact between the air and the bitumen is carried out in the invention by using high turbulence. The reactor used in the invention generates turbulence by using mixing devices of rotor-stator type that generate the shear forces: the bitumen and the air are forced through the reactor in such a way that strong shear forces and much mixing is directed to those with a good dispersion of the air as a result that is very advantageous for the contact. In the preparation method of the invention, the reactor used to improve the contact of the air- bitumen mixture, the so-called rotating apparatus, is a so-called power homogenizer, which is a mixing device of the type wherein the mixing is achieved ■ by one or more rotor-stator pairs, alone or in series.
It has been shown by tests in Bench scale that by the method of the invention the retention time needed can be reduced to a submultiple of the actual one. A shorter retention time and a better contact means an essentially smaller invest¬ ment in oil or gas and thus a diminishing of the safety risks and less use of air.
By the invention the bitumen and the air react considerably faster than with the actual bubble column technique, wherein mixing is not used and also faster than by means of conventional mixing devices. The total rate of the reactions is the essential factor. Coarsely, the rate of the reactions depends on two factors:
– the rate of the transfer of the oxygen from air to the bitumen – the rate of the reactions between oxygen and the bitumen.
The transfer of the materials can be further improved by influencing on the following things:
– the conditions in the reactor
– the construction of the reactor
– the amount of the reactors (different reactor couplings).
In the oxidation reactor the oxygen of the air and the bitumen react so that the oxygen molecule cleaves a part off from the long chained molecule thus forming short chained hydroperoxides and on the other hand long chained compounds containing oxygen. These long chained molecules containing oxygen tend to combine thus forming still more long chained molecules. This is a wished reaction as the bitumen can be processed in a wished way depending on how heavy bitumen is wished. For more detailed information reference is made to the following literature:
The rate of the transfer of the materials can in other words be influenced on by the construction of the reactor, in other words by selection of dimensions and amounts of rotor-stator pairs and by selecting the dimensions of the reactor. In other words the existing turbulence in the reactor can very much be influenced on by the inner construction of the reactor. Different cogs, guiding means for streams, pump effects and other things are factors by means of which the function of the reactor can be designed in the wished way.
A raising of the temperature in the reactor increases the diffusion coefficient and thus also the conversion coefficient. A suitable temperature in the invention is 180-240° C, but 150-300° C is a possible working range.
An increased amount of air accelerates the oxidation of the bitumen as then there is more oxygen in the reactor for a given amount of bitumen, whereat the amount of oxygen diffused into the bitumen is bigger. A suitable volume ratio liquid/air is 2/1 -1/50 when the reactor is pressurized.
An elevated of the pressure increases the amount of air in the reactor as air is an comprimable gas. The total time of the reactions will then be reduced and higher amounts of bitumen can be processed with the same device. The volume of the bitumen does not essentially change as an effect of the pressure. The inner overpressure of the power homogenizer to be used in the invention is as high as possible e.g. >. 6 bar, preferably j> 20 bar. The mechanical strength of the apparatus sets a limit for the overpressure. The small apparatus of the invention has lead to that use of high overpressures is possible and more safe.
The distribution of the retention time of the product and thus the product properties can be influenced on with an inner circulation in the apparatus, wherein the return is carried out from the outside of the reactor back to the inside. The curing of the product can be increased with circulation.
Also the material transfer in the reactor increases with an increasing turbulence. A suitable circumferential speed of the rotor is appr. 1-100 m/s.
The reactor can either be coupled so that the supply will come to the outer periphery of the reactor or to the middle of the periphery (peripheries) of the reactor. The turbulence in the reactor can be increased by supplying the bitumen and the air to the periphery of the reactor. If the air and the bitumen are fed to the middle of the periphery (peripheries) of the reactor, the need of pumping and comprimation energy is less. Also the apparatus costs are then lower.
It is possible to couple the oxidation reactors of the bitumen in several manners. Several reactors can e.g. be coupled in series and/or in parallel. It is also possible to couple reactors with the supplies to the middle of the periphery (peripheries) and to the outer periphery in different ways to each other. The best embodiment is the so-called “once through drive” wherein the bitumen and the air are flowing only once through the reactor system.
In another embodiment, both the bitumen and the air (rest air or fresh air) are pressurized (pumped/comprimated) and further are fed to the following reactor. Then it is question about series connection of two or more reactors.
Yet in another embodiment there are used a reactor of mixing container type that works continuously or batchwise. The advantages of the invention in production scale
The advantages of the invention in production scale have been evaluated in the following.
Safety
– in possible leakage situations the bitumen volume is considerably smaller as the pumping of the bitumen to the homogenizer can be interrupted at once and the bitumen content of the homogenizer is considerably smaller than that of a conventional blowing reactor. Also the gas volume is considerably smaller of this reason which means a smaller explosion and fire risk
– furthermore, thanks to this, there is a smaller amount to be safety blown; a simpler and more reliable safety blowing system. – as the reactions are more effective, the oxygen content is considerably lower than in the oxidation air exhaust gases which improves the fire safety.
Quality costs
– The starting of the production in full scale is appr. Vi – 1 days faster than the actual one;
(12 – 24 h x capacity) smaller amounts of heavy fuel oil with a high sulphur content is produced
– a smaller amount of products outside the quality requirements in disturbance situations
– the total quality of the product is better than previously;
– The control of the process is faster; the guiding is improved, the product is “right at once”. Economy
The investment in apparatus to the reactor of the invention is considerably cheaper than the bubble column.
– Less exhaust gases to be treated because of a more effective use of air
– the work hygienic drawbacks decrease for the final user because of the less volatility of the product.
In the following the invention is described by means of a test example which is not meant to restrict the invention.
EMBODIMENT EXAMPLE
To make sure that the invention works, the main components of the test appar- atus constructed are:
The flow graph of the apparatus is presented in figure 5. 2. Performance of the tests
The flow capacity of the liquid of the apparatus is 10 – 80 1/h. Other so-called guiding parameters that have an influence on the so-called oxidation capacity and the product properties and their influencing direction and strength has been studied e.g. in the following way:
0 – 6 bar (abs)
Distillated ECO-300 road bitumen raw material has been used as test material.
The tests have mainly carried out by the so-called “once through” principle which means that the bitumen oil to be oxidated has been fed only once through the homogenizer and the product samples to be analyzed have been taken from the process immediately after the outlet of the homogenizer and after the gas exhaust container.
The change between the penetration value of the product and the penetration value of the fed material, the so-called delta penetration, has been used as the measure of the oxidation capacity.
An appr. 5000-fold material transfer has been achieved with the test apparatus compared with the bubble column method of prior art. In other words, the same change of penetration is achieved with the new method in ca 1-3 seconds which with the prior art method was achieved by a retention time of ca 2 hours.
The above comparison is illustrated in figure 1.
The important factors affecting the oxidation capacity have been the inner pressure of the homogenizer and the flowing directions of the material in the homogenizer. The inner pressure increases the partial pressure of the oxygen and improves the diffusion of gas into the bitumen (figure 2).
The inner oxidation capacity, the so-called change of penetration in the “once through” drive has been achieved by means of overpressure and by changing the flowing directions of the materials to the opposite with respect to a conventional flowing direction. See figure 3a wherein the flowing direction is normal and figure 3b wherein it is changed. The bitumen oil is lead in in point 1 and the air in point 2. The product is coming out from point 3.
The effect of the above-mentioned parameters has been illustrated in figure 2.
The effect of the retention time on the oxidation capacity has been calculated by means of the inner volume of the homogenizer and the liquid flow of the apparatus. It can be stated about the results that when the liquid flow is increased to a level at which the calculated retention time is less than appr. one second, the oxidation capacity of the test apparatus begins to decrease. A graphi- cal presentation is shown in figure .
