WO1996011242A1

WO1996011242A1 - Method of decreasing the fuming tendency of bitumen

Info

Publication number: WO1996011242A1
Application number: PCT/SE1994/000942
Authority: WO
Inventors: André GASTMANS
Original assignee: Nynas Petroleum AB
Current assignee: Nynas Petroleum AB
Priority date: 1994-10-07
Filing date: 1994-10-07
Publication date: 1996-04-18
Anticipated expiration: 1997-04-07

Abstract

A method of decreasing the fuming tendency of bitumen is disclosed which method comprises treating the bitumen with overheated steam and preferably also with air for an extended period at an elevated temperature.

Description

Method of decreasing the fuming tendency of bitumen

The present invention relates to a method of decreasing the fuming tendency of bitumen.

Certain bitumen of naphthenic origin are inclined to form fumes while being used in asphalt mixes. These fumes are sometimes called "blue smoke". They can be of considerable inconvenience to asphalt workers.

The fumes find their origin in the following aspects: First, there is the total amount of residual bitumen after straight run distillation. The more bitumen of a specific penetration grade (ASTM D5) yielded after distillation the lower also the end distillation temperature. As a consequence, not all the light or middle boiling ends have been distilled off or evacuated, When using such a bitumen in asphalt mixes, a kind of continued distillation in thin bitumen layers around the aggregates will occur. The amount of residual light-ends in the bitumen is manifested in a high loss on heating at 163°C for 5 hours (ASTM D 1754-1976) . Evidently, the higher the loss the more fuming the bitumen. Another factor to consider in this connection in the straight run distillation process is the residence time of the bottom residual product in the vacuum tower. The shorter this residence time the higher the amount of molecules with a lower boiling point. The conse¬ quence is again an increased fuming potential manifested in a high loss on heating.

In order to improve the distillation and evacuation of light molecules, overheated steam in the bottom of the vacuum tower is used in industrial processes to make azeotropic mixtures between the steam and the relatively light hydrocarbons. The relatively light hydrocarbons which are still in the bitumen at the bottom of the tower are in a way present at such a temperature that they should already be in gaseous state. However, the solubility power of the surrounding bitumen liquid is so great that the evaporation of the molecules is not done in the short time available in the distillation unit. The use of overheated steam releases these molecules and brings them in gaseous state out of the liquid bitumen.

This treatment results in a lower loss on heating (tested according to ASTM D 1754) . The use of overheated steam is well known from literature (vide e.g. "Le petrόle-Raffinage et Genie Chimique", P. Whithier ed. , Technip 1972). The residence time at the bottom of the vauum tower, however, is not increased by introducing steam at said bottom. The end effect on fuming is a reduction but the result is far from ideal.

Another origin of fume formation is the reaction with oxygen due to contact with air at the mixing temperature (150° to 185°C) at the mixing plant. The bitumen, covering the aggre¬ gates, is present in the form of a very thin layer (thickness 2-25 μm) . The surface area which is in contact with air is important. The impact of this oxidation reaction is a kind of cracking or dealkylation of the bitumen. The cracking effect can be described as a splitting off of side chains present in complex molecules. These cracked-off side chains have a boiling point which is lower than that of the parent mole¬ cules. Due to the enormous surface area of the aggregate covered by the thin layer of bitumen during the mixing process not only the newly-formed low boiling ends but also less low boiling molecules distill off to form fumes.

A lot of attempts have proven that the loss on heating (tes¬ ted according to ASTM D 1754) can be reduced by introducing steam at the bottom of the vacuum distillation tower. The distillation effect is improved but the effect of deal- kylation and accordingly the origin of an important factor of blue smoke formation has not been removed. The observations on blue smokes have provoked some prescrip¬ tions with respect to the total amount of particles per volume of air. The Occupational Health and Safety Association (OSHA) (US) made a proposal of a maximum of 5 mg of particles per m³ of air. The amount of fume around asphalt mixes is evidently linked to the mixing temperature and the duration of the contact exposure to air. The higher the temperature the higher will be the amount of particles per ³ of air and the duration of the contact with air at a certain asphalt temperature will have a corresponding effect.

It is an object of the present invention to provide a method by which a considerable improvement in the decreasing of the fuming tendency of bitumen can be achieved in comparison with the prior art methods discussed above.

This object is achieved according to the present invention by means of a method which is characterized in that the bitumen is treated with overheated steam and preferably also with air for an extended period at an elevated temperature.

The method according to the present invention hence utilizes a controlled set of influence factors, viz. time, tempera¬ ture, steam and, preferably also, oxygen (air) .

In the conventional process the residence time at the bottom of the vacuum tower (and hence the duration of the treatment with steam) is only in the order of 10 to 25 seconds at a temperature of around 340°C. According to the present in- vention the duration of the treatment with steam is increased more than tenfold. At the same time an increased amount of overheated steam and, preferably, a controlled amount of air are introduced.

These combined actions according to the present invention take place in a separate reactor, either after the bitumen leaves the bottom of the vacuum tower or when bitumen out of storage tanks are adequately heated up again.

According to one aspect of the method of the present inven- tion the treatment with overheated steam and/or air is carried out at a temperature within the range of from 200°C to 350°C, preferably between 250°C and 320°C.

According to another aspect of the method of the present invention the treatment is carried out with a mixture of air and overheated steam and/or with air and overheated steam in separate steps, overheated steam, possibly in admixture with air, being used in the last step.

Generally the volume relation between air and overheated steam in the overall treatment will be 0 to 70% (v/v) of air to 100 to 30% (v/v) of overheated steam, preferably 0 to 50% (v/v) of air to 100 to 50% (v/v) of overheated steam, calcu¬ lated on the total volume of air and overheated steam at 1 ata (steam) .

The addition of the controlled amount of air speeds up the reaction. However, it should be noted that the amount of air must be kept low for not damaging the binding characteristics of the bitumen. Indeed the reason for adding small amounts of air is to speed up a dealkylation of the most susceptible side chains without attacking the polar materials of the bitumen.

The total volume of overheated steam plus air, calculated at 1 ata, will generally be within the range of from 200 to 900 1/kg bitumen, preferably 300 to 600 1/kg bitumen.

The overall duration of the treatment with air and overheated steam will generally be within the range of from 30 minutes to 4 hours, preferably between 45 minutes and 3 hours. The invention will now be further described by means of a number of non-limitative working examples.

Example l

In a reactor, a bitumen of Venezuelan origin having a pene¬ tration value of 183 (xO.l mm) was treated at 250°C by passing 125 1 air/kg bitumen/h therethrough during 30 min followed by steaming for 2 h with 160 1 steam at 1 ata/kg bitumen/h at the same temperature. The exact pressure could not be measured. Simulated distillation of the remaining bitumen after this time is given in Table 1 below.

Example 2

In a reactor, a bitumen of Venezuelan origin having a pene¬ tration value of 181 (xO.l mm) was treated at 250°C by passing 160 1 steam at 1 ata/bitumen/h therethrough during 1 h. The exact pressure could not be measured. This treatment was followed by a treatment with air for h at the same temperature using 125 1 air/kg bitumen/h, followed by a treatment with steam for 2 h under the same conditions as above. Simulated distillation of the final residual bitumen is given in Table l below.

Example 3

In a reactor, a bitumen of Venezuelan origin having a pene¬ tration value of 178 (xO.l mm) was treated at 250°C with a mixture of air and steam as follows: 160 1 steam at 1 ata/kg bitumen/h (the exact pressure could not be measured) together wiht 125 1 air/kg bitumen/h during . After this treatment the bitumen was further treated for 2 h with 160 1 steam at 1 ata/kg bitumen/h. The exact pressure could not be measured. Simulated distillation of the final residual bitumen is given in Table 1. Comparative test

The amounts of volatile products in the products of Examples 1 to 3 above and in a bitumen of Venezuelan origin having a penetration value of 183 ("Bitumen 183") were investigated with a simulated distillation test on pure bitumen using a gas chromatography equipment (Perkin Elmer 8500) . In this test a small amount of bitumen is heated up gradually. An inert carrier gas brings the particles released at that temperature to burn at the end of the gas chromatography column and to integrate and express in volume calculated on the initial sample.In order to avoid a blocking up of the gas chromatography column, a pre-column was attached. This pre- column can be cleaned by backflushing.

The following results were obtained.

Table 1

Cummulative simulated distillation of bitumen (% on 100% (w/w) bitumen)

Up to ^*C Ex 1 Ex.2 Ex-3 Bitumen 183

360 0 0 0.013 0.089

370 0.014 0 0.028 0.148

380 0.056 0.012 0.054 0.256

390 0.160 0.066- 0.101 0.456

400 0.370 0.203 0.192 0.804

410 0.738 0.475 0.360 1.362

420 1.307 0.932 0.632 2.205

H 430 2.170 1.669 1.082 3.327

440 3.261 2.638 1.718 4.731

450 4.639 3.897 2.608 6.368

460 6.200 5.333 3.726 8.126

470 7.853 6.854 5.006 10.156

480 9.714 8.550 6.551 12.682

490 11.518 10.167 8.074 15,270

500 13.375 11.821 9.615 17.986

510 15.470 13.668 11.327 20.846

620 17.209 15.199 12.693 23.576

530 19.279 17.024 14.425 26.740

540 21.349 18.555 16.192 29.902

550 23.481 20.448 18.437 32.742

Claims

1. Method of decreasing the fuming tendency of bitumen, characterized in that the bitumen is treated with overheated steam and preferably also with air for an extended period at an elevated temperature.

2. Method according to claim 1, characterized in that the treatment is carried out a temperature within the range of from 200°C to 350°C, preferably between 250°C and 320°C.

3. Method according to any of claims l and 2, characterized in that the treatment is carried out with a mixture of air and overheated steam and/or with air and overheated steam in separate steps, overheated steam, possibly in admixture with air, being used in the last step.

4. Method according to claim 3, characterized in that the volume relation between air and overheated steam in the overall treatment is 0 to 70% (v/v) of air to 100 to 30%

(v/v) of overheated steam, preferably 0 to 50% (v/v) of air to 100 to 50% (v/v) of overheated steam, calculated on the total volume of air and overheated steam at 1 ata (steam) .

5. Method according to any of claims 3 and 4, characterized in that the total volume of steam plus air, calculated at 1 ata, is 200 to 900 1/kg bitumen, preferably 300 to 600 1/kg bitumen.

6. Method according to any of claims 3-5, characterized in that the overall duration of the treatment is between 30 minutes and 4 hours, preferably between 45 minutes and 3 hours.