Bitumen production

Asphalts (Bitumen – fossil resin from Latin) are solid, semi-solid or liquid water-insoluble  hydrocarbons with a small impurity of their oxygen, sulfurous and nitrogenous derivatives. Asphalts can be natural and artificial which are mainly derived from the rests of oil (asphaltum oil) distillation, cracking and clearing of oils (asphaltums, extracts).

        To make a grade classification of commercial asphalts depending on their quality various test methods have been elaborated and are applied now. These methods are authorized by the standards of different countries.

 

 

 

 

Properties of asphalts

Key parameters describing asphalt is penetration, ductility, softening point, viscosity, friability temperature. 

Penetration is a parameter describing the depth of a needle penetration into asphalts at the certain operating conditions. Penetration indirectly characterizes the degree of asphalts hardness. The higher the asphalt penetration at the designed softening point and asphalt friability temperature at the designed penetration, the higher its heat resistance is. To derive asphalts with a high heat resistance one can  make an appropriate selection of raw materials, process method and operating conditions.

 Ductility (extensibility) is an ability of asphalt to be stretched in a string. This parameter indirectly characterizes asphalt adherence and is related to the nature of its components as well.   The softening point is the temperature at which asphalts transit from a relatively solid state into a liquid one. The test is most frequently carried out by “the ring and ball method”.(RaB). The technique of the softening point determination is not scientifically grounded but it is widely used in  practice.

 Viscosity provides with a more detailed data on asphalts state at the different operating temperatures compared with the penetration and softening point . It can be easily measured in a short term at any production temperature required and with asphalt application. It is advisable that at the other equal parameters asphalt had the greatest viscosity at the maximum operating temperature and had a temperature-viscosity curve as more flat as possible.

 Friability temperature is the temperature at which the material is damaged as affected by the short-term applied load. Friability temperature characterizes behavior of asphalt in a road surface: the  lower the temperature, the higher the quality of asphalt cement is. The asphalts oxidized have a lower friability temperature than other asphalts of the same penetration.

Element structure of asphalts is approximately as follows: 80-85 % of weight - carbon; 8,0-11,5 % of weight - hydrogen; 0,2-4,0 % of weight - oxygen; 0,5-7,0 % of weight - sulfur; 0,5-0,7 % of weight - nitrogen.

With carrying out the componential structure analysis there is usually a determination of oils contents in asphalt (the sum of naptha, naphthene and aromatic connections), pitches (high-molecular connections of cyclic and heterocyclic structure of a high condensation degree), asphaltenes, paraffin waxes.

Oils reduce hardness and asphalts softening point , increase fluidity and vaporizability.

Pitches are intermediate substances between oils and asphaltenes. They provide asphalt with  hardness, plasticity and extensibility.

Asphaltenes are the products of pitches condensation. The rise of the asphaltenes content in asphalt increases its hardness and softening point.

Paraffin waxes enhance asphalts friability at the reduced temperatures.

Asphalt structure depends on the oil nature, structure of the initial raw materials and on the manufacturing techniques It is different for asphalts of the identical softening point derived from various oils.

To determine oil suitability for the asphalts production the formulas stated below are used:

А+С - 2,5*P≥8 - it is most suitable for the asphalt cement production;

0≤А+С - 2,5*P≤8 - it is suitable for reception of asphalt of some road marks;

А+С - 2,5*P≤0 - reception of asphalts of building, roofing, insulating marks only is possible.

 A- asphaltenes content, % wt.;

C - pitches content, % wt.;

P - paraffin waxes content, % wt.

Industrial production of asphalts

The best grades asphalts oxidized are received from the rests of high resinous low in paraffin oils, mainly from asphaltum oil.

The major factors which influence the asphaltum oil oxidation process are: the nature of raw materials - oil, initial asphaltum oil softening point, the content of oils in it, paraffin-naphthene connections, asphaltenes in it, temperature, air rate, oxidizing period, pressure and liquid level in a reactor.

Operating temperature. The higher the oxidation temperature, the faster the process is. But at a too high temperature the reactions of carbenes and carboids cyclization are accelerated. The rests received from high resinous asphalt and mixed oils are oxidized at 250 - 280°С, the rests of paraffin oils - at 270 - 290°С. Taking into consideration the nature of raw materials and asphalts properties required it is necessary to select a corresponding oxidation temperature; for most of the raw materials with due account for the economic feasibility it is close to 250°С.

  Pressure. Increase of pressure in a reaction zone promotes the oxidation process intensification and quality improvement of the asphalts oxidized. Asphalt cements are inappropriate to receive in a columnar reactor at the pressure of above 0,4 MPa owing to a sharp decreasing of asphalts extensibility. Oxidation under pressure enables to use raw materials with a low oil content and with all that to derive asphalts which have high extensibility, penetration and plasticity interval parameters.

 Air rate. The rate of air, its degree of dispergation and distribution on cross section of an oxidation column influence the intensity of process and asphalts properties to a great extent. With increase of the air rate by 1 ton of raw materials up to a certain point (1,4 cubic meters / mins) the process efficiency raises, then at the further increase the degree of air oxygen use worsens and the efficiency is reduced; heat resistance of the asphalts oxidized thus raises.

In industrial practice the process of oil residual fractions oxidation by air is carried out in the units of different type: batch still, tubular soaking reactors and hollow columns of continuous action.

Oxidation in the still - a hollow cylinder with a small operational area height-to-diameter ratio is  carried out on at old installations or with production of low-tonnage asphalts grades. This method is used abroad as well. 

Oxidation in a tubular reactor - a reactor with a vertical pipes arrangement - occurs in a turbulent air stream. Air movement and raw materials oxidized is direct-flow. Liquid-gas mixture reacted flows from a reactor to the vaporizer where it is separated into gases and liquid. Gases outgo from the vaporizer top for disposal and the liquid phase – asphalt-is pumped out from the vaporizer bottom in the tank farm.

Oxidation in vertical vessels . For the last years piped oxidation columns are widely applied as continuous asphalt unit reactors. Continuous oxidation column is characterized by a high efficiency, simple design, it can be operated easily while in service. Several identical columns at the unit provide with flexibility in the operation which is very important at a wide assortment of the asphalts derived and its seasonal fluctuations. The advantages of the oxidation process in vertical vessels are also an opportunity of thermal oxidation behavior regulation due to the change of temperature of the raw materials flowing into columns, low pressure compressors application and a high automation level.

Residual asphalts are almost not made in Russia . The characteristic features of residual asphalts as opposed to the asphalts oxidized are: a relatively high density, high hardness and tear resistance, temperature sensitivity. Weather-proof residual asphalts are derived from high resinous (asphalthene) oils.

To receive residual asphalts only certain oil grades can be used – of naphtene and naphtene and aromatic base i.e. heavy oils with a small paraffin content

Residual asphalts production is based on the atmospheric vacuum distillation of first-quality oils. Asphalt is carried off as a commercial product from the bottom of AVDU (Atmospheric-Vacuum Distillation Unit). In some cases there is an additional vacuum column specifically for asphalt reception at the AVDU. With regulation of the selection process conditions it is possible to derive residual asphalts with a various penetration.

Precipitated asphalts (asphaltums) are derived in the course of asphaltum oil deasphaltizing. Deasphaltizing process conditions (thermal gradient in an extraction column, a propane/raw materials ratio) are regulated in relation to the asphalt quality required. In a such process deasphalted oil (the raw material for catalytic cracking, hydrocracking) is already a by-product. It is common usage to apply paraffin or mixed based oils inappropriate for the direct asphalts production. The process enables to create a greater variety of raw materials resources of the asphalt production.

Asphalts from the asphaltums derived from deasphaltizing contain less paraffin-naphthene base connections and more pitches and asphaltenes. It causes their less penetration, plastic range and larger extensibility, friability temperature and cohesion compared with asphalts of the same softening point received by asphaltum oil oxidation of the same oil.

In asphalts production the process combinations are used as well. For example, further oxidation of  the residual asphalts, compounding of the asphalts overoxidized or precipitated.

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