AIR BLOWING OF BITUMEN (ASPHALT) PROCESS

SOURCES OF ASPHALT (BITUMEN) 

• Asphalt is obtained as the ultimate bottom product of a vaccum distillation column(VDU) .
•  The residums obtained from VDU may still contain some oil, but further distillation of it serves no use at all.

• These residums are rich in asphalts and form the starting materials for air blown bitumen process.
• Asphalts are also obtained on a large scale from deasphalting units.
• Reduced crudes, waxy distillates, lube oils and long residums are usually deasphalted for getting quality lube oil.

CHEMICAL STRUCTURE OF ASPHALT

• Asphalts do not have general representative formula and are very complex in nature.
•  A sample of asphalt contains :

                 Oil                       :       35 to 50 %

                 Rosins                :        5 to 20 %

                 Asphaltenes     :         20 to 30 %

                 Acids                  :        upto 10 %

AIR BLOWING OF BITUMEN PROCESS

•  Air blowing of bitumen is mostly done in batch units.
• In this operation, asphalts from various sources are mixed and heated to a temperature of 200-210 degrees and sent to a reactor.
• The reactor may be horizontal or vertical type, usually made of mild steel , of a capacity to hold 1000 tons of charge stock .
• These reactors are fitted with air distributors at the bottom and also cooling and heating coils.
• During oxidation the temperature may increase rapidly, hence as a precautionary measure provision for cooling is made available.
• The reactor should be maintained at a temp. of 200 to 320 degrees , and air rate of 0.5 - 1.5 (meter cubes) at a pressure of 1.1 to 1.2 kgs/cm2 per hour, per ton of charge stock.
• Blowing time lasts for 10 to 14 hours depending upon the required consistency of bitumen.
• Gases are allowed to escape into refinery flare up or combustion system after being stripped with water.
• The product, air blown bitumen is obtained from the bottom of the reactor.
• The oxidation is also possible with sulfur but the cost of the process is very high.

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Enthalpy(H) - Entropy(S) Diagram OR Mollier Diagram

MOLLIER DIAGRAM 

The Mollier Diagram is named after a german Mechanical Engineer Richard Mollier. It is a graphical representation of steam tables, in which Specific Entropy is plotted along the ordinate(X-axis) and Specific Enthalpy along the abscissa(Y-axis). The diagram is divided into two portions by a somewhat horizontal line termed as Saturation Curve.The lower portion(i.e., wet stream region) contains the values of wet stream, whereas the upper portion (i.e.,superheated steam region ) contains the values of superheated stream. 

A Mollier diagram has the following lines:-

     1.    Dryness Fraction lines

     2.    Constant Specific Volume lines

     3.    Constant Pressure lines

     4.    Constant Temperature lines

Dryness Fraction lines :-

These lines are drawn in the wet stream region.i.e., only below the saturation curve (which represents dryness fraction equal to unity). These lines represent the condition of wet steam between various values of enthalpy and entropy. The dryness fraction lines are slightly curved in horizontal direction.

Constant Specific Volume Lines :-

These lines are drawn in both wet steam region and superheated steam region. These lines represent the specific volume of steam between the various values of enthalpy and entropy. The lines are straight in the wet steam region, i.e., below the saturation curve, but are curved upwards in the superheated region i.e., above the saturation curve.

Constant Pressure Lines :-

These lines are also drawn in both wet steam region and superheated steam region. These lines represent the pressure of steam between the various values of enthalpy and entropy. The pressure lines are also straight in the wet steam region, i.e., below the saturation curve, but are curved slightly upwards in the superheated region i.e., above the saturation curve.

Constant Temperature Lines :-

These lines are drawn only in the superheated steam region i.e., above the saturation curve. These lines represent the temperature of steam between the various values of enthalpy and entropy . The temperature lines are slightly curved in the horizontal direction.

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