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.