Chapter 18, Problem 1P

To determine

To describe:

The process of distillation by which asphalt cement is produced from crude petroleum and the process of obtaining asphalt binders which is used to coat highly siliceous aggregates.

Explanation of Solution

Asphalt cement is obtained after the segregation of lubricating oils. It's semi-solid hydrocarbon with certain physio-chemical characteristics which make it a good cementing agent.The processes of refining that are used to obtain petroleum asphalts are mainly of two types:

1. Fractional distillation.
2. Destructive distillation.

Fractional distillation:

It includes the following steps:

1. Fractional distillation comprises of segregation of different materials in crude petroleum without substantial changes in their chemical composition.
2. It includes removal of different types of volatile materials in the crude oil by gradually increasing temperatures by using steam or by vacuum.
3. The crude petroleum at high temperatures gets separated in the distillation tower and gets collected on the successive trays.
4. The temperature is increased by steam or by vacuum process.

Destructive distillation:

It includes the following steps:

1. Destructive distillation also known as cracking includes application of enormously high temperature and pressure which results in the chemical composition of the materials.
2. Destructive distillation involves intense heat as high as 1100^OF and high pressure up to 735 lb/in² to produce effective changes in the chemical composition of the materials.
3. This process is mainly used when high quantities of the light fractions of materials are required.
4. The asphalt cement from this process is not used in pavement because it is more vulnerable to climate changes comparative to that produced from the fractional distillation.

The process of obtaining asphalt binders that can be used to coat highly siliceous aggregates can be defined as follows:

   1. Asphalt binders are formed from the asphalt cement, by breaking the asphalt cement into minute particles, usually 100 to 250 in penetration range and spreading them in water with an emulsifier.

   2. The small particles that are produced are electrically like charged, and hence they don't merge.

  3. Asphalt emulsions mainly consist asphalt around 55% to 70 % by weight with an emulsifying agent and a stabilizer in some cases.

  4. These emulsions are classified into three categories, which are anionic, cationic and nonionic. Out of these, cationic emulsions are more effective with the electronegative aggregates for example those having high percentages of siliceous material.