Conversion

The separation processes described above are based on differences in physical properties of the components of crude oil. All petroleum refineries throughout the world employ at least crude oil distillation units to separate naturally occurring fractions for further use, but those which employ distillation alone are limited in their yield of valuable transportation fuels. By adding more complex conversion processes that chemically change the molecular structure of naturally occurring components of crude oil, it is possible to increase the yield of valuable hydrocarbon compounds.

Purification

Before petroleum products can be marketed, certain impurities must be removed or made less obnoxious. The most common impurities are sulfur compounds such as hydrogen sulfide or the mercaptans — the latter being a series of complex organic compounds having as many as six carbon atoms in the hydrocarbon radical. Apart from their foul odour, sulfur compounds are technically undesirable. In motor and aviation fuels they reduce the effectiveness of antiknock additives and interfere with the operation of exhaust-treatment systems. In diesel fuel they cause engine corrosion and complicate exhaust-treatment systems. Also, many major residual and industrial fuel consumers are located in developed areas and are subject to restrictions on sulfurous emissions.

Most crude oils contain small amounts of hydrogen sulfide, but these levels may be increased by the decomposition of heavier sulfur compounds during refinery processing. The bulk of the hydrogen sulfide is contained in process-unit overhead gases, which are ultimately consumed in the refinery fuel system. In order to minimize noxious emissions, most refinery fuel gases are desulfurized.

Other undesirable components include nitrogen compounds, which poison catalyst systems, and oxygenated compounds, which can lead to colour formation and product instability.

Exercise 7. Fill in the gaps using the list of words and word combinations below.

separation hydrocarbon sulfur conversion purification absorption

Basic refinery processes include separation, conversion and ____. The function of the separation process is to separate many types of _____ present in crude oil into fractions. The main processes of separation are distillation, ___ and crystallization. ___ processes are based on differences in physical properties of the components of crude oil. _____ processes chemically change the molecular structure of the components of crude oil. In diesel fuel _____ compounds can cause engine corrosion and complicate exhaust-treatment systems.

Exercise 8. Match these words with their definitions.

1. Refinery a) a liquid that can change a solid substance into liquid

2. Fractional distillation b) plant, designed to process specific

crude oils into selected products

3. Reflux c) part of the condensed liquid

4. Sidestreams d) process employed to recover valuable light

components from the vapors that leave the top of

crude-oil or process-unit fractionating columns within the refinery

5. Bitumen e) the primary process for separating the

hydrocarbon components

6. Absorption f) intermediate products

7. Solvent g) the residue remaining after vacuum distillation

Exercise 9. Decide whether the following statements are true (T) or false (F) in relation to the information in the text. If you think a statement is false, change it to make it true.

1. ___ Fractional distillation units are much larger than modern crude oil distillation units employed in chemical and other industries.

2. ___ The liquid absorbs heavier constituents from the vapour and gives up lighter constituents to the vapour phase.

3. ___ In comparison with vacuum distillation fractional distillation allows for distilling heavier materials at lower temperatures.

4. ___ The separation processes are based on differences in physical properties of the components of crude oil.

5. ___ The crystallization of wax from lubricating oil fractions is essential to make oils suitable for use.

Exercise 10. Answer the following questions.

1. What is refinery designed for?

2. What are the main refinery processes?

3. What do crude oil distillers separate?

4. Tell about the processes after crude oil is withdrawn from storage tanks?

5. What happens with a small amount of uncondensed gas?

6. What distinguishes fractional distillation from simple distillation columns?

7. Where does unvaporized oil entering the column flow?

8. Why is steam injected into the bottom of the column?

9. What is the difference between vacuum and fractional distillation?

10. How may bitumen be used?

11. What is the purpose of absorption?

12. What is the purpose of crystallization?

Exercise 11. Put the verbs in brackets into correct grammar form.

1. Basic refinery processes (to perform) three functions.

2. A crude oil distiller (to separate) crude oil into fractions.

3. As demand for automotive fuel rose, methods for continuous distillation of crude oil (to develop).

4. Reflux (to pump) back into the top of the column and (to descend) from tray to tray.

5. In order to reduce the partial pressure of the hydrocarbons and assist in the separation steam (to inject) into the bottom of the column.

Exercise 12. Read and translate the text.