DEFINITION :

Naphtha is a general term used to describe special boiling point spirits having a boiling range of approximately 30 to 170 deg. C

NOMENCLATURE

Naphtha is widely used in fertilizer plants and petrochemical industries as a feed stock. It is a highly volatile product, manufactured from crude oil by direct atmospheric distillation and by catalytic cracking of heavy residues. There are two types of Naphtha marketed namely, High Aromatic Naphtha (HAN) and Low Aromatic Naphtha (LAN) known as Naphtha (Petrochemical). Naphtha essentially consists of paraffinic, naphthenic and aromatic Hydrocarbons. The presence of Aromatic Hydrocarbons in Naphtha is very critical especially when it is used in fertilizer plants. In fact, the design of a fertilizer plant may entirely depend upon the composition of Naphtha available or a refinery has to produce Naphtha according to the needs and specifications demanded by a fertilizer plant. This is one of the reasons that IS Specifications for Naphtha has been withdrawn.

Naphtha is used as  a fuel in fertilizer plant reformers where high temperatures are required. It is also used as a fuel for steam generation in the plants where reforming is done with the help of steam. Some gas turbines for power generation have also been installed recently which will require Naphtha as fuel.

AROMATICS

The most important criterion for the selection of Naphtha as a feed stock in a fertilizer plant, is its aromatic content. This is because, in a fertilizer plant, Naphtha is reformed to carbon dioxide and hydrogen in presence of a catalyst which are used for the manufacture of Urea and Ammonia. Aromatics are basically resistant to reformation and hence, high temperatures are to be maintained or more time will be required to complete reformation which results in loss of production. Aromatics also burn with smoke releasing a fine soot of carbon which gets coated on the catalyst thus, reducing catalyst activity which in turn, reduces production. When Naphtha with high aromatic content is used in a petrochemical plant, say for example manufacture of Ethylene, production will decrease since it is difficult to convert aromatic hydrocarbons to Ethylene. Hence, low aromatic Naphtha is preferred in these type of industries.

OLEFINS

These are unsaturated hydrocarbon compounds and their presence in Naphtha would lead to :

a. Possible gum formation in presence of air.

b. Increased coking in the pre-heater tubes of the hydro-desulphurization section.

c. Increased hydrogen consumption in the hydro-desulphurization section.

Therefore, the olefin content in Naphtha is kept to the minimum and in manufacturing specification, it is limited to max. 1% on volume basis.

SULPHUR

Sulphur poisons the nickel catalyst used in the reformer in fertilizer plants, resulting in the deactivation of catalyst thus affecting the production patterns. Large amounts of expensive catalyst may have to be replaced due to sulphur poisoning of catalyst. Hence, an upper limit on the amount of sulphur in Naphtha is specified. At times, sulphur limit is prescribed owing to the design parameters of the plant. The maximum limit 0.15% by wt. prescribed in the specification is acceptable to most of the fertilizer and petrochemical industries in India.

DISTILLATION

The higher the distillation range, the longer will be the carbon chain length. In a fertilizer or petrochemical plant, the catalyst is designed to reform hydro-carbons having a particular carbon chain length only. In case the carbon chain length exceeds the limit, the catalyst may not be in a position to act on it effectively. This is the reason why there is a limit on distillation range. In addition to the above, distillation range ensures uniform quality of the product throughout.

TRACE ELEMENTS

The trace elements like lead, vanadium, sodium etc. in Naphtha may fuse with catalyst at high temperature that they encounter in the reformer. This results in the deactivation of catalysts. Hence, it is desirable to have Naphtha free of these trace metals.

However, these trace metals concentration cannot be controlled in the Refinery.