11.1 Introduction

In this lecture, we present an overview of light end processing followed with gas processing and polymerization processes in the refinery.

Gas fraction is produced from various units. Some of them are

• • Crude distillation unit

• Catalytic cracking unit

• Catalytic reforming unit

• Hydrocracking unit

• Coking unit

The light end streams are classified as

• • Streams rich in Butane: Sold as calor gas or LPG. Used internally for blending and alkylation units
  (isobutane only).

• Streams rich in Propane.

• Light ends rich in olefins.

We have already studied alkylation and isomerisation as important gas processing operations. Now we will study the additional units namely gas processing and polymerization units.

11.2 Gas processing technology (Figure 11.1)

The objective of gas processing is to produce ethane and methane.

The produced ethane and methane is to serve later for fuel gas or hydrogen production.

The gas processing section consists mainly of two different sections.

Absorption using Naphtha and kerosene:

• • First, collected gases are compressed to be fed to an absorption. 

• The purpose of naphtha is to absorb heavier hydrocarbons in the gas fraction.  These are C3s and C4s in the feed stream. To carry out absorption, first the gases are cooled and fed to a phase separator to facilitate early separation of lighter and heavier fractions. 

• From the phase separator two streams emanate namely a gas stream and a liquid stream. 

• The gas stream is fed to an absorber unit where naphtha is used as a solvent to absorb the left heavier hydrocarbons in the gas. 

• The naphtha rich with hydrocarbons is fed to the phase separator so as to stabilize the naphtha stream.

• The gas from the absorber is fed to a second absorber where lean oil (such as kerosene) is used as a solvent to absorb any heavier hydrocarbons other than the methane and hydrogen.  Eventually, fuel gas is produced as the gas product from this absorption.  The other product from the absorber is the rich oil stream.

Naphtha rich stream processing:

• • The liquid product from the phase separator is fed to a deethanizer which generates ethane rich stream as the top product. This stream is recycled back to the gas processing unit i.e., mixing with the feed and subjected to cooling followed by phase separator.

• The bottom product from deethanizer is naphtha rich stream with butanes and propanes.

• This stream is subjected to fractionation using debutanizer, depropanizer and deisobutanizer to obtain propane, isobutane and naphtha. The stabilized naphtha can be used for absorption purposes.

11.3 Olefin Polymerization

• • Olefin polymerization to yield polymer gasoline is primarily carried out to obtain polymers with good octane numbers.

• The octane number of the polymer gasoline product is not greater than the octane number of the products produced from reforming   and alkylation. Instead, comparatively poor quality product is obtained. But for the sake of enhancing octane number polymerization is   carried out.

• On the other hand, polymer gasoline has more vapour pressure than the corresponding alkylation products.   Therefore, in both ways,   polymer gasoline product quality is lower than that obtained from the alkylation unit.

• Typical feedstocks for polymerization process are C₃ and C₄ olefins that are obtained from catalytic cracking.

• The end product from polymerization reactor is a dimer or a trimer of the olefins.