4.1 Introduction

In this lecture, a brief overview of various refinery processes is presented along with a simple sketch of the process block diagram of a modern refinery.  The sketch of the modern refinery indicates the underlying complexity and the sketch is required to have a good understanding of the primary processing operations in various sub-processes and units.

4.2 Refinery flow sheet

We now present a typical refinery flowsheet for the refining of middle eastern crude oil.  There are about 22 units in the flowsheet which themselves are complex enough to be regarded as process flow sheets.  Further, all streams are numbered to summarize their significance in various processing steps encountered in various units.  However, for the convenience of our understanding, we present them as units or blocks which enable either distillation in sequence or reactive transformation followed by distillation sequences to achieve the desired products.
The 22 units presented in the refinery process diagram are categorized as

1. Crude distillation unit (CDU)
2. Vacuum distillation unit (VDU)
3. Thermal cracker
4. Hydrotreaters
5. Fluidized catalytic cracker
6. Separators
7. Naphtha splitter
8. Reformer
9. Alkylation and isomerisation
10. Gas treating
11. Blending pools
12. Stream splitters

A brief account of the above process units along with their functional role is presented next with simple conceptual block diagrams representing the flows in and out of each unit.

 

a. Crude distillation unit

The unit comprising of an atmospheric distillation column, side strippers, heat exchanger network, feed de-salter and furnace as main process technologies enables the separation of the crude into its various products.  Usually, five products are generated from the CDU namely gas + naphtha, kerosene, light gas oil, heavy gas oil and atmospheric residue (Figure 4.1a). In some refinery configurations, terminologies such as gasoline, jet fuel and diesel are used to represent the CDU products which are usually fractions emanating as portions of naphtha, kerosene and gas oil.  Amongst the crude distillation products, naphtha, kerosene have higher product values than gas oil and residue.  On the other hand, modern refineries tend to produce lighter components from the heavy products.  Therefore, reactive transformations (chemical processes) are inevitable to convert the heavy intermediate refinery streams into lighter streams.

Operating Conditions : The temperature at the entrance of the furnace where the crude enters is 200 – 280°C. It is then further heated to about 330 – 370°C inside the furnace. The pressure maintained is about 1 barg.

 

b. Vacuum distillation unit (VDU)  

The atmospheric residue when processed at lower pressures does not allow decomposition of the atmospheric residue and therefore yields LVGO, HVGO and vacuum residue (Figure 1b).  The LVGO and HVGO are eventually subjected to cracking to yield even lighter products.  The VDU consists of a main vacuum distillation column supported with side strippers to produce the desired products.  Therefore, VDU is also a physical process to obtain the desired products.

Operating Conditions : The pressure maintained is about 25 – 40 mm Hg. The temperature is kept at around 380 – 420°C.