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
Crude distillation unit (CDU)
Vacuum distillation unit (VDU)
Fluidized catalytic cracker
Alkylation and isomerisation
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.
c. Thermal cracker
Thermal cracker involves a chemical cracking process followed by the separation using physical principles (boiling point differences) to yield the desired products. Thermal cracking yields naphtha + gas, gasoil and thermal cracked residue (Figure 4.1c). In some petroleum refinery configurations, thermal cracking process is replaced with delayed coking process to yield coke as one of the petroleum refinery products.
Operating Conditions : The temperature should be kept at around 450 – 500°C for the larger hydrocarbons to become unstable and break spontaneously. A 2-3 bar pressure must be maintained.
For many refinery crudes such as Arabic and Kuwait crudes, sulfur content in the crude is significantly high. Therefore, the products produced from CDU and VDU consist of significant amount of sulfur.Henceforth, for different products generated from CDU and VDU, sulfur removal is accomplished to remove sulfur as H2S using Hydrogen.The H2 required for the hydrotreaters is obtained from the reformer unit where heavy naphtha is subjected to reforming to yield high octane number reforme product and reformer H2 gas. In due course of process, H2S is produced. Therefore, in industry, to accomplish sulfur removal from various CDU and VDU products, various hydrotreaters are used.
In due course of hydrotreating in some hydrotreaters products lighter than the feed are produced. For instance, in the LVGO/HVGO hydrotreater, desulfurization of LVGO & HVGO (diesel) occurs in two blocked operations and desulfurized naphtha fraction is produced along with thedesulfurized gas oil main product (Figure 1 f). Similarly, for LGO hydrotreating case, along with diesel main product, naphtha and gas to C5 fraction are obtained as other products (Figure 4.1e). Only for kerosene hydrotreater, no lighter product is produced in the hydrotreating operation. It is further interesting to note that naphtha hydrotreater is fed with both light and heavy naphtha as feed which is desulfurized with the reformer off gas. In this process, light ends from the reformer gas are stripped to enhance the purity of hydrogen to about 92 % (Figure 4.1d). Conceptually, hydrotreating is regarded as a combination of chemical and physical processes.
Operating Conditions: The operating conditions of a hydrotreater varies with the type of feed. For Naphtha feed, the temperature may be kept at around 280-425°C and the pressure be maintained at 200 – 800 psig.
e. Fluidized catalytic cracker
The unit is one of the most important units of the modern refinery. The unit enables the successful transformation of desulfurized HVGO to lighter products such as unsaturated light ends, light cracked naphtha, heavy cracked naphtha, cycle oil and slurry (Figure 1i). Thereby, the unit is useful to generate more lighter products from a heavier lower value intermediate product stream. Conceptually, the unit can be regarded as a combination of chemical and physical processes.
The gas fractions from various units need consolidated separation and require stage wise separation of the gas fraction. For instance, C4 separator separates the desulfurized naphtha from all saturated light ends greater than or equal to C4s in composition (Figure 4.1g). On the other hand, C3 separator separates butanes (both iso and n-butanes) from the gas fraction (Figure 4.1j). The butanes thus produced are of necessity in isomerization reactions, LPG and gasoline product generation. Similarly, the C2 separator separates the saturated C3 fraction that is required for LPG product generation (Figure 4.1k) and generates the fuel gas + H2S product as well. All these units are conceptually regarded as physical processes.
Operating Conditions: Most oil and gas separators operate in the pressure range of 20 – 1500 psi.
g. Naphtha splitter
The naphtha splitter unit consisting of a series of distillation columns enables the successful separation of light naphtha and heavy naphtha from the consolidated naphtha stream obtained from several sub-units of the refinery complex (Figure 4.1n). The naphtha splitter is regarded as a physical process for modeling purposes.
Operating Conditions: The pressure is to be maintained between 1 kg/cm2 to 4.5 kg/cm2. The operating temperature range should be 167 – 250°C
As shown in Figure 4.1O, Heavy naphtha which does not have high octane number is subjected to reforming in the reformer unit to obtain reformate product (with high octane number), light ends and reformer gas (hydrogen). Thereby, the unit produces high octane number product that is essential to produce premium grade gasoline as one of the major refinery products. A reformer is regarded as a combination of chemical and physical processes.
Operating Conditions : The initial liquid feed should be pumped at a reaction pressure of 5 – 45 atm, and the preheated feed mixture should be heated to a reaction temperature of 495 – 520°C.
i. Alkylation & Isomerization
The unsaturated light ends generated from the FCC process are stabilized by alkylation process using iC4 generated from the C4 separator. The process yields alkylate product which has higher octane number than the feed streams (Figure 4.1r). As isobutane generated from the separator is enough to meet the demand in the alkylation unit, isomerization reaction is carried out in the isomerization unit (Figure 4.1q) to yield the desired make up iC4.
j. Gas treating
The otherwise not useful fuel gas and H2S stream generated from the C2 separator has significant amount of sulfur. In the gas treating process, H2S is successfully transformed into sulfur along with the generation of fuel gas (Figure 4.1m). Eventually, in many refineries, some fuel gas is used for furnace applications within the refinery along with fuel oil (another refinery product generated from the fuel oil pool) in the furnace associated to the CDU.
Operating Conditions: Gas treaters may operate at temperatures ranging from 150 psig (low pressure units) to 3000 psig (high pressure units).
k. Blending pools
All refineries need to meet tight product specifications in the form of ASTM temperatures, viscosities, octane numbers, flash point and pour point. To achieve desired products with minimum specifications of these important parameters, blending is carried out. There are four blending pools in a typical refinery. While the LPG pool allows blending of saturated C3s and C4s to generate C3 LPG and C4 LPG, which do not allow much blending of the feed streams with one another (Figure 4.1t). The most important blending pool in the refinery complex is the gasoline pool where in both premium and regular gasoline products are prepared by blending appropriate amounts of n-butane, reformate, light naphtha, alkylate and light cracked naphtha (Figure 4.1u). These two products are by far the most profit making products of the modern refinery and henceforth emphasis is there to maximize their total products while meeting the product specifications. The gasoil pool (Figure 4.1v) produces automotive diesel and heating oil from kerosene (from CDU), LGO, LVGO and slurry. In the fuel oil pool (Figure 4.1w), haring diesel, heavy fuel oil and bunker oil are produced from LVGO, slurry and cracked residue.
l. Stream splitters
To facilitate stream splitting, various stream splitters are used in the refinery configuration. A kerosene splitter is used to split kerosene between the kerosene product and the stream that is sent to the gas oil pool (Figure 4.1h). Similarly, butane splitter splits the n-butane stream into butanes entering LPG pool, gasoline pool and isomerization unit (Figure 4.1p). Unlike naphtha splitter, these two splitters facilitate stream distribution and do not have any separation processes built within them.
With these conceptual diagrams to represent the refinery, the refinery block diagram with the complicated interaction of streams is presented in Figure 4.2. A concise summary of stream description is presented in Table 4.1.