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
h. Reformer
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.