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.
d. Hydrotreaters
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.
f. Separators
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.