PTQ Q4 2023 Issue

distillation tower do not meet the required specifications of transportation fuels such as gasoline, the fractions must be processed in other units, such as fluid catalytic cracking (FCC), hydrocracking, and many other units. When not used as fuels, these cuts are sent to olefin plants as feeds. Olefin plant feeds must meet certain con - taminant levels and boiling ranges. Ethylene plants can tolerate a wide range of feed properties when properly designed. Therefore, some of the refinery operations can be bypassed when used for olefin production. Field con - densates are often used as cracker feeds. These are much lighter than crudes but contain some characteristics similar to crude. 7 For example, the distillation curve of an Algerian condensate feed is also shown in Figure 1. When these condensates are cracked without proper heater design, the heater run lengths are very short. The Algerian condensate shown in Figure 1 is success- fully cracked in many plants only when it is successfully vapourised in the convection section coils without coking. It also has some unwanted contaminants (Hg & As) at high levels, which cause problems in the recovery section. If not properly handled, they will damage the recovery section. Therefore, for every crude, the effect of the contaminants with respect to olefin plant operation is important. Without a detailed compositional analysis, it cannot be predicted which crude is suitable for steam cracking. Typically, light crudes have low levels of contaminants. Super light crudes are almost like field condensates. Super light crudes and condensates have a high fraction (>60%) of naphtha boiling range material (<200ºC) in the whole crude and a small fraction of residue (<5%) that boils above 525ºC. Inspection of many crudes shows that many light and super light crudes are potential candidates as olefin plant feeds. When sulphur is present at high levels, a larger acid gas (H₂S) removal system must be used in the ethylene plant. In addition, other sulphur species such as mercaptans and thiophenes act as catalyst poisons for the hydrogenation reactors within the ethylene plant, making the ethylene plant more expensive. Often high sulphur crudes contain other contaminants such as high micro carbon residue (MCR) and high levels of basic nitrogen species. Though high sulphur can be removed in the ethylene plant, it may not be worthwhile spending extra Capex in the ethylene plant for a large acid gas removal system. It is better to condition the feed in the refinery and reduce the sulphur content. When there is no refinery near the petrochemical complex, it is preferable to consider crudes with less than 0.5 wt% sulphur for ethylene production due to economic reasons. The basic crude-to-chemicals scheme outlined in this discussion can be used for any type of crude. Crude to chemicals scheme Many light crudes with low sulphur can be economically processed to produce ethylene. High residue-contain- ing crudes cause short heater run length. What are good feeds to produce olefins? As shown in Figure 2 , normal paraffins are the best feeds to produce olefins, especially ethylene. Normal paraffins produce very high ethylene and propylene yields. Isoparaffins produce lower ethylene

800

Arab Superlight Bonny Light Arab Medium

700

600

LAPA-Hvy Alg. Cond.

500

400

300

200

100

Volume distilled, %

and tube metal temperature, which limits ethylene produc- tion. Periodically, the heater must be steam/air decoked. When the frequency of cleaning is increased, it may not be economical to crack that feed. Typically, Lummus SRT heaters run between 30 and 60 days before a cleaning is required. When contaminants are present in the feed, the run length between decoking can be reduced significantly. Cracking is a homogeneous reaction and hence is independent of coil surface-to-volume ratio, but coking reactions are surface dependent. Though chem- ical reaction engineering principles can be used to improve the run length to some extent, feeds or reactor designs that result in short heater run lengths are not preferred. The difference between traditional feeds such as naphtha or gasoil and crude is the run length of the heater. Typically, when crudes are used as feeds, the run length can be a matter of hours. Therefore, crudes were not considered good feeds for olefin production in traditional coil technology. In addition, each type of feed requires optimum processing conditions; hence, even with good-quality crude, the ethylene yield will not be high if optimum conditions are not chosen. How do we achieve maximum performance with crude cracking? Crudes Unlike ethane or propane, crude is not a single species but a mixture of species boiling from very low temperatures (~10°C) to very high temperatures (>700°C) with compo - sitions that vary widely. Typical distillation curves for some light and heavy crudes are shown in Figure 1 . Material boil- ing above 525°C is typically called residue. They cannot be cracked in pyrolysis heaters since the residue feeds cannot be vapourised completely, and the residue deposits as coke. These crudes are currently sent to a refinery crude dis - tillation unit (CDU) after going through a desalter and then separated into different cuts, such as naphtha, kerosene, diesel, VGO, and residue. These cuts are made to specifi - cations (such as specific gravity, distillation range, and sul - phur) so that these cuts can be used as fuel or processed in downstream units. When the fractions from the crude Figure 1 Different types of crudes and Condensate: 1-Arab Superlight (API=49.6, MCR=0.48 wt%), 2-Bonny Light (49.6, 1.24), 3-Arab Medium (30.9, 5.4), LAPA Brazil (22.7, 7.0); Dashed line-Algerian Condensate (70.1, 0.0009)

PTQ Q4 2023