PTQ Q4 2023 Issue

ethylene yield. The residue contains a significant portion of asphaltene molecules. Asphaltene is a class of high molecular weight com- pounds containing many heteroatoms (S, N, O, and met - als). Considerable research has been done in recent years to understand these compounds.¹ 0 These are multi-ring compounds with very high molecular weight and are pres- ent in peptised form in the crude. Depending upon ther- modynamic conditions, they will separate out and cause coking. By removing these species from crude, processing the remaining portion of crude is easier. These molecules should not enter the cracking heater. There is a direct relation between the asphaltene content of crude and MCR. Therefore, potential crudes with high MCR should be avoided. Conversion of asphaltenes rather than rejection through processes such as solvent deas- phalting (SDA) forms the basis of special reactor platforms (liquid circulation or slurry bubble) and catalyst systems, which form part of the new crude-to-chemicals scheme. Crude to olefins Processing more than one cut of crude and cracking them optimally in different heaters maximises olefins. For each species, there is an optimum cracking condition that will give maximum ethylene yield or maximum ethylene + pro- pylene or other combinations. Note that the cracking reac - tion proceeds via a free radical mechanism, and hence the initiation rate controls the overall rate. Up to certain tem- peratures, both ethylene and propylene yields increase with increasing coil outlet temperature. The mechanism dictates that the largest olefin reaches the maximum first and then the smaller olefins. Ethylene is primarily formed from ethyl radical decomposition, which is the main reaction in the thermal cracking of all hydrocarbon species. It can be shown one cut of crude from C 5 to 525ºC boiling range will not produce optimum olefin yields for a light crude. Calculations show 2-3 HOPS stages are rea - sonable, and often two stages are sufficient. In the first stage, the cut end point is selected around 160- 240ºC (pseudo-naphtha cut) and in the second stage, a

Cracking Heater convection section

Crude

FPH

UMPH

DSSH

Dilution steam

Residue/liquid

Lights through VGO cut

LMPH

To Radiant section

Figure 5 One stage HOPS unit for crude cracking

and select appropriate boiling (cut) points, depending on the crude properties. One or two stages are adequate for most feeds. The bottom stream of the final HOPS unit (residue) is sent to fuel or for further processing as it is not processed in the radiant coils for olefin production. The amount of residue removed depends upon the crude quality. Controlling this residue portion is critical since this material primarily pro- duces coke during the vapourisation process in the convec- tion section, radiant section and transfer line exchangers. The amount of dilution steam and the level of superheat dictate the maximum possible cut point for the crude. Rejecting the residue from processing in the cracking heat- ers leads to improved run length and better once-through

350-525ºC end point (pseudo-gas oil cut) is selected depending upon the crude and the scope of the pro- ject. When three cuts are produced (three-stage HOPS), naphtha, gas - oil and VGO cuts are produced. The heaviest portion (residue) is rejected. Generally, residue is mixed with fuel oil produced in the pyroly- sis unit and sold as a product. As an example, the properties of different cuts are shown in Table 1 for Arab Light crude. Typically, naphtha feed will produce 30 wt% ethylene and 5 wt% fuel oil. Gasoil will produce 26 wt% ethylene and 16 wt% fuel oil, and VGO will produce more than 20 wt% fuel oil yield, and the residue is not cracked. Even when the crude is

Different ideal cuts of Arab Light crude

Cut# Name

Naphtha & light

AGO cut 160-350

VGO cut 350-525

Residue 525C+

Cut point, ºC

<160 18.83

Yield of crude, wt%

32.77

24.80

23.61

S.G.

0.7112

0.8322

0.9162

1.0133

Sulphur, wt% Nitrogen, wt% Metals, ppm MCR, ppm Paraffins, wt%

0.04 0.00 0.00 0.00

0.83 0.00 0.00 0.00

2.64 0.06 1.68 0.33

4.16 0.25

63.50 16.55

74.79 16.78

Naphthenes, wt% Aromatics, wt% Polyaromatics, wt% C 7 Asphaltenes, wt%

8.43 0.00 0.00 15.4

8.76 0.00 13.6

52.09

>60 3.84

0.20

H-Content, wt%

Table 1

PTQ Q4 2023