Feb-2024

Crude to chemicals: Part 2

Part 1 covered the basics of crude-to-chemicals. Part 2 explains how hydroprocessing technology can be used to convert any crude to chemicals to maximise yields.

Kandasamy M Sundaram, Ujjal K Mukherjee, Pedro M Santos and Ronald M Venner
Lummus Technology

Article Summary

Saudi Aramco Technologies Company, Lummus Technology, and Chevron Lummus Global (CLG) conducted several years of research to develop an improved thermal crude-to-chemicals technology known as Thermal Crude to Chemicals (TC2C). This proprietary technology can produce high chemicals yields while extending the feedstock range beyond just very light crudes or condensates typically considered. The research and subsequent commercialisation of the technology involved the following vital steps:
• Very detailed componential analysis of crudes and heavy oils
• Development of specialised separation devices to separate the crude into fractions for optimised processing without having to utilise energy and Capex-intensive crude atmospheric and vacuum distillation
• Utilisation of commercially proven integration of fixed bed, ebullated bed, and slurry reactor systems in crude conditioning so that the products from the crude conditioning section could be routed directly to the steam cracker
• Development of unique catalyst systems for the fixed bed and ebullated reactors that would provide the right amount of hydrogenation without overcracking to naphtha, LPG, and light ends
• Rigorous testing of the impact of varying amounts of pyrolysis fuel oil recycled from the recovery section. TC2C successfully upgrades the pyrolysis fuel oil to steam cracker reactor feed.

Throughout the development, particular attention was paid to minimising equipment count (Capex), energy input, carbon footprint, emissions, catalyst deactivation rate, and the reactor fouling rates for various feeds. Overall, hydrogenation improved the steam cracking reactor feed quality.

The integrated crude conditioning/steam cracking reactors/recovery systems formulate the integrated TC2C technology.

Crude analysis and conditioning
There have been many attempts to characterise crude through detailed compositional analysis.1,3 In the lower carbon numbers, the total number of n-paraffins, i-paraffins, naphthenes, and aromatics is reasonable and easily identified. With increasing carbon numbers, the number of possible compounds increases exponentially. Boduszynski3 started  evaluating crude using detailed compositional analysis. It is well known that diverse compounds with similar molecular weight cover a broad boiling range. Boduszynski evaluated compounds with a hydrogen deficiency or ‘Z’ value. He showed that complete hydrogenation and ring opening with no change in carbon number could consume vastly different amounts of hydrogen depending on the ‘Z’ value. The general formula is:

CnH2n+Z

where
Z = 2-2*(R+DB)
n = number of carbon atoms
R = number of rings,
DB = number of double bonds,
Z = hydrogen deficiency.

In all crudes, the hydrogen deficiency increases with boiling point with a higher concentration of condensed rings, as previously shown in Part 1 (PTQ, Q4 2023). Typically, the highest boiling fractions in crude (containing what is broadly termed asphaltenes) are the most difficult to convert to transportation fuels or petrochemical feedstocks. Indeed, the analysis of asphaltenes using advanced techniques formed part of the research.

In TC2C, a naturally abundant n-paraffin-rich light stream is separated with a novel separation device such that it eliminates heavier molecules from the product that is routed to the steam cracker, as previously shown in Part 1. This step uses dilution steam to vapourise the light cut. Bottoms from the separation device are routed through another separation device that separates a heart cut with carbon number varying between 20 and 35, depending on the TC2C variant. The heart cut is sent for fixed bed hydroprocessing to remove nitrogen and sulphur, hydrogenation of aromatics, ring opening and hydrocracking. The catalyst systems are carefully selected to optimise the molecular profile for subsequent processing.

The heaviest fraction of the crude with a carbon number exceeding 35 is routed to a liquid circulation (LC) reactor with either extrudate or slurry catalyst. These reactors have small online catalyst addition and withdrawal capabilities and can run continuously for more than five years. LC reactor information can be found elsewhere.2 The liquid circulation reactors convert the asphaltene and recycle pyrolysis oil from the ethylene plant to lighter components that are hydrotreated/hydrocracked to suitable steam cracker feed.
This system ensures no heavy polynuclear aromatics (HPNA) reach the steam cracker. Some known structures that impede the full conversion of residue hydrocracked VGO are shown in Figure 1. Through extensive analysis of commercial data from residue hydrocracking and tailored pilot plant tests, residue hydrocracking has an increased concentration of double bond equivalent (DBE) value of 15+ compounds. For pure hydrocarbons, DBE=C+1-H/2, where C is the number of carbon atoms, and H is the number of hydrogen atoms. It represents the level of unsaturation or hydrogen deficiency.

The measured DBE of a typical ethylene plant pyrolysis fuel oil and a residue are shown in Figure 2. Typical pyrolysis fuel oil characteristics are shown in Table 1. Within the integrated hydrocracking system, the catalysts system and operating conditions are carefully controlled such that DBE is restricted to 15 or lower in the effluent. Slurry hydrocracking utilising a very special catalyst can increase the conversion of residue to more than 97%. The addition of pyrolysis fuel oil to the residue feed increased the residue conversion significantly. The remaining unconverted oil is filtered and sent over a fixed bed reactor system to meet IMO-compliant very low sulphur fuel oil (VLSFO) specifications (<0.5 wt% sulphur). Thus, TC2C ensures that no part of the converted crude is wasted while maximising the yield of chemicals. Changes in DBE before and after an LC technology are shown in Figure 3. High DBE value species are almost reduced to zero.