Nov-2023

Crude to chemicals: Part 1 - The basic concept of crudes

Reactor platforms with novel heat integration designed around advanced separation techniques and catalyst systems achieve 75% yields from selected crudes

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

Article Summary

Steam crackers traditionally use ethane, LPG, and naphtha as feedstock. Occasionally, heavier hydrocarbons such as gasoils and unconverted oil from hydrocracking units have been utilised. Lummus Technology, CLG (Chevron Lummus Global), and Saudi Aramco Technologies Company have conducted intense joint research over several years to understand the implications of steam cracking crudes heavier than very light crudes and condensates. The research involved understanding the distribution of molecular species within the crude, including the very complex structures in asphaltenes.

Advanced separation techniques avoiding conventional crude atmospheric/vacuum distillation systems, appropriate catalysts, and integrated reactor platforms with novel heat integration permit the consistent yield of high-value chemicals from crude to increase to higher than 75%. Strategic hydrogen addition optimises the molecular species for optimum cracker yields and maximises ethylene plant heater run lengths.

One important result that contributed to the successful commercialisation award in 2020 was the elimination of low-value stream production, such as pyrolysis gasoils, even while processing heavier hydrocarbon feeds. Heat integration, operability and reliability were key parameters in the development, as was the significant reduction of overall Capex, Opex, plot space, and CO₂ footprint.

Olefins and on-demand fuels production
Annually more than 200 million metric tons of ethylene are produced worldwide. Most of the ethylene comes from the thermal cracking of hydrocarbons, such as ethane, LPG, naphtha, and hydrocracked vacuum gasoil (VGO). Steam crackers produce very high-purity products suitable for downstream polymer and chemicals production (for example, 99.95% pure ethylene). Stringent feed specifications are imposed when the feed is destined for steam cracking. Thermal cracking, denoted as steam cracking in the industry, is one petrochemical process that uses very high energy and emits significant quantities of CO₂.

A typical naphtha cracker emits from 1.2 to 1.8 kg CO₂/kg C₂H₄. When the ethylene plant is integrated with a refinery, the feeds, products, and energy (especially steam/electricity) are exchanged between the units for improved efficiency. LPG and naphtha feeds used in the cracker are usually obtained from crude by separation in a distillation train, an energy-intensive process. Do we really need crude/vacuum distillation to obtain feeds to produce olefins?

Novel methods employed to process crude for producing olefins while bypassing the refinery will be further discussed. This reduces the energy, Opex, Capex, and CO₂ emissions. Although, in principle, any crude can be used as feed, some are more economically attractive than others. A primary optimisation parameter is to maximise ethylene, propylene, and other valuable chemicals while minimising fuels but maintaining the flexibility to produce on-demand fuel products.

Ethylene production
Most ethylene is produced by thermally cracking hydrocarbons mixed with dilution steam in the vapour phase at low pressures/high temperatures in tubular reactors without catalyst. The product recovery section consists of compression, many distillation columns, and cryogenic fluids as coolants. Fired heaters are used to supply the energy to the cracking reactions. Heat is recovered primarily as super high pressure (SHP) steam (100+ bar) that is used to drive the compressors in the recovery section. Modern cracking furnaces achieve more than 94% overall thermal efficiency.

Direct crude cracking has been studied since the 1960s but has never been commercialised due to poor economics.1 With continued research and development in various disciplines, features such as the proprietary Short Residence Time (SRT)/low-pressure drop coils led to superior olefin selectivity and reduced feed consumption. Ethylene capacity has increased from 25 KTA per furnace in the 1960s to 300 KTA per furnace today, and single-train plant capacity has increased from 300 KTA to more than 2,000 KTA. The feeds have not changed, but the once-through yields have improved substantially. The latest innovations in crude cracking to improve refinery economics (TC2C and heavy oil processing scheme, HOPS) are discussed at recent conferences.^2-4

Fundamentally, thermal cracking proceeds via free radical mechanism;5,6 hence, low hydrocarbon partial pressure and short residence time favour higher olefin selectivity. However, during this process, side reactions leading to the formation of coke occur. Coke, a solid phase, deposits on the walls of the reactor and increases coil pressure drop and tube metal temperature, which limits ethylene production. 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 chemical 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 compositions that vary widely. Typical distillation curves for some light and heavy crudes are shown in Figure 1. Material boiling 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 distillation 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 specifications (such as specific gravity, distillation range, and sulphur) so that these cuts can be used as fuel or processed in downstream units. When the fractions from the crude 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.