PTQ Q4 2023 Issue

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

S team crackers traditionally use ethane, LPG, and naph- tha as feedstock. Occasionally, heavier hydrocarbons such as gasoils and unconverted oil from hydroc- racking units have been utilised. Lummus Technology, CLG (Chevron Lummus Global), and Saudi Aramco Technologies Company have conducted intense joint research over sev- eral years to understand the implications of steam cracking crudes heavier than very light crudes and condensates. The research involved understanding the distribution of molec- ular 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 chem- icals from crude to increase to higher than 75%. Strategic hydrogen addition optimises the molecular species for opti- mum cracker yields and maximises ethylene plant heater run lengths. One important result that contributed to the success- ful 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 exam- ple, 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 indus- try, 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 produc- ing 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 hydrocar- bons mixed with dilution steam in the vapour phase at low pressures/high temperatures in tubular reactors without catalyst. The product recovery section consists of com- pression, 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 dis - ciplines, features such as the proprietary Short Residence Time (SRT)/low-pressure drop coils led to superior olefin selectivity and reduced feed consumption. Ethylene capac- ity 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 radi- cal 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

PTQ Q4 2023