To produce polymer-grade ethylene and propylene, a butadiene rich C4 cut, an aromatic C6–C8 rich raw pyrolysis gasoline and a high-purity hydrogen by steam pyrolysis of hydrocarbons ranging from ethane to vacuum gasoils (VGOs). Progressive separation applied for concept in the fields either front-end or back-end hydrogenation is used in steam cracking.
For either gaseous (ethane/propane) or liquid (C4/naphtha/gasoil) feeds, this technology is based on Technip’s proprietary pyrolysis furnaces and progressive separation. This allows processing of olefins at low energy consumption with a particularly low environmental impact. The progressive separation concept is applied for either front-end hydrogenation or back-end hydrogenation.
• The front-end hydrogenation corresponds to a front-end deethanizer for an ethane cracker or a front-end depropanizer for the heavier feedstocks both are placed at the third-stage discharge of the cracked-gas compressor.
• The back-end hydrogenation corresponds to a front end; the tower is placed at fifth-stage discharge.
Hydrocarbon feedstocks are preheated (also to recover heat) and then cracked by combining with steam in a tubular pyrolysis furnace at an outlet temperature ranging from 1,500°F to 1,600°F. The furnace technology can be either an SMK type (for gas cracking) or GK6 type (for liquid cracking). The GK6 type design can be oriented to a high olefins yield with very flexible propylene/ethylene ratios, or to a high BTX production. This specific approach allows long run length, excellent mechanical integrity and attractive economics.
The hydrocarbon mixture at the furnace outlet is quenched rapidly in the transfer line exchangers (TLEs) or selective line exchangers (SLEs), generating high-pressure steam. In liquid crackers, cracked gas flows to a primary fractionator, after direct quench with oil, where fuel oil is separated from gasoline and lighter components, and then sent to a quench water tower for water recovery (to be used as dilution steam) and heavy gasoline production (end-point control). In a gas cracker, cracked gas flows to a quench water tower for water recovery and removal of tars. A caustic scrubber placed at the third-stage discharge of the cracked-gas compressor removes acid gases. The compressed gas at 450 psig is dried and then chilled.
For an ethane cracker, a single demethanizing stripping system operating at medium pressure is implemented; the overhead is recycled back to the cracked gas compressor.
For a liquid cracker, a double demethanizing stripping system operating at medium pressure and reboiled by cracked gas, minimizes the refrigeration required (heat integration) as well as the investment for separating methane (top) and C2+ cut (bottoms). A dual column concept (absorber concept: Technip’s patent) is applied between the secondary
demethanizer overheads, and the chilled cracked gas minimizes ethylene losses with a low-energy requirement. The high-purity hydrogen is produced in a cold box.
The bottoms from the demethanizers are sent to the C2 cut treatment for ethylene purification.
The C2 splitter is operating as a heat pump. The tower can be arranged as open heat pump integrated with the ethylene refrigerant when the front-end hydrogenation system or closed heat pump operating with the propylene refrigerant for the back-end hydrogenation scheme.
The residual ethane from the C2 splitter is recycled for further cracking. Polymer-grade propylene is separated from propane in a C3 splitter. The residual propane is either recycled for further cracking or exported. C4s and light gasoline are separated in a debutanizer.
Gas expansion (heat recovery) and external cascade using ethylene and propylene systems supply refrigeration. The main features of Technip’s patented technology are:
• Optimization of olefins yields and selection of feedstocks
• Reduced external refrigeration in the separation sections
• Auto-stable process, heat integration acts as feed forward system
• Simple process control; large usage of stripper/absorber towers (single specification) instead of distillation towers (antagonistic top and bottom specifications).
Economics: Ultimate range of ethylene yields vary from 83% (ethane) to around 25% (VGOs), 35% for intermediate full-range naphtha. These correspond to the respective total olefins yields (ethylene and propylene) from 84% (ethane) to 38% (VGOs), and 49% for intermediate fullrange naphtha. The specific energy consumption range is 3,100 kcal/kg ethylene (ethane) to 5,500 kcal/kg ethylene (GO) and 4,700 Kcal/Kg ethylene for an intermediate full-range naphtha.