To produce polymer-grade ethylene and propylene by thermally cracking hydrocarbon feedstocks (ethane through hydro-cracked residue). Shaw’s key process technologies are:
1. Ultra-selective cracking (USC) furnaces—Selective pyrolysis with proprietary quench exchanger systems
2. Ripple tray and vapor flute—High capacity with fouling minimization for quench oil and quench water towers
3. Advanced recovery system with heat-integrated rectifier (ARS/HRS)—Energy efficient cold fractionation.
The following description and diagram are given for liquid feedstock steam cracking. Fresh liquid feed as well as recovered ethane and propane are sent to USC furnaces (1). Contaminant removal may be installed on the fresh feed if required. A portion of the cracking heat may be supplied by gas turbine exhaust as preheated combustion air. Pyrolysis occurs at temperatures and residence time requirements specific to the feedstock and product requirements. The USC technology utilizes a number of radiant coil designs for the cracking furnaces to reduce residence time and coil pressure drop and to maximize ethylene yield.
Rapid quenching preserves olefin yields and the heat of quenching is used to generate high-pressure steam. Lower temperature heat is recovered for the production of dilution steam. Pyrolysis fuel oil and gasoline byproducts are recovered in the quench oil and quench water systems (2). Cracked gas (C4 and lighter) is compressed (3), scrubbed with caustic to remove acid gases and dried prior to fractionation. C3 and lighter components are separated from the C4 and heavier components in the low fouling front-end dual pressure depropanizer (4). Overhead vapor of the high-pressure depropanizer is hydrogenated to remove acetylene, methyl acetylene and propadiene (5) and then routed to the HRS and demethanizer systems (6). The demethanization system includes a turbo-expander for energy efficiency and greater hydrogen recovery. Alternatively, the acetylene can be extracted as a high-purity product (8).
The ARS minimizes refrigeration energy by using distributed distillation and simultaneous heat and mass transfer in the HRS system.
Two C2 streams of varying composition are produced within the ARS/HRS. The heavier C2/C3 stream is deethanized (7) and the C2 overhead stream is fed directly to a low-pressure ethylene-ethane fractionator (9), which is integrated with the C2 refrigeration system (9). Polymer-grade ethylene product is taken from the overhead from the ethylene-ethane fractionator.
C3s from the dual pressure depropanizer system are combined and may require further hydrogenation to remove methyl acetylene and propadiene (10). Either polymer-grade or chemical-grade propylene can be produced overhead from a propylene-propane fractionator. The propylene-propane fractionator can either be a high-pressure system that is
condensed by cooling water or a low-pressure system that utilizes a heat pump (11).
C4 and heavier byproducts are further separated in a sequence of distillation steps. Ethane and propane are typically recycled to pyrolysis. Refrigeration is typically supplied by a cascade ethylene/propylene refrigeration system.
Advantages of ARS technology are:
1. Reduced chilling train refrigeration requirements due to chilling/pre-fractionation in the HRS system.
2. Reduced methane content in feed to the demethanizer, which reduces the demethanizer condenser duty and refrigeration loads.
3. The dual feed ethylene fractionator (lower reflux ratio) reduces refrigeration loads and energy consumption.
4. Reduced refrigeration demand via the use of an integrated heat pump on the ethylene-ethane fractionator.
Economics: Once-through pyrolysis yields range from 57 wt% (ethane, high conversion) to 28 wt% (heavy hydrogenated gasoils) ethylene. Ultimate yields for ethylene of 85% from ethane feedstock and 32% from liquid feedstock are achieved. The ethylene plants with USC furnaces and an ARS/HRS recovery section are known for high reliability, low energy consumption, short startup time and environmental compliance.
Licensor: The Shaw Group