Case Study: A renewable energy plant converting 1000 TPD (dry) urban green waste can generate 600 BPD of Fischer-Tropsch fuels and 16.5 MW of export electricity to yield 466 bbl/day of "green diesel" or syngas and 166 bbl/day of Naphtha.
The following process and key components are described for the purpose of meeting the above requirements.
The sweet syngas from the LO-CAT unit will be fed to the Fischer-Tropsch (FT) reactor where the syngas will be in contact with iron-based catalyst to produce liquid hydrocarbons. The Fischer Tropsch process consists of a feed charge compressor, Sulfur guard reactor, FT reactor, steam drum, wax settlers, cold/hot separators, catalyst activation, distillate stabilizer, secondary wax filtration and spent wax/catalyst handling.
Syngas Feed Compressor & FT Reactor
Syngas feed is compressed via FT Feed Compressor from 350 psig to 448 psig then heated by the Feed Gas Heater upstream of Sulfur guard reactor. The Sulfur guard reactor will remove Hydrogen Sulfide (H2S) and Carbonyl Sulfide (COS) from the syngas feed to be less than 0.5 ppmv. Once the feed is cleaned up, the syngas is directed to the FT reactor. In the FT reactor, Hydrogen and CO react over an iron-based catalyst to produce a mixture of light hydrocarbon gas, middle distillate, heavy hydrocarbon (wax) and water. The reaction is exothermic, and the heat released is used to generate MP steam in the steam coils inside the FT reactor.
The heavy hydrocarbon (wax) product is extracted as slurry from the side of the FT reactor by use of four (4) FT Reactor Primary Settlers. The primary settlers are designed to extract a wax product from the slurry while the remaining slurry reenters the FT reactor. The wax is let down in pressure and enter Wax Flush Drum. Then the flushed gas is sent to the spent wax/catalyst handling process while the crude wax is sent to the secondary Wax Filtration System. Once the crude wax is filtrated through Secondary Wax Filter Package, the crude wax product is sent to storage tank and to the Hydrocracker for further processing.
FT Overhead Separation
The FT overhead consists of light and middle distillate, oxygenates, produced water, CO2 and un-reacted syngas. The overhead gas is cooled and condensed in two consecutive 3 phase separators (hot/cold). The distillates are cooled in two stages in order to prevent wax from solidifying in the cold separator section. The light and middle distillates are sent to the Distillate Stabilizer. The produced water with dissolved oxygenates streams from both separators are directed to the FT water surge drum then sent to the waste water treatment. The cold separator overhead called FT tail gas will be directed to the Gas Turbine.
FT products yields:
- Stabilized Distillate: 230 barrel/day
- Crude Wax: 370 barrel/day
- MP FT Steam: 56,190 lb/hr
- FT Fuel Gas: 2401 lb/hr
The light and middle distillates as well as the overhead produced in the catalyst activation reactor are directed to the Distillate Stabilizer. Additionally, the flush gas from the Wax Flush Drum that is condensate in the Wax Stabilizer is sent to the Stabilizer. The Distillate Stabilizer is a packed column that includes a Distillate Stabilizer Reboiler, Distillate Stabilizer Overhead Condenser and a Distillate Stabilizer Reflux Drum. Stabilized Distillate product is directed to either Product Upgrading (UOP Unionfiner) or storage tank. The overhead gas product is routed to the fuel gas header. The produced water is sent to FT Water Surge Drum then routed to the waste water treatment.
The Activation Reactor is designed to supply 1/20th of the total catalyst in the main FT reactor. There are two activation cases: Partial Load, supply a 1/20th catalyst load to the FT reactor and Makeup Load, will supply a 1/40th catalyst load to the FT reactor. One activation cycle will occur every other day. Fresh un-activated catalyst is mixed with activation fluid from the Hydrocracker or storage tank in a Blending Vessel installed with a low shear mixer. Then the blended catalyst is pumped to the activation reactor via Catalyst Transfer Pump. The Activation unit is a smaller version of the FT unit with similar design. Due to the high temperature (475°F) requirement, HP (900 psig) steam is used to heat the Boiler Feed Water (BFW). The reaction is exothermic, and the heat released is used to generate MP steam in the steam coils inside the Activation Reactor.
The heavy hydrocarbon (wax) product is extracted from the side of the Activation Reactor by use of Dynamic Settlers. The wax is let down in pressure and introduced into the Wax Flush Drum.
The overhead gas is cooled and condensed in a 3-Phase Separators. The FT liquid product is directed to the Distillate Stabilizer. The produced water is sent to the FT Water Surge Drum then sent to the waste water treatment. The Separator overhead called FT Tail Gas will be directed to the fuel gas header.
The activated catalyst is extracted from the side of the Activation Reactor then the pressure is let down and introduced into the FT Reactor.