Light Fischer-Tropsch Liquid (LFTL) and Wax Hydrocarbon are processed in the Product Upgrading Unit. Product upgrading includes a Unionfining Process Unit and a Unicracking Process Unit.
Unionfining is a hydrotreating process to stabilize the LTFL light-distillate and mid-distillate feeds from the Fischer-Tropsch (FT) process by reducing the oxygen and olefin content.
Unicracking is a hydrocracking process that converts the heavy wax feedstock long-chain molecules into shorter-chain products. Common fractionation facilities process the Unionfining and Unicracking products into "green diesel" and Naphtha fuel fractions.
UOP is the technology licensor for the hydroprocessing units to generate the desired products. Unionfining and Unicracking are integrated into a system for achieving the targeted product parameters. UOP also established the necessary fractionation operations to recover the naphtha and diesel fractions with recycle of any heavier unconverted oil (UCO) back to the Unicracker.
The following process configuration and operating parameters are selected to illustrate maximizing diesel production: 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.
Unionfining process will be utilized to upgrade the LFTL feed by reducing the oxygen and olefin content. The Unionfiner utilizes two reaction stages in order to upgrade the feed. The first stage is comprised of four beds in series to perform the majority of the oxygenate conversion and olefin saturation. Effluent from the first reaction stage is processed in the second stage with a more active catalyst to complete the de-oxygenation to less than 200 ppm.
LFTL is received directly from the Fischer-Tropsch Distillate Stabilizer or from storage. The feed is passed through a Feed Coalescer and Feed Filter to remove any contaminants and potential water condensation that may have occurred during storage. After initial filtration and water removal, the LFTL arrives at the Feed Surge Drum. The drum is kept under a nitrogen gas blanket to ensure the exclusion of oxygen in the feedstock. The surge drum also provides liquid holdup volume to protect the reactors and feed pump from upstream upsets.
LFTL is transferred to the reactor via Charge Pump which is spared. Sulfiding agent is injected into the feed upstream of the Charge Pump to maintain catalyst activity. LFTL is injected with a quantity of hydrogen soak gas to minimize polymerization and preheated with reactor effluent in the heat exchanger. Recycle hydrogen is heated in the heat exchanger, combines with the pre-heated LFTL and is fed to the first stage reactors consisting of four beds in series. Additional hydrogen is injected between each bed.
Reactor effluent is cooled by heat exchange with recycle gas, fresh feed and flash drum liquid. After final cooling is performed by the heat exchanger, the effluent is sent to the Water Separator for removal of water from the first stage reactors.
Water Separator off-gas and intermediate liquid are preheated separately with stage 2 reactor effluent. Intermediate off-gas is preheated in the heat exchangers. Final temperature controlled heating of the off-gas is performed with electric Intermediate Recycle Gas Heater. Intermediate liquid after pre-heat in Intermediate Feed Heat Exchanger is combined with the heated intermediate recycle gas and sent to the single bed second stage Reactor.
After providing pre-heat to the intermediate recycle gas and intermediate liquid feed, stage 2 reactor effluent is cooled and flows into the Cold Separator. Flash gas is separated from the liquid hydrocarbon and water phases. A water-hydrocarbon interface level is maintained in the separator. Water and hydrocarbon phases are individually transferred to the Cold Flash Drum.
Water separated from the hydrocarbon phase in the Cold Flash Drum is routed to wastewater treatment. Flash drum hydrocarbon liquid is preheated with the stage 1 reactor effluent, and then flows to the fractionation section. Flash gas from the Cold Flash Drum is combined with the Cold Separator vapor and sent to the Recycle Gas Compressor Suction Drum.
Recycle gas from the suction drum flows to the Recycle Gas Compressor which is spared. After compression to 1049 psig, the flash gas combines with makeup hydrogen and is split into two streams. The first stream flows to the heat recovery network for pre-heat with reactor effluent, then to electric Recycle Gas Heater prior to entering Reactor No.1 at the required temperature. The second stream from the recycle gas compressor discharge is injected between the other beds of the stage 1 reactor as previously mentioned.
Unicracking process converts the FT wax and heavier-than-diesel fraction from the hydrotreated LFTL into naphtha and diesel fractions. Objective is to maximize the diesel fraction in the product. Hydrotreated LFTL heavy material along with unconverted material from hydrocracking is fed from the common fractionation section where the entire back-end cut portion (690oF+) is designated as Unconverted Oil (UCO) and recycled to the Unicracker inlet for complete conversion.
FT Wax can be received directly from the Fischer-Tropsch Secondary Wax Filtration Package or from storage. UCO containing LFTL heavy material and unconverted Unicracking compounds is received directly from the Product Fractionator or from storage. Unlike the LFTL, there is no feed coalescer because the wax and UCO are maintained at hot enough temperatures to prevent water droplet formation. The combined FT Wax and UCO from storage is filtered in 2 Filters and routed to the Feed Surge Drum. The drum is kept under nitrogen blanket to prevent oxygen migration into the process. Liquid holdup is provided to stabilize flow to the reactors and minimize disturbances from upstream excursions.
Heavy feed materials are transferred to the reactor with Charge Pump is spared. Sulfiding agent is also added into the Unicracking feed stream to maintain catalyst activity. Reactor feed is preheated with Hot Separator vapor in the heat exchanger and with reactor effluent in a series of heat exchangers. Recycle gas from Recycle Gas Heater is combined with the pre-heated reactor feed and fed to a series of 5 reactors. Five separate reactor beds operating in series are provided with recycle gas added as quench between the beds to control the temperature resulting from the exothermic reactions.
Reactor effluent after providing pre-heating for the feed stream and recycle gas is sent to the Hot Separator. Vapor and liquid components are separated at high temperature and pressure. Hot Separator liquid is sent to the Hot Flash Drum. Hot Separator vapor is cooled by transferring heat to the reactor feed, stripper feed and recycle gas. Cooled vapor is water washed, final cooled in the Hot Separator Vapor Condenser and routed to the Cold Separator.
The Hot Flash Drum reduces the pressure of the Hot Separator liquid releasing most of the remaining hydrogen. Hot Flash Drum liquid is sent to the fractionation section. Vapors are cooled in the heat exchanger and sent to the Cold Flash Drum.
Cold Separator is a three-phase separator to split vapor from the hydrocarbon and water liquid phases. A hydrocarbon/water interface is maintained to separate the two liquid phases. Cold separator gas comprised principally of hydrogen is sent to the Recycle Compressor Suction Drum for reuse. Water and hydrocarbon liquids are transferred individually to the Cold Flash Drum for additional separation.
Cold Flash Drum provides additional three-phase separation of reactor effluent. Off-Gas from the Cold Flash Drum is sent to Mix Drum for use as Fuel Gas. Water is separated and sent to wastewater treatment. Hydrocarbon is fed to the Stripper in the fractionation section after pre-heat with Hot Separator vapor in the heat exchanger.
Recycle Gas after passing through the Recycle Compressor Suction Drum is fed to Recycle Gas Compressor for reuse. After compression to 1120 psig, the recycle gas is combined with fresh make-up hydrogen and sent to the Unicracker reactors. One stream is routed to Reactor No. 1 for mixing with the feed. The other stream as mentioned previously is fed as quench gas to the other Reactors for controlling the temperature of the exothermic reactions.
Makeup Gas Compressor with associated Suction Drums and Coolers provides the makeup gas for the Unionfining and Unicracking units. The source of the makeup gas is produced Hydrogen from the Pressure Swing Adsorption (PSA) purification unit. Makeup hydrogen is introduced to the Unionfining unit downstream of the 2 Unionfing Recycle Gas Compressors and to the Unicracking unit downstream of the 2 Unicracking Recycle Gas Compressors.
Common fractionation facilities are provided to separate the Unionfining and Unicracking intermediate hydrocarbon materials into Naphtha and Diesel primary products. Cold Flash Drum hydrocarbon streams from the Unionfining and Unicracking units are fed to the top portion of the Stripper. Hot Flash Drum liquid from Unicracking enters the middle portion of the Stripper. Medium pressure steam is injected into the bottom of the stripper to remove light ends from the streams. Overhead vapor from the Stripper is cooled in Stripper Condenser. Vapor is separated from the condensed liquids in Stripper Overhead Receiver and sent as off-gas to the Mix Drum. Water separated in the receiver flows to wastewater treatment. The Stripper operates on total reflux so the entire hydrocarbon stream is returned to the column.
Stripper bottoms are fed to the Product Fractionator for separation into final products. Feed is pre-heated with fractionator bottoms in the heat exchanger and then the Fractionator Feed Heater raises the temperature to 690°F. Hot feed then enters the Product Fractionator tower and is separated into products. The Product Fractionator produces Naphtha and Diesel products with the bottoms designated Unconverted Oil (UCO) which is recycled to the Unicracker. Diesel product is side-stripped.
LP steam is fed to the bottom of the tower to separate diesel and Naphtha components from the heavy UCO materials.
Overhead vapor from the tower is condensed in the Fractionator Condenser. The overhead stream flows to the Fractionator Receiver. The reflux drum is padded with fuel gas and provides pressure control of the Product Fractionator. A portion of the Naphtha leaving the Fractionator Receiver is returned to the tower as external reflux. The remainder is the Naphtha product, cooled with the heat exchanger and sent to storage. Water is collected in the receiver boot and routed to wastewater treatment.
Distillate material is drawn from the Product Fractionator and enters the Diesel Stripper. The Diesel Stripper is equipped with a reboiler heated by Product Fractionator bottoms liquid. Overhead vapors are returned to the Product Fractionator. Diesel product is withdrawn from the bottom of the Diesel Stripper, cooled in the heat exchanger and sent to storage.
Product Fractionator bottoms after providing the heat source for the Diesel Stripper Reboiler is sent to Heavy Polynuclear Aromatic (HPNA) adsorption beds. HPNAs formed in the hydrocracking process if not reduced in concentration can result in fouling of equipment and shortened catalyst life. The HPNA Adsorber consists of two beds filled with activated carbon operating in series. As a bed is depleted, the carbon will be replaced and the spent material sent off-site for regeneration. After HPNA removal, the fractionator bottoms will pre-heat the Product Fractionator feed as previously mentioned and be returned to the Unicracker for additional conversion. Products are pumped to storage at 466 BPD for Diesel and 166 BPD for Naphtha.