Introduction
Distillate or Diesel Hydrotreaters, also referred to as Hydrodesulfurization Units, remove organic sulfur, nitrogen, halide, oxygenate, and metals compounds from diesel boiling range feed. Hydrotreaters achieve this by passing feed over a bed of hydrotreating catalyst in the presence of hydrogen and at high temperatures and relatively high pressures. Organic Sulfur and Nitrogen Compounds must be removed from diesel product to reduce SOX and NOX emissions from diesel engines during combustion. In the diesel hydrotreating process, organic sulfur, nitrogen, and oxygen compounds are converted to H2S, Ammonia, and Water, respectively.
C4H9SH + H2 -> C4H10 + H2S
C5H5N + 5H2 -> C5H12 + NH3
C6OH +H2 -> C6H12+ H2O
Metal contaminants are trapped on the reactor catalyst surface, which in addition to coking, deactivates the catalyst overtime and necessitating catalyst change-out. Because these reactions take place in the presence of hydrogen, hydrotreating also saturates olefin and aromatic compounds in the feed.
Diesel Hydrotreater
Feed and Feed Pre-Heat
Diesel from numerous units in the refinery e.g. Crude/Vac, FCC, Coker, etc. are combined with makeup and recycle hydrogen. The combined feed is directed to the feed preheat section of the plant where it is exchanged with hot reactor effluent to raise the mixture to optimum feed heater inlet temperature. This decreases the duty (energy) requirement of the fired feed heater-saving on natural gas or fuel oil which fires the heater. In the fired heater, the feed passes through the fired heater’s tubes where it is raised to the required reactor inlet temperature to promote the hydrotreating reactions, 300 to 400C in the unit depicted above.
Reactor
The hydrotreating reactions are exothermic. As a result, a temperature rise is observed across the reactor catalyst bed. To prevent a runaway, or uncontrolled, extremely high temperature reactions in the catalyst bed, reactor bed temperatures are closely monitored. A runaway results in extremely high temperatures that could potentially result in a serious safety hazard for refinery personnel as well as catalyst and equipment damage as temperatures exceed the specifications of the metallurgy of the reactor vessel. The refiner has several handles to control the rate of reaction: Feed Quality, H2/HC Ratio or H2 Makeup, Reactor Temperature, Recycle, Quench, LHSV, and Catalyst.
Feed Quality
Feed quality to the diesel hydrotreater, DHT, varies with the refinery’s current crude mix, the type of upstream unit feeding the diesel hydrotreater, and the operating mode of the upstream units. Refinery process units do not operate in a vacuum-they are dependent on the operating mode of the entire refinery. If the refinery is running sweet crudes, less that .5% sulfur, the amount of sulfur in the feed to a DHT will be much lower than when the refinery is running sour crude. Another concern is the amount of crack stock in the feed to the unit. Cracked stock is product from a thermal or catalytic cracking unit like a delayed coker or FCC. These units process the heavier cuts from the crude unit like gas oil and residue which contain more difficult to process sulfur, nitrogen organic compounds as well as a higher amount of metals. Diesel direct from the crude unit, referred to as straight run diesel, results in a much lower exotherm (temperature rise) across the reactor bed because 1) it is derived direct from the lower sulfur portion of the crude mix and 2) it contains fewer olefinic , or unsaturated, straight-chain compounds. Olefins react rapidly with hydrogen (to saturate), an exothermic reaction that results in a large heat release. Another consideration is operation of the fractionation sections of the upstream units. If the end point of the light cycle oil, LCO, from the FCC’s Main Column increases (meaning heavier material is drawn from the diesel/LCO draw from the column and sent to the DHT), a higher amount of more difficult to hydrotreat material will be sent to the DHT.
H2/HC Ratio
The desulfurization, denitrogenation, and deoxygenation reactions consume hydrogen. Hydrogen, H2, also saturates olefins and aromatics and prevents coking or condensation of hydrocarbon on hydrotreating catalyst. Think of coking essentially as hydrocarbons burning and leaving carbon residue on the active sites of the catalyst preventing catalyst from carrying out the required hydrotreating reactions or catalyst deactivation. H2/HC ratio is the amount of Makeup+ Recycle Hydrogen divided by barrels of feed. Hydrogen is a feed source for several units in the refinery. Refineries are often H2 limited or do not have enough hydrogen to meet the needs of all H2 consumers. Hydrogen is generated in the refinery’s Catalytic Reforming or Steam Methane Reforming Units or purchased from an outside provider. Hydrogen is costly for the refinery to produce or purchase. As such, the H2 to HC ratio must be optimized to not only meet the needs of the hydrotreating reactions and to maintain catalyst life but also to ensure profitability for the refinery. To prevent waste of Hydrogen, Separator Hydrogen is recycled back to the feed of the hydrotreater in addition to normal H2 make-up. Some refineries also cascade hydrogen purge from one hydrotreater to another if the hydrogen content is high enough to meet the units needs.
Reactor Temperature
Reactor Temperature is a variable in daily unit operation as well as long term catalyst activity monitoring. Operations can control the heater coil outlet temperature of the furnace and quench, described below. Several thermocouples are inserted throughout the catalyst bed to monitor catalyst activity and look for hot spots due to channeling and poor catalyst loading. Poor distribution of hydrocarbon through the bed results from channeling which leads to reactor internals damage or temperature runaway.
The Weight Average Bed Temperature is used by unit engineers to monitor catalyst deactivation. WABT increases SOR to EOR (Start of Run to End of Run) due to coking or metals deposition on catalyst. Operators should not operate the unit to WABT in daily operation. Temperatures from individual thermocouples throughout the catalyst bed should be considered and monitored for hot spots. Still, because of the distance between thermocouples, it is still possible to have undetected hot spots.
Recycle
Recycling hydrotreated product from the fractionation section back to the feed can also be used to control the severity of hydrotreating operation. Recycling back the product reduces the amount of highly contaminated and thus highly reactive feed to the unit. However, this also limits the capacity or the amount of feed that can be processed in the unit.
Quench
Some Distillate Hydrotreaters have the option of directing hydrogen directly in between beds in multiple catalyst bed reactors. This hydrogen serves as a means of cooling to the reactors in high severity operations as well as additional hydrogen for the treating reactions.
LHSV
Liquid Hourly Space Velocity is the volumetric feed rate divided by catalyst volume. Higher LHSV means a higher feed rate to the unit, means a greater amount of reactive material to the catalyst, leading to a greater temperature rise across the catalyst. This also leads to earlier catalyst deactivation.
Catalyst
Hydrotreating Catalyst is typically made of Nickel-Molybdenum. Catalyst is selected by the refinery to meet a variety of feed, product, and operating goals e.g. typical feed quality and desired operating severity, medium or Ultra Low Sulfur Diesel operating mode, and length of catalyst life.
Gas Separation
Effluent from the reactor passes through the feed effluent exchanger and enters a fin fan exchanger where the effluent is cooled to separator temperature, 35C per the above diagram. Organic Chlorides combined with Ammonia from the denitrogenation reactions to for ammonium salts which could deposit on the tubes of the fin fan exchanger and, in addition to reduced cooling/heat transfer, cause lead to corrosion. To prevent this, water is combined with the reactor effluent which washes the salts away. Once the material is cooled, it enters the separator where the gas phase hydrogen and light end material (typically C1-C4) is separated or flashes off from the now liquid phase product. This light end C1-C4 material comes from the small amount of hydrocracking that also occurs in addition to the hydrotreating reactions. This number is small typically .4-.6 wt% of feed in most operations. The separator sets the pressure for the unit. Typically two pressure control schemes are used: 1) the Makeup hydrogen rate is maintained to set unit pressure while the HPS vent to the sour gas system floats or the HPS vent to the sour gas system is used to control pressure while make-up hydrogen floats. Remember Hydrogen Partial pressure and pressure of a DHT are essentially synonymous as it makes the greatest contribution to the pressure of the unit. Products from the Separator are typically Recycle Hydrogen (Unless Once Through), Vent Gas to the Sour H2 System or Fuel Gas, Water if there is a water boot, and Liquid Hydrocarbon. The Recycle Hydrogen in the diagram above is directed to an amine contactor to scrub for H2S and Ammonia before directing it to the recycle compressor suction to pressure the gas back to the feed or the reactor as quench.
Fractionation
The liquid from the separator is directed to the unit’s fractionation section where products are prepared for downstream units or product blending. The stripping column depicted above is total condensing meaning that no overhead product is drawn from the top of the tower other than light ends which are sent to the refinery’s gas plant. This light end material is removed not only to rid diesel product from any dissolved H2S and ammonia but also to allow the diesel to meet other specifications like flash for blending. There may also be a naphtha overhead product that is drawn from the overhead in partial condensing towers (reflux plus product draw).
In the drawing above, the stripping column is heated or reboiled by exchanging a portion of the bottoms material with hot oil or steam and returning that material to the bottom of the tower. When feed enters the tower, lighter material vaporizes and rises through the trays of the column while the heavier material flows down the column. Some valuable diesel product material is also vaporized and entrained with lighter material which exits the top of the column. This vapor is cooled to liquid phase in a condenser, collected the overhead receiver. Uncondensed light ends are vented to sour gas treatment and the heavier liquid is refluxed back to the column to improve separation. Cooled refluxed higher distillate concentrated liquid flows down over the upper column trays and comes in contact with rising vapors. Due to vapor liquid equilibrium discussed in earlier posts, heavy material flows back down the column leading to product purification. Light end material leaves the overhead drum and is directed to the refinery’s sour gas plant. Product diesel is drawn from the tower bottoms and sent to tankage or blending.
