Distillation Columns & Reactors
What Are Distillation Columns
Distillation columns are the most recognizable pieces of equipment in any oil refinery. These tall, cylindrical pressure vessels perform fractional distillation, a separation process that exploits the difference in boiling points between the various hydrocarbon compounds present in crude oil. A single barrel of crude contains hundreds of different hydrocarbon species ranging from light gases (methane, ethane) with boiling points well below 0 degrees C to heavy residual molecules that do not vaporize until temperatures exceed 550 degrees C.
The basic principle is straightforward: crude oil is heated in a fired heater (furnace) until a significant portion vaporizes, then this partially vaporized feed enters the column at a feed zone. Inside the column, vapor rises upward while liquid flows downward under gravity. At each tray or packing section, the rising vapor contacts the descending liquid, and a mass transfer process occurs: lighter (lower boiling point) components preferentially move from the liquid phase into the vapor phase, while heavier (higher boiling point) components condense from the vapor into the liquid. This repeated contact across many stages achieves the separation of crude into distinct product fractions.
The overhead vapor from the top of the column is condensed in an air-cooled or water-cooled condenser and collected in a reflux drum. A portion of this liquid is returned to the column as reflux (to improve separation efficiency), while the remainder is drawn off as overhead product (typically LPG and light naphtha). Side draws at intermediate levels withdraw kerosene, diesel, and gas oil fractions. The heaviest material exits from the bottom of the column as bottoms product (atmospheric residue in a crude distillation unit).
Distillation is an energy-intensive operation. A large crude distillation unit processing 200,000-400,000 barrels per day consumes substantial quantities of fuel in the preheat furnace and requires significant cooling capacity for the overhead condenser. Optimizing column performance through proper tray design, reflux ratios, and heat integration is a critical aspect of refinery engineering.
Atmospheric Distillation Column Schematic
Types of Distillation Columns
Atmospheric Distillation Unit (CDU)
The Crude Distillation Unit (CDU) is the first and largest processing unit in virtually every oil refinery. It operates at or slightly above atmospheric pressure (typically 1 to 2 atm gauge at the top). Crude oil is preheated through a series of heat exchangers (recovering heat from product streams) and then enters a fired heater where it is heated to approximately 340-370 degrees C. At this temperature, roughly 40-60% of the crude vaporizes.
The partially vaporized crude enters the column at the flash zone, located roughly one-third of the way up from the bottom. Below the flash zone, a few stripping trays and a steam stripping section help remove light components from the liquid residue. Above the flash zone, the rectifying section contains 30 to 50 trays where the rising vapor is progressively cooled and fractionated.
A large CDU in a world-scale refinery handles 100,000 to 400,000+ barrels per day through a column 6 to 12 meters in diameter, making CDUs among the largest columns in any plant. The main column typically contains 30 to 50 trays and operates at 1.0 to 1.8 atm at the top, with a feed temperature of 340-370 degrees C. Products withdrawn include LPG, light naphtha, heavy naphtha, kerosene, diesel, atmospheric gas oil, and atmospheric residue. Two to three pumparound circuits (intermediate cooling loops) are installed to remove heat at multiple levels, improving the overall energy efficiency of the unit.
Vacuum Distillation Unit (VDU)
The Vacuum Distillation Unit processes the atmospheric residue (the bottoms product from the CDU) to recover additional distillate products that cannot be separated at atmospheric pressure without thermally cracking the molecules. By reducing the pressure to 25 to 100 mmHg absolute (0.03 to 0.13 atm), the boiling points of the heavy hydrocarbons are lowered sufficiently to allow vaporization without excessive temperatures.
The VDU operates with an overhead pressure of 25-50 mmHg absolute and a flash zone pressure of 50-100 mmHg absolute. Feed temperature is held at 390-420 degrees C, limited to prevent thermal cracking. The column produces light vacuum gas oil (LVGO), heavy vacuum gas oil (HVGO), and vacuum residue. Internally, the VDU uses predominantly packed sections (structured packing) rather than trays because packing delivers lower pressure drop per theoretical stage, which is critical in vacuum service.
Vacuum columns are distinctive for their very large diameter (up to 14 meters) in the flash zone area and their use of steam ejectors or liquid ring vacuum pumps to maintain the low operating pressure. The overhead system handles non-condensable gases and steam that are drawn out of the column.
The LVGO and HVGO products from the VDU are primary feedstocks for downstream conversion units such as the FCC (fluid catalytic cracker) and the hydrocracker.
Tray Columns
Tray columns use horizontal plates (trays) installed at regular intervals inside the column shell. Each tray provides a stage where vapor and liquid contact each other, promoting mass transfer. The three main tray types are:
Sieve Trays are the simplest and most economical design. They consist of a flat metal plate with numerous small circular perforations (typically 5 to 12 mm diameter). Vapor passes upward through the holes, bubbling through the liquid layer on the tray. Liquid flows across the tray and over a weir at the outlet side, then descends to the tray below through a downcomer. Sieve trays have good efficiency at design conditions but are sensitive to turndown: at low vapor rates, liquid can weep through the holes, and at high vapor rates, the tray can flood.
Valve Trays are an improvement over sieve trays that provide better turndown flexibility. Each hole is covered by a movable metal cap (valve) that lifts as vapor flow increases and closes as it decreases. This self-adjusting mechanism maintains good vapor-liquid contact across a wider range of operating conditions (turndown ratios of 4:1 or better). Common proprietary designs include Koch-Glitsch V-1 Ballast trays and Sulzer MVG trays.
Bubble-Cap Trays are the oldest tray design, featuring a riser (chimney) over each hole with a cap placed over the riser. Vapor travels up through the riser, reverses direction under the cap, and is dispersed into the liquid through slots in the cap periphery. Bubble-cap trays provide a positive liquid seal (no weeping at any vapor rate) and excellent turndown capability, but they are more expensive, have higher pressure drop, and lower capacity than sieve or valve trays. They are now used primarily in specialized services (e.g., chemical reactors, small-diameter columns, or services requiring absolute prevention of liquid drainage).
Packed Columns
Packed columns use beds of packing material instead of trays to provide surface area for vapor-liquid contact. The liquid is distributed over the top of the packing and flows downward as a thin film, while vapor flows upward through the void spaces. Packing is divided into two categories:
Random Packing consists of individual packing elements dumped randomly into the column. Raschig rings (simple cylinders, now largely superseded) were the original form. Modern random packing includes Pall rings with internal fingers for improved capacity, IMTP (Intalox Metal Tower Packing) saddle-shaped elements with high surface area, and Nutter rings with an open structure for very low pressure drop. Random packing is cost-effective for small to medium columns and moderate efficiency requirements. Typical sizes range from 25 mm to 90 mm, with smaller sizes providing better efficiency but higher pressure drop.
Structured Packing consists of corrugated metal or wire mesh sheets assembled into blocks that are stacked inside the column. The corrugations create defined flow channels that promote thin-film flow and efficient mass transfer with very low pressure drop. Leading products include Mellapak (Sulzer) corrugated sheet metal packing with crimped surfaces, Flexipac (Koch-Glitsch) in a similar corrugated sheet design, and BX gauze packing (wire mesh) for very high efficiency in vacuum columns.
Structured packing is the preferred choice for vacuum distillation because its pressure drop per theoretical stage is only 1-3 mmHg, compared to 5-8 mmHg for trays. This lower pressure drop is required in vacuum service where maintaining low absolute pressure across the column height is critical to prevent thermal cracking of heavy hydrocarbons.
Tray Types Comparison
Column Internals
Beyond the primary trays or packing, distillation columns contain several types of internal components that are required for proper operation.
Trays: Efficiency Comparison
Tray selection depends on the specific service requirements. This comparison summarizes the key trade-offs:
| Parameter | Sieve Tray | Valve Tray | Bubble-Cap Tray |
|---|---|---|---|
| Cost | Lowest | Moderate | Highest |
| Efficiency | 70-80% | 75-85% | 65-75% |
| Capacity | High | High | Moderate |
| Pressure drop | Low | Low-Medium | High |
| Turndown ratio | 2:1 | 4:1 | 5:1+ |
| Fouling resistance | Good | Moderate | Good |
| Maintenance | Easy | Moderate | Difficult |
| Weeping tendency | High at low load | Low | None |
Valve trays are the most widely used type in modern refineries due to their good balance of efficiency, capacity, and turndown flexibility. Sieve trays are preferred when cost is the primary driver and the operating range is narrow. Bubble-cap trays are reserved for special applications requiring absolute prevention of liquid drainage or very high turndown.
Packing: Random vs. Structured
| Parameter | Random Packing | Structured Packing |
|---|---|---|
| Pressure drop | Moderate | Very low |
| Efficiency (HETP) | 0.5-1.0 m | 0.2-0.5 m |
| Capacity | Moderate | High |
| Cost per stage | Lower | Higher |
| Best application | Atmospheric services, scrubbers | Vacuum distillation, revamps |
| Sensitivity to maldistribution | Lower | Higher |
HETP (Height Equivalent to a Theoretical Plate) is the key metric for packing performance. Structured packing achieves much lower HETP values, meaning fewer meters of packed height are needed per theoretical stage. This is particularly advantageous in vacuum columns where minimizing overall pressure drop is required.
Liquid Distributors and Collectors
Proper liquid distribution is critical for packed column performance. If the liquid is not evenly distributed across the column cross-section at the top of each packed bed, the packing efficiency deteriorates significantly.
| Distributor Type | Design | Typical Application |
|---|---|---|
| Trough distributor | V-notch or orifice-type troughs that gravity-feed liquid across the column area | Moderate to high liquid rates |
| Pipe distributor | Perforated pipes (ladder-type or spray ring) | Low liquid rates and smaller columns |
| Spray nozzle | Pressurized spray in specialized applications | Quench sections (prone to maldistribution) |
Collectors (or collector trays) are installed below packed beds to gather liquid before it is redistributed to the next packed section below. They also serve as draw-off trays for side product withdrawal.
Feed Distributors
The feed distributor introduces the feed stream into the column at the flash zone. In crude distillation units, the feed enters as a two-phase mixture (vapor and liquid), so the distributor must separate these phases and direct the vapor upward and the liquid downward without excessive entrainment. Common designs include gallery-type distributors and vane-type flash zone internals.
Chimney Trays
Chimney trays are specialized collection trays with cylindrical risers (chimneys) that allow vapor to pass upward while collecting all descending liquid. They appear at total liquid draw-off points (pumparound draw trays), at feed trays in some configurations, above packed beds for liquid collection, and where a liquid seal is needed for side-draw circuits.
Reactors in Downstream Processing
While distillation columns separate crude oil into fractions, reactors transform those fractions into higher-value products through chemical reactions. Refinery reactors operate under demanding conditions of high temperature, high pressure, and corrosive environments, often in the presence of hydrogen and specialized catalysts.
Fluid Catalytic Cracking (FCC)
The FCC unit is one of the most important conversion units in a refinery. It breaks down heavy gas oil fractions (from the VDU) into lighter, more valuable products; primarily gasoline, light cycle oil (LCO), and olefins (propylene, butylenes). The process uses a finely powdered zeolite-based catalyst at temperatures of 480-540 degrees C and near-atmospheric pressure.
The FCC unit has two main vessels:
Riser Reactor: The feed (heavy gas oil, typically 340-560 degrees C boiling range) is injected at the base of a vertical pipe (riser) where it contacts hot regenerated catalyst. The catalyst-to-oil ratio is typically 5:1 to 8:1 by weight. The endothermic cracking reactions occur in 2-4 seconds as the mixture travels up the riser at high velocity. At the top of the riser, cyclones separate the spent catalyst from the hydrocarbon vapors. The vapors proceed to the main fractionator for product separation.
Regenerator: The spent catalyst, now coated with coke (carbon deposits from the cracking reactions), flows down a standpipe into the regenerator vessel. Here, air is blown through the catalyst bed at 650-730 degrees C to burn off the coke, restoring the catalyst’s activity and providing the heat needed to drive the endothermic cracking reactions. The flue gas (CO2, N2, excess O2) exits through cyclones at the top of the regenerator.
The continuous circulation of catalyst between the riser and regenerator creates a self-sustaining heat balance: the heat released by burning coke in the regenerator provides the energy consumed by cracking in the riser.
Hydrocracking
The hydrocracker is a high-severity conversion unit that processes heavy vacuum gas oil or other heavy feeds in the presence of hydrogen and a dual-function catalyst (combining hydrogenation and cracking functions). Unlike FCC, hydrocracking operates at very high pressure and produces a higher proportion of middle distillates (diesel, jet fuel).
Hydrocracking conditions are among the most severe in refinery service. Reactors run at 350-430 degrees C and 100-200 bar (1,500-3,000 psig), with hydrogen partial pressures of 80-170 bar. The catalyst is amorphous silica-alumina or zeolite loaded with Ni-Mo or Ni-W metals, and LHSV ranges from 0.5 to 2.0 h^-1.
The reactor is a thick-walled pressure vessel containing multiple catalyst beds with inter-bed quench systems (cold hydrogen injection between beds to control the exothermic reaction temperature). Reactor wall thickness can exceed 250 mm (10 inches) due to the combination of high pressure, high temperature, and hydrogen service. Materials of construction are critical; the reactor shell is typically fabricated from 2-1/4 Cr-1Mo or 2-1/4 Cr-1Mo-1/4V steel with an internal stainless steel (Type 347 SS) weld overlay or lining to resist hydrogen attack and corrosion.
Hydrocracking achieves very high conversion rates (80-99% per pass in recycle configurations) and produces high-quality products with low sulfur, low nitrogen, and high cetane number (for diesel) or high smoke point (for jet fuel).
Catalytic Reforming
Catalytic reforming converts low-octane heavy naphtha into high-octane reformate (a key gasoline blending component) and produces hydrogen as a valuable byproduct. The process uses a Pt (platinum) or Pt-Re (platinum-rhenium) catalyst on an alumina support to promote dehydrogenation of naphthenes to aromatics, isomerization of paraffins, dehydrocyclization of paraffins to aromatics, and (to a lesser extent) hydrocracking of paraffins.
Reforming typically uses 3 to 4 fixed-bed reactors in series, with fired heaters (inter-heaters) between each reactor to reheat the process stream. The reactions are predominantly endothermic, and the temperature drops 30-60 degrees C across each reactor.
Reactor inlet temperatures sit at 480-530 degrees C and pressures range from 3.5 to 35 bar, depending on whether the unit is semi-regenerative or continuous regenerative. The H2/HC molar ratio is maintained at 3:1 to 8:1, and the catalyst is Pt/Al2O3 or Pt-Re/Al2O3, chlorided for acidity.
Modern continuous catalytic reforming (CCR) units (licensed by UOP or Axens) operate at lower pressure (3.5-10 bar) with continuously moving catalyst that circulates through the reactors and a regeneration section. This allows higher severity operation and higher reformate octane (RON 100-104).
Hydrotreating
Hydrotreating (or hydrodesulfurization, HDS) is the most common catalytic process in refineries. Virtually every intermediate product stream (naphtha, kerosene, diesel, gas oil) is hydrotreated to remove sulfur, nitrogen, oxygen, and metals before further processing or blending into finished products.
The hydrotreater uses a trickle-bed reactor where liquid feed flows downward over a fixed catalyst bed while hydrogen gas flows co-currently downward. The catalyst is typically Co-Mo/Al2O3 or Ni-Mo/Al2O3. Operating temperatures range from 300 to 400 degrees C at pressures of 30-100 bar, with hydrogen consumption of 10-300 Nm3/m3 of feed depending on sulfur content and severity. The treated product exits with reduced sulfur content, below 10 ppm for ultra-low sulfur diesel.
The reactor vessel is simpler than a hydrocracker; lower pressure and temperature mean thinner walls and less demanding metallurgy. However, the catalyst volume can be substantial (hundreds of cubic meters for large diesel hydrotreaters).
FCC Unit Simplified Schematic
Reactor Types Comparison
| Reactor Type | Process | Temperature (C) | Pressure (bar) | Key Products | Catalyst |
|---|---|---|---|---|---|
| FCC Riser | Catalytic cracking | 480-540 | 1.5-3 | Gasoline, LPG, LCO, olefins | Zeolite (Y-type) |
| Hydrocracker | Hydrocracking | 350-430 | 100-200 | Diesel, jet fuel, naphtha | Ni-Mo or Ni-W on SiO2-Al2O3/zeolite |
| Reformer | Catalytic reforming | 480-530 | 3.5-35 | High-octane reformate, H2 | Pt or Pt-Re on Al2O3 |
| Hydrotreater | Hydrodesulfurization | 300-400 | 30-100 | Low-sulfur products | Co-Mo or Ni-Mo on Al2O3 |
| Coker Drum | Delayed coking | 480-510 | 2-6 | Coke, coker naphtha, gas oil | None (thermal) |
| Visbreaker | Thermal cracking | 440-500 | 5-15 | Reduced-viscosity fuel oil | None (thermal) |
A few patterns stand out in this data. Hydrocrackers operate at by far the highest pressures, requiring the thickest vessel walls and most demanding metallurgy. FCC units are unique in using a fluidized catalyst system rather than a fixed catalyst bed. Reformers are the only units that are net hydrogen producers; all other catalytic units consume hydrogen. Thermal processes (coking, visbreaking) rely on temperature alone to break molecular bonds and use no catalyst.
Standards and Specifications
The design, fabrication, and inspection of distillation columns and reactors are governed by a full set of codes and standards:
| Standard | Description |
|---|---|
| ASME Section VIII, Div. 1 & 2 | Pressure vessel design rules for column shells and reactor vessels. Division 2 (Alternative Rules) is commonly used for thick-walled reactors to optimize wall thickness |
| API 934-A | Materials and fabrication of 2-1/4Cr-1Mo and 3Cr-1Mo steel heavy-wall pressure vessels for high-temperature, high-pressure hydrogen service |
| API 934-C | Materials and fabrication of 2-1/4Cr-1Mo-1/4V steel heavy-wall pressure vessels for high-temperature, high-pressure hydrogen service |
| API 941 | Nelson curves for steels in high-temperature hydrogen service (HTHA avoidance) |
| ASME Section II, Part D | Material properties (allowable stresses, physical properties) used in vessel design calculations |
| API 660 | Shell-and-tube heat exchangers (applies to reboilers and condensers associated with columns) |
| NACE MR0103 / ISO 17945 | Materials for resistance to sulfide stress cracking in refinery equipment |
| UOP / Axens / CLG | Licensor-specific standards for proprietary reactor internals, catalyst support systems, and quench hardware |
| API 579-1 / ASME FFS-1 | Fitness-for-service assessment of in-service equipment (evaluation of flaws, corrosion, HTHA damage) |
| AWS D1.1 / ASME IX | Welding qualification and procedures |
Materials Selection
Materials selection for distillation columns and reactors depends on the operating temperature, pressure, corrosive environment, and hydrogen partial pressure.
Column Shells
For atmospheric and vacuum distillation columns operating at moderate temperatures and pressures, carbon steel (SA-516 Gr. 70) is the standard shell material. The design temperature for a CDU column ranges from about 150 degrees C at the top to 400 degrees C at the bottom. In the hotter lower sections, or where the crude oil contains high concentrations of naphthenic acid (high Total Acid Number, TAN), the shell may be upgraded or clad with higher-alloy materials:
| Material | Typical Application |
|---|---|
| 5Cr-0.5Mo (Type 501) | Naphthenic acid corrosion resistance |
| Type 410S SS cladding | Severe naphthenic acid service |
| Type 316 SS (with Mo) cladding | Combined naphthenic acid and sulfidation environments |
| Type 317L SS or alloy 625 cladding | Most aggressive services |
Vacuum column shells are typically carbon steel since they operate at low absolute pressure (thin walls) and moderate temperatures.
Trays and Column Internals
Trays and internals are thin-gauge components (2-4 mm thick) that must resist corrosion without the benefit of significant corrosion allowance.
| Material | Typical Service |
|---|---|
| Type 410 SS (12Cr) | Standard for most refinery distillation; good sulfidation resistance at moderate temperatures |
| Type 304 SS (18Cr-8Ni) | Higher corrosion resistance needs such as amine service and sour water strippers |
| Type 316 SS (16Cr-12Ni-2Mo) | Naphthenic acid service or chloride-enhanced corrosion environments |
| Alloy 2205 (duplex SS) | Particularly aggressive corrosion environments |
| Monel 400 | Overhead condensation zones with severe HCl dew-point corrosion |
Reactor Vessels
Reactor vessels for hydrocracking and hydrotreating operate under severe conditions of high pressure (100-200 bar), high temperature (350-450 degrees C), and hydrogen-rich environments. The combination of these factors makes material selection critical.
The standard shell material is 2-1/4Cr-1Mo (SA-336 F22), which provides good creep strength at elevated temperatures and resistance to hydrogen attack. Temper embrittlement is a concern, requiring tight control of tramp elements (P, Sn, As, Sb) per API 934-A. For new hydrocracker reactors, 2-1/4Cr-1Mo-1/4V (SA-336 F22V) has largely replaced conventional F22. The vanadium addition raises strength and creep resistance, allowing thinner walls and lighter vessels for the same design conditions, covered by API 934-C. A third option, 3Cr-1Mo (SA-336 F21), sees occasional use as an alternative to the 2-1/4Cr grades.
Internally, reactors are lined with Type 347 SS weld overlay as standard practice. The overlay is applied in two layers: a first high-dilution layer followed by a second layer meeting full 347 SS chemistry. This lining provides corrosion resistance and acts as a hydrogen permeation barrier. Type 321 SS serves as an alternative to 347 SS in some services, while alloy 625 weld overlay is reserved for the most aggressive conditions with high H2S partial pressure and ammonium bisulfide corrosion.
High-Temperature Hydrogen Attack (HTHA)
High-temperature hydrogen attack is a damage mechanism unique to equipment operating in hydrogen service above approximately 200 degrees C. Atomic hydrogen diffuses into the steel and reacts with carbon to form methane (CH4) at grain boundaries. Since methane molecules cannot diffuse out of the steel, they accumulate and create internal voids and micro-fissures that eventually lead to catastrophic failure.
The API 941 Nelson Curves provide guidance on the minimum alloy content required for a given combination of temperature and hydrogen partial pressure. Carbon steel is limited to approximately 200 degrees C in significant hydrogen partial pressure. Adding 1Cr-0.5Mo extends the safe operating envelope somewhat, while 2-1/4Cr-1Mo allows operation up to approximately 450 degrees C at high hydrogen pressures. The Nelson Curves have been revised multiple times as field failures revealed that earlier curves were non-conservative, particularly for carbon steel and C-0.5Mo steel. C-0.5Mo has been effectively removed from hydrogen service as a result.
Design Considerations
Number of Theoretical Stages
The number of theoretical stages (or trays) required for a given separation is determined by the relative volatility of the key components and the desired product purity. The McCabe-Thiele method (for binary systems) or rigorous simulation (for multicomponent systems, using software such as Aspen HYSYS, PRO/II, or UniSim) is used to determine the stage count. A typical CDU requires 30 to 50 theoretical stages; a debutanizer may need only 30 to 40.
The actual number of physical trays is the theoretical stages divided by the tray efficiency (typically 0.70 to 0.85 for valve trays). For packed columns, the required packing height equals the number of theoretical stages multiplied by the HETP.
Reflux Ratio
The reflux ratio (L/D, the ratio of liquid returned to the column as reflux to the liquid withdrawn as distillate) is a critical operating parameter that controls separation quality. Increasing the reflux ratio improves separation but increases energy consumption (more heat input at the reboiler and more cooling at the condenser).
The minimum reflux ratio corresponds to an infinite number of stages; the minimum number of stages corresponds to total reflux (no product withdrawal). The optimum design reflux ratio is typically 1.1 to 1.5 times the minimum reflux ratio, balancing capital cost (number of trays) against operating cost (energy).
Flooding and Weeping
Flooding occurs when the vapor velocity through the column is too high, causing excessive liquid entrainment upward or backup of liquid in the downcomers. Flooding leads to a sharp loss of separation efficiency and must be avoided. Column diameter is sized to keep the vapor velocity below the flooding velocity (typically operating at 75-85% of flood).
Weeping occurs at low vapor rates when liquid drains through the tray perforations instead of flowing across the tray to the downcomer. This short-circuits the vapor-liquid contact and reduces tray efficiency. Valve trays resist weeping better than sieve trays.
Column Diameter
Column diameter is determined primarily by the vapor load (vapor volumetric flow rate). The maximum allowable vapor velocity depends on the tray type, tray spacing, liquid load, and fluid physical properties (density, surface tension). The Souders-Brown correlation is commonly used to estimate the maximum vapor velocity:
V_max = C_sb x sqrt((rho_L - rho_V) / rho_V)
where C_sb is the Souders-Brown coefficient (dependent on tray spacing and system properties), rho_L is the liquid density, and rho_V is the vapor density. The column diameter is then calculated to give an actual vapor velocity of 75-85% of V_max.
Large crude distillation columns can reach diameters of 10-14 meters, making them among the largest pressure vessels fabricated. Transportation logistics often govern the maximum shop-fabricated diameter; columns larger than approximately 6-7 meters must often be field-erected.
Reactor Wall Thickness
Hydrocracking reactor wall thickness is driven by the combination of high design pressure (150-200 bar), high design temperature (450 degrees C+), and the need for adequate material properties in hydrogen service. Using ASME Section VIII, Division 2 (which allows higher allowable stresses through more rigorous analysis), a typical hydrocracker reactor might have a shell inside diameter of 3.5 to 5.0 meters, wall thickness of 200 to 300 mm (8 to 12 inches), total weight of 500 to 1,500 tonnes including internals and catalyst, and an overall length of 15 to 30 meters.
These reactors are among the heaviest single-piece equipment items in a refinery and require specialized fabrication, heat treatment (post-weld heat treatment, PWHT), and transportation.
Post-weld heat treatment (PWHT) for Cr-Mo reactor vessels is a critical step. For 2-1/4Cr-1Mo-1/4V steel, PWHT temperatures are carefully controlled (typically 705 plus or minus 14 degrees C) to achieve the proper balance of strength, toughness, and temper embrittlement resistance. The entire PWHT cycle (including heating, holding, and cooling rates) can take 48 to 72 hours for a large reactor.
Applications
Oil Refineries
Distillation columns and reactors are the backbone of every oil refinery. The Crude Distillation Unit (CDU) is the primary atmospheric column that processes all incoming crude oil; every refinery has at least one. The Vacuum Distillation Unit (VDU) takes the atmospheric residue and recovers LVGO and HVGO as cracking feed, and is present in all complex refineries. The FCC (Fluid Catalytic Cracking) unit converts heavy gas oil to gasoline and olefins and is often called the “heart” of a gasoline-oriented refinery. Hydrocracking serves the same feed but converts it to middle distillates (diesel, jet fuel), making it dominant in diesel-oriented refineries across Europe and Asia. Catalytic reforming upgrades naphtha octane for gasoline blending and produces hydrogen, appearing in virtually all refineries. Multiple hydrotreaters throughout the refinery remove sulfur from naphtha, kerosene, diesel, and VGO feeds. Smaller distillation columns such as debutanizers, depropanizers, and stabilizers separate light ends (C3, C4) from heavier liquid products, while amine regenerators strip H2S from rich amine solution as part of the sulfur recovery system.
Gas Processing Plants
Natural gas processing facilities use distillation columns extensively for NGL (natural gas liquids) recovery and fractionation. A deethanizer separates ethane from heavier NGL components (C3+), a depropanizer separates propane from C4+ components, a debutanizer separates butanes from C5+ (natural gasoline), and a de-isobutanizer separates isobutane from normal butane. NGL fractionation trains may include all of these columns operating in series.
These columns typically operate at higher pressures (10-30 bar) than refinery crude columns because the components being separated are lighter and more volatile. Tray columns (valve trays) are standard for NGL service.
Petrochemical Plants
Petrochemical facilities rely on both distillation and reactor technology. Ethylene crackers (steam crackers) use tubular furnaces (pyrolysis reactors) to thermally crack ethane, propane, or naphtha into ethylene, propylene, and other olefins; the cracked gas is then separated in a train of distillation columns operating at cryogenic temperatures down to -100 degrees C. Aromatics complexes use catalytic reforming to produce BTX (benzene, toluene, xylenes), followed by extractive distillation or liquid-liquid extraction to recover high-purity aromatics. Polymerization reactors (slurry, gas-phase, or solution types) produce polyethylene, polypropylene, and other polymers. These are distinct from refinery reactors but share some design principles. Methanol synthesis reactors, either fixed-bed or slurry, operate at 50-100 bar and 200-300 degrees C over Cu-ZnO-Al2O3 catalyst.
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