Inside the CDU: The furnace–column handshake that sets a refinery’s profits

The crude distillation unit (CDU, the front‑end atmospheric distillation block) is where refineries separate incoming crude into gas, naphtha, kerosene, gas oils, and atmospheric residue, after desalting to remove water, salts, and particles to protect downstream equipment (IntechOpen). The scale is vast: global refining capacity in 2023 reached ~103.5 million barrels a day (Offshore Technology), while Indonesia’s state refineries account for roughly 1.0–1.1 Mbbl/d—about 90% of national capacity (Pertamina).

PT Pertamina’s six refineries (RUs II–VII) process ≈1 million bpd and added ~126 kbpd from 2019–2023 via debottlenecking and upgrades (Pertamina; Pertamina). What those units actually make depends on crude quality: a mid‑density crude (API ~35°, SG≈0.84; API gravity indicates crude density; SG is specific gravity) typically yields ~38% gasoline (light+heavy naphtha), 12% kerosine, ~36% distillate (gas oil), and ~6% bottoms; heavier crudes (e.g., SG≥0.88) swing toward >50% distillate plus residue and less naphtha (Engineering Toolbox). Small changes in furnace outlet temperature or reflux ratio can shift yields of high‑value fractions by a few percent.

Design documents for the crude‑preheat and desalter block routinely interface with supporting site utilities. Where those interfaces are specified, teams reference site standards for supporting equipment for water treatment, such as water treatment ancillaries, to keep boundaries clear without altering CDU thermodynamics.

Fired heater duty and temperature window

The fired heater (“heater” or “furnace”) is central to the CDU, supplying heat to vaporize part of the crude feed. After preheat to about ~250°C, the crude is taken to ~350–380°C across convection and radiant sections (IntechOpen). Tube‑furnace designs are most common, with target outlet temperature typically 330–385°C, set to vaporize roughly 70–80% of the crude so the feed enters the column near vapor–liquid equilibrium (IntechOpen).

One case study at ~2 bar heating Bonga crude from 250°C to 380°C produced ~103,104 kg/h of vapor and 35,055 kg/h of liquid (≈75% vapor by mass), underscoring the need to hit the cut‑point precisely (IntechOpen; IntechOpen). Under‑heating yields too much heavy bottoms; over‑heating risks thermal cracking and coking, with a safe limit around ~385°C in the radiant coils (IntechOpen).

Furnace efficiency and controls

Heaters run on refinery fuel gas or fuel oil, with combustion efficiency typically 85–95%. Excess air, flue‑gas losses, tube fouling, and radiation losses are the controllables. Upgrading burner management and adding CO/O₂ trim can cut fuel use 5–10% and trim CO₂ a few percent; one study reported a ~5% fuel cut (~0.74 m³ oil/day), 3–5% CO₂ reduction, and ~1.5% higher thermal efficiency after a CDU furnace control upgrade (MDPI).

Heat recovery is part of the energy story: stack economizers and feed/effluent preheat trains can reclaim roughly 10–15% of feed enthalpy via pinch exchangers, lowering fired duty. In large CDUs (100–250 kbpd), the heater typically supplies >90% of internal reboiling duty; annual fuel use often exceeds 10¹¹–10¹² kJ. Typical duty scales with load—on the order of ~40–50 MW for a 100 kbpd unit. Indonesian refineries must meet emissions limits (NOₓ/SOₓ), so modern burners (ULNB or LNB; ultra‑low‑NOx/low‑NOx designs) are often specified in new or revamped furnaces.

Tube fouling and corrosion are plant realities discussed in heater operations. In that broader reliability context, refiners differentiate mechanical, operational, and chemical approaches; for reference, categories such as scale inhibitor and corrosion inhibitor are part of refinery supply chains, separate from the core CDU heat balance.

Column internals: trays or packing

Inside the column shell, separation is driven by countercurrent vapor–liquid contact. Industrial distillation uses either trays (plates) or packings, with roughly a 50/50 split worldwide in column count (Chemical Engineering). Conventional atmospheric CDU towers typically use trays; vacuum towers more often use packing because of pressure‑drop sensitivity, with fouling, turndown, and liquid rates also in the mix (Chemical Engineering).

Tray hydraulics and performance

Tray columns create discrete stages: vapor bubbles through tray decks into liquid pools, then flows down via downcomers. Typical tray spacing runs ~0.6–0.9 m in large towers; a crude column may have 20–30 active trays above the feed (rectifying section) and 20–30 below (stripping), depending on separation goals (IntechOpen; Chemical Engineering). Types include bubble‑cap (wide rangeability, higher pressure drop), valve (adjustable, moderate turndown), and sieve trays (lower cost, weeping risk) (Chemical Engineering).

Trays tolerate fouling and offer predictable stage efficiency—often 40–70% of theoretical per tray for crude fractionation—but they impose higher pressure drop because vapor must bubble through liquid on each stage (Boardman; Chemical Engineering).

Packed sections and vacuum service

Packed columns replace plates with media that deliver large surface area per volume for continuous film contact. Options range from random packings (Raschig rings, Berl saddles) to structured packings (corrugated/perforated sheets), with modern structured designs achieving very high surface area (thousands m²/m³) at low resistance (Chemical Engineering). Packed towers deliver much lower pressure drop—critical in vacuum, where even small ΔP at ~100–200 mbar can crush boilup—so CDU vacuum sections often use structured packing (Boardman).

Drawbacks include sensitivity to liquid maldistribution and fouling/foaming. In practice, many CDUs use trays for the atmospheric column and packing for the vacuum tower. For the same separation, packed towers can require fewer height‑equivalent stages (lower HETP), but can flood at elevated liquid rates; tray towers handle capacity changes and solids better, at the cost of pressure drop and energy (Chemical Engineering).

Sizing, cuts, and pressure profile

An atmospheric tower for a large CDU may stand 30–60 m tall and 5–8 m in diameter. Overhead product rate sets vapor traffic and diameter; tray spacing, downcomer area, and weir design follow hydraulic calculations. Bubble‑cap trays can be used for wide turndown, though valve/sieve trays are more common for simple CDU duties.

Pressure is modest: furnace outlet and bottom section run about ~1–3 bar(a); the top sits near atmospheric to slightly below to boil naphtha overhead. Overhead is condensed and partly refluxed; a naphtha reflux ratio of 3–5:1 is common case‑by‑case. Typical temperature landmarks: ~130–150°C just under the condenser (naphtha), kerosene around 200°C, diesel ~300°C. Stripping steam or water wash may be used near the bottom to improve recovery, especially for kerosene/diesel.

Where project specifications touch high‑pressure side‑services or sample stations, teams keep those utilities distinct from the tower core; as a reference point, high‑pressure housings in industrial service are a separate specification item (see steel filter housings), not a column internal.

Yields, energy, and economics

A 1°C tweak in cut‑point can shift yields by tenths of a percent; a 5–10°C rise might increase heavy gas oil and reduce naphtha by ~1–2%. On a 100 kbpd unit, that swing means thousands of barrels per day, making furnace tuning economically significant. The CDU is also one of the site’s biggest energy consumers: a well‑integrated 100 kbpd unit might need ~3–5 GJ/tonne of crude feed (including fuel and steam). Roughly half the heat value of the crude ends up in products; the rest is lost to cooling. The furnace duty spans tens of MW, burning ~10–30% of the crude’s energy content. Efficiency measures such as O₂ trim mirror the control upgrade gains noted earlier (fuel down ~5% and CO₂ down 3–5%; thermal efficiency up ~1.5%) (MDPI).

Accurate utilities metering and dosing are handled outside the column proper; when chemical dosing is part of a plant’s utility scope, engineers specify it separately (see dosing pump considerations) to avoid conflating CDU mass balance with utility injection points.

Emissions and regulatory context

Refineries in Indonesia must comply with emissions standards (Ministerial regulations). Flue gases from CDU furnaces must meet NOₓ and CO limits (often <150 ppm NOₓ for new furnaces). Particulate (PM) control is addressed via selective catalytic reduction or burner design as needed. Product quality regulations (Euro IV/V sulfur and olefins limits) drive CDU cut‑points and downstream hydrotreaters; recent Pertamina RFCC/hydrotreating projects align with third‑party specifications and national fuel standards.

Capacity growth and technology trends

Yields track crude gravity: light crudes can exceed 50% yield above ~180°C, while heavy crudes can produce >20% atmospheric residue, per publicly available yield correlations (Engineering Toolbox). Indonesia has been expanding CDU capacity—e.g., Balikpapan’s CDU rose from 125 to 150 kbpd—and targets ~1.3 Mbpd by 2025 (Pertamina; Pertamina). Globally, modest net capacity growth is expected by 2025, with Asia‑Pacific leading new CDU builds (Offshore Technology).

Efficiency R&D emphasizes heat integration—up to and including multi‑effect distillation and heat pumps—to trim reboiler duty, alongside advanced process control (MPC, neural nets) to stabilize operation under variable feed (MDPI). Simulations (e.g., ASPEN HYSYS on Basrah crude) indicate coordinated furnace‑boiler control can shave fuel a few percent at full load. Decarbonization goals are pushing fuel gas recovery and lean combustion; CO₂ costs make every percentage point of fuel saved count.

Internals innovation and practical limits

Designers are fielding high‑capacity tray designs (e.g., finger‑trays for >1000 kt/h vapor) and ultra‑high‑efficiency structured packings. Vacuum towers may employ structured coil‑wound packings to manage very low flows. Tray manufacturers report that cracking and fouling physics still cap furnace temperature and UOP gravity cutoff; current upgrades emphasize better alloy coils and fouling‑resistant packings.

Bottom line: furnace–column synergy

The CDU works when the heater vaporizes the right fraction of crude (≈70–80%) at high efficiency and the column internals provide enough area for clean splits. Well‑designed units recover >95% of crude into saleable fuel streams with minimal fuel use. Small changes in heater temperature, tray count, packing type, or reflux have measurable impacts on yields and energy—so designs should be anchored by data, from furnace heat balances to tray hydraulics (IntechOpen; MDPI).

Sources: Pertamina; Offshore Technology; Engineering Toolbox; IntechOpen; Chemical Engineering; MDPI; Pertamina; Pertamina; Boardman; Boardman; Chemical Engineering; IntechOpen; IntechOpen; Pertamina; Pertamina; MDPI