1. Role of the Debutanizer in NGL Processing
The debutanizer (DeC4) is a distillation column that separates butanes (iC4 and nC4) from pentanes and heavier hydrocarbons (C5+). It occupies the fourth position in the NGL fractionation train, receiving the C4+ bottoms product from the depropanizer and producing mixed butanes overhead while sending natural gasoline (C5+) to the bottoms.
The feed to the debutanizer is the depropanizer bottoms stream, which typically contains trace C3, isobutane (iC4), normal butane (nC4), isopentane (iC5), normal pentane (nC5), hexane (C6), and heavier components (C7+). The overhead product is a mixed butane stream (iC4 + nC4) used for LPG blending, alkylation feed, or petrochemical feedstock. The bottoms product is natural gasoline (C5+), a valuable product used for gasoline blending, diluent for heavy oil transport, and condensate stabilization.
NGL Fractionation Train Sequence
| Column | Top Product | Bottom Product | Key Separation |
|---|---|---|---|
| Demethanizer (DeC1) | C1 (sales gas) | C2+ (NGL) | C1/C2 |
| Deethanizer (DeC2) | C2 (ethane) | C3+ (NGL) | C2/C3 |
| Depropanizer (DeC3) | C3 (propane) | C4+ | C3/iC4 |
| Debutanizer (DeC4) | C4 (mixed butanes) | C5+ (natural gasoline) | nC4/iC5 |
| Butane Splitter | iC4 (isobutane) | nC4 (normal butane) | iC4/nC4 |
Product Disposition
| Product | Typical Composition | End Uses |
|---|---|---|
| Overhead — Mixed Butanes | iC4 + nC4, max 2 mol% C5+ | LPG blending, alkylation feed, petrochemical cracker feed, aerosol propellant |
| Bottoms — Natural Gasoline (C5+) | iC5, nC5, C6, C7+ | Gasoline blending, heavy oil diluent, condensate stabilization, export |
If further fractionation of the overhead butane stream is required, a downstream butane splitter (iC4/nC4 column) separates isobutane from normal butane. Isobutane is a premium alkylation feedstock, while normal butane is used for gasoline RVP blending or LPG. The debutanizer column itself operates at moderate pressures and temperatures, making it one of the more straightforward columns in the NGL train from both a design and operational standpoint.
2. nC4/iC5 Key Component Separation
The key separation in the debutanizer is between n-butane (nC4, the light key) and isopentane (iC5, the heavy key). The relative volatility of nC4 to iC5 is defined as:
Where KnC4 and KiC5 are the vapor-liquid equilibrium K-values at column conditions. The normal boiling point of nC4 is 31°F and iC5 is 82°F, giving a boiling point difference of approximately 51°F. This relatively large spread results in favorable relative volatility values of 1.8–2.5, making the nC4/iC5 separation easier than the C3/iC4 separation in the depropanizer at comparable pressures.
K-values can be estimated using the Wilson K-value correlation for preliminary design:
Where Pci and Tci are the critical pressure and temperature of component i, ωi is the acentric factor, and P and T are the operating pressure and temperature. For more rigorous work, the Peng-Robinson or Soave-Redlich-Kwong (SRK) equations of state are preferred.
Relative Volatility vs. Operating Pressure
| Pressure (psig) | Column Temp Range (°F) | α(nC4/iC5) | Separation Ease |
|---|---|---|---|
| 75 | 100–250 | ~2.4 | Easiest |
| 100 | 115–275 | ~2.2 | Easy |
| 125 | 130–300 | ~2.0 | Moderate |
| 150 | 140–325 | ~1.8 | More difficult |
Operating pressure for the debutanizer is typically selected in the range of 75–150 psig, which is significantly lower than the depropanizer (200–300 psig). The lower operating pressure is advantageous because it increases relative volatility, reduces the required number of theoretical stages, and lowers the reboiler temperature. The pressure is generally set by the ability to condense the overhead butane product with available cooling media (air or cooling water).
The geometric mean relative volatility across the column is used in shortcut design calculations:
With αavg values typically in the range of 2.0–2.3, the debutanizer requires fewer theoretical stages than the depropanizer for equivalent product purity, resulting in a shorter and less expensive column.
3. Column Design Using FUG Shortcut Method
The Fenske-Underwood-Gilliland (FUG) shortcut method is used for preliminary debutanizer sizing, providing estimates of minimum stages, minimum reflux, and actual stage count before rigorous simulation. This is the same methodology applied to the deethanizer and depropanizer columns in the NGL train.
Fenske Equation — Minimum Stages
The Fenske equation calculates the minimum number of theoretical stages at total reflux:
Where xLK,D and xHK,D are the light key and heavy key mole fractions in the distillate, and xHK,B and xLK,B are the corresponding mole fractions in the bottoms. For a typical debutanizer producing a butane product with maximum 2 mol% C5+ and a natural gasoline bottoms with maximum 2 mol% C4, Nmin is typically 6–10 theoretical stages.
Underwood Equation — Minimum Reflux
The Underwood equations determine the minimum reflux ratio (Rmin) at infinite stages. The root θ is found from:
Where q is the feed quality (q = 1 for saturated liquid, q = 0 for saturated vapor). The value of θ lies between the relative volatilities of the light key and heavy key. Then the minimum reflux is obtained from:
Gilliland Correlation — Actual Stages
The Gilliland correlation (Molokanov form) relates the actual reflux ratio to the actual number of stages:
The Molokanov approximation provides the closed-form solution:
Kirkbride Equation — Feed Tray Location
The Kirkbride equation estimates the optimal feed tray location by computing the ratio of rectifying to stripping stages:
Where NR and NS are the number of rectifying and stripping stages, z values are feed compositions, x values are product compositions, and B/D is the bottoms-to-distillate molar flow ratio.
Typical Debutanizer Design Parameters
| Parameter | Typical Range |
|---|---|
| Operating pressure | 75–150 psig |
| Overhead temperature | 120–160°F |
| Bottoms temperature | 300–400°F |
| Minimum theoretical stages (Nmin) | 6–10 |
| Actual trays | 25–40 |
| Tray efficiency | 70–85% |
| R/Rmin | 1.1–1.3 |
| Reflux ratio (R) | 1.0–2.5 |
| Feed condition (q) | 0.5–1.0 (mixed to saturated liquid) |
| Column diameter | 3–10 ft |
| Tray spacing | 24 in |
The higher relative volatility of the nC4/iC5 system compared to the C3/iC4 system means that the debutanizer typically requires fewer trays (25–40 actual) than the depropanizer (30–45 actual) for equivalent product purity. The lower reflux ratio also means a smaller column diameter for the same throughput. As with the depropanizer, the feed is typically at or near its bubble point (q ≈ 1.0) since it arrives as the hot bottoms stream from the upstream column.
4. Condenser and Overhead System
The debutanizer overhead temperature of 120–160°F at typical operating pressures of 75–150 psig is well above ambient conditions in virtually all climates. This makes air-cooled condensers almost always feasible for the debutanizer, offering significant capital and operating cost savings compared to shell-and-tube cooling water systems.
Condenser Configuration
The debutanizer uses a total condenser, producing a liquid mixed-butane overhead product and a liquid reflux stream returned to the column. The total condenser is standard because the butane product is stored and transported as a pressurized liquid. A reflux drum (accumulator) is installed between the condenser and column to provide liquid holdup for reflux flow control and surge capacity.
The condenser duty is calculated from:
Where R is the reflux ratio, D is the distillate rate (lb/hr), and λavg is the average latent heat of condensation (BTU/lb) at overhead conditions.
Butane Product Quality
The overhead mixed-butane product must meet specification limits, typically a maximum of 2 mol% C5+ content. Product quality is monitored by online gas chromatograph or periodic laboratory analysis. The butane product exits the reflux drum as a pressurized liquid and is routed to pressurized storage (bullets or spheres), pipeline, or truck/rail loading facilities.
Air vs. Water Cooling Economics
| Parameter | Air-Cooled Condenser | Shell-and-Tube (CW) |
|---|---|---|
| Capital cost | Higher (larger surface area) | Lower (compact) |
| Operating cost | Fan power only | Pumping + water treatment + makeup |
| Approach temperature | 15–30°F above ambient dry-bulb | 10–15°F above CW supply |
| Hot-day performance | Reduced capacity | Relatively stable |
| Maintenance | Fan belts, motors, fin cleaning | Tube fouling, water chemistry |
| Typical selection | Most inland NGL plants | Coastal or water-rich sites |
For most midstream NGL facilities, air-cooled condensers are the preferred choice for the debutanizer due to lower total lifecycle cost and elimination of cooling water infrastructure. In hot climates where ambient temperatures exceed 100°F, a combined air-cooled condenser with a trim water cooler may be used to ensure adequate subcooling on design hot days.
5. Reboiler Design and Energy Integration
Reboiler Design
The debutanizer reboiler provides the heat input necessary to vaporize the nC4 component from the bottoms liquid, driving the separation. The reboiler duty is related to the condenser duty by the overall energy balance:
The factor of approximately 1.1 accounts for the feed enthalpy contribution and ambient heat losses (typically 3–5% for insulated columns). The more rigorous expression is:
Debutanizer bottoms temperatures range from 300–400°F depending on operating pressure. This temperature range is well suited for medium-pressure steam (150 psig, saturation temperature 366°F) as the heating medium, providing adequate driving force for heat transfer while avoiding the need for high-pressure steam or fired heaters.
Reboiler Type Selection
Kettle reboilers are the most common type used in debutanizer service. Key advantages include:
- Kettle reboiler — Most common choice; provides clean vapor-liquid separation within the reboiler shell, handles varying heat loads well, and is straightforward to size and maintain
- Thermosiphon reboiler — Used when lower residence time is desired (thermal cracking concern for heavy feeds) or when plot space is limited; requires adequate static head from column bottoms for natural circulation
- Forced circulation reboiler — Occasionally used for fouling services or when the bottoms product contains wax-forming components
Heating Media Comparison
| Heating Medium | Supply Temperature | Advantages | Considerations |
|---|---|---|---|
| Medium-pressure steam (150 psig) | 366°F | Constant temperature, high U-value, widely available | Water treatment, condensate return piping |
| Hot oil (Dowtherm, Therminol) | 350–500°F | No freezing risk, precise temperature control | Pumping cost, fluid degradation over time |
| Process heat exchange | Varies | Energy recovery, lowest operating cost | Tight heat balance required, limited turndown |
Energy Integration in the NGL Train
The debutanizer offers several opportunities for heat integration with other columns in the NGL fractionation train:
- Feed preheat: The hot debutanizer bottoms (C5+ natural gasoline at 300–400°F) can preheat the debutanizer feed or other process streams, reducing reboiler duty by 10–20%
- Stacked column arrangement: The depropanizer and debutanizer can be thermally coupled or operated in a stacked configuration, where the depropanizer bottoms flow directly into the debutanizer, reducing total energy consumption by 15–25%
- Condenser heat recovery: The debutanizer condenser rejects heat at 120–160°F, which can be used for glycol regeneration preheating, utility water heating, or amine solution preheating
- Cross-column integration: The debutanizer reboiler duty can potentially be supplied by the depropanizer condenser in a heat-pumped arrangement, though the temperature overlap is marginal and requires careful economic evaluation
Typical Duties by Feed Rate
| Feed Rate (MBPD) | Condenser Duty (MMBTU/hr) | Reboiler Duty (MMBTU/hr) | Reflux Ratio |
|---|---|---|---|
| 10 | 3–6 | 4–7 | 1.2–1.8 |
| 25 | 8–15 | 10–18 | 1.2–1.8 |
| 50 | 18–30 | 20–35 | 1.5–2.0 |
| 100 | 35–60 | 40–70 | 1.5–2.0 |
These values are approximate and depend strongly on feed composition, operating pressure, product purity specifications, and the degree of heat integration. The debutanizer generally has lower specific energy consumption (BTU per gallon of product) than the depropanizer due to the higher relative volatility and lower required reflux ratio. Detailed process simulation is required for final engineering design.
References
- GPSA, Chapter 16 — Hydrocarbon Recovery
- GPA Standard 2140 — Liquefied Petroleum Gas Specifications
- Kister, H.Z. — Distillation Design, McGraw-Hill
- ASME Boiler and Pressure Vessel Code, Section VIII, Division 1
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