1. Purpose and Position in NGL Train
The depropanizer (DeC3) is a distillation column that separates propane (C3) from butanes and heavier components (C4+). It occupies a central position in the NGL fractionation train, receiving the C3+ bottoms product from the deethanizer and producing specification-grade propane overhead while sending the C4+ bottoms to the debutanizer.
The light key (LK) component is propane (C3H8) and the heavy key (HK) component is isobutane (iC4H10). The overhead product is HD-5 propane meeting GPA 2140 specifications, which limits C4+ content to a maximum of 2.0 mol%. The bottoms product is the C4+ stream that feeds the debutanizer for further separation into butane and natural gasoline fractions.
NGL Fractionation Train Sequence
| Column | Top Product | Bottom Product | Key Separation |
|---|---|---|---|
| Demethanizer | C1 (sales gas) | C2+ (NGL) | C1/C2 |
| Deethanizer | C2 (ethane) | C3+ (NGL) | C2/C3 |
| Depropanizer | C3 (propane) | C4+ | C3/iC4 |
| Debutanizer | C4 (butanes) | C5+ (gasoline) | C4/C5 |
HD-5 Propane Product Specifications (GPA 2140)
| Parameter | Specification |
|---|---|
| Propane (C3) minimum | 90.0 mol% |
| Butanes and heavier (C4+) maximum | 2.0 mol% |
| Ethane (C2) maximum | 5.0 mol% |
| Propylene maximum | 5.0 mol% |
| Vapor pressure at 100°F | 208 psig max |
The C3/iC4 separation is considered relatively easy compared to the upstream deethanizer (C2/C3) split. The normal boiling point difference between propane (−44°F) and isobutane (+11°F) is approximately 55°F, providing favorable relative volatility. Importantly, the depropanizer overhead temperature is warm enough to allow air cooling or cooling water condensation at typical operating pressures, eliminating the need for the refrigeration systems required by the deethanizer.
2. C3/iC4 Relative Volatility
Relative volatility is the key thermodynamic parameter governing separation difficulty. For the depropanizer, the relative volatility of propane to isobutane is defined as:
Where KC3 and KiC4 are the vapor-liquid equilibrium K-values for propane and isobutane at column conditions. The relative volatility depends primarily on pressure and temperature: higher pressure reduces α and makes separation more difficult, while lower pressure increases α and improves separation efficiency.
Relative Volatility vs. Operating Pressure
| Pressure (psig) | Column Temp Range (°F) | α(C3/iC4) | Separation Ease |
|---|---|---|---|
| 150 | 90–180 | 3.0–3.5 | Easiest |
| 200 | 110–210 | 2.6–3.1 | Easy |
| 250 | 125–235 | 2.3–2.8 | Moderate |
| 300 | 140–260 | 2.0–2.5 | More difficult |
The geometric mean relative volatility across the column is used in shortcut design methods:
K-values for the depropanizer are typically estimated using the Wilson, Peng-Robinson, or Soave-Redlich-Kwong (SRK) equations of state. The Wilson K-value correlation provides a convenient first approximation for light hydrocarbons and is widely used in preliminary design. For multicomponent feeds, the presence of normal butane (nC4) as a second heavy key must also be considered, since nC4 is less volatile than iC4 and tends to concentrate in the bottoms, slightly improving the apparent separation.
Compared to the deethanizer (αC2/C3 = 2.5–4.5), the depropanizer α values are somewhat lower, but the absence of cryogenic operating temperatures and refrigeration requirements makes the depropanizer a less expensive column per unit of separation achieved.
3. Column Design: Stages and Reflux
The Fenske-Underwood-Gilliland (FUG) shortcut method is used for preliminary depropanizer sizing, providing estimates of minimum stages, minimum reflux, and actual stage count before rigorous simulation.
Fenske Equation — Minimum Stages
The Fenske equation calculates the minimum number of theoretical stages at total reflux:
For a typical depropanizer with 98% propane recovery overhead and 2% maximum C4+ in the distillate, Nmin is typically 8–14 theoretical stages, depending on feed composition and operating pressure.
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). Then the minimum reflux is:
Gilliland Correlation — Actual Stages
The Gilliland correlation relates the actual reflux ratio (R) to the actual number of stages (N):
The Molokanov approximation provides a convenient closed-form solution:
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 Depropanizer Design Parameters
| Parameter | Typical Range |
|---|---|
| Operating pressure | 200–300 psig |
| Overhead temperature | 100–130°F |
| Bottoms temperature | 220–280°F |
| Minimum theoretical stages (Nmin) | 8–14 |
| Actual trays | 30–45 |
| Tray efficiency | 70–85% |
| R/Rmin | 1.1–1.3 |
| Reflux ratio (R) | 1.5–3.5 |
| Feed condition (q) | 0.5–1.0 (mixed to saturated liquid) |
| Column diameter | 3–12 ft |
| Tray spacing | 24 in |
The feed condition (q-line) significantly affects column performance. A subcooled or saturated liquid feed (q ≥ 1.0) increases internal liquid traffic in the stripping section, improving stripping efficiency but increasing reboiler duty. A partially vaporized feed (q < 1.0) reduces reboiler duty but requires more rectifying stages. In most NGL plants, the depropanizer feed from the deethanizer bottoms is at or near its bubble point (q ≈ 1.0).
4. Condenser and Overhead System
A major advantage of the depropanizer over the upstream deethanizer is the condenser design. At typical operating pressures of 200–300 psig, the overhead temperature is 100–130°F — warm enough for conventional air-cooled or cooling water condensation, eliminating the propane refrigeration system required by the deethanizer.
Condenser Configuration
The depropanizer typically uses a total condenser, producing a liquid overhead product (HD-5 propane) and a liquid reflux stream returned to the column. The total condenser is preferred because the propane product is marketed as a liquid (LPG). A reflux drum is provided between the condenser and the column to separate the condensed liquid, maintain reflux flow control, and provide surge volume for stable operation.
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.
Condenser Type Comparison
| Condenser Type | Overhead Temp | Cooling Medium | Advantages | Typical Use |
|---|---|---|---|---|
| Air-cooled (fin-fan) | 110–130°F | Ambient air | Low operating cost, no water treatment | Warm climates, lower pressures |
| Shell-and-tube (CW) | 100–120°F | Cooling water | Compact, precise temperature control | Coastal plants, water-rich sites |
| Combined air/trim CW | 100–130°F | Air + cooling water | Best efficiency, meets hot-day design | Most common in NGL plants |
Reflux Drum Sizing
The reflux drum is typically sized for 5–10 minutes of liquid holdup based on the total liquid flow (reflux + distillate product). The drum operates at the condenser outlet pressure and must provide adequate vapor-liquid disengagement if any non-condensables are present. Standard L/D ratios of 3:1 to 5:1 are used, with the drum oriented horizontally.
Propane Product Handling
The overhead propane product exits the reflux drum as a pressurized liquid. It may be routed directly to pressurized storage spheres, pipeline, or truck loading facilities. Product quality is monitored by an online C4+ analyzer or periodic laboratory gas chromatograph analysis to ensure compliance with GPA 2140 HD-5 specifications.
5. Reboiler and Energy Integration
Reboiler Design
The depropanizer reboiler provides the heat input necessary to vaporize the C3 component from the bottoms liquid, driving the separation. The reboiler duty is approximately:
Where Qfeed accounts for feed enthalpy relative to column conditions and Qloss represents ambient heat losses (typically 3–5% of reboiler duty for insulated columns). The bottoms temperature of 220–280°F is well within the range of low-pressure steam (50 psig, 298°F) or hot oil systems.
Heating Media Comparison
| Heating Medium | Supply Temperature | Advantages | Considerations |
|---|---|---|---|
| Low-pressure steam (50 psig) | 298°F | Constant temperature, high U-value | Water treatment, condensate return |
| Hot oil (Dowtherm, Therminol) | 300–400°F | No freezing risk, wide temp range | Pumping cost, fluid degradation |
| Process heat exchange | Varies | Energy recovery, lowest operating cost | Tight heat balance, turndown limited |
The most common reboiler types for depropanizer service are:
- Kettle reboiler — Most common choice; provides clean vapor-liquid separation, handles varying heat loads, and is simple to operate and maintain
- Thermosiphon reboiler — Used where lower residence time is desired or when plot space is limited; requires adequate static head from the column bottoms for natural circulation
Energy Integration in the NGL Train
The depropanizer offers significant opportunities for heat integration within the NGL fractionation train. Because the depropanizer condenser operates at 100–130°F and rejects substantial heat, while the deethanizer reboiler requires heat input at 200–350°F, direct heat exchange between these units is not straightforward. However, several integration strategies are commonly employed:
- Feed preheat: The hot depropanizer bottoms (C4+ at 220–280°F) can preheat the deethanizer or depropanizer feed, reducing reboiler duty
- Condenser heat recovery: The depropanizer condenser heat can be used to preheat utility water, glycol regeneration, or other low-temperature duties in the plant
- Stacked column arrangement: In some designs, the depropanizer and debutanizer are thermally coupled or stacked to reduce total energy consumption by 15–25%
- Heat pump configuration: Mechanical vapor recompression (MVR) can be applied where electricity is inexpensive, using the overhead vapor as a heat source for the reboiler after compression
For detailed reboiler sizing methodology including heat transfer coefficients and tube layout, see the Reboiler Sizing Fundamentals guide.
Typical Duties by Feed Rate
| Feed Rate (MBPD) | Condenser Duty (MMBTU/hr) | Reboiler Duty (MMBTU/hr) | Reflux Ratio |
|---|---|---|---|
| 10 | 5–8 | 6–10 | 2.0–2.5 |
| 25 | 12–20 | 15–25 | 2.0–2.5 |
| 50 | 25–40 | 30–50 | 2.0–3.0 |
| 100 | 50–80 | 60–100 | 2.0–3.0 |
These values are approximate and depend strongly on feed composition, operating pressure, product purity specifications, and the degree of heat integration. Detailed process simulation is required for final engineering.
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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