Sizing and Selection of Kettle Reboilers for Distillation ColumnsKettle reboilers are among the most widely used shell-and-tube reboiler types in distillation operations. They serve as the primary heat source that vaporizes part of the column bottom liquid to generate reflux and drive separation. This article explains the principles, design criteria, sizing calculations, selection considerations, operational issues, and practical tips for choosing and sizing kettle reboilers to meet performance, safety, and reliability requirements.
1. What is a kettle reboiler and when to use it
A kettle reboiler is a shell-and-tube heat exchanger in which the tube bundle is submerged in a liquid pool (the “kettle”) of column bottoms. Heat is supplied on the shell side (commonly by steam), or sometimes through the tubes (fired heater or hot oil). The liquid in the kettle boils around the tubes, producing vapor that is routed back to the column. Advantages include:
- Good liquid residence time and vapor disengagement (reduced entrainment).
- Easy handling of fouling fluids because the bundle can be lifted for cleaning.
- Simple level control and mild liquid mixing, suitable for liquid mixes with solids or polymerizing components.
Typical applications: atmospheric and vacuum distillation bottoms, systems with fouling tendencies, columns requiring large hold-up, and services where gentle vaporization is desired.
2. Key design objectives
- Provide required reboil duty (Q) to meet column boil-up and separation needs.
- Achieve adequate heat transfer coefficient and heat transfer area while avoiding excessive temperature driving forces.
- Maintain acceptable liquid level and vapor disengagement to prevent carryover.
- Avoid tube-side or shell-side pressure drops that interfere with utilities or process constraints.
- Ensure reliability, accessibility for maintenance, and compliance with codes (ASME, TEMA).
3. Basic design inputs and parameters
Essential inputs for sizing and selection:
- Distillation column boil-up rate (kg/s or kmol/h) and required reboil duty Q (kW or kcal/h).
- Bottoms composition, physical properties (boiling point, molecular weight, vapor pressure), and fouling tendency.
- Operating pressure (column bottom and shell-side steam pressure if applicable).
- Available heating medium (steam — pressure/temperature, hot oil, thermal fluid, fired).
- Allowable terminal temperature approach and allowable fouling resistance.
- Required materials of construction (corrosion, temperature limits).
- Layout constraints (vertical space for kettle, maintenance access).
- Utilities and allowable steam pressure drop or condensate return arrangement.
4. Thermal design and sizing methodology
- Determine required reboiler duty Q:
- Q = Lb * (hb,vap – hb,liq) + latent heat to vaporize boil-up stream
More practically, Q = V * λ_avg where V is vapor flowrate (kg/s) and λ_avg is average latent heat.
- Select heating medium and log-mean temperature difference (LMTD):
- For single-phase hot medium to boiling liquid, use LMTD: ΔT_lm = (ΔT1 – ΔT2) / ln(ΔT1/ΔT2)
where ΔT1 = T_hot,in – T_boil and ΔT2 = T_hot,out – T_boil. - For steam condensers (flowing condensing steam), the steam temperature is nearly constant; ΔT ≈ T_steam – T_boil. LMTD simplifies accordingly.
- Estimate overall heat transfer coefficient (U):
- For kettle reboilers with steam condensing on shell side and boiling on tube side (or vice versa depending on configuration), the overall U often ranges:
- Steam condensing shell / boiling liquid: U ≈ 1500–6000 W/m²·K depending on fouling and fluid properties.
- Hot oil or thermal fluid services: U lower, often 200–800 W/m²·K.
- Calculate U from resistances: 1/U = 1/h_shell + R_fouling_shell + ln(d_o/d_i)/(2πk_tube) + R_fouling_tube + 1/h_tube.
- Compute heat transfer area A:
- A = Q / (U * ΔT_lm)
- Choose tube count and layout (tube length, pitch, diameter) to meet A and allow proper vapor disengagement and liquid volume.
- Check velocities and pressure drops:
- For tube-side: ensure liquid/vapor velocities avoid excessive vibration, erosion, or high pressure drop. Typical tube velocities depend on service; avoid >2–3 m/s for liquid, and consider two-phase flow correlations for boiling inside tubes.
- For shell-side: if steam condensing, pressure drop is usually small; if shell-side liquid flows, ensure acceptable flow rates.
- Kettle volume and liquid level:
- Kettle must provide sufficient residence time for fouling, allow vapor disengagement, and accommodate fluctuations. Typical holdup volumes depend on column design but often target several minutes’ residence at design flow to stabilize compositions.
- Include space for tube bundle removal (prefer split kettle or removable cover).
5. Mechanical and layout considerations
- Orientation: vertical shell with vertical tube bundle is common; allows bundle lifting and drainage.
- Tube bundle removal: design for liftable bundles (bolted manway or handhole) for cleaning.
- Nozzle placement: vapor outlet located above liquid surface with baffles or disengagement space; feed and return nozzles positioned to avoid jetting and ensure good mixing.
- Supports and thermal expansion: provide saddles, expansion joints, or floating heads if differential expansion expected.
- Materials: choose corrosion-resistant alloys for bottom services (e.g., stainless steels, nickel alloys) where sour water, chlorides, or caustic exist.
- Code compliance: ASME Section VIII for pressure vessels; TEMA and API guidelines for exchanger layouts.
6. Fouling, maintenance, and operational issues
- Fouling: kettle reboilers often handle fouling better than U-tube designs because of accessibility. However, fouling reduces U and increases required area. Specify fouling factors and design spare area or plan for offline cleaning.
- Carryover/entrainment: ensure steam/water separators and sufficient disengagement space to avoid liquid carryover to the column.
- Level control: stable level control loop crucial; sudden changes affect boil-up. Typical control uses level transmitter and modulates steam valve.
- Scale and corrosion monitoring: sample bottoms, inspect periodically, consider on-line fouling monitors or heat flux sensors.
- Start-up and shutdown: avoid thermal shock; ramp heat input gradually; ensure venting for air removal during startup to prevent air-bound pockets.
- Two-phase flow instabilities: if boiling occurs inside tubes, anticipate flow oscillations; select tube-side geometry and flow rates to minimize.
7. Common selection trade-offs
- Steam vs. thermal fluid:
- Steam: higher U, simpler control with condensate return, but limited by available steam pressure/temperature.
- Thermal oil: enables higher tube temperatures or non-condensing heating, but lower U, larger area, possible fire risk.
- Kettle vs. U-tube or thermosyphon:
- Kettle: good for fouling, solids; larger holdup; easier maintenance.
- Thermosyphon: more compact and efficient for natural circulation; less holdup; not ideal for solids/fouling.
- Bundle type:
- Fixed tubesheet: simpler, cheaper, but harder to clean tube outside.
- Pull-through or floating head: easier cleaning and differential expansion management; higher cost.
Use a short comparison table:
| Option | Pros | Cons |
|---|---|---|
| Steam-heated kettle | High U, simple condensate handling | Requires steam availability/pressure |
| Thermal fluid kettle | Works at high temp, no steam | Lower U, larger area, fire risk |
| Kettle (vs thermosyphon) | Good for fouling, easy maintenance | Larger footprint, higher holdup |
| Fixed tubesheet bundle | Simpler, lower cost | Harder cleaning, thermal expansion limits |
| Floating head bundle | Easy cleaning, handles expansion | Higher cost, more complex seals |
8. Example sizing (simplified)
Assume:
- Required Q = 1,000 kW
- Heating medium: saturated steam at 10 bar (T_steam ≈ 184 °C)
- Boiling temperature of bottoms T_boil = 120 °C
- Assume U = 1500 W/m²·K and negligible steam temperature drop
ΔT ≈ 184 − 120 = 64 °C
A = Q / (U * ΔT) = 1,000,000 W / (1500 * 64) ≈ 10.4 m²
If using 25 mm OD tubes (area per tube ≈ π*d*L; with L = 4 m: Atube ≈ π*0.025*4 ≈ 0.314 m²), required tube count ≈ 10.4 / 0.314 ≈ 33 tubes. In practice choose more tubes accounting for fouling, layout, baffles, and safety margin; perhaps 40–60 tubes with appropriate pitch.
Note: This is a simplified illustration — full design requires detailed property data, two-phase correlations, pressure-drop checks, and safety factors.
9. Design checks and verification
- Thermodynamic equilibrium: confirm boiling point vs column pressure and heating medium temperature; ensure adequate driving force at design and minimum loads.
- Pressure drop and flow regime: verify two-phase flow behavior and that pumps/steam supply handle design conditions.
- Mechanical integrity: stress and nozzle reinforcement calculations per ASME, seismic and wind loads, supports.
- Safety: relief sizing for vapor or overpressure, emergency drains, and safe venting.
- Control and instrumentation: level control, temperature monitoring, pressure gauges, and interlocks.
10. Practical tips and industry best practices
- Always design with a fouling allowance and specify cleanable bundles.
- Prefer steam heating for most kettle reboiler services unless process conditions require thermal oils.
- Provide adequate disengagement space and baffles to reduce entrainment to % of vapor flow.
- Use conservative U values for initial sizing; validate with vendor thermal ratings.
- Include inspection ports and lifting arrangements for bundle removal.
- Coordinate with column designer on allowable reboil temperature approach and column bottom specs.
- Conduct a vendor comparison (thermal duty, area, headspace, maintenance features) rather than specifying a single manufacturer.
Kettle reboilers are robust, maintainable solutions for many distillation bottom duties. Proper selection balances thermal performance, fouling tolerance, mechanical design, and operational considerations. For a detailed engineered design you’ll need full process stream properties, operating envelopes, fouling factors, and layout constraints to produce final heat-exchanger specifications, pressure-drop calculations, and mechanical drawings.
Leave a Reply