reboiler-design

name	reboiler-design
description	Reboiler design — types (kettle, thermosiphon vertical/horizontal, forced circulation, once-through), heat duty calculation, boiling heat transfer (Mostinski, Chen/Palen correlation), nucleate boiling vs. convective boiling, critical heat flux (CHF) in reboilers, circulation ratio (thermosiphon driving head), shellside vs. tubeside boiling, fouling allowance (TEMA), reboiler sizing (shell diameter, tube count, U [W/m²·K]), TEMA type selection, and ASME/TEMA standards.
metadata	{"priority":7,"promptSignals":{"phrases":["reboiler design","reboiler heat exchanger","thermosiphon reboiler","kettle reboiler","column reboiler","boiling heat exchanger"],"minScore":3}}

Reboiler Design — Complete Skill

Reboiler Types

Kettle Reboiler (TEMA Type K)

Configuration: Shell larger than tube bundle; liquid level maintained above bundle; vapor disengagement space in shell Tubes: horizontal; liquid fed from column sump; vapor returned to column bottom; liquid overflow over weir Shell diameter ≥ 1.5 × bundle diameter (to allow vapor disengagement)

Characteristics: Stable operation; wide turndown; good vapor quality; easy CHF avoidance Higher cost (large shell); low residence time; widely used for vacuum distillation, fouling services Typical vapor quality (exit): 20–40% wt; 80% of duty from nucleate boiling

Sizing: Bundle area: A = Q / (U × LMTD × F) Shell diameter: D_s based on bundle + vapor space: D_s = 1.3–1.5 × D_b_estimated + vapor disengagement Minimum shell diameter from Souders-Brown for vapor velocity: u_max = K × √((ρ_l - ρ_v)/ρ_v); K = 0.06–0.15 m/s

Thermosiphon Reboiler (Vertical — Most Common)

Configuration: Vertical shell-and-tube; liquid from column sump flows up through tubes or shellside; heated; two-phase mixture returns to column Natural circulation driven by density difference between liquid in downcomer and two-phase mixture in tubes

Driving head: ΔP_driving = ρ_l × g × H_liquid - ρ_2ph × g × H_tubes [pressure difference driving flow; H = height] ρ_2ph = ρ_l × (1-x) + ρ_v × x [two-phase density; x = vapor quality]

Circulation ratio: CR = total flow / vapor generated = 1/(1-x_exit_quality) Typical x_exit = 0.20–0.35 (20–35% vapor by weight) → CR = 3–5 Low CR → potential stagnation, dry-out; High CR → good wetting but more pipe and pump cost (for forced)

Process side: tubes (tubeside boiling most common for vertical thermosiphon) or shellside (TEMA E or G shells)

Horizontal Thermosiphon

Shell horizontal; tubes horizontal; liquid enters, two-phase exits to column: Shellside boiling; tubes carry heating medium (steam or hot oil) Lower height (less elevation head) → smaller driving force → less self-circulation More tube-bundle effect; better for viscous liquids; larger shell

Forced Circulation Reboiler

Pump circulates liquid through horizontal or vertical exchanger; returns two-phase to column: Used when: viscous liquid (thermosiphon doesn't circulate), wide boiling range (dP insufficient), close approach temp, fouling service Design Q = ṁ × c_p × ΔT + ṁ × x × λ [sensible + latent heat portion] Pump head: ΔP_circ ≥ ΔP_friction + ΔP_two_phase [includes two-phase pressure drop in exchanger]

Once-Through Reboiler

100% vaporization; no recirculation; liquid enters, pure vapor exits: For clean services with single-component or narrow boiling range Avoids concentration effects; simple piping; used in chemical and refinery columns

Heat Transfer Correlations

Nucleate Boiling (Mostinski Correlation)

For shellside kettle or pool boiling: h_nb = 0.00417 × P_c^0.69 × q^0.7 × F_p [W/m²·K; q = heat flux [W/m²]; P_c = critical pressure [kPa]] F_p = 1.8 × (P_r)^0.17 + 4 × (P_r)^1.2 + 10 × (P_r)^10 [P_r = P_op/P_c = reduced pressure]

Rohsenow (pool boiling, tubeside): q = μ_l × h_fg × [g(ρ_l-ρ_v)/σ]^0.5 × [c_p,l × ΔT_sat / (C_sf × h_fg × Pr_l^r)]³ [as in pool-boiling skill]

Chen Correlation (Convective Boiling — Tubeside Thermosiphon)

Two-component superposition: h_tp = S × h_nb + F × h_l [Chen; S = suppression factor; F = enhancement factor; h_l = Dittus-Boelter liquid-only] F = f(1/X_tt); S = f(Re_tp) Re_tp = Re_l × F^1.25; S = 1/(1 + 2.53×10⁻⁶ × Re_tp^1.17) X_tt = [(1-x)/x]^0.9 × (ρ_v/ρ_l)^0.5 × (μ_l/μ_v)^0.1

Palen Correlation (Shell-Side Boiling — Kettle/TEMA K)

For bundle boiling (multiple tubes in pool): h_b = h_nb × F_b [F_b = bundle boiling factor = 1.0–3.0 depending on tube pitch and geometry] F_b accounts for convection in bundle due to rising bubbles (enhances h_nb) Typical: F_b = 1.5–2.0 for triangular pitch at typical bundle sizes

Overall U for reboiler: 1/U = 1/h_process + r_fi + t_wall/k_wall + r_fo + 1/h_utility [r_f = fouling resistance TEMA table] Typical U for steam reboiler (liquid organics): 800–1,500 W/m²·K Water as utility: 400–700 W/m²·K

Critical Heat Flux (CHF) in Reboilers

CHF Prevention

Shellside CHF (kettle reboiler): Zuber-Kutateladze: q_CHF = 0.149 × h_fg × ρ_v × [σ × g × (ρ_l-ρ_v)/ρ_v²]^(1/4) [free surface; conservative] For bundle: q_CHF,bundle = q_CHF,free × F_bundle_factor [F_bundle ≈ 0.6–0.8 for typical bundle] Design: operating flux q_design ≤ 0.7 × q_CHF (40% safety margin minimum)

Thermosiphon (tubeside): CHF depends on quality, mass flux, and pressure (Bowring or Katto-Ohno correlation) Higher mass flux → higher CHF; design for x_exit < 30% (avoids critical quality region)

Consequence of CHF: Dry-out → wall temperature spike → fouling acceleration, tube failure; prevent by proper flux and quality control

Reboiler Sizing Procedure

Design Steps

1. Heat duty: Q = V_top_vapor × λ [W; V_top = column vapor rate; λ = latent heat] Or from column simulation: Q = F_bottom × h_bottom - F_feed × h_feed - F_top × h_top [enthalpy balance]

2. Utility: Steam: Q = ṁ_steam × λ_steam; condensing temperature = T_sat(P_steam); LMTD with constant T Hot oil: LMTD = (ΔT_hot - ΔT_cold) / ln(ΔT_hot/ΔT_cold); F-correction for configuration

3. Required area: A = Q / (U × LMTD) [m²; LMTD = effective mean temperature difference; F included] Add area: A_design = 1.1–1.2 × A_required (10–20% overdesign allowance)

4. Tube count and shell diameter: Select tube OD (19 mm or 25 mm standard), length (1.8 m, 2.4 m, 3.0 m, 4.8 m, 6.0 m) N_tubes = A / (π × D_o × L_tube) Shell diameter: from TEMA tube count tables for selected tube pitch and layout (30°, 45°, 60°, 90°)

5. Shell type (TEMA): Kettle: TEMA type K; Vertical thermosiphon: TEMA type E (one-pass) or F (two-pass); Horizontal: TEMA type J or X

6. Verify flux: q = Q / A_actual [W/m²]; compare with 0.7 × q_CHF

7. Two-phase pressure drop (thermosiphon): Martinelli-Chisholm or Lockhart-Martinelli for two-phase multiplier φ² ΔP_total = ΔP_1ph_entry + φ² × ΔP_1ph + ΔP_acceleration + ΔP_gravity Check against available thermosiphon driving head

Fouling and TEMA Standards

TEMA Fouling Resistances

Service	r_f [m²·K/W]
Steam (clean)	0.000088
Steam (industrial)	0.000176
Light hydrocarbon	0.000176
Heavy hydrocarbon	0.000352
Crude oil	0.000528
Cooling water (treated)	0.000176

TEMA classes: Class R (refinery/chemical): most stringent; highest quality materials and fabrication Class C (commercial): general use Class B (chemical/general industrial): intermediate

Standards and References

Standard	Scope
TEMA (Tubular Exchanger Manufacturers Association)	Standards for shell-and-tube HX (fouling, tolerances, types)
ASME VIII Div. 1	Pressure vessel code for shell, tubesheet, and heads
API 660	Shell-and-tube heat exchangers for general refinery service
API 661	Air-cooled heat exchangers (for fin-fan alternative)
HEDH (Heat Exchanger Design Handbook)	Comprehensive boiling and two-phase correlations

Output

Provide: service (column; overhead vapor rate [kg/hr]; bottom product flow; pressure P [bar]; system T_sat [°C]; boiling range [°C]), heat duty Q [kW] (from enthalpy balance or simplified latent heat), utility selection (steam P [bar]; T_sat [°C]; or hot oil T_in/T_out [°C]; ṁ_utility [kg/hr]), LMTD [°C] and F-correction factor, estimated U_overall [W/m²·K] (h_process [W/m²·K] by correlation; h_utility; r_f from TEMA), required heat transfer area A [m²], reboiler type selection (kettle/vertical thermosiphon/forced; justification), tube geometry (OD × gauge × length [mm]; N_tubes; pitch; layout; shell diameter D_s [mm]; TEMA type), operating flux q [W/m²] vs. q_CHF [W/m²] (safety margin ≥ 30%), thermosiphon check (circulation ratio CR; x_exit [%]; driving head ΔP_avail vs. ΔP_friction [Pa]), fouling allowances (TEMA r_f total [m²K/W]; U_clean vs. U_fouled), and applicable standard (TEMA, ASME VIII, API 660).