Kettle & Thermosiphon Reboiler Sizer (TEMA K-Shell / API 660)

Reboilers are specialized shell-and-tube heat exchangers designed to boil column bottoms liquid, generating the ascending vapor stream that strips volatile components up a distillation column. Operating at the highest temperature and pressure in the distillation column system, reboilers handle heavy, high-boiling residues and represent a major capital and energy investment.

Under API Standard 660 and TEMA Section 5, reboiler design requires simultaneous optimization of nucleate boiling heat transfer, tube bundle vapor escape geometry, and phase separation to avoid catastrophic operational pitfalls such as film boiling burnout, liquid droplet carryover, and natural circulation hydraulic stalling.

Liquid boiling occurs across four distinct thermodynamic regimes: natural convection, nucleate boiling, transition boiling, and film boiling. Optimal process design requires operation strictly within the high-performance nucleate boiling regime, where vapor bubbles form at discrete microscopic surface cavities and detach rapidly, producing heat transfer coefficients reaching 1,500 to 5,000 W/m²·K.

However, if the operating heat flux exceeds the Critical Heat Flux ($q''_{max}$), vapor bubbles merge into a continuous vapor blanket covering the tubes. In film boiling, the thermal resistance skyrockets, causing heat transfer to collapse and tube wall temperatures to spike. Our engine evaluates the Mostinski and Kutateladze-Zuber critical heat flux and enforces API 660 safety rules ($q'' \le 0.70 \cdot q''_{max,bundle}$).

The TEMA Type K kettle reboiler features an eccentric enlarged shell surrounding a submerged horizontal tube bundle. The upper shell volume serves as a gravity disengagement space where liquid droplets fall out of the rising vapor stream before entering the column return nozzle.

The maximum permissible vapor velocity ($u_{v,max}$) is determined by the Souders-Brown correlation ($u_v = K_{SB} \sqrt{(\rho_L - \rho_V)/\rho_V}$). An internal overflow weir plate maintains a minimum 50 mm liquid cover over the bundle top, while liquid spilling over the weir collects in a downstream residue compartment sized for 1.5 to 3 minutes of residence time.

Unlike kettle reboilers, vertical thermosiphons operate via natural density-driven circulation without a mechanical pump. Cold, dense liquid in the column bottoms downcomer pipe ($\rho_L$) exerts hydrostatic pressure that pushes liquid into vertical tubes.

As the liquid ascends through the heated tubes, it flashes into a low-density two-phase vapor-liquid froth ($\bar{\rho}_{tp}$). The natural driving head ($\Delta P_{driving} = [\rho_L \cdot g \cdot H_{sump}] - [\bar{\rho}_{tp} \cdot g \cdot L_{tube}]$) balances total loop friction (inlet downcomer, tubes, acceleration, and return riser). Stable natural circulation requires a driving safety factor $\ge 1.05$.

Scenario: Size a TEMA K-shell kettle reboiler boiling 25,000 kg/h of isobutane/n-butane at 16.0 bar ($T_{boil} = 110.0^\circ\text{C}$) using saturated steam at 4.5 barg ($T_{sat} = 155.0^\circ\text{C}$).

1. Heat Duty ($Q$):
   $$Q = \frac{25,000 \times 295}{3600} = 2,048.6\text{ kW}$$
2. Driving Temperature & Area Sizing:
   $$\Delta T_m = 155.0 - 110.0 = 45.0^\circ\text{C}$$
   Mostinski nucleate pool boiling yields $h_{nb} = 3,180\text{ W/m}^2\cdot\text{K}$, giving dirty overall coefficient $U_{dirty} = 1,180.6\text{ W/m}^2\cdot\text{K}$.
   Required surface area: $A_{req} = 38.56\text{ m}^2$. Selecting 102 U-tubes ($25.4\text{ mm}$ OD, $4.88\text{ m}$ length) gives $A_{installed} = 39.72\text{ m}^2$ ($+3.0\%$ margin).
3. Critical Heat Flux Check:
   Bundle burnout limit: $q''_{max,bundle} = 92,842\text{ W/m}^2$.
   Operating flux: $q'' = 51,577\text{ W/m}^2$ (Flux ratio $= 0.556 \le 0.70$ • Compliant).
4. Kettle Shell Diameter:
   Bundle diameter $D_b = 430\text{ mm}$. Sizing enlarged shell for Souders-Brown velocity ($u_{v,max} = 0.171\text{ m/s}$) yields $D_s = 850\text{ mm}$ ($D_s / D_b = 1.98$) with $365\text{ mm}$ vapor freeboard ($> 250\text{ mm}$).