Coker debutanizer column bottom reboilers (Figure 1) are vulnerable to what is broadly identified as polymerization fouling due to their high operating temperatures and the significant diolefin content in the cracked naphtha feed. Industry experience has led many operators to limit the inlet temperature of the reboiler heating medium, usually heavy cycle oil (HCO) or heavy coker gas oil (HCGO), to an arbitrary value no greater than 288 °C (550 °F), thereby inhibiting the maximum skin temperature on the boiling side to effectively mitigate the polymerization fouling mechanism. Skin temperature refers to the temperature of the fluid-facing fouling layer surface. If no fouling layer is present, Tskin = Twall.

Figure 1. Simplified process flow for a debutanizer column reboiler
Figure 1. Simplified process flow for a debutanizer column reboiler

However, cooling a liquid heating medium incurs costs associated with a cooling stage prior to the reboiler and the increased pump rate to maintain the required heat duty at the cooler stream temperature. Accordingly, the design approach would benefit from a rigorous determination of the maximum hot inlet condition that avoids excessive skin temperatures, rather than the continued reliance on an arbitrary limit. This type of assessment is possible using HTRI's incremental shell-and-tube exchanger thermal rating software, Xist®.

A recirculating thermosiphon reboiler, detailed below, has a hot oil inlet temperature of 290 °C (554 °F) and suffers accelerated fouling in the upper part of the bundle. The exchanger geometry is summarized in Figures 2 – 4.

Figure 2. Xist exchanger drawing
Figure 2. Xist exchanger drawing
Figure 3. Xist tube layout and geometry input data
Figure 3. Xist tube layout and geometry input data
Figure 4. Xist 3D bundle drawing showing baffle configuration above and below longitudinal baffle
Figure 4. Xist 3D bundle drawing showing baffle configuration above and below longitudinal baffle

An Xist thermal model was developed for the reboiler. Part of the Xist Output Summary is shown in Figure 5. The calculated overdesign is 13%, and the hotside pressure drop is at the allowable limit. The Shellside Flow Regime Map (Figure 6) indicates that the shellside flow is well mixed in all but the first baffle spacing, where no fouling was observed, so the tubes are expected to remain well wetted. This result is important because phase separation would drive up the skin temperatures of exposed tubes to an excessive degree. The maximum coldside skin and wall temperatures listed in the Xist Final Results are 185.6 °C (366.08 °F) and 209.8 °C (409.64 °F), respectively. These maxima occur near the outlet nozzle closest to the front header, as shown in Figure 7. The operator believes that polymerization is exacerbated when the skin temperature exceeds 200 °C (392 °F).1 Since the skin temperature is below this limit for the fouled design condition, one might conclude that the fouling risk is averted, but fouling resistance constitutes over 50% of the total thermal resistance (Figure 5, line 34), and good design practice calls for assessment of unit performance under clean conditions.

Re-running the Xist thermal calculation with zero fouling factors yields an overdesign of 130% and a maximum skin/wall temperature of 215.6 °C (420.08 °F). The control strategy for this unit involves throttling the hot oil flow rate to maintain the design duty at clean start-up conditions. An additional Xist run with a reduced hot oil flow rate, corresponding to start-up operation, yields a calculated maximum skin temperature of 205.8 °C (402.44 °F) (Figure 8).

Figure 5. Xist Output Summary for fouled design conditions
Figure 5. Xist Output Summary for fouled design conditions
Figure 6. Xist Shellside Flow Regime Map for fouled design conditions
Figure 6. Xist Shellside Flow Regime Map for fouled design conditions
Figure 7. Xist 3D incremental cold side skin temperatures for fouled design conditions
Figure 7. Xist 3D incremental cold side skin temperatures for fouled design conditions
Figure 8. Xist 3D incremental coldside skin temperatures for clean start-up conditions
Figure 8. Xist 3D incremental coldside skin temperatures for clean start-up conditions

To reduce the maximum skin temperature without lowering the hot oil inlet temperature, the operator proposed a cocurrent flow configuration (Figure 9) for evaluation in Xist. The maximum skin temperature calculated for this configuration was 197.6 °C (387.68 °F), located in the lower half of the bundle, with temperatures significantly reduced over the upper tuberows, as shown in Figure 10. By implementing a cocurrent flow configuration, corroborated by the Xist incremental results, the operator was able to mitigate the rapid polymerization fouling problem and prolong the operating cycle between cleaning stages, with a positive impact on the profitability of the overall coker unit.

Figure 9. Modified cocurrent flow configuration
Figure 9. Modified cocurrent flow configuration
Figure 10. Xist 3D incremental cold side skin temperatures for clean start-up condition with cocurrent flow configuration
Figure 10. Xist 3D incremental cold side skin temperatures for clean start-up condition with cocurrent flow configuration

Footnote

1 This anecdotal threshold is for illustrative purposes only and should not be considered a recommended upper limit for this application. The actual skin temperature above which polymerization can become severe is dependent on several factors that affect the diolefin concentration in the naphtha feed.