When dynamic relief calculations give unexpected results
Case studies discuss avoiding pitfalls in column relief estimates using dynamic modelling
Ben Leviton and Harry Z Ha Fluor Canada Ltd.
R elief load estimation for distillation column sys- tems is a common challenge in engineering design. The unbalanced heat (UBH) method 1 is commonly employed in the grassroots design of a project. The relief loads predicted by this method are generally considered conservative and tend to incorporate a safety margin into the relief system design. More rigorous methods, including steady state and dynamic simulation, are often reserved for revamp and debottleneck projects where the acceptable column relief load is limited by existing equipment.⁴ Unlike UBH, dynamic simulations can model changing conditions over time and incorporate system volume con- tributions. This allows for better characterisation of the relief stream, including depletion of light ends, and a more accurate evaluation of exchanger duties at relief conditions. Dynamic models also allow the engineer to evaluate the impact of instrumented control systems where a normal response has the potential to increase the relief load. The column relief loads predicted by dynamic simulation are often promising in eliminating potential modifications to an existing pressure safety valve (PSV) or flare system. While a dynamic relief analysis will typically yield a smaller relief load compared to the equivalent UBH calcula- tion, occasionally a detailed dynamic analysis will ‘uncover’ a relief load greater than the preliminary UBH calculation. This article presents a series of case studies where this occurred for a variety of reasons, such as control system complexities, unexpected thermodynamic phenomena, and invalid assumptions. Unbalanced heat approach The UBH method 1 employs a heat and material balance around the column envelope at relieving conditions in order to estimate excess heat input. The relief rate is calculated from the excess heat divided by the latent heat of the relieving material. The excess heat calculation considers the enthalpy of each stream at relieving pressure, typically assuming all product stream compositions remain constant. An end- less supply of relieving material is assumed to be available (typically represented by the liquid in the top tray of the column). Normally, no credit is taken for the following mit- igating factors:
• Compositional changes, including depletion of light components. • Accumulation of mass within the system volume as pres- sure increases. • Hydraulic limitations. • Overhead cooling before the reflux drum is flooded. Aside from typical over-estimation of relief loads, UBH is also subject to the following key limitations: u UBH can over-estimate or under-estimate relief loads for systems where the column energy balance is sensi- tive to minor compositional changes. In these systems, the enthalpy of bottoms stream is typically affected by the presence of light components which skews the calculated relief load. Some examples are: • Stripping columns that remove absorbed components from a solvent (for instance, a sour water stripper). • Columns with a low overhead-to-bottoms flow ratio (for instance, a stabiliser). v UBH is not normally suitable for complex systems such as: • Reactive distillation columns. • Columns with relief occurring near the critical region. • Scenarios with significant transient effects, such as a major upset upstream affecting feed conditions. The key assumptions that go into a UBH relief load are subject to engineering judgment. As a result, two engineers analysing the same system may arrive at different relief load estimates. For example, credit for ‘reboiler pinch’ (i.e. a reduc- tion in reboiler duty due to reduced log mean temperature difference (LMTD) as the column bottoms temperature rises) is often applied on a case-by-case basis. Similarly, the engi- neer may incorporate a compositional change (for example, bottoms composition changing to approach feed composi- tion during relief) if there is reason to expect it to occur and worsen the relief load. Thus, the UBH method is conserva- tive so long as due consideration is given to each underlying assumption based on a strong mental model of the system. Dynamic approach A dynamic model can be employed to simulate an upset event and predict the resulting relief profile as it changes over time. This method can account for transient effects
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PTQ Q3 2025
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