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Dynamic column simulation delivers reduced relief load

Dynamic modelling of a gasoline hydrodesulphurisation column simulates the column relief profile during power failure

Fluor Canada
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Article Summary
Effective 1 January 2017, the US EPA Tier 3 regulation set a standard of 10 wppm sulphur content in gasoline. Although the EPA granted refiners a transition period (between 2014-2019) to comply with the new standards, as well as incorporating a credits trading programme,1 the compliance deadline is imminent as revamp projects typically take years to complete.

The gasoline desulphurisation (GDS) distillation column is the key column in GDS units, typically generating the single largest relief load within the unit. One of the challenges in GDS revamp projects to comply with Tier 3 is the potential increase in GDS column relief loads, posing a risk to pre-existing pressure safety valves (PSV) and flare sub-headers being under-sized for the new mode of operation.

The column relief load is typically estimated by the unbalanced heat (UBH) method. The conventional UBH method is generally conservative and leads to over-
sizing of the column relief and flare system, per API Standard 521 (Section 4.3.3, 2014).2 The UBH method establishes a heat and material balance around the column at relieving conditions to estimate the excess heat within the system.3 The relief load is then determined by dividing the excess heat by the latent heat of vaporisation of the relieving material. However, there are limitations in utilising the UBH method. Shortcomings of the UBH method are assumptions related to:
•    An unlimited amount of light components capable of being relieved
•    A constant latent heat of vaporisation on column contents
•    A constant relief temperature.

The dynamic model accounts for the depletion of light components over time and estimates the latent heat of vaporisation and changes in relief temperature with time as column compositional changes occur. Accounting for these changes over time provides the opportunity for evaluating additional parameters, such as the partial loss of the column reboiler duty due to decreased LMTD, or the depletion of liquid inventory in the column. Both parameters have the potential to impact the resulting column relief load. For those distillation columns having widespread boiling range material, dynamic simulation has the potential to significantly reduce column relief loads when compared to those predicted using the conventional UBH method or steady-state simulation.

To better understand relief scenarios and behaviours associated with a GDS column, dynamic simulation is employed to simulate the column relief profile over time. A dynamic model has been developed for the gasoline hydrodesulphurisation (CDHDS) column to simulate the column relief profile during global power failure scenarios. The simulation estimates fluid properties and compositional changes over the period of relief, while tracking the column temperature and pressure relationship based on heat input and heat removal from the column. The model also accounts for actual/residual cooling effects of the condensers, the system volume contribution to column pressure rise, the composition changes inside the column during the relieving event, and the depletion of inventory in the attached vessels and piping. The increment in reaction heat under relieving conditions is also simulated with the pressure rise based on the available results of reaction kinetics. Such rigorous modelling enabled accurate determination of column relief loads which, in turn, assist with accurate determination of project risk and scope whilst permitting the avoidance of unnecessary project costs and potential project delays.

This article showcases the advantages of using dynamic simulation for column relief analysis over the UBH method. The methodology applied to the GDS column can also be used for simple distillation columns without reactions.

Process system and dynamic model set-up
The sulphur species in a refinery gasoline pool typically reside within fluid catalytically cracked (FCC) naphtha. A review of the gasoline pool in the US indicated that FCC naphtha contributed to 98% of total sulphur in the pool, despite the fact that FCC naphtha only contributed to 36% of the pool by volume.4 As such, the focus of this article is on hydrotreating FCC naphtha in a GDS unit.

The commercially licensed technologies for gasoline hydrotreating include ExxonMobil’s SCANfining, Axens’ Prime G+, and CBI Lummus’s CDHydro + CDHDS processes (Song 2003). This article uses CBI Lummus’s CDHydro + CDHDS process as an example for study. Korpelshoek and Rock showed that the CDHydro + CDHDS process is a selective desulphurisation process, where light catalytic naphtha (LCN) is separated in the CDHydro column and only medium and heavy catalytic naphtha (MCN + HCN) are hydrotreated in the CDHDS column.5 Figure 1 shows the processing scheme of the CDHydro + CDHDS process.

In the CDHydro column, mercaptans are combined with dienes, going through catalytic thioetherification reactions and forming heavier unsaturated sulphides. The unsaturated sulphides, together with the medium catalytic naphtha (MCN) and heavy catalytic naphtha (HCN) are sent to the CDHDS column for desulphurisation. Mild hydrogenation of C5 diolefins and isomerisation of light olefins occur in the upper section of the CDHydro column. As a result, the sulphur content in light catalytic naphtha (LCN) can be reduced to 1 wppm 
or less.

The CDHDS column is a reactive distillation column where the MCN and HCN are hydrotreated and separated. The separation of MCN and HCN allows further hydrotreating of the HCN as required. The CDHDS column is the most complex column of its kind for relief analysis, where the hydrodesulphurisation reaction, distillation and supercritical relief are all presented during the column relief condition. Using the conventional UBH method, over-conservative relief loads are obtained for global upset scenarios, as the method fails to accurately predict the actual column relieving characteristics, especially under supercritical relief conditions. A dynamic simulation is developed in Hysys to simulate the actual relieving behaviour for both total power failure (TPF) and partial power failure (PPF) relief scenarios. To capture the changes in feed and the facets of systematic change during the relief event, the dynamic simulation model included:
•    CDHydro column, complete with overhead system
•    CDHydro column reboilers
•    CDHDS column, complete with overhead system
•    CDHDS column furnace reboiler
•    Heat integration between CDHydro and CDHDS columns
•    Feed preheating of the CDHDS column.

The following assumptions and configurations are considered in model development:
•    The recycle gas loop is modelled in a simplified way such that the compressor shuts down during the studied relief event – power failure.
•    Equipment geometries are modelled with actual dimensions to represent the system volumes.
•    Static head calculations are enabled with elevations of equipment and piping applied.
•    Control valves are modelled with actual Cv values.
•    Control parameters are tuned to provide stable operation, prior to an ‘event’.
•    Pumps are modelled with actual pump curves.
•    Piping actual frictional losses are simulated.
•    Columns having multiple PSVs with staggered set pressures are assumed to have PSVs fully open at 16% above the initial set pressure, per API Standard 521.

The HDS reactive column is modelled with a typical distillation column for reaction and separation, and a sump for bottom liquid hold-up. To simplify the model, hydrotreating reactions are modelled with simple molecular shifting based on actual product yields. The reaction heat is modelled by energy streams for all catalytic beds in the column. The catalytic beds are modelled with equivalent theoretical trays of separation. The trayed column section in the CDHydro is modelled with the actual number of trays, with a tray efficiency assigned. Tray spacings are adjusted to match the actual height within each of the columns. The liquid hold-up in 
packed beds is assumed to be 5-6% based on the vendor information and modelled by adjusting the tray weir height.
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