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Apr-2014

Accurate prediction of tower relief

A comparison of conventional and dynamic simulation methods applied to a project to debottleneck a deisobutaniser

HARRY Z HA, ABDULLA HARJI and JONATHAN WEBBER
Fluor Canada Ltd
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Article Summary
Refineries and upgraders are often required to increase throughput beyond their initial nameplate capacity. Existing bottlenecks must be identified and mitigated before the unit can process more feed. With increased throughput, relief loads are recalculated to ensure that the corresponding process units are protected. When conventional calculation methods are used, cases arise where the newly calculated relief loads exceed the existing capacity of pressure safety valves (PSV) on columns, or the capacity of the existing flare system. For a debottlenecking project, adding PSVs or modifying an existing flare system can be costly and impractical due to constraints on available downtime. Conventional methods for calculating relief loads are generally conservative1 and lead to over-sizing of the relief and flare system. Dynamic simulation provides an alternative method to better define the relief system and improve understanding of what happens during relief.2

Dynamic simulation is rigorous and can model many processes, contributing to a reduction in relief loads that the steady state-based unbalanced heat method cannot. The dynamic model simulates the fluid inventory hold-up and predicts the time before relief pressure is reached. This time before relief can be used to take credit for operator intervention to prevent/mitigate the relief load. Unlike the steady state based models, which assume an unlimited amount of light components being relieved, the dynamic model accounts for 
the depletion of light components and estimates changes in heat of vaporisation and temperature with time, which can lead to a partial loss of reboiler duty due to decreased log mean temperature difference (LMTD) or depletion of liquid inventory in the column sump.

Dynamic simulation can take credit for the column overhead condenser duty before the accumulator floods, reducing the relief load through condensation. Typically, credits are not given to the control valve responses on over-pressure protection in either conventional or dynamic calculations. In cases to consider favourable responses of control valves to reduce relief loads, dynamic simulation has the advantage of simulating the response over the time of incident, to ensure that any credits taken are appropriate within that time frame. Dynamic models also simulate interactions among the processing units and provide a more realistic overall picture of the response in various relief scenarios. Dynamic simulation can also help identify potential design modifications to reduce the relief load potential which conventional methods cannot address.

In this study, relief loads for an existing deisobutaniser column have been calculated using both the conventional unbalanced heat method and dynamic simulations under various relief scenarios. As a result of debottlenecking, the feed to the column was increased by 18%. Calculated relief loads from both methods are compared to each other and against the existing PSV capacity. The advantages of dynamic simulations are pronounced when an existing column relief system is being rechecked for debottlenecking.

Dynamic simulation model set-up
The deisobutaniser is one of several columns in the refinery alkylation unit that remove the light ends from alkylates, to enhance the octane number of gasoline. As Figure 1 shows, the deisobutaniser has 72 trays; two feed streams enter the column at trays 64 and 33. The column is equipped with two baffled, once-through thermosiphon reboilers at the bottom and air-cooled condensers on the overhead. The heating medium for the reboilers is medium pressure steam (150 psig).

The bottom tray temperature is cascaded to the flow controller on the condensate from the reboilers. The column sump level is controlled by the alkylate product flow from the column bottom. The column pressure rides on a push-pull pressure controller on the accumulator. The accumulator level is controlled by the overhead liquid product flow.

A dynamic simulation in Hysys was set up based on the configuration shown in Figure 1 and on the latest information on existing equipment, piping, and instrumentation (datasheets, P&IDs, mechanical drawings, and isometric drawings of piping). The dimensions of equipment and piping were input into the simulation with the exact elevations. Control valves were simulated with reported volume flows and openings from the datasheet. PID controllers were used to compute the controller actions. Relief valves are installed on the overhead line and a spreadsheet function was used to calculate the required relief area as a function of time. The PSV sizing was based on the API Standard.2 In particular, baffled thermosiphon reboilers were installed with an imaginary column flash zone and a sump to achieve the designed boil-up ratio and circulation (see Figure 2). The details of this configuration were important in calculating the detailed hydraulics between the column and the thermosiphon reboilers. This allowed simulation of the circulation and boil-up changes resulting from the loss of static head during column relieving. Condensate levels in the reboilers were not simulated and the reboilers were assumed not to be flooded, allowing for a conservative reboiler duty for relief load estimation.

The dynamic model was then tuned to match the performance of steady state simulation for operational conditions (P/T), feed and product rate and quality, and the duties of reboilers and condensers. In the evaluation of the load from an individual relief device, no favourable response was allowed by any control instrument that would reduce the relief load according to API standard guidelines. If the normal controller response could act to reduce the relief load, then the control valve is assumed to remain in its last position before the upset.

Based on relief analysis using conventional methods, two relieving scenarios were identified as the potential governing relief cases for the deisobutaniser: total power failure (TPF) and reflux failure (RF). The assumptions for each scenario are summarised as follows:

Total power failure (site-wide power failure)
• Feed from the alkylation reactors stops
• Feed from OSBL (mixed isobutanes) stops
• Overhead product/reflux pump stops
• Column bottom product continues on level and pressure as LC fails in position
• Medium pressure steam to reboiler, flow controls fail in position (no credit taken)
• 25% of the normal duty is assumed available for air cooling condensers based on natural draft; however, the duty of overhead condensers is considered to be zero once the accumulator is flooded
• PSV back pressure is assumed 40% of the PSV set pressure as per preliminary hydraulic calculation of the flare header
• PSV set pressure is 140 psig and 110% overpressure is used for PSV sizing
• Liquid level in the accumulator is at 70 vol% before the incident (normal operation).

Reflux failure (reflux pumps fail)
• Feed from alkylation reactors continues as normal
• Feed from OSBL (mixed isobutanes) continues as normal
• Overhead product/reflux pump stops
• Column bottom product continues on level and pressure as LC fails in position
• Medium pressure steam to reboiler, flow controls fail in position (no credit taken)
• Air cooler fans are assumed available until the accumulator is flooded (then the condenser will be lost)
• PSV back pressure is assumed 40% of the PSV set pressure
• PSV set pressure is 140 psig and 110% overpressure is used for PSV sizing
• Liquid level in the accumulator is at 70 vol% before the incident (normal operation).
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