Oct-2012

Dynamic simulation: a tool for engineering problems

Dynamic simulation can be used to solve engineering problems that require an in-depth understanding of transient processes

DIPANJAN BHATTACHARYA, HARIBABU CHITTIBABU, JESSE MUMM and JALEEL VALAPPIL
Bechtel

Article Summary

Dynamic simulation is an established tool for evaluating and understanding transient physical and chemical processes. Commonly accepted for the development of operator training simulators (OTS), recent improvements and awareness have shifted the emphasis towards using dynamic simulation as a process engineering tool. Process industry professionals have always known that dynamic interactions in heat, mass and momentum transfer are captured imperfectly by steady-state process models. Improved dynamic models have the capability to accurately represent the real-world transient behaviour of fluids, catalysts, controls and equipment. Obvious benefits include predicting design limitations over a wide range of operating envelopes and overall validation of design in terms of performance, operability and safety.

Dynamic process models are utilised to conduct multiple what-if scenarios with only minor modifications. For a pressure relief scenario, for example, the dynamic model can be used to evaluate multiple relieving cases such as fire, blocked discharge, utility failure, runaway reaction or control valve failure. With the help of an event scheduler, multiple process upset scenarios can be programmed into the same model and then executed separately. Event schedulers are a standard feature of commercial dynamic simulation programs.

While steady-state simulation models predict discrete process conditions, they fail to evaluate the wide range of potential transient conditions ignored by steady-state modelling. Say, for example, there are two hypothetical cases, Case A and Case B, and that each case represents different operating conditions for the same process. A steady-state model would be able to predict the conditions at either Case A or Case B, but would not shed any light on the intermediate conditions as the process transitions from A to B. A dynamic model, on the other hand, will predict the entire trajectory of the process as it moves from Case A to Case B. Traditionally, batch and semi-batch processes have been viewed as attractive candidates for dynamic simulation (multiple transient conditions). We know that continuous processes undergo a range of transient responses to daily perturbations caused by climactic events, equipment, control systems, start-up and turndown situations as well as human factors. Impacts of continuous process upsets are no less significant than those associated with non-continuous processes.

Currently, Bechtel uses dynamic simulation extensively as an engineering tool for designing gas processing, pipelines and petroleum refineries. Dynamic simulation has evolved to provide significant operability, safety and cost benefits for our natural gas liquefaction business. Some of the studies and analyses carried out in Bechtel are:
• Compressor studies, including anti-surge system design and settle-out pressure calculations
• Analysis of start-up, turndown and low flow conditions
• Gas turbine or motor start-up analysis
• Fuel gas system analysis
• Relief and depressurisation system design
• Inlet facility control behaviour analysis for gas separation plant or LNG
• Distillation column controllability studies
• Hydraulic analysis for low- pressure tank network systems
• HAZOP and operability studies
• Controller tuning and control valve actuator stroke rate design.

This article discusses some examples of dynamic simulation applications in brief.

Case study 1:
Hydrocracker/hydroprocessing unit depressurisation studies
In hydrotreaters and hydrocrackers, controlled depressurisation is an important safety feature. Depressurisation essentially lowers the reactor and separator loop pressure. The objective of depressurisation is to propagate the system towards a safe shutdown and to prevent any runaway situations or thermal excursions. Controlled depressurisation protects equipment and minimises the potential for temperature excursions caused by exothermic chemical reactions. Automated depressurisation can be actuated by several malfunctions in the unit, as listed below:
• Power failure
• Hydrogen failure
• Fire in the unit
• Reaction runaway
• Major leak in the reactor or furnace.

The objective of conducting dynamic simulations for the hydroprocessing unit shown in Figure 1 is to:
• Size the depressurising valve for different relieving scenarios
• Provide peak temperatures and pressures in the heat integrated reactor and stripper circuit over a one-hour period. This is used for validating metallurgy selection. Depressurisation conditions can often be the controlling case for temperature and pressure design limits for the heat exchangers, drums and piping in the reactor loop
• Determine downstream stripper/fractionator feed enthalpy and operational conditions to calculate relief load. The dynamic simulation model may also be extended to determine stripper/fractionator relief load.
To simulate the problem effectively, it is critical to understand the events during depressurisation. The following happens during the depressurisation scenario:
• The unit feed stops and the depressurisation valve opens
• The reactor effluent cooling is reduced because the cooling medium, which is the feed, stops
• The reactor outlet temperature will start increasing and the reactor effluent flowing to the separator will see an increase in temperature and decrease in pressure.

For this study, power failure was considered as the worst case. During this scenario, feed and recycle gas failure were considered and the system was depressurised. The depressurisation valve was sized to bring down the system pressure to a certain specified value over a period of 15 minutes.