Profit More from Process Simulation

The technology can improve a plant's bottom line in numerous ways.

By Grant Stephenson, Peter Henderson and Henry Schindler, Honeywell Process Solutions

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Most chemical companies view a dynamic process simulator as a valuable training tool that helps operators reduce errors in the control room, thus avoiding potential upsets and unplanned downtime. Keeping production running smoother and longer clearly boosts profitability.

However, the financial benefits of such a simulator don't have to stop — or for that matter, start — at operator training. In fact, they can begin much earlier than initial training and continue throughout the plant's lifecycle. Unfortunately, many plants overlook several uses of simulation technology that can greatly improve overall safety and process reliability while reducing capital and operating costs and environmental impact.

. . . [A]voiding a single incident, especially if it carries worker safety consequences, provides a strong economic justification.

Pre-Construction Benefits
Typically 60% of capital investments are made during a plant's early design phases. At these stages simulation models can help drive down project cost, sustain schedules and manage risk:

Capital cost optimization. Steady-state process simulation enables evaluating various operating scenarios to solve equipment-rating problems. Benefits include minimizing investment and avoiding process bottlenecks. Dynamic simulation also can aid in cutting capital costs, by minimizing overdesign of equipment while still meeting the needs for process dynamics. For example, dynamic simulation was used to determine the optimal size of a water system providing dilution to batch digesters to accommodate sequencing.

Control system development. Dynamic simulation helps create realistic operating scenarios that are better for assessing control system configuration than the traditional static checkout method. It allows engineers to evaluate whether:

• operator station configurations present a clear picture of the process;
• alarms effectively bring the operator's attention to a potential upset;
• regulatory controls sustain reliable operating conditions; and
• safety systems keep the plant from straying into risky scenarios.

Engineering verification. Studies completed prior to capital investment and initial operations allow engineers to optimize equipment design, ensure that reliability and safety are considered, and confirm operational readiness. When combined with operator training, these studies can help identify any shortcomings with the distributed control system (DCS) or logic (interlocks and emergency shutdown systems) configuration.

It's best to use the simulator to review plant startup or emergency procedures because weaknesses there can significantly impact production. This review might take a few weeks, with additional time likely required to enhance the system. The operations team should lead the effort — but process and control engineers also should be involved, as they will have to respond to operations' feedback. Improving behavior of complex systems may require some iterations. However, it's certainly better to invest time during earlier project phases rather than during initial operations.

For example, many plants rely on large expensive compressor systems whose damage can quickly cost tens of millions of dollars in equipment and loss of profits during downtime. In addition, if a large compressor is severely damaged, replacement may require a two-to-three-year leadtime. It's therefore critical that compressor protection systems are properly designed and correctly operating. By dynamically modeling these systems, engineers can study potential trip events and identify any flaws, or ensure the systems are designed right and fully protected.

Another valuable use of simulation is for checking whether an existing flare system will suffice for a plant expansion. Detailed studies of unit depressurizing, including dynamic models of the flare system, can determine the existing relief system's ability to handle new loads. In many cases such studies have shown the current relief system is adequate — thereby obviating expansion of the relief and flare system and thus providing savings that can amount to tens of millions of dollars.

Developing optimal strategies. Dynamic simulation, with its ability to predict how the plant will behave during startup, is an effective way to create and validate layouts, procedures and control strategies. It can identify errors prior to capital investment — in the office and off the project's critical path, mitigating project risks, delays and the cost of on-site resolution. This step must involve design/control engineers and process engineers.

Plants can apply different control strategies to achieve the same production results. But which strategy or combination of them won't undermine process stability? A liquefied natural gas (LNG) plant noticed strong interaction between its five LNG trains and steam-powered boilers after startup. When one train tripped the boilers the entire plant went down. Dynamic simulation enabled engineers to rectify the problem through decoupling and process redesign. Doing such a simulation before startup could have avoided the difficulty altogether.

In another instance, a different company in the early design stages of an LNG project requested to see models for a refrigerant recycle loop. In less than 30 minutes, it realized there wouldn't be enough propane to top off the initial refrigerant inventory prior to production — and thus the startup procedure needed revision. This is a good example of a company improving its plans for startup before training a single operator.

For a gas project in New Zealand, development and use of a dynamic model identified numerous opportunities for process improvement. Simulation also allowed thorough testing of initial process design, control, logic, graphics and operation procedures prior to commissioning. Operators used a replica of the actual DCS to control the simulator.

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