Optimising gas processing assets using process modeling tools

According to Deloitte, between now and 2035, an estimated $205 billion will be poured into gas infrastructure capital assets in North America. Most midstream owners and operators have limited resources to conduct any of the engineering of these assets in-house. Nonetheless, they will be under increasing pressure to handle feasibility studies and project oversight in house, which are the two essential aspects of any project that is required to keep construction and project costs under control. Midstream engineering work will continue to be outsourced to engineers & consultants (E&Cs), and continue to be extremely dynamic. The experience to date of the E&Cs is that client requirements change and, therefore, project teams need to be extremely flexible in design and redesign of projects particularly early in the projects’ lifecycle (Robertson, Elliot, 2011).
 
Fortunately, process modeling has advanced and fundamentally evolved in such a way that the most current and advanced software is well suited to meet the technical and business needs of this asset build out, and this innovation continues.
 
Here, we will discuss the evolution of process engineering software from the modeling and design of specific parts of the gas system to broader modeling and optimisation of the entire gas processing system (incorporating gathering, separation, dehydration, acid gas treatment, pipeline hydraulics, and liquefaction).
 
Minimising energy use
One interesting innovation has been the incorporation of energy analysis within a gas processing and production system model. Traditionally, process models are used to design and optimise one portion of a gas asset – such as a dehydration system, a gas-to-liquids (GTL) system or a system of compressors. This model achieves a good design for one portion of a system. However, when different design or operating objectives need to be addressed, such as energy use optimisation, emissions management, or carbon footprint minimisation, models need to be combined or separate models need to be built. Energy minimisation for example is usually achieved by use of pinch technology, a methodology usually in the domain of specialists in that arena. The recent innovation known as ‘Activated Energy’ embeds sophisticated pinch analysis inside the process simulator to allow a process engineer to easily compare and evaluate energy use and carbon footprint of process alternatives without being an expert in pinch analysis.  
 
In another variation of energy optimisation, Process Ecology (Holaboff, James, 2013) employs such a modeling approach to develop strategies to ensure compliance with air emissions permits in Alberta for glycol dehydration units operated by Encana and by Devon Energy. Process modeling studies enable the operators to match monitoring data to process models, accurately model facility performance, and identify optimal operating strategies that reduce glycol circulation rates and energy consumption, and ensure that emissions stay within permitted limits. This design analysis additionally achieved hard dollar savings of about $30,000 per dehydrator unit.

Optimising heat exchanger selection and sizing
Another innovation has been the incorporation of high fidelity heat exchanger models within the process modeling environment. Heat exchangers are often critical design elements of gas processing facilities. Incorrectly designed gas processing systems, from a heat exchanger point of view, can result in wasted energy and reduced performance. Conversely, over design of heat exchangers, especially high performance heat exchangers, such as plate fin exchangers, can result in very significant excess capital costs. Incorporation of energy minimisation and heat exchanger network design and optimisation within the overall modeling environment, can easily lead to savings of 10-30% in energy use and a resulting reduction in greenhouse gas emissions.  
 
Another crucial innovation has been the close integration of rigorous cost estimating conceptual models with the process simulation model, also known as ‘activated economics’. The ability to determine a design’s capital and operating cost within a reasonable accuracy envelope (usually +/- 20 to 30 % to achieve authorisation for expenditure) at an early stage of design provides the engineer with the ability to discuss both technical alternatives and economic feasibility early in the project when there is still project flexibility to make changes. Also, it provides the engineer with the tools to quickly handle the engineering change process when the client’s business and design criteria change, typically driven by changes to on-the-ground gas supply and other operating parameters.
 
This flexibility is especially valuable given the dynamic gas marketplace dynamics involving fast moving business opportunity on both the supply and sale side as well as capital project financing opportunities. In a recent Gulf Coast natural gas liquids project (NGL), Burns and McDonnell was able to employ process modeling integrated with conceptual economics models to rapidly evaluate a number of technical alternatives, understand the capital and operating cost impacts, communicate these with the client, and arrive at a final design that saved the client significant capital cost. Additionally, these designs achieved an increase in gas processing capacity still within the constraints of the existing infrastructure. These significant improvements were all accomplished in an environment where owner’s criteria changed several times (Robertson, Elliott, 2011)
 
Kuwait Oil Company also used this “activated economics” innovative technology and saved $20 million dollars on a gas processing retrofit project.  The operator had already selected a unit revamp approach, but a process engineer was able to convince management to briefly stay the project to perform a comparative economic analysis of a new unit instead of a revamped unit. The process model and economic model were extremely instrumental in changing the company’s direction towards implementation of the non-intuitive but less costly approach (Kapavarapu, V, 2011).
 
Innovations in acid gas cleaning modeling
Acid gas cleaning is another important aspect of the engineering and operation of gas processing facilities. Benefits of accurate and correct design include capital costs (column sizing), amine costs, energy costs, and regulatory compliance and operability. The latest innovations in process modeling have applied rigorous and accurate rate-based distillation to a completely new and more accurate approach to modeling acid gas stripping. Also, accurate models that can predict performance of packed columns (which achieve significant capital cost savings) and heat stable salt formation (which impact operating efficiency and uptime) have been introduced. The technology is currently in the early adopter phase.
 
Dynamic modeling to improve compressor operability
One of the operational integrity risks in a gas processing network are compressor units. Compressors are subject to a variety of instabilities and upset conditions. Dynamics models of compressors are a valuable tool in understanding upset and operational risk scenarios. Dynamic modeling has been made progressively easier to adopt by integrating dynamics closely within steady state simulators and introducing getting started guides and compressor sample models to help engineers to model their compressor installations without starting from scratch.
 
The payoff
As mentioned, midstream and E&C organisations that are using these new innovative advances in process modeling are achieving concrete results in terms of fast project cycle times (as much as 30% time savings), better use of capital (in many cases, identifying, within tight project deadlines, alternatives that can save 10-20% capital investment), and significant incremental energy efficiency in gas processing and transportation. An added bonus has been an improved ability to project emissions compliance risks and reduced carbon footprints.
 
Future directions
The innovations described above all provide powerful evaluation and optimisation methods to midstream companies and their E&Cs to integrate design and operations. Operators of gas processing networks are increasing employing process modeling to improve the operations of their networks, from the wellhead to the sale point.
 
References:
Holaboff, James (2013). Aspen Optimize Global Conference, May 2013, Boston MA.
Robertson, Elliott (5/2011). Capital Cost Optimization of an NGL Facility Revamp, Aspen Optimize Global Conference, May 2011, Washington DC.
Kapavarapu, Venkata M.R. (10/2011). Process Optimization Using Simulation and Integrated Economics, MEPEC Conference, October 2011, Bahrain.
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The author is the Senior Principal Business Consultant (Engg) APAC at AspenTech