Back to Basics
Incorporate Rigorous Heat Exc Exchanger hanger Models in Simulation Ron Beck Tom Ralston Shelby Hegy
Aspen Technology, Inc.
Embedding a rigorous heat exchanger model within an established simulation environment can help engineers increase process yields, while minimizing heat exchanger energy consumption and capital expenditures.
H
eat exchangers — both their design and operational performan performance ce — play play a major major role in the the energy energy use, product yield, and protability of most process plants. plants. For example, example, heat heat exchan exchangers gers that perfor perform m heating heating,, reboiling, and condensation are intrinsic to the separation performanc performancee of distil distillati lation on columns columns and and hence hence the the overall overall performanc performancee of the the process process.. Most projects that use heat exchangers follow a sequential workow — the process engineer models the process, a thermal specialist designs the heat exchanger from a heat transfer viewpoint, and a mechanical designer determines the manufacturability and maintainability maintainability of the design. In addition, a specialist in pinch analysis may look at the overall heat exchanger network from a heat integration point of view. view. With this many players involved in the design process, optimization is often not possible on a time-limited project. Consequently, Consequently, the process engineer is often tasked with the overall process optimization, assuming a feasible heat exchanger design. One major challenge faced by the process engineer responsible for designing a new process or optimizing the operation of an existing one is managing the interactions among several different software applications. Traditionally Traditionally,, separate software tools are used to simulate the overall process, integrate the heat exchanger network, and simulate the heat exchangers. However, a unied environment that enables process owsheet analysis, heat exchanger network optimization, and rigorous heat exchanger design and simulation enables the process engineer to examine the tradeoffs among achieving the optimal process process yield, yield, reducin reducing g energy energy consumptio consumption, n, and minimizi minimizing ng
Copyright © 2014 American Institute of Chemical Engineers (AIChE)
heat exchanger capital expenditures. These tradeoffs play a key role in maximizing operational protability and increas ing reliability. Rigorous heat exchanger models can signicantly increase the delity and accuracy of the process model, which benets both design and operations. In design, more alternatives can be considered more quickly. quickly. In operations, operational risks and maintenance issues can be identied and addressed. Having rigorous heat exchanger models in simulation software can signicantly streamline the workow of conceptual engineering — resulting in better designs. While heat exchanger design tools have been connected with process simulators at an elementary level for at least ve years, recent advances in software architecture and methods have enabled much closer and more fundamental integration between rigorous heat exchanger models and process simulators. simulators. This enhanced enhanced and fundamentally fundamentally new new level of integration and visualization is the subject of this article.
What is a rigorous model? When using rigorous heat exchanger models, the process engineer is employing the same heat exchanger software tools that the thermal designer uses. The main difference is that the process engineer is presented with an interface that enables him or her to complete a rigorous model without considering all of the design details, which a thermal designer typically evaluates comprehensively later. Process engineering training generally incorporates basic
CEP
July 2014
www.aiche.org/cep 21
Back to Basics
education in heat exchanger design, and many computer based, online training resources are also readily available. These tools enable the process engineer to create a design that is sufcient for the conceptual design phase. A thermal specialist should be consulted as necessary to ensure that the design choices being made are feasible. A rigorous exchanger model is based on an exchanger whose geometry has been fully specied. Table 1 lists some of the geometric parameters required for specifying the geometry of a shell-and-tube heat exchanger. In addition, the rigorous model employs a complete thermal and hydraulic representation of exchanger behavior based on the dened geometry and the process conditions within the device. This requires a sophisticated model with the ability to handle single-phase and multiphase applications. If you are optimizing the performance of an existing process, a complete geometric specication of the existing exchangers will be available. If you are designing a process, you will need to dene a suitable design to meet each exchanger’s process requirements. By allowing process engineers to develop preliminary heat exchanger designs in the context of the process (without leaving the process simulator), the process modeler will be able to explore the initial heat exchanger design decisions, understand the best design(s), and then update the process parameters for that heat exchanger in order to more accurately conduct the process modeling. This can identify: • areas where nding a feasible exchanger design is difcult and may necessitate changes in the process conguration • situations where proposed heat exchangers could introduce operational risks, such as tube vibration or rapid fouling • situations where alternative exchanger types could improve yields and/or reduce capital cost.
These explorations can be performed before the process ow diagram and the heat and material balances have been nalized. This reduces the number of iterations between the process engineering group and the thermal designer, thereby enabling consideration of more design options within a tight project schedule. However, what is described here is more than simply accessing a second model. It is the integration of the two models at a level such that the rigorous heat exchanger model is embedded within the simulator, and the models are solved simultaneously. Sensitivity analysis studies can be run to examine changes in both the heat exchanger and other related aspects of the process.
Initial modeling in the process flowsheet When an exchanger is added in a process simulation during conceptual design, the user typically species the required inlet and outlet conditions for the streams that exchange heat. Alternatively, the process engineer may specify the heat exchanger’s inlet conditions and required thermal duty (Table 2). In addition to the thermal requirements, the maximum allowable pressure drop is usually specied for each process stream. This simple, so-called shortcut, method provides enough information for the simulator to calculate overall heat and material balances. It does not, however, provide sufcient information to predict how an exchanger will respond to changes in process stream conditions or stream compositions. And, it cannot indicate how large an exchanger will need to be, how much it will cost, how much plot space it will require, or what impact it will have on piping or foundations. All of these details require a rigorous exchanger model. Table 2. These specifications can be used to define a heat exchanger unit operation in a process simulation.
Hot stream pressure drop allowance Table 1. These details are typically specified to define the geometry of a shell-and-tube heat exchanger in a rigorous model.
Cold stream pressure drop allowance Hot stream outlet temperature
Number of shells in series
Hot stream outlet temperature decrease
Number of shells in parallel
Hot outlet – cold inlet temperature difference
Shell configuration ( e.g., TEMA Type E, F, G, etc.)
Hot stream outlet subcooling
Shell diameter (ID and OD)
Hot stream outlet vapor fraction
Tube length
Hot inlet – cold outlet temperature difference
Number of tubes and tube passes
Cold stream outlet temperature
Tubesheet layout
Cold stream outlet temperature increase
Number of baffles
Cold stream outlet superheat
Baffle pitch and cut
Cold stream outlet vapor fraction
Shell and tube materials of construction
Exchanger duty
Setting plan
Hot/cold outlet temperature approach
22
www.aiche.org/cep
July 2014 CEP
Copyright © 2014 American Institute of Chemical Engineers (AIChE)
Embedding a rigorous heat exchanger model Follow these steps to employ rigorous heat exchanger models within the process simulator. (Some of the details may change depending on the software used, but the fundamental workow will be similar.) 1. Build the process owsheet (or model), and place heat exchanger unit operations as needed to achieve the desired process conditions for key synthesis and separation operations within the simulation. 2. Converge the process model using the shortcut method. 3. Identify the key heat exchangers — those that will have the biggest impact on the design in terms of process heat and costs — and convert one or more of them to a rigorous model (Figure 1). Although a thermal specialist will still need to nalize the design, a process engineer can quite easily — especially with experience — create a preliminary design using the rigorous heat exchanger software. 4. Use the software’s optimization capabilities to identify the designs with the lowest capital costs. This optimization mode enables the process engineer to home in on a design close to what the thermal specialist will arrive at later. 5. Re-solve the process model that now incorporates the selected rigorous heat exchanger model(s) and explore a range of operating scenarios, including turndown and excess throughput. These can be run as sensitivity analyses within the simulation environment.
6. If necessary to improve modeling efciency, convert the rigorous models back to basic shortcut models based on the process specications that resulted from the rigorous heat exchanger models. 7. After reviewing the process schema and completing process modeling, provide the heat exchanger les to the thermal designer, who then will verify and complete the design to ensure that it meets cost and operability objectives. If necessary, the thermal designer will also further tune individual exchanger designs.
Developing the rigorous model’s geometry Rigorous heat exchanger models are typically created using heat exchanger design and rating software, which is available from a small number of recognized process engineering software vendors. The most critical concern for accurate design or simulation is the validity of that software’s models and correlations. This can often be judged by assessing the research on which these models are based and the models’ industrial acceptance. Other important considerations are the quality of the integration between the exchanger design software and process simulator, the ability to produce exchanger designs optimized for capital expense (rather than simply heat-transfer area), and the ability to undertake a comprehensive automated search of possible designs. The following steps outline how to optimize a rigorous exchanger model in simulation:
p Figure 1. This rigorous shell-and-tube heat exchanger model is being specified within the process simulation environment.
Copyright © 2014 American Institute of Chemical Engineers (AIChE)
CEP
July 2014
www.aiche.org/cep 23
Back to Basics
Step 1. Dene streams, temperatures, and pressures. First, dene the stream owrates, inlet and outlet temperatures, and operating pressures. When the simulator and the exchanger design model are integrated and running simultaneously, the process simulation run will automatically populate the relevant elds with these data. The designer should review the stream data, and can override parameters if necessary. Step 2. Specify properties and property-estimation methods. Next, dene the physical properties character izing the various streams. The process simulator and the exchanger design and rating software should use the same physical property data and estimation methods to ensure that the heat exchanger design meets the duty requirements. In the integrated, concurrent modeling approach, all required physical properties are provided by the process simulator to the rigorous heat exchanger model, which ensures consistency.
Step 3. Specify the exchanger geometry. Once physical property data have been entered, the next step is to dene the exchanger geometry, starting with the exchanger’s conguration. For a shell-and-tube exchanger, this entails specifying the shell type and whether the hot stream will be on the shellside or the tubeside. Default values for the tube outer diameter, tube pitch, tube pattern, bafe type, exchanger material of construction, etc., provided by the software are generally sufcient. Users commonly specify preferred head congurations and stream fouling resistances, as well as recommendations to avoid certain design options like shell expansion bellows, based on in-house engineering standards or an industry standard such as that of the Tubular Exchanger Manufacturers Association. Step 4. Simulate performance in the rigorous model mode. This will provide insight into the exchanger’s suitability for the process requirements. More importantly, the engineer can switch between heat exchanger types, for example from shell-and-tube to air-cooled. This allows multiple design alternatives to be considered quickly. An overdesigned exchanger requires additional capital expenditures and may suffer excessive fouling or be difcult to control. A process with an under-designed exchanger will fall short of its required yields and/or consume excess energy, with consequent effects on protability. One recent innovation displays the fouling suscepti bility and vibration risk information for each rigorously modeled heat exchanger on a type of dashboard. This information can be used to explore design alternatives that reduce these operational issues. Some of the available software programs used for heat exchanger design can perform more advanced optimization studies based on the initially selected design.
Improvements in process simulators bring heat exchanger operation risks and warnings to the attention of the process engineer.
p Figure 3. The operational risks of a p Figure 2. A simple heat exchanger model (top) requires less specifica-
tion than a rigorous heat exchanger model (bottom). Use rigorous models when more insight into the process is required.
24
www.aiche.org/cep
July 2014 CEP
heat exchanger can be displayed on a dashboard. The display can help process engineers understand how design decisions will impact maintenance problems, cost, frequency, and equipment lifetime.
Copyright © 2014 American Institute of Chemical Engineers (AIChE)
Step 5. Re-solve the process model. Once a rigorous exchanger design has been optimized for the process, the optimum design is automatically incorporated into the owsheet. The overall simulation can be run again with the more-accurate representation of exchanger behavior. This may reveal issues in exchanger performance or in interactions between unit operations that the process modeler can identify and address. Step 6. Move from thermal to mechanical design. The other important element in ensuring a practical exchanger design is to conrm with the mechanical design group that the proposed thermal design is feasible from a fabrication standpoint. Traditionally, this involves passing the entire proposed thermal design to the mechanical designer. The mechanical design group can ensure, for example, that the proposed arrangement of the bafes within the shell is consistent with the nozzle locations and the shell ange and tubesheet thicknesses, as well as whether the tubes can be successfully welded. Often, the mechanical designer suggests changes to the design. The most advanced approaches to heat exchanger design offer a close integration between the thermal and mechanical design, making it faster and easier for the thermal and mechanical teams to collaborate and arrive at the nal design. In some organizations, the thermal team works directly with the mechanical team throughout the entire process. Alternatively, it can be very effective for the thermal designer to undertake preliminary mechanical design before the mechanical team gets involved.
When to use rigorous vs. shortcut models Rigorous exchanger models provide insight into exchanger performance in the context of the process, whether for verifying the design for operability issues such as excessive temperature, pressure, or vibration risks, or for troubleshooting plant operation (Figure 2). This is because
the rigorous model provides a highly detailed representation of exchanger performance, including prediction of operational risk conditions like ow-induced vibration and critical heat ux. The rigorous exchanger model can simulate off-design conditions so that the engineer can evaluate whether serious issues might result from changes in process ows, temperatures, pressures, or stream compositions. (Figure 3 provides an example of one way operational risks can be visualized.) However, shortcut models may provide an adequate rst approximation in some cases, such as when there is a need to reduce simulator convergence time or achieve convergence of large complex owsheets. Table 3 compares the types of process simulation applications for which shortcut methods and rigorous heat exchanger models are generally preferred.
Rigorous exchanger modeling in practice Process simulators have been helping manufacturers as well as engineering and construction rms design and operate plants and reneries across many industries for over 30 years. Similarly, exchanger design and rating software has been available since the late 1970s. However, only the recent integration of process simulation and rigorous exchanger modeling tools has allowed for higher-delity process simulations. Many companies have been able to reap the benets of this integrated technology to increase process yield, throughput, and energy efciency, and to reduce downtime. A gas plant revamp. A large engineering and construction (E&C) company serving the oil and gas industry uses rigorous exchanger models (of shell-and-tube, air-cooled, and multistream plate-n exchangers) integrated within the Aspen HYSYS oil-and-gas process simulator to per form debottlenecking and revamp studies. A revamp study of a gas plant had a goal of increasing plant capacity by
Table 3. Shortcut methods and rigorous heat exchanger models are typically preferred in these process simulation applications. Shortcut
Rigorous
Conceptual process modeling
Evaluating process behavior at off-design conditions
Trying to converge large complex models
Troubleshooting operations
Precursor to rigorous exchanger modeling
Verifying and validating exchanger design
Sensitivity studies where exchanger performance has minimal impact
Comprehensive sensitivity analysis Comparing different exchanger types Revamp studies to estimate cost, weight, and plot requirements Troubleshooting and debottlenecking studies Energy optimization studies Fouling studies Optimization of cleaning schedules Article continues on next page
Copyright © 2014 American Institute of Chemical Engineers (AIChE)
CEP
July 2014
www.aiche.org/cep 25
Back to Basics
Increasing the collaboration between process engineers and thermal experts helps create process designs that are optimized for exchanger performance. 20%. The E&C engineers believed that replacing some of the shell-and-tube heat exchangers in the existing installation with plate-n units should be considered. Because of the scale and complexity of the facility, evaluating the options was very challenging. The engineers rst developed a complete model of the process. Then, they introduced rigorous heat exchanger models and evaluated process scenarios in which both shell-and-tube and plate-n exchangers were used, in various combinations and with various ratings. The alternative scenarios were run with the design objective of increasing gas yield by 20%. The rigorous models helped them understand how different congurations would inuence the overall process yields, energy consumption, and other key specications. Using rigorous models, the engineers determined that only two of the three critical exchangers in the process required replacement — a capital expenditure savings over replacing all three exchangers — and that the plant revamp had a one-month payback period. Additionally, through integration of the rigorous heat exchanger models within the simulation model, they were able to identify better design alternatives for the portion of the process being studied that would not otherwise have been visible, and were able to communicate this to the facility owner and achieve the desired yield improvement at a low capital cost.
RON BECK is the Director of Product Marketing for aspenONE Engineering at Aspen Technology, Inc. (Email:
[email protected]). His experience includes 10 years as a project manager in an engineering and technology consultancy and over 15 years of experience implementing engineering solutions for the process industries. For the past seven years he has marketed process modeling, heat exchanger, and economic evaluation products at AspenTech. He graduated from Princeton Univ. with a degree in biology. TOM RALSTON is the Product Director for the Aspen Exchanger Design and Rating suite of software at Aspen Technology, Inc. (Email:
[email protected]). He has over 20 years of experience working with software for the design and optimization of heat exchangers. His involvement in the heat exchanger arena includes a key role at HTFS prior to and after its acquisition by Aspen Tech. In that role, he has had a long-time relationship with heat transfer groups at many engineering and construction companies and with owner-operators globally. He has been a driving force in improving the technical scope, ease of use, and integration of the software suite. SHELBY HEGY is the Product Marketing Manager for the Aspen Exchanger Design and Rating suite of tools at Aspen Technology, Inc. (Email:
[email protected]), where she supports the sales group selling to fabricators. She holds a BS in chemical engineering from the Univ. of Minnesota and has been with AspenTech for just over one year.
26
www.aiche.org/cep
July 2014 CEP
Planning and performing preventive maintenance in a refnery. A major oil producer and renery operator developed a monitoring and cleaning scheduling tool for a renery preheat train using the methods described in this article. Its engineers used rigorous shell-and-tube exchanger models embedded within the Aspen HYSYS process simulator to create a model of existing reneries, calibrated the models against historical operating data, and then used this validated model as a predictive tool to calculate fouling resistances and assess the impact of the fouling resistances on the process performance. The model was constructed with a spreadsheet-style interface that could be used by the plant operators. This allowed the operators to optimize cleaning schedules, avoid over-cleaning, and ultimately increase process uptime, which improved plant protability. The use of rigorous heat exchanger models that are tuned to accurately represent the operating conditions of an existing exchanger is a common application of the combined modeling tools, and typically achieves concrete and measurable results.
Closing thoughts Incorporating rigorous exchanger models into process simulation is a recent technological advance that has had a signicant impact on the process industries. Increasing the collaboration between process engineers and thermal experts helps create process designs that are optimized for exchanger performance and generates signicant capital and operating expenditure savings. Rigorous exchanger models provide accurate information about the thermal and hydraulic performance of an exchanger through sophisticated incremental calculations that incorporate full exchanger geometry specications, rather than just stream conditions or an exchanger duty, in the simulator. This allows process-engineering contractors to produce more fully optimized process designs with a very effective means of assuring that serious operational risks are avoided. For the process operator, the integration of exchanger models into process simulation supports better optimization of operations, while avoiding operational risks that may otherwise be hidden. For the heat exchanger manufacturer, rigorous process models are a better means of predicting situations in which the heat exchanger will fall short of user requirements, and provide a sound framework for exploring remedial measures. The chemical and rening industries can look forward to further innovations in this area as companies continue to search for opportunities to expose key performance indicators to the process designers, heat exchanger fabricators, and plant operators, as well as better ways to enable fully CEP optimized designs. Copyright © 2014 American Institute of Chemical Engineers (AIChE)
