1. Field of the Invention
The present invention relates in general to the field of energy recovery systems, and in particular, to systems, program product, and methods related to synthesizing a heat exchanger network for a process or cluster of processes including a plurality of hot process streams to be cooled and a plurality of cold process streams to be heated.
2. Description of the Related Art
Many different types of processes consume multiple steam levels and electricity to obtain an output result, or to produce a required product or compound. For large-scale processes that, for example, consume significant amounts of fuel steam, it is preferable to optimize the consumption of energy through careful operation, design or reconfiguration of the plant and the equipment used. Further, in some industrial manufacturing processes, specific streams of material flows need to be supplied to different types of equipment and machinery at specific temperatures. These material flows may need to be heated or cooled from an original starting or supply temperature to a target temperature. This, in turn, will require the consumption of steam to heat specific streams and consumption of water, for example, to cool down specific streams.
The total energy employed or consumed by the industrial manufacturing processes can be optimized to a global minimal level, for example, through careful placement and configuration of specific material streams with respect to one another. There may be, for example, the potential for hot streams that require cooling to be placed in proximity with cold streams that require heating. Streams having thermal energy already present that need to be removed (waste heat) or streams that need to have heat added can be associated with one another to optimize the energy consumption of the process. A network of heat exchangers can be synthesized to provide a medium for utilizing this waste heat to provide heat to those streams that need to have heat added. This heat exchanger network can be a very important sub-system in any new plant.
As such, the heat exchanger network synthesis problem has arguably been one of the most studied problems in the field of process synthesis in the last four decades. The systematic synthesis of heat exchangers network, however, has proven to be a challenging task. During the last three decades a considerable number of methods have been proposed and utilized in commercial software and/or academia. These methods are referenced in the two famous review papers of T. Gundersen and L. Naess, “The Synthesis of Cost Optimal Heat Exchanger Networks,” Computers and Chemical Engineering, vol. 12, pp. 503-530 (1988), and of Kevin C. Furman and Nikolaos V. Sahinidis, “A Critical Review and Annotated Bibliography for Heat Exchanger Network Synthesis in the 20th Century,” Industrial Engineering & Chemistry Research vol. 41, pp. 2335-2370 (2002).
Other methodologies include mathematical programming-based methods. Although such methods have been in academia since the late eighties, they are still not widely used on a large scale in industrial applications for several reasons. The academics claim that the reasons behind this are: (1) that the computational requirements of such methods are substantial, especially for large problems; and (2) that the resultant solution, in general, can not guarantee globality. These two reasons might be considered the most important obstacles, but there are also other very important ones. Other significant obstacles include the black box nature of the methods, the assumptions regarding problem economics, the types of heat exchangers used in the network (shell & tube, twisted tube, plate and frame types, etc.), the need to know the several utilities types and temperatures beforehand, and the non-inclusive nature of the “transshipment model” used for streams matching and superstructure application. Use of the transshipment model can be seen clearly in superstructures that produce networks that exhibit the structures in which utilities heat exchangers are always at the terminals of the network. In superstructure construction where it is required for the designer to know ahead of time just how many times a stream or one of its branches are going to meet another stream, however, the transshipment model is inadequate as it does not include or account for various situations, such as, for example: those in which it would be beneficial to allow the optimization process to select the utility types and supply temperatures to be used; those in which it would be beneficial for one or more streams to change their identities; and those in which it would be beneficial for one or more utilities streams to effectively become process streams, and so on, or consider the effect of including such possibilities on such streams superstructures.
The state-of-the-art software widely used in industry for initial synthesis of the heat exchange network (HEN) includes, for example, an AspenTech Inc. product known as Aspen Pinch, a Hyprotech Inc. product known as HX-NET (acquired by AspenTech), a KBC product known as Pinch Express, and a UMIST product known as Sprint, which attempt to address the heat exchanger network synthesis problem, systematically, using the well known pinch design method, followed by an optimization capability that optimizes the initial design created by the pinch design method through use of streams split flows in streams branches and the global network heat recovery minimum approach temperature as optimization variables in a non-linear program to recover more waste heat, shift loads among heat exchangers to remove small units, redistribute the load among units, and optimize surface area, of course, always within the constraints of the topology determined using the pinch design method. The pinch design method, followed by the optimization capability method, or combination of methods, has seen wide spread acceptance in the industrial community due to its non-black box approach. That is, the process engineer is in the feedback loop of the design of the heat exchangers network such that process engineer can take design decisions that can change with the progress of the design.
Recognized by the inventor, however, is that in all applications of near pinch and multiple pinches problems to the above software applications, their respective calculations render a larger than optimal number of heat exchange units. Also recognized is that, in addition, software applications that use the pinch design method or that use the pinch design method as a basis for its initial design followed by the optimization option for branches and duties can not handle certain situations/constraints/opportunities that can render better economics, for example, from energy, capital, or both points of view, which means that some superior network designs will never be synthesized using such applications. For example, such software applications do not systematically handle or allow for: stream-specific minimum approach temperatures; situations in which a hot stream is matched with a hot stream and/or a cold stream is matched with one or more cold streams; or situations in which a hot stream is partially converted to a cold stream and/or a cold stream is partially converted to hot stream.
Accordingly, recognized by the inventor is the need for an improved method, system, or technique that can address any or all of the above optimization issues, particularly during the design stage, and which can minimize energy and capital costs for waste heat recovery through application of a systematic process prior to the actual design, construction or modification of actual plant and equipment. Particularly, recognized is the need for a new method in grassroots applications that can render in all cases, a network design including a number of the exchanger units that is less than or an equal number of heat exchanger units for the networks synthesized using the pinch design method, even when combined with heat exchanger duty and branch optimization options currently implemented in commercial software, for all types of problems, i.e., to include pinched problems, problems with near pinch applications, as well as multiple pinches problems, that need both heating and cooling utilities, and problems that need only cooling or only heating utility (called threshold problems).
Still further, recognized by the inventor is that such goals can be realized by employing a method, system, and program product which solves each of such problems, for example, as a single problem, rather than decomposing the problem into multiple separate problems such as, for example, an above-the-pinch problem, a below-the-pinch pinch problem, and an at or near the pinch problem, as is performed by the above described pinch applications, especially for problems that exhibit multiple pinches, pinch problems with near pinch applications, and threshold problems. Where the pinch design method performs matching at the pinch point, e.g., at a medial point along the temperature scale extending between maximum and minimum target and supply temperatures, and moves up on the temperature scale to complete the sub-problem above the pinch point, and then starts again at the pinch point and moves down at the temperature scale to complete the sub-problem below the pinch point, which can result in unnecessary constraints solved by splitting of streams and which can correspondingly result in a network with an excessive number of units, the inventor has recognized that by performing matching between the hot streams and utilities with the cold streams beginning, for example, at the highest temperature or temperature interval on the temperature scale and then proceeding from that point, top to bottom, the streams can be matched at the same temperature interval (where the temperature approach between the hot and cold streams are minimum), which can allow the balance/difference between the supply of the heat and the demand of the cold to be compensated for by a utility or utilities with the lowest possible supply temperature. It is further recognized that such approach can minimize the energy “quality” loss or the “degradation” in energy quality.
Also recognized by the inventor is that, rather than merely employing streams splitting to satisfy problem feasibility for matching, which results from a decomposition of the problem, streams splitting can instead be employed upon user request to reduce energy quality degradation due to undesirable matching of a hot stream at a certain temperature interval at the process sink region with one or more cold streams at lower temperature intervals.
Further, recognized by the inventor is that it is not only unnecessarily, but imprudent, to treat threshold problems that do not have pinch constraints as a pinch problem merely to generalize the pinch design method for handling all types of problems, as is the case in the pinch design method, because doing so creates a constrained situation in a problem that does not have such constraints. Such unnecessary addition of constraints resultingly necessitates splitting of streams at the factious pinch point again to satisfy the matching criteria at the pinch according to the pinch design method rules, which correspondingly results in a network with an excessive number of heat exchanger units. Accordingly, recognized by the inventor is the need for methods, systems, and program product that solve the threshold problems without treating such threshold problems, which do not have a pinch/constraint, as a pinched problem, and thus, can resultingly reduce the number of required heat exchanger units to a number below that of networks synthesized using the pinch design method.
It is further recognized by the inventor that it would be beneficial if the heat exchanger network design, according to such methods, systems, and program product, were also such that the network was configured to be “easily-retrofitable” in future times to allow for growth and/or for contingencies, for example, due to dramatic changes in energy prices. Notably, it is not believed that the pinch design method could adopt retrofitability during the design stage as it does not have a systematic method to select an optimal set of stream specific minimum temperature, either in general, or based upon a trade-off between capital and energy costs, in particular, and because its pinch design philosophy starts the design of the network only after selecting an optimal network global minimum approach temperature using, for example, the “SUPERTARGET” method which targets for both energy consumption and the heat exchanger network area at the same time. Even by repeating such sequential philosophy using the global minimum temperature approach, the resulting new network structure would not be expected to consistently resemble the previous network structure, in class, and thus, would result in a requirement for an undue expenditure in network reconciliation efforts, to try to form a continuum of common-structure heat exchanger network designs which can be used to facilitate user selection of a physical heat exchanger network structure satisfying both current user-selected economic criteria and anticipated potential future retrofit requirements and corresponding physical heat exchanger network development and facility surface area of allotment based upon such selected design.