Process Heat Exchanger Network Research Articles

Optimal integration of organic Rankine cycles into process heat exchanger networks: A simultaneous approach

Organic Rankine cycles can produce power from various low-to-medium temperature heat sources efficiently, and may be integrated into industrial processes to recover low-grade waste heat for improved energy efficiency. This paper presents a mathematical programming model for the synthesis of organic Rankine cycle-integrated heat exchanger networks. The model is based on an organic Rankine cycle representation of four alternative configurations and a modified stage-wise superstructure, where heat exchange between process streams and organic Rankine cycle streams can take place in all the stages. In addition, the flowrate and operating temperatures of the working fluid are treated as variables, and its thermodynamic properties (e.g. enthalpies and pump/turbine outlet temperatures) are correlated as functions of the temperatures. This allows the organic Rankine cycle and the heat exchanger network to be optimised simultaneously, with the objective of maximising the net power output or minimising the overall energy cost. Two literature examples are used to illustrate the proposed approach. The results show that apart from producing power, the organic Rankine cycle can reduce the cold utility consumption by up to 26% when integrated with a process. Furthermore, the overall energy cost can be minimised by increasing the net power output optimally (by 54.6%), despite increased hot (+38.9%) and cold utility requirements (+19.5%). The proposed approach is thus useful in assessing the benefits from process integration of organic Rankine cycles.

Optimal Integration of Organic Rankine Cycles into Process Heat Exchanger Networks

This paper presents a mathematical programming model for the synthesis of an optimal heat exchanger network (HEN) in a process incorporating an organic Rankine cycle (ORC), which is used for improved heat recovery. The model is based on a modified stage-wise superstructure, where the process and ORC streams can exchange heat in all the stages. In addition, the evaporation and condensation temperatures of the ORC working fluid are treated as variables, and its thermodynamic properties (e.g. enthalpies and temperatures) are correlated as functions of evaporation and condensation temperatures. This allows the ORC operating conditions and the HEN to be optimised simultaneously, with the objective of maximising net power output or minimising overall energy cost. A literature example is presented to demonstrate the application of the proposed model. Results show that, besides producing shaft work, the ORC can further reduce the cold utility requirement. Furthermore, the overall energy cost can be minimised by increasing ORC power production, at the expense of increased hot and cold utility requirements. The proposed approach is thus useful in assessing the benefits from optimal ORC integration in the background process.

An extended energy saving method for modification of MTP process heat exchanger network

Pinch Technology has been widely adopted and considered to be one of the most successful techniques in process energy integration. But according to limitation and defect of pinch analysis in heat exchanger retrofit, a new conceptual diagram is proposed for super-ambient and sub-ambient temperature process. This diagram is based on heat flow rate as a function of temperature. By using this diagram, it can be achieve to the concepts such as: 1- retrofit of the HEN for energy saving, 2- the best location of hot and cold utility, 3- The actual amount of required hot and cold utility. Composite curve, the grand composite curve and the energy transfer diagram have been constructed for analysis of the HEN of a methanol to propylene plant. In this novel design it was attempted to improve the HEN’s efficiency of MTP process and get the maximum economic return. Annual profit percentage reach to 13.2% by improving HEN via bridge method. Results showed that utilities consumption have been decreased significantly in the novel design by bridge method (about 9.3%) to pinch analysis (5.0%). Finally, it can be said, bridge analysis provides more detail about heat savings modifications, than traditional pinch analysis.

A new method for optimization of Solar Heat Integration and solar fraction targeting in low temperature process industries

Today, process industries are responsible for a large portion of world's energy demand. Accordingly, in this section, replacing fossil fuels with renewables can have a great effect on total energy consumption and CO2 emission of the world. Using Heat Integration concepts, a general procedure for integration of solar heat into processes is generated. This procedure provides a tool for designers to find the best integration scenario and solar fraction targets. Also, it can help economic optimization of Solar Heat Integration by calculating the solar fraction targets for a certain amount of capital investment. Then, a new index for evaluation of existing designs is presented. Finally, the case of an organic distillation plant was investigated for Solar Heat Integration. Using the proposed procedure, the best place for solar heat exchangers on the process heat exchanger network and the solar fraction target was found. Annual simulations suggested that with current collector technologies, a payback period of 7–9 years is reachable. It was predicted that with further developments in collector technologies and more restrictions on CO2 emission, Solar Heat Integration for this case will eventually be profitable.

Retrofit of Integrated Refrigeration Systems

Existing integrated refrigeration systems are decomposed into a refrigeration cycle and a process Heat Exchanger Network (HEN). To optimize the refrigeration cycle, the cycle is extended first to include all potential refrigeration options. Besides the complex refrigeration cycle, these options also generate a special HEN. A systematic method is proposed to optimize the complex cycle and the special HEN simultaneously. This method can give marginal prices of the refrigeration used as utilities in the process HEN. To improve the process HEN, the existing methods for HEN retrofit are used fully, based on the marginal prices of utilities from the refrigeration cycle. A strategy is proposed to co-ordinate the optimisation of the refrigeration cycle and the retrofit of the process HEN. Two case studies show how the proposed methods can retrofit the integrated refrigeration systems efficiently.

A novel approach for retrofit and operations management of existing total sites

A new methodology is developed for analysis of total sites which expedites the energy retrofit and operations management. The directions for improvement of site efficiency, and their limits are first identified by conducting a utility system analysis termed as the top-level analysis. The results are represented graphically on the novel “power generation efficiency curves.” These curves enhance the understanding of performance of an existing utility system, and thus enable the identification of the most promising modifications is the retrofit of process heat exchanger networks (HENs). A significant advantage of the approach is that it minimises the amount of data required, a big obstacle in total site retrofit, without compromising on the optimality of the results. The power generation efficiency curves also prove useful for optimisation of utility system operations to reduce the energy costs on a site. An industrial case study is solved for demonstration.

Total site heat integration using the utility system

Plant sites are normally divided into regions consisting of the individual production processes and the utility system(s). While heat integration within the individual processes has been common for a long time, applications of heat recovery between processes, as well as systematic techniques for this, are only recently being reported. A significant difference when implementing the new approach is that the steam system provides the medium for indirect heat transfer between the processes, so the existing utility lines can be used. In order to minimize the overall cost of such schemes, the fuel consumption and power generation in the utility system need to be considered with the capital cost (mostly heat exchangers) to be invested in the processes. The task requires a proper costing approach for the steam usage, so that the duties and number of levels of steam can be best determined. This paper shows a graphically based procedure for such an optimization which uses the targeting methods of Pinch Technology. Heat integration between different processes on a plant is normally carried out through the utility system. Getting the cost optimum solution can be a very complex problem when the capital cost for the heat exchanger networks and the fuel and power costs in the utility system have to be considered simultaneously. By using exergy analysis for steam costing, the resulting steam costs can act as an interface between the utility plant and the processes. With this cost interface, the subsequent design of the utility system and process heat exchanger networks can be treated as two separate tasks. The energy-capital tradeoff for the heat recovery in each process regions is now carried out in two stages. Firstly, a large number of steam levels is used for the energy/capital tradoff in order to generate an ideal utility profile which decides the minimum total cost target. Next, only a few of these steam levels are selected to arrive at a practical utilities placement. This result is normally very close to the minimum cost target given earlier from the ideal profile. Once the optimum steam levels and their duties are obtained, design of the utility system can be carried out.