(527b) Shared and Practical Approach to Conserve Utilities in Eco-Industrial Parks

Authors: 
Nair, S. K., National University of Singapore
Guo, Y., National University of Singapore
Mukherjee, U., University of Waterloo
Karimi, I. A., National University of Singapore
Elkamel, A., University of Waterloo
Eco-Industrial Park (EIP) is a community of manufacturing and service facilities in close proximity. These enterprises collaborate with each other for improving the social, environmental, and economic performance of the whole complex in a sustainable manner by utilizing each otherâ??s resources efficiently, and sharing facilities, services, and utilities (Boix et al., 2015; Côté & Cohen-Rosenthal, 1998; Lowe et al., 1996). Examples of such successful industrial symbiosis are Jurong Island (Singapore), Kalundborg (Denmark), and Rotterdam (Netherlands). EIPs such as Jurong Island in Singapore are interested in assessing the potential for inter-plant heat integration among its inhabitants. The aim of such an enterprise wide heat exchanger network system is to exploit the synergistic heating/cooling needs and reduce total utility costs. However, there are many practical challenges in the implementation, construction, and operation of such an enterprise wide network (Chew et al., 2013) that need to be addressed while synthesizing inter-plant heat exchange network (HEN).

(1) The inter-plant heat exchange network synthesis (HENS) methodology should involve all the capital and operating expenses. Since, the exchanger and pipeline costs are comparable in case of long distance transport, inter-plant piping and transport costs cannot be ignored in configuring an EIP-wide HEN.

(2) The independent and diverse profit-making enterprises should be willing to participate and collaborate in such an EIP-wide HEN. Hence, there should be attractive economic benefits and fairness guaranteed to motivate the enterprises for such collaboration.

(3) The exchangers in an EIP-wide direct heat integration must have real physical locations. Furthermore, if a plant hosts a heat exchanger, it must bear the added responsibilities of maintaining and operating that exchanger, and guaranteeing the safety, security, and confidentiality of any guest streams.

(4) Operational disturbances in one plant shouldnâ??t propagate to other plants, as the plants should be able to operate independently.

In this work, we present a novel strategy for EIP-wide HENS that addresses most of the issues highlighted above. We propose a central, shared, mutually agreed location for the HEN. The participating enterprises can appoint a third party logistics provider to manage and operate such a facility. We have considered pumping costs in the operating costs and the pipeline, pump and heat exchanger costs in the capital investment. By optimizing the HEN location, we can minimize the transport and pipeline costs. In our proposed model, we allowed the cooling (heating) of a hot (cold) stream both before entering and after leaving the HEN. To give operational flexibility and control to the parent company, existing in-plant utility heaters/coolers will be retained. Such an arrangement offers several advantages in terms of flexibility, control, cost, safety, security, confidentiality, accountability, operation, and management. We then propose a more practical and rational cost-benefit sharing arrangement. The contribution of an enterprise to capital expenses (CAPEX) and operating expenses (OPEX) is proportional to the utility cost savings it enjoys through the shared HEN. This ensures fairness among the enterprises as each enterprise enjoys an identical rate of return.

The EIP-wide HEN must trade off capital and operating costs versus utility savings. Therefore, we develop a mixed-integer non-linear programming (MINLP) model to maximize the net present value (NPV) of an EIP-wide HEN. The superstructure-based formulations allow sub-streams and non-isothermal mixing in a stage (Huang & Karimi, 2013; Yee & Grossmann, 1990). Since we wish to maximize the net present value of the shared HEN, discounted flows and rate of return has been included in the cost equation. Our model allows arbitrary cost expressions for exchangers with both fixed and variable components and cost multipliers for material and installation considerations. The single HEN location allows the full flexibility of multiple heat exchanges among streams from different plants. We have also improved the efficiency of formulation by adding additional constraints for eliminating redundant or complicated network designs. We have limited the number of sub-streams in a stage, avoided repeat identical matches, and included lower limits on exchanger areas and temperature drops to avoid exchangers with small-duty or small-area. If the HEN looks profitable (NPV > 0), each enterprise will enjoy annual savings. In return, it must share the various costs of the HEN in proportion to its annual savings.

Our proposed model for the case study in Hiete et al. (2012) offers an NPV of $5.71 million with a utility savings of $1.74 million/a for the EIP. The piping and pumping costs are major costs in this shared HEN, which restates their importance in EIP-integration. Enterprise E1 contributes 67.4 % to the CAPEX and OPEX, as it enjoys the most savings. In the game theory based formulation in Cheng et al. (2014), enterprise E2 trades streams to E1 and E3, but pays nothing for the exchangers. In contrast, our optimal HEN apportions costs on a more rational and practical basis of utility savings. Since E1 gains the most from the EIP-integration with a utility cost savings of $126 456/a ($29 411 /a for E2 and $71 183 /a) for E3, it contributes the largest share of the costs (E1:55.7 %, E2:13.0 % and E3:31.3 %). Like the case study of direct heat integration in Wang et al. (2015), our HEN also achieves a total utility savings of 95 %, which is the thermodynamic limit. Wang et al. (2015) did not clearly indicate the locations of their heat exchangers (or equivalently the streams being transported). E2 contributes 90.5 % of investment costs, as it supplies the cold streams that would otherwise require expensive hot utilities. Hence, solutions are fair, as companies saving expensive heating utilities will have to contribute more in the investment costs.

Heat integration has been employed over the years in several chemical processes, in particular refineries and petrochemical. As discussed, EIP-wide heat integration poses several unique economic, design, logistics, operations, and safety challenges. But, the utility costs for refineries and petrochemical plants can be huge every year, which is an incentive to reduce these costs in spite of the various challenges. In fact, modern EIPs like Jurong Island in Singapore and Jubail in Saudi Arabia are very much interested in assessing the potential for inter-plant heat integration. The methodology presented here paves the way for assessing and implementing heat integration for such â??futuristicâ? systems.

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