Kwok Yuen Cheung

Short-term site-wide maintenance scheduling

Abstract Preventive maintenance is essential for every chemical production site to prevent failure and accidents, however, it upsets material and utility flows inside the site and also causes production loss. In order to minimize the loss, maintenance of each plant unit has to be carefully scheduled together with considerations on site-wide material and utility balances. This will involve both production and utility systems, and indeed is a very complicated problem. A scheduling strategy is then employed to handle the problem efficiently. It divides the scheduling into two steps, long-term and short-term. Long-term maintenance scheduling determines the combination of plant shutdown in a period of 2–5 years. Base upon the long-term schedule, a short-term maintenance scheduling optimizes the exact timing of plant shutdown, overhaul, inspection and startup within a maintenance period of 4–10 weeks. Short-term maintenance scheduling involves pre-set utility and material demand profiles during a plant shutdown, overhaul and startup making it a very challenging task. In this paper, a multi-period mixed integer linear programming (MILP) model, a site-model, is proposed as an aid to optimize short-term site-wide maintenance schedule. A special formulation is also developed to deal with the pre-set utility and material demand profiles in the site-model.

Total-site scheduling for better energy utilization

Abstract Improving energy efficiency is one of the most important issues for clean production. In a chemical production site, production and utility plants are highly interactive and are all required to perform preventive maintenance to reduce production losses and accidents due to equipment failures. To minimize its impact on production and utility systems during routine maintenance, maintenance scheduling has to be done carefully with consideration of site-wide utilities and material balances. In this paper, a multi-period mixed integer linear programming model, namely a site model, is proposed for generating optimum long-term schedules for plant maintenance as well as production. The model simultaneously takes into account product demands, plant and equipment capacities, site-wide material and utility balances, etc., to optimize a maintenance schedule for a period of two years. The model can be used to maximize site-wide energy efficiency during normal and shut-down maintenance periods, optimise long-term electricity contracts, inventory strategies and purchasing of intermediate materials. A site model contains eight production plants and a utility plant with two boilers and three turbines is used for case studies.

Multi-site utility integration—an industrial case study

Abstract Mitsubishi Kasei and Mitsubishi Petrochemical merged to become Mitsubishi Chemical in 1994 making it the largest chemical company in Japan. This allows two original companies’ production sites, especially for those are physically built next to each other, to improve themselves by multi-site integration. Mitsubishi Yokkaichi production plant site is one of the typical examples, which consists of three individual plant sites in the old company structure. Each plant site contains a utility plant to generate steam and electricity for chemical production. After the company merge, connecting steam and electricity among plant sites have been carried out for better flexibility, efficiency as well as capability of the utility systems. Besides these, there are still many other improvement alternatives. A site-model, which includes all three utility plants and production units, was then developed to explore further opportunities. In this paper, applications at Mitsubishi Yokkaichi production site are presented to illustrate the features of the site-model.

Heat exchanger network optimization with discontinuous exchanger cost function

Heat exchanger network synthesis with a stagewise superstructure gives rise to a mixed integer nonlinear problem. This problem is further complicated if discontinuous cost functions are considered. Three different formulations namely Big-M (BM), Convex-Hull (CH) and Multi-M (MM) are used to formulate the discontinuous cost functions. The BM formulation is believed to be the simplest but least efficient among the three methods. The CH formulation, on the other hand, performs better than the BM method by disaggregating the exchanger area into sub-areas. Due to the disaggregation of the exchanger area, the CH formulation requires more continuous variables but produces a tighter relaxation than the BM method. Instead of disaggregating the exchanger area, the MM formulation disaggregates the BM parameter for each pair of cost segments produces the same relaxation as the CH formulation for problems. Since the MM formulation does not need to disaggregate exchanger areas, it requires less continuous variables than the CH formulation. In this paper, performances in terms of problem relaxation, objective values and computing times of the three formulations are compared.

Electricity Contract Optimization for a Large-Scale Chemical Production Site

Abstract This paper presents a study on selecting and optimizing electricity contracts for a large-scale chemical production site, which requires electricity importation. Two common types of electricity contracts are considered, Time Zone (TZ) contract and Loading Curve (LC) contract. A multi-period linear programming model, Site-Model, is adopted for the contract selection and optimization. This model includes all site-wide information, such as, product demands, material and utility balances. Therefore, an optimal contract for maximizing the total site profit can be determined.

Marginal values analysis for chemical industry

Abstract The paper proposes a systematic method called “Marginal Values Analysis” (MVA) to calculate marginal values of streams and process sections in a chemical production site. In the MVA, the marginal values are defined into three types to represent the marginal profit, production cost and product value of a stream. With these precise definitions, the marginal values can be used for locating process bottlenecks, pricing utilities, intermediate materials and energy flows. They can also be applied on developing site-wide management strategies, assisting business decisions. A general linear programming model, site-model, that includes all the heat and mass balances and interactions of a chemical production site is adopted for generating the marginal values. By proper formulations in the site-model, all the three marginal values of a stream can be obtained simultaneously. A case study of investment decision-making on a utility system is presented to illustrate the capability of MVA on debottlenecking.