مجله مدیریت تولید و عملیات
سال دوازدهم شماره 1 (پیاپی 24، بهار 1400)

This article aims to develop an integrated production-inventory-vehicle routing model for deteriorating products in a green supply chain considering environmental impacts. The objective is to maximize the profit of selling products considering the costs of production, setup, inventory holding, distribution, vehicle routing, transportation, fuel, and environmental pollutants.

 Intending to integrate the supply chain policies, the developed model aims to maximize the total profit of producers and wholesalers in a horizontal collaboration. The model is formulated to mixed-integer linear programming (MILP). A set of deteriorating products with a shelf life is assumed. The quality of the products decreases over time, and therefore, there is a fixed deterioration rate in each period. A discounting policy is considered for producers and wholesalers. The model is coded using IBM ILOG CPLEX. Three examples with different dimensions are solved in two centralized and decentralized versions. The sensitivity of demand and holding cost parameters are analyzed. 

The solving results of the model in both centralized and decentralized cases underlined increasing profits by integrating the supply chain decisions. By comparing the model’s solving results in both centralized and decentralized cases, the sales revenue of the wholesalers in the decentralized case increased by more than 30% in all three samples when a price discount was possible. However, the revenue from the wholesalers' regular sales was more concentrated than those of decentralized case. Compared to the decentralized case, the manufacturers' revenues from discounted and regular sales in the centralized case also increased by more than 10% and 70%, respectively.

Although the demand and many other parameters such as cost and production rate are affected by many exogenous factors, and hence, are not deterministic in reality, demand was assumed deterministic. The problem was solved for small and medium-size samples. In real cases, the development of appropriate algorithms for solving large size samples is recommended.

 The basis of the proposed model is a study on a group of agricultural products from production to distribution, which can be applied for a real case study. This research influences business and the economy. It also proposes a model to build an effective collaboration between manufacturers and wholesalers to increase profitability.

This research tries to integrate decision-making in a supply chain and improve profitability. As a result, business improves and customer satisfaction increases. The model considers the waste of the deteriorating products, i.e. the destructive waste effects on the environment. Fuel consumption as a factor that harms the environment and depletes nonrenewable resources is also recognized. Moreover, possible price discounting can lead to profit increase, waste decrease, and satisfaction of economic or low-income customers. Considering the mentioned factors are substantial in a sustainable supply chain.

This paper presents a mathematical model (MILP) for a supply chain of deteriorating items such as agricultural products. Due to the existing unequal and disproportionate volume of deteriorating products transporting between producers and wholesalers, the transportation fleet is considered heterogeneous. The possibility of a price discount policy is assumed to increase the demand level and total profit and decrease overall waste. Environmental considerations in the form of a reduction in fuel consumption and pollutants in routing are also recognized. The centralized and decentralized cases were also compared.

Despite their achievements, the industrial parks have had a devastating effect on the environment and sometimes did not address social welfare issues. Therefore, it is necessary to improve their design based on the characteristics of sustainable development and ecological principles. One of the effective measures in this regard is Industrial Symbiosis (IS). It is a set of integrated plants aiming to exchange the waste of each plant as the raw material of another plant. Although several models have been presented for the optimization of water or energy exchange, one of the purposes of this research is to simultaneously exchange raw materials and waste in solid, liquid, and gas types. Considering the social characteristic of sustainable development is another necessity of this research that has been less discussed. Also, a significant portion of the waste needs to be recycled and cannot be exchanged directly between plants. Therefore, another purpose of this research is to increase the productivity of the model considering material flow between plants and recovery centres, which did not exist in previous

 In this research, a two-objective mixed-integer linear programming model is proposed considering the characteristics of sustainable development, which simultaneously enables the exchange of all raw materials and waste. Providing recyclable waste exchanges via recovery centres is another strength of the proposed model. The first objective is to minimize the economic costs of the IS network, and the second objective is to maximize social welfare. The environmental characteristic is also controlled by model constraints. Finally, to validate the proposed model, it has been implemented in one of the industrial parks of Alborz province. Also, to solve the model, the ε-Constraint method has been used.

 According to the model’s results, most of the plant's waste was replaced by the input raw materials of other plants, while before IS, all plant waste was disposed and all input materials were supplied through fresh materials. Therefore, the findings of the proposed model are as follows:reducing the volume of disposed and useless plant waste and its costs; reducing the volume of fresh input raw materials plants and its costs; creating 23 job opportunities through the establishment of centres to recover; compensating for lost working days due to the reduction of environmental pollutants to achieve the social characteristics of sustainable development; and improving the characteristics of sustainable development in the industrial park compared to the time before IS.

 This research, like other studies, has assumptions and limitations in model development. For example, the transmission path of all three types of material and waste was considered the same. The feasibility of situations trade-off and the use of waste of recovery centres was not studied. Also, the expectations of the park and the plants were considered in an integrated manner. Therefore, the following subjects are suggested to improve and develop the model in future studies:design of IS network using bi-level models; development of a model for the use of waste from the recovery centre; for example, methane gas or sludge treatment plant; model development by considering seasonal plants through reliability inventory models and intermittent flows; development of a model for locating and allocating unusable waste in the current network to the new plant; and development of a model based on the different transmission paths for material and waste flow in solid, liquid, and gaseous according to the research assumption 3.

One of the most significant applications of this paper is the simultaneous optimization of exchange material and waste in three types of solid, liquid, and gaseous in IS network to achieve the characteristics of sustainable development. Also, reducing the cost of input material and waste disposal compared to the time before IS are the economic advantages of this research. Social implications - One of the aims of this paper was to reduce the environmental and social impact of the industrial park to achieve the characteristics of sustainable development. For example, reducing the volume of fresh input raw materials and disposed waste plants and job creation were the environmental and social advantages of this study.

The innovations of this research include the following:simultaneous optimization of material and waste flow in three types: solid, liquid, and gaseous; considering the social characteristic of sustainable development; and considering the exchange between recovery centres and plants to use the waste in need of recovery.

Customer retention is formed by focusing on existing customers to build long-term relationships with them. In the end, this relationship will result in the promotion of businesses. One of the influencing factors in improving customer satisfaction and establishing long-term relationships with them is service recovery. This study aims to evaluate the effect of the recovery strategy, recovery time, and recovery status of employees in employee-related failures on customer’s after-purchase behaviour.

 The statistical population of this study consisted of the customers of the agriculture bank (Keshavarzi) in Tehran. First, a semi-structured interview was conducted with 50 customers from different branches of the agriculture bank to identify the most frequent type of failure. Then a sample of 392 people was selected using a stratified sampling method. The research data was gathered by taking the questionnaire after its validity and stability were confirmed by some experts and professors. Data analysis was performed in the first part using content analysis; the second part was conducted using general linear modelling (GLM).

Findings indicated that the type of recovery strategy does not affect the satisfaction level and word of mouth. Also, customer loyalty has a direct effect due to the type of failure. Recovery time based on the type of failure is directly related to the satisfaction level, customer loyalty, and word of mouth. For the failure type, the recoverer's personality has a direct impact on the satisfaction level. It also does not affect customer loyalty and word of mouth. Also, recovery strategy along with recovery time significantly affect customer loyalty due to the type of failure.

The main limitations of this research was the lack of segregation of recovery based on the type of service used by the customer. This non-segregation can range from simple services such as a password change in the cash cards to advanced and sensitive service such as issuing a warranty. Lack of control over the customer's previous relevant experience of recovering similar breaches by the bank or similar competitors is also an ignored issue in this study. Finally, ignorance of different protest methods is another research limitation. A protest can have various forms (verbal expression of objections to complaints to legal authorities), which can be another limitation of this research. Investigating the post-recovery implication of failure (the degree of importance to the customer) in customer satisfaction is suggested as a future study subject. The customer's previous experience in failure recovery in the same organization or competing organizations is another suggestion.

 Given that the position of the recoverer, both directly and interactively with time, affects customer satisfaction and loyalty, service providers must select the most appropriate person to implement recovery strategies in the case of failure. Organizations need to train their employees with job-related and behavioural skills. It empowers them in responding appropriately to different customer needs, different personalities, and different situations. Unlike customers who complain to higher authorities, others are less likely to request compensation from the organization. In this way, executives can devote more time to the organization's strategic plans. According to the effect of time, both directly and in combination with position and strategy on customer satisfaction and loyalty, the service provider should carefully select the recovery time. Recovery strategies, such as apologizing, should be done rapidly. Other types of recovery strategies are employed after the service completes. Accordingly, in the general policies of banks and especially the target bank of this research and considering specific customers, separate units should be considered for the position of the recovering person. Based on the type of service failure, this person should choose the best option (apology or justification) at the best time (immediate or after service).

According to the results of this study, apologies or justifications are not always appropriate. Bank managers should consider adaptation to different situations and holding highly critical power in defining the position of recovering people. As a result, this action can create positive associations in the customers' minds, and to some extent, can cover the service failure by an optimal strategy at the right time. Accordingly, the position of individuals and their requests are recognized.

Service recovery seems more important than delivering customer service. So this study has tried to cover customer dissatisfaction by examining the position of the recoverer, recovery strategy, and recovery time. In other words, customers are the most significant capitalists of any business, and their satisfaction and efforts to create maximum satisfaction should be a priority of any business.

Today, due to the breadth, diversity and multiplicity of investment projects, decision makers and investors are faced with the issue of choosing the most profitable project. The economic rate of return (ERR) index is one of the proper indices that has been presented to determine the profitability of the projects. However, the ERR index has important shortcomings that have led to serious limitations in its use. In this regard, this paper introduces a new measure called the reliable economic rate of return (RERR), which has originated from ERR measure, but it has resolved the drawbacks of ERR index. On the other hand, this paper proposes a new solution by considering the values of the cash flow in terms of fuzzy numbers. After calculating the fuzzy economic rate of return, the profitability potentiality degree of the investment projects can be determined as well as their reliability based on the net present value (NPV) method.

  This paper proves that if we equate the initial cash flow of investment projects with the sum of the negative values ​​of the relevant cash flow stream, then the RERR value would be greater than ‘-1’. In this regard, this paper proposes a solution to modify the initial and final values ​​of the investment cash flow stream, merely without changing middle values. Thereby, the NPV of the modified cash flow does not change. On the other hand, it is sometimes difficult to estimate the values of cash flows as a crisp number, which may lead to a wrong decision in accepting or rejecting projects. For this purpose, this paper considers the values of cash flow stream as fuzzy numbers and describes a solution for calculating the fuzzy RERR using the α-cut method and the RERR index. The proposed solution is compatible with the NPV method and also has fewer calculations than the other methods. This paper also proposes a method that calculates the profitability potentiality degree of projects and provides more comprehensive information to the decision makers by examining the intervals obtained from cutting the fuzzy values of the projects and presenting a new and reliable solution.

Using the @RISK software, it was found that the estimation of fuzzy return rate using RERR index and α-cut method is very close to the simulation result. It confirms the appropriate accuracy of the proposed solution. Also, the results obtained from the software in determining the profitability of projects indicated that the proposed solution is more accurate than the methods of ‘comparison of the expected intervals’ and ‘strict exceedance possibility’. It is also fully compatible with the NPV method.

In this research the market rate is assumed constant, while, in practice it can vary over the periods. Defining the market rate as an inconstant parameter, using the robust optimization approach and investigating the subject of this paper under other uncertainty environments are future study suggestions to both researchers and practitioners.

One of the most important applications of this study is in risky projects wherein estimating the values of cash flow stream as a crisp number is difficult. The results of this paper help decision makers in reducing risk and determining the project profitability. It is worth mentioning that the solution proposed in this paper can be used for all projects with any cash flow stream value.

 Failure to make the correct decision regarding the acceptance or rejection of the projects could have major negative consequences for the companies, organizations and even society. This paper helps decision makers, investors and organizations to determine the projects profitability that could benefit organizations and society.

This paper proposes the RERR measure as a profitability index such with several favorable properties: (ⅰ) it exists and is unique, (ⅱ) it is NPV-consistent in accepting/rejecting decisions, (ⅲ) it provides values greater than ‘-1’, (ⅳ) it takes into account all of the project’s outflows which may be regarded by some practitioners as an appropriate indicator of the overall investment. Furthermore, this paper proposes an appropriate method for calculating the fuzzy RERR and a new method for determining the profitability potentiality degree of the investment projects based on the NPV method.

Companies operating in the petrochemical industry have faced many problems due to unbalanced and unsuccessful performance in predicting and controlling environmental complexities in developing and even maintaining market share and using cost-effective systems. This study aims to model and simulate the dynamics of the production system and the cost price of petrochemical products in Takht-e-Jamshid Petrochemical Company (TJPC) as a start-up company; and suggest solutions for reducing the cost price.

According to the complexity of the system and its inherent dynamism, System Dynamics (SD) approach has been applied to model and simulate the production system of TJPC using the time series data of the company. Furthermore, different structural and behavioral tests and sensitivity analyses have been performed to validate the model.   

Five strategies were developed based on the simulation and sensitivity analysis results and the planners' viewpoints. They were i) increasing the production budget of raw materials and reducing the budget of production capacity expansion; ii) increasing the research and development budget; iii) increasing the percentage of profit allocation to employees’ rewards out of the accumulated profit of the company; iv) increasing the budget of purchasing spare parts and equipment; and v) increasing the shareholders' capital to launch development plans. In addition, individual and combined strategies were incorporated into the model's structure. According to the results, implementing the four combined solutions reduced the cost and increased the company's profit in the simulated time horizon, simultaneously and significantly.

Some factors affecting the production and cost systems were considered as exogenous variables. For example, fluctuations in the selling prices of goods and purchasing price of raw materials, arising from various sources such as changing exchange rates, inflation rate, and political relations, were not considered in the model. However, by the model's sensitivity analysis, the effect of each change on the system behavior is predictable.

The petrochemical industry has the highest volume of exports and added value of hydrocarbon resources. It produces millions of tons of polymer products and plays a crucial role in supplying feed to downstream industries, not relying on exports and preventing the sale of crude in the country. Therefore, providing solutions to reduce costs in this industry will benefit both the government and the people.

Reducing costs and increasing profits in the petrochemical industry will lead to the country's economic growth and reduce poverty in the society, consequently. Reducing costs and increasing profits in the petrochemical industry result in national economic growth. They also reduce poverty in society, consequently.

There is limited research in identifying cost reduction strategies in the petrochemical industry. The focus of most studies in other industry sectors is on reducing public costs in products' production and consumption cycle to seek cost reductions in marketing, distribution, and after-sales. The developed model addresses the dynamics of the complex economic system of a petrochemical company via modeling and simulation. Therefore, the developed system dynamics framework provides a guideline for modeling the cost reduction strategies in the petrochemical industry.

Proper pricing of products or services can have a significant impact on corporate earnings. Therefore, this study aims to address the optimization of price and production quantity to maximize the total profit using game theory in the coffee supply chain (composed of suppliers, retailers and customers) in Isfahan city.

 The applied model focuses on competition among members of the chain in probabilistic space. It is based on discrete selection models in which the impact of inventory costs and routing is analyzed on the pricing model simultaneously. In this model, competition between coffee importers (because coffee is not produced in the country) is studied in the probable space. Due to complexity in computation, it could not solve problems with a large number of customers. Therefore, the approximation of the binomial distribution by the normal distribution is used, an approximate model is proposed. Finally, the equilibrium point is determined by solving a system of nonlinear equations using the Newton method.

The maximum profit in non-cooperative game conditions is achievable based on the appropriate demand function. Finally, the equilibrium price was calculated. Findings indicated that the computational time of the approximate model did not change with increasing the number of customers, and the model can determine the optimal price and production quantity. The model results implied that the time of solving the approximate model used did not change with the increasing number of customers, and the model was able to determine the optimal quantity and price.

In the present study, the equilibrium behaviour of suppliers, retailers and customers in a two-tier supply chain of the coffee supply and distribution industry was investigated in Isfahan city. A price competition model was proposed, and ten active companies were studied in the Isfahan market. Moreover, the computational speed diagram indicated that the computational time in the original function increased with increasing the number of customers, but this time in the approximation function did not react to the increase in the number of customers and remained constant.

In the proposed model of the present study, competition among coffee suppliers was investigated in a probabilistic environment in which retailers acted as intermediaries and had no effect on pricing. Holding and shortage costs were also taken into account. Also, routing costs were approximated using the proposed Himovich model. Then, a pricing model was proposed. The proposed model could not solve problems with a large number of customers due to complexity in computation.

The advantage of the proposed model over basic models is in considering the inventory costs, which has not been considered yet in price competition models based on discrete selection models. Another significant feature of the price competition problem is the type of demand function. In this paper, to study a competitive model, a stochastic demand function was developed based on the logit model. The advantage of the introduced demand function is that it considers shortage and inventory holding costs in a one-period model. Also, in the game theory studies, usually with the increasing number of customers, the model is computationally complicated and less effective to solve real-world problems. For this reason, in this study, the normal distribution approximation was used instead of the binomial distribution.