A simulation-optimization approach for integrated sourcing and inventory decisions

In this paper, we study a generalized vendor selection problem that integrates vendor selection and inventory replenishment decisions of a firm. In addition to vendor-specific procurement and management costs, we consider inventory replenishment, holding, and backorder costs explicitly to meet stationary stochastic demand faced by the firm. Our goal is to select the best set of vendors along with the optimum inventory decisions at each plant of the firm so that we minimize the system-wide total costs and achieve desired service and reliability levels. Due to uncertainties inherent in the problem related to demand observed by the firm, quality provided by vendors, and disruptions observed by vendors, we utilize a simulation-optimization approach to solve the problem. More specifically, we build a discrete-event simulation model to evaluate the objective function of the problem that works in concert with a scatter search-based metaheuristic optimization approach to search the solution space. Computational results not only provide managerial insights and measure the significance of intangible factors in the vendor selection process but also highlight the importance of computational tools such as simulation-optimization for the vendor selection problem.     

Production of hydrogen over bimetallic Pt–Ni/δ-Al2O3: II. Indirect partial oxidation of LPG

Indirect partial oxidation (IPOX) of a 75:25 propane:n-butane mixture, which was used as a model for LPG, was studied over the bimetallic 0.2 wt%Pt–15 wt%Ni/δ-Al₂O₃ catalyst in 623–743 K temperature range. The effects of steam to carbon ratio (S/C), carbon to oxygen ratio(C/O₂) and residence time (W/F (g cat-h/mol LPG)) on the hydrogen production activity, selectivity and product distribution were studied in detail. The results are compared with the results obtained in the IPOX of pure propane. An Increasing Temperature Program (ITP) was applied during all experiments and the results showed that the presence of n-butane in the feed enhances hydrogen production activity and selectivity. Considering the well established distribution network of LPG and the superior performance of the bimetallic Pt–Ni catalyst in the IPOX of LPG, Pt–Ni system seems a very promising catalyst alternative to be used in commercial fuel processors.     

Simulation-Based Identification of Possible Locations for Mobile Cranes on Construction Sites

Identifying spatial-conflict free locations of mobile cranes that could minimize delays associated with crane relocation can result in productivity and safety improvements on construction sites. Existing approaches for identifying possible crane locations are based on two-dimensional (2D) work envelopes created by reasoning about the lift capacities of a crane during operations. Since spatial conflicts related to crane operations typically occur in three dimensions and during any period of operation, representing possible crane locations based on such 2D work envelopes can result in identifying some locations as good when in fact they might result in conflicts and missing possible locations that might be feasible. This paper presents an approach that determines possible locations of mobile cranes based on discrete-event simulation of crane operations incorporating dynamic behaviors of cranes. This approach starts with identifying a search space for possible crane locations by reasoning about a reachability radius of a crane determined by crane characteristics and the weight of the load to be carried. Later, it reduces the search space through boom-line intersection tests. For the remaining locations, it checks for potential spatial conflicts between building components and cranes moving in three dimensions and across time. Validation studies show that the developed approach can accurately identify possible locations for mobile cranes that minimize the relocation of mobile cranes and avoid potential spatial conflicts.     

Capacitated Refueling Station Location Problem with Traffic Deviations Over Multiple Time Periods

In this paper, we consider the problem of locating refueling stations in a transportation network via mathematical programming. The proposed model is applicable for several alternative fuel types and is particularly suitable for hydrogen fuel. We assume that a central planner, a hydrogen manufacturer or a government agency, determines the locations of the refueling stations for a given intra-city transportation network while accounting for multi-period travel demand, nonlinear refueling station operational cost, and the deviations of travelers from their shortest routes to refuel. Incorporating demand patterns over multiple periods allows us to account for both short- and long-term variation in hydrogen refueling demand (the former due to time of day, and the latter future hydrogen fuel cell vehicle growth). It also helps us model the changes in user preferences (station and route choices) and traffic conditions over different periods. To account for refueling station operational cost in making investment decisions, we introduce a staircase marginal cost function. In addition, the model explicitly considers station and route choices of travelers as they may deviate from their original paths to refuel, incurring additional costs and affecting the number and locations of refueling stations. We formulate this problem as a multi-period, mixed-integer model with constant link travel time and staircase operational cost at refueling stations. We applied two well-known solution algorithms, branch-and-bound and Lagrangian relaxation, to solve the problem. Our analysis shows that although we are able to solve the refueling station location problem to optimality with branch-and-bound, the Lagrangian relaxation approach provides very good results with less computational time. Additionally, our numerical example of Mashhad, Iran demonstrates that locating refueling stations with considering multi-period traffic patterns (as opposed to single-period) results in minimum network-wide traffic congestion and lower user and agency costs over a planning horizon.     

Interval-indexed formulation based heuristics for single machine total weighted tardiness problem

In this paper, we solve the single machine total weighted tardiness problem by using integer programming and linear programming based heuristic algorithms. Interval-indexed formulation is used to formulate the problem. We discuss several methods to form the intervals and different post-processing methods. Then, we show how our algorithm can be used to improve a population of a genetic algorithm. We also provide some computational results that show the effectiveness of our algorithm. Many aspects of our heuristic algorithm are quite general and can be applied to other scheduling and combinatorial optimization problems.     

Synthesis of β-myrcene-based macroporous nanocomposite foams: Altering the morphological and mechanical properties by using organo-modified nanoclay

Organo-modified nanoclay incorporated high internal phase emulsions (HIPEs) were successfully used for the preparation of macroporous nanocomposite foams. Due to the aim of obtaining mechanically improved foams, HIPEs were prepared by using a monomer mixture composed of β-myrcene and ethylene glycol dimethacrylate. Accordingly, two groups of macroporous nanocomposite foams were synthesized depending on the nanoclay type. The morphological analysis demonstrated that the pore openness of the resulting nanocomposites were significantly improved due to the decrease in the average cavity size and increase in the interconnected pore size. In terms of mechanical properties, it was found that filling 1 wt% of nanoclay which is surface modified by hydrogenated tallow lead to a 33% of increment in the compression modulus, as compared to the neat foam. However, loading 5 wt% of nanoclay having octadecylamine and aminopropyltriethoxysilane surface groups caused only 11% of increment in the compression modulus, as compared to the neat foam.     

Scalable fabrication of high-performance graphene/polyamide 66 nanocomposites with controllable surface chemistry by melt compounding

Scalable and ease fabrication of high-performance graphene reinforced polyamide 66 (PA66) nanocomposites by melt-mixing were achieved by selecting ideal graphene reinforcement having high C/O ratio. In this study, single-layer amine functionalized reduced graphene oxide and multi-layer thermally exfoliated graphene oxide (TEGO) were used to investigate the influence of surface chemistry and dispersion state on crystallization behaviors, mechanical, and thermal properties of graphene reinforced PA66 nanocomposites. Both types of graphenes acted as nucleating agent but TEGO showed the better performance due to its intercalated structure formation mechanism and efficient viscous flow during melting. Mechanical results indicated that 0.5 wt% TEGO based PA66 nanocomposite showed the highest tensile properties by increasing tensile modulus and tensile strength up to 45% and 16.1%, respectively. In addition, TEGO reinforced nanocomposites showed more stable viscoelastic behavior by reaching a plateau at high temperatures and restraining long-range motion of polymer chains.     

Petroleum asphaltene-derived highly porous carbon for the efficient removal of pharmaceutical compounds in water

Journal of Porous Materials - Highly porous carbons (PCs) were successfully developed through KOH chemical activation process from petroleum asphaltene for the efficient removal of pharmaceutical...     

A Mixed Mode Neural Network Circuitry for Object Recognition Application

A general purpose Conic Section Function Neural Network (CSFNN) circuitry in Very Large Scale Integration (VLSI) has been designed for an object recognition application. CSFNN is capable of making open and closed decision regions by combining the propagation rules of Radial Basis Functions (RBF) and Multilayer Perceptrons (MLP) on a single neural network with a unique propagation rule. Chip-in-the-loop learning technique was used during the training process. Utilizing mixed-mode hardware techniques, the inputs of the network and the feedforward signals are all analog while the control unit and storage of the network parameters are fully digital. CSFNN circuitry architecture is problem independent and consists of 16 inputs, 16 hidden layer neurons and 8 outputs. Inheriting the merits of CSFNN, the circuitry has good recognition performance on several objects with invariance to pose, lighting, and brightness. The designed hardware achieved a good recognition performance by means of both accuracy and computational time comparable to CSFNN software.