Optimal Residence Times for a Batch Biomass-to-Methane Conversion System

Biomass conversion processes have the potential for satisfying approximately 25% of the national demand for methane gas. At the present time, very little analytical work has been done to optimally design and operate the production facilities associated with these processes. This study was motivated by the high cost of these systems and the attendant need for efficient operation. While in storage, biomass decays (exponentially) over time, and gas production declines (exponentially) with time. The basic operational problem is to determine the optimal residence times for batches in the anaerobic digester to maximize total production over a fixed planning horizon. This paper provides analysis to characterize the form of the optimal policy and presents an efficient algorithm for obtaining the solution. An example problem, based upon realistic biomass and digester data, is used to illustrate the algorithm as well as the behavior of the optimal solution.inventory/production, applications, perishable/decaying items

Fill-Rate Optimization in a One-Warehouse N-Identical Retailer Distribution System

This paper examines the system fill-rate of a one-warehouse N-identical retailer distribution system as a function of warehouse and retailer safety stock. Using the approximation model of Deuermeyer and Schwarz (Deuermeyer, B. L., L. B. Schwarz. 1981. A model for the analysis of system service level in warehouse/retailer distribution systems: the identical retailer case. Chapter 8 in Multi-Level Production/Inventory Control Systems: Theory and Practice. North-Holland, Amsterdam.) we examine the problem of maximizing system fill-rate subject to a constraint on system safety stock. Optimal safety stock policy is characterized to be the intersection of a fill-rate policy line and the safety stock budget line. Properties of fill-rate policy lines are given. These properties may be used to provide managerial insight into system optimization and as the basis for heuristics. One heuristic, the vertical heuristic, based on the corresponding deterministic version of the problem, yields trivially simple and near-optimal policies.inventory/production, multi-echelon inventory system, fill-rate optimization