Generalization of Scheduling Models for Batch Plants and Pipeless Plants

Abstract

Unlike in batch operations where significant progress has been made, there is no adequate research work done on optimal operations of pipeless plants. Pipeless plants were initially explored as an alternative production setup with potential applications in production of paints, adhesives, lubricants, food, pharmaceuticals, and fine chemicals. The concept of pipeless plants requires a renewed momentum to explore if they could become competitive. Unlike in the literature, where researchers have presented different aspects of scheduling models for pipeless plants through different models, in this work the focus is to develop a unified/generalized approach whereby the advancements in the batch scheduling literature can be readily extended to pipeless plants. We explore how batch scheduling models can be extended and generalized for the optimal operation of pipeless plants with fixed design, layout, and routing aspects. The important modifications and additions made to the batch scheduling models for handling scheduling of pipeless plants include: (i) modifications in state−task−network representation to bring in vessel, product, and equipment identities, leading to a "generalization" of models for batch and pipeless plants; (ii) treatment of moveable vessels as states, and not as separate resources, which are consumed only by the first unit and subsequently released only by the last unit in the production sequence for each product; and (iii) elimination of material balances and batch size aspects to simplify the models and reduce the problem size. Emphasis has been laid on development of a systematic procedure for effective STN representation for pipeless plants leading to unification of scheduling models for batch plants and pipeless plants. Two scheduling models are presented with varying problem size and complexity to explore the best suitable model for the optimal operation of pipeless plants. The proposed models have been validated on two benchmark problems, drawn from the literature, involving multiproduct and multipurpose plant features.

Simultaneous Scheduling and Routing of Pipeless Plants in Different Layouts

“…In this work, we assume that design details are specified a priori, that is, the available quantity of processing stations and movable vessels are specified by an upper-level problem, but their layout is still a decision variable. The problem statement for integrated scheduling and routing problem of pipeless plants is specified as follows: 16,17,20 Either scheduling time horizon is specified or final product demand is given along with processing times for each task in respective processing stations, and their production sequence. The layout (linear, or circular or herringbone) and capacity of processing stations are specified along with required amounts of raw materials.…”

“…The development of STN representation of pipeless plants for scheduling purposes has been presented by Shaik 16 and Shaik and Mathur 17 where they proposed two cases, constrained and unconstrained, depending on the quantity of available vessels and production demand. For constrained cases, i.e., when the quantity of vessels (N V ) is less than the production demand (N B ), that is, N V < N B , then vessels are exclusively considered as states (S V ).…”

“…15 A mixed integer linear programming (MILP) framework has been developed to address vessel movement between stations based on local dispatch rules. 6 Recently, a unified method for scheduling of pipeless plants and batch plants has been proposed 16,17 for specified design and layout of processing stations. They did not consider vessel routing aspects.…”

“…20 State-task-network (STN) representation is developed incorporating splitting of states and tasks based on demand and vessel allocation. 16,17,20 Conventional material balances and capacity constraints are replaced by a sequencing constraint to reduce problem size. 17 scheduling and routing for pipeless plants is described.…”

“…16,17,20 Conventional material balances and capacity constraints are replaced by a sequencing constraint to reduce problem size. 17 scheduling and routing for pipeless plants is described. In Section 3, the proposed mathematical formulation is presented in detail.…”