using recipe centric hybrid simulation to drive the ... - Semantic Scholar

USING RECIPE CENTRIC HYBRID SIMULATION TO DRIVE THE TOOLS USED IN THE LIFE CYCLE OF PRODUCTS MANUFACTURED IN BATCH MODE. Steven M. Clark and Girish S. Joglekar Batch Process Technologies, Inc. 1291 Cumberland Avenue W. Lafayette, IN 47906

Abstract: The recipe information describes the sequence of operations and the details of each operation associated with the manufacture of a product. In the early product development stage, the main objective is to identify operating parameters to give the best overall product yield. In the next stage, the recipe is verified on a pilot scale and expanded to include additional steps such as material preparation, cleaning, set-up, shared resources and so on. In the final stage the recipe is implemented in a manufacturing facility to make the desired product. BATCHES, a hybrid simulator, provides a framework for developing detailed recipes and use them in analyzing a wide range of problems related to batch processes. A recipe evolves along with the product throughout its life cycle. In this paper, the use of the simulator in scale-up, design and operation of a batch plant is illustrated. Keywords: Simulation, Batch Processes, Recipe management Introduction The manufacturing instructions for making a product in batch mode constitute its recipe. The recipe defines the sequence of operations performed from raw material to finished product. In addition, it provides the details of carrying out each operation such as the sequence of steps within each operation, material and resource requirements for each step, duration of each step and so on. Furthermore, specific rules to be followed by the production personnel are also part of a product recipe. In the early stages of product development, the synthesis route is identified. Typically, this is an iterative process where various conversion and separation technologies are evaluated on laboratory scale with the objective of minimizing the estimated product cost. The selection of the synthesis route is the basis for the first recipe definition for a product. In the next phase of evolution, the product is made on a pilot plant. In the

final phase, the product is either made in an existing facility or a new production facility is constructed to manufacture that product. In each stage of its evolution, a product recipe can be modified as more knowledge is gained about the processing steps. Of course, based on the knowledge gained at each stage, it is also possible that the selected synthesis route may become economically infeasible and a new route may have to be identified and tested. Typically, a variety of computational tools are used during product development and manufacturing. The tools are selected to provide specific answers at each stage of evolution. For example, during synthesis route selection, detailed process dynamics models may be used to optimize process parameters. During manufacturing, a scheduling package may be used for planning and day to day scheduling, or a plant wide simulation model may be

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2 used to study the effect of equipment breakdown on overall schedule. For designing a plant, you may want to study the impact of cycle time of a particular stage on pumping costs and process throughput. The ability to accurately represent the underlying process and manufacturing recipes is crucial in using a tool to solve problems related to batch processes. If the necessary details are lacking, the results generated by a tool may not be very reliable. At present, various tools used for analyzing batch processes have their own unique constructs for representing the underlying process. Due to the requirements of its underlying methodology, each tool may use different levels of abstraction of the process. A significant portion of the problem formulation is devoted to translating the recipe information. Since a common repository of recipe information is typically not available, the same information exists in multiple formats and levels of detail suitable to each application. Aside from being very inefficient, this is prone to errors if the data is not kept up to date. The publication of the batch control standards S88 was the first attempt to formalize a structure for defining recipe information. By proposing a common terminology, the ANSI/ISA-S88 (1995) standards bring uniformity to recipe descriptions and facilitate the exchange of information. Until now, the standards have been accepted and implemented by only a few batch control system vendors. The vendors providing computational tools used in decision support have been even slower in adopting them. There significant benefit to specifying the recipe information in a standardized format and then using it to solve the problems related to batch processes. Hybrid simulators have the flexibility to perform detailed process dynamics computations accompanied by discrete changes in the state. Additionally, the time advance mechanism of these simulators can handle purely discrete systems as well. Thus, in principle, hybrid simulators can provide the backbone for the decision support tools used in all stages in a product’s life cycle. In conjunction with the simulation engine, however, a structure would be required for specifying the recipe details and available equipment, and for executing the decision made during the operation of a batch process. BATCHES, a hybrid simulator, provides a framework for building general, scalable recipes. The recipes evolve as the products evolve from laboratory scale to the plant floor. In this paper, the use of general recipes and simulation in doing scale-up and scheduling is illustrated. Developing General Recipes A general recipe describes product-specific processing information, and is created without specific knowledge of process equipment that will be used to manufacture the product (ANSI/ISA-S88, 1995). In contrast, a lower level recipe such as master recipe results from the application of a general recipe to specific process equipment. The details of different recipe types and their

requirements are described in the standards. A general recipe identifies raw materials, relative quantities and processing conditions. The development of a general recipe begins at the laboratory scale where the synthesis route and the characterization of the main processing step are done. The use of a uniform framework in the recipe development during a product’s lifecycle significantly reduces the time required to bring the product to the marketplace. For example, the recipe of a product consisting of three operations, REACT, PREP and SEPARATE, is given in Figure 1. PROCESS_1 A

REACT CHARGE_A

HCL

DI NACL

CHARGE_HCL

SEPARATE

HEAT

XFER1

REACT

XFER2

EMPTY1

AGITATE1

PREP

CUT1

CHARGE_DI

CONC

CHARGE_NACL

EMPTY

WASTE2 SOLVENT

AGITATE EMPTY

PRODUCT

Figure 1: An example of a general recipe. The details of each step are as follows: REACT Operation: Charge 1.0 kg of raw material A and add HCL 3 times the amount of A. Heat the contents until the temperature reaches 350 K. Maintain the temperature until the conversion of A is 90%, marking the end of the reaction step. PREP Operation: Prepare salt solution by mixing deionized water 5 times the amount of A and salt 3 times the amount of A. SEPARATE Operation: Transfer the batches from reactor and salt solution. Agitate and heat until the mixture reaches bubble point. Remove the lights as the first cut. Separate the solvent from the mixture until solvent mass fraction goes below 0.001. Remove the final product. Being a laboratory scale recipe, the durations of some of the elementary steps, such as charge or transfer, may not be important at this level, however the steps are a part of the overall recipe. For other elementary steps, such as REACT, CUT1, the condition that marks the end of each step is given in the recipe. Also, the batch size of each step is tied to the amount of raw material A charged during the CHARGE_A step of the REACT operation. Therefore, to apply this recipe to a pilot plant, only the amount of raw material A needs to be changed in accordance with the available equipment. The transfer durations are based on the batch size and flow rates. The

3 durations of HEATING, CUT1 and CONC elementary steps are influenced by the jacket temperature and overall heat transfer coefficients of the processing vessels. A pilot plant is used as an intermediate step for gaining more insight into the manufacturing of a product on a commercial scale. In that regard, more details may get added to the recipe, such as, demand for shared resources, cleaning and setup associated with each operation, placement of intermediate storage, rejection and/or reprocessing of off-spec material, and so on. For example, the demand for hot water used in HEATING could be batch size and time dependent. All process equipment may have to undergo cleaning in place (CIP) after each batch using a shared CIP system. Due to the restrictions on packaging line operation, an intermediate storage tank may be necessary after the SEPARATE elementary step. Downstream operations such as packaging and warehousing are also studied in a pilot plant. Pilot plant studies often provide the basis for the preliminary design of some of the co-ordination control algorithms implemented in a production facility. For example, one batch of PREP operation must be initiated for each REACT batch so that the material becomes available just in time for the SEPARATE operation. The modeling constructs of the BATCHES simulator are ideally suited for using general recipes as outlined in the batch control standards (BATCHES, 2002). Thus, if general recipes already exist, the simulation model can be set up to interface with it and extract the appropriate information for use in applications. On the other hand, a BATCHES recipe model can be translated into a general recipe for use by batch control systems. Plant Model A plant model is the result of applying the detailed general recipe models to either an existing or a proposed production plant. As in the case of a pilot plant model, the amounts and durations get scaled where appropriate. A plant model can play an important role in a wide range of applications. Product Allocation: The first decision made in producing a product commercially is the selection of the plant where that product would be made. This decision is influenced by such factors as the projected demand over a selected time horizon, the other products made and the idle capacity at the facility, the economics of replacing existing products with the new product, and so on. By applying the recipe model of the new product in conjunction with the recipe models of existing products to all available and suitable production facilities, the product allocation decision can be made. The complexity increases as the number of products and the available facilities increases.

Typically, the product allocation decisions are not based on in-depth analysis. The main reason is that there is not enough knowledge base for the new product. Thus, only simple indicators are considered while making these decisions, for example, available capacity, and suitability of equipment. Applying the general recipe to each potential production facility is valuable in building the knowledge base for a new product. The objective of the single product studies is to identify bottlenecks, evaluate alternatives to balance the average throughput of each stage and reduce the overall cycle time. The average throughput of a stage depends of the batch size, number of equipment items in a stage and the equipment batch cycle time. Some of the alternatives identified in single product studies may require additional capital outlays. The cost of process changes becomes a part of the overall product allocation decision. After the single product studies, various product mixes are tried at various facilities to make the product allocation decisions. After the allocation decision is made, the new product is added to the slate of products made at a given facility. The general recipe is modified to account for any dependencies on the sequence in which products are made. Also, changes are made to those elementary steps whose durations are equipment dependent. If the process equipment has constraints due to connectivity, size and material of construction, then additional rules must be incorporated into the model so that the right equipment assignment decisions are made. Establishing the operability of each production facility for the modified product slate is crucial in product allocation decisions. Plant Level Applications Although considerable progress has been made in the recent years in the area of enterprise wide decision support systems, reliable plant level decision support tools are not readily available. Consequently, operating decisions continue to be made based on judgment and experience, resulting in either schedules that cannot be met or productivity deterioration due to excessive slack in the process. There is an industry-wide push for achieving schedule conformance at highest productivity levels possible. As batch controls systems and data historians gain wider acceptance, up to date process recipe data will become available for wider use. The key elements of a shop-floor level, simulation based scheduling system are discussed in this section. Scheduling system: The schematic diagram of a scheduling system that generates high quality schedules and allows you to evaluate schedule conformance is given Figure 2. The general recipes derived from the process provide information for the batch control system as well as the process model generator. The data historian stores the historical data about the process state at predefined

4 frequency. The Process Snapshot block uses the data to initialize the model at a given point in time. The data historian also provides the data for evaluating the schedule conformance. As part of the standard operating procedure, the scheduling system is invoked at a specific time, for example, 8:00 am every Monday. Using the snapshot and the heuristics, the system generates a schedule for the selected time horizon, typically three weeks. Of course, the schedule is subject to change as deemed appropriate. The schedule is used by the batch control system to initiate operations in the process according to the starting times generated.

Process

Historian

Snapshot

Heuristics

General Recipe

Plant Model

Simulator

Demand Batch Control Modify Process Modify Recipe Modify Rules

Schedule No

Sched OK?

Compare Schedule

Figure 2: Schematic diagram of a shop-floor level scheduling system The schedule conformance is evaluated on a daily basis. The predicted start times of operations are compared with the actual start times. If significant deviations are detected, two actions are triggered. First, a new schedule is generated based on the current process state. Additionally, the deviations are further analyzed to determine the root causes. The analysis may result in changes to the process, the general recipes or the rules for operating the process. New schedules may also be generated because of unexpected events in the process, for example, failure of a batch, equipment failure, changes in demand and so on. The schedules also generate the raw material consumption profiles and cumulative production profiles. These are used by the ERP systems for ordering raw materials and shipping finished products. To facilitate the use of the system, a user interface was created that allowed the shop-floor personnel to set up a model and make a run with minimum external data input. The results generated by the system are reliable because it uses a detailed model and heuristics. Process Improvements: Finding opportunities to improve throughput is an ongoing activity in any production facility. The process improvement studies are typically done iteratively by identifying a bottleneck and taking actions to eliminate it.

The bottleneck locations keep changing during this iterative process. The changes may be at the plant level such as adding or resizing equipment, or at the recipe level such as changing operating conditions or cycle times. As the changes are implemented, the general recipes and the simulation model can be kept up to date. Thus, the application tools always use the latest information. Conclusions The general recipe for a product evolves from product development at laboratory scale to detailed manufacturing instructions at production scale. The recipe forms the core of the information used for such diverse applications as scale-up and shop floor level scheduling system. In this paper, the use of the recipe information in conjunction with a hybrid simulator, BATCHES, in solving a variety of problems was illustrated. The use of detailed models provides very accurate and reliable information for making decisions. Additionally, the models can be easily changed and kept up to date with changes to the process or recipes. References ANSI/ISA-S88.01-1995 Standard. “Batch Control Par1: Models and Terminology”, The International Society for Measurement and Control, 1995. Higgins, K. T. (2002). Recipes for Integration. FoodEngineering, May, p27-31. BATCHES (2002). BATCHES Users’ Manual, Batch Process Technologies, Inc., W. Lafayette, IN.