Modeling, Scheduling, Debottlenecking, and Economic Evaluation of Integrated Batch Chemical Processes: Part I

Demetri P. Petrides, Intelligen, Inc.
Pericles T. Lagonikos, Schering-Plough Research Institute

Contents

Introduction
Benefits of Batch Process Simulation
Debottlenecking Batch Operations
Environmental Impact Assessment

Introduction (Back to Top)
Batch process simulation is a powerful analysis tool for professionals involved in the development, design, and operation of integrated batch chemical processes. It enables process engineers and scientists to describe complex and integrated batch chemical processes in the form of computer models. Once the computer model representation of a process is completed, engineers can conduct "experiments" to better understand the behavior of the real system under various equipment configurations and operating conditions. If these experiments were to be performed in the laboratory, they would require a significant investment of time and money.

Given a product and a desired annual production rate (plant throughput) batch process simulation generally endeavors to answer the following questions: What are the required amounts of raw materials and utilities? What is the required size of process equipment and supporting utilities? What is the total capital investment? What is the manufacturing cost? How long does a single batch take? How many batches can we carry out in a year? During the course of a batch, what is the demand for various resources (e.g., raw materials, labor, utilities, etc.)? What is the total amount of resources consumed? Which process steps or resources constitute bottlenecks? What can we change to increase throughput? What is the environmental impact of the process (i.e., amount and type of waste materials)? Which design is the "best" among several plausible alternatives?

Benefits of Batch Process Simulation (Back to Top)
The primary motivation for performing batch process simulation depends on the type of product, the stage of development, and the size of the investment. For commodity-like, low price chemicals, simulation typically aims at minimizing the manufacturing cost. For high-priced chemicals, engineers use simulation to improve process characteristics (environmental impact, safety, flexibility, operability) and accelerate commercialization (reduce the time-to-market). Figure 1 shows a pictorial representation of the objectives and benefits from the use of simulation tools at the various stages of the commercialization process.

• Idea Generation. At the early stages, when product and process ideas are conceived, process simulation is used for project screening/selection and strategic planning.
• Process Development. The goal at this stage is to improve upon the process characteristics that are often neglected yet difficult to modify in a later stage: reduce environmental impact, improve resource utilization, increase process flexibility. Simulation tools enable engineering teams tackling the above issues to function cooperatively with minimal duplication or loss of data. Being able to experiment on the computer with alternative process setups and operating conditions reduces the costly and time-consuming laboratory and pilot plant effort. Furthermore, since such tools pinpoint the most cost-sensitive areas—the economic "hot-spots"—of a complex process, they can be used to judiciously focus further lab and pilot plant studies. The result is an accelerated and cost effective process development.
• Facility Design. With process development near completion at the pilot plant level, simulation tools are used to systematically design and optimize the manufacturing facility. Issues of operational flexibility, safety, and process scheduling must be considered at this stage; simulation tools greatly facilitate and improve the outcome of these tasks.
• Manufacturing. During product manufacturing, simulation tools are primarily used for plant debottlenecking, process scheduling, and overall plant optimization. For existing batch manufacturing facilities, when throughput needs to be increased, either the equipment capacity of a processing step or a utility supply becomes a bottleneck. Simulation tools that are capable of tracking equipment utilization for overlapping batches can identify bottleneck candidates and guide the user through the debottlenecking effort.

The next two sections elaborate on the role of process simulation in debottlenecking batch operations and minimizing environmental impact.

Debottlenecking Batch Operations (Back to Top)
Many specialty chemical plants operate in batch (cyclical) mode. Several upstream and downstream process steps are required to convert the raw materials into the purified final product. Each step is usually initiated after the previous step is completed. However, a new batch is usually initiated before the previous one is fully completed. The maximum possible overlap between consecutive batches is determined by the process step that has the longest cycle time (usually the reaction steps). For a given plant, to increase annual production we reduce the time between consecutive batches till we reach the limit set by the longest process step. At that point, the only way to increase throughput is by adding extra capacity to the bottlenecking equipment or by introducing multiple processing lines that operate in a staggered mode.

Let us define batch equipment capacity utilization (BECU) as the product of equipment uptime (EU) and step equipment capacity utilization (SECU):

BECU = EU x SECU

EU = (Total time equipment is utilized per batch)/(Effective plant batch time)

SECU = (Fraction of equipment capacity utilized during a step)

Effective plant batch time is the time between consecutive plant batches. The value of EU is always in the [0, 1] range. An EU value of 0.7 indicates that a unit is 70% of the time occupied and 30% idle. SECU is unity for units that fully utilize their capacity during a cycle (e.g., membrane filters, disk-stack centrifuges, etc.). SECU is less than one for units with storage capacity (e.g., reactors and storage tanks) that do not operate at full capacity. For instance, if a vessel is 60% full during its operation, then, its SECU is 0.6. If we plot the value of BECU for each piece of equipment involved in batch production (Figure 2), the resulting bar graph directly shows all equipment bottleneck candidates.

In addition to allocating appropriate equipment, process steps require various utilities (e.g., WFI, various types of steam, cleaning materials, power, etc.) and labor. Utilities and labor (called resources in the rest of the chapter) are usually required at the same time by several steps of multiple overlapping batches at various rates. Figure 3 shows typical instantaneous and cumulative demands for a resource. As we increase throughput in a batch plant, the demand for resources increases. If a resource demand exceeds its maximum availability rate, we reach a resource-related bottleneck. Very often, adjustments in scheduling can be made that eliminate this type of resource bottleneck. Another type of resource-associated bottleneck can be reached if the total consumption (cumulatively) of a given resource rises over its available quantity. In this case, the only way to eliminate the bottleneck is by installing extra capacity for that resource.

Simulation tools that can calculate equipment utilization and resource demand as a function of time can greatly facilitate the debottlenecking effort in a batch manufacturing plant. During the design stage, the same tools can be used to judiciously size process equipment and utilities.

Environmental Impact Assessment (Back to Top)
Chemical plants generate a wide range of liquid, solid, and gaseous waste streams that require treatment prior to discharge. The cost associated with waste treatment and disposal has skyrocketed in recent years due to increasingly stricter environmental regulations. This cost can be reduced through minimization of waste generation at the source. However, generation of waste from a chemical process is a function of the process design and the manner in which the process is operated. Thus, reducing waste in an industrial process requires intimate knowledge of the process technology, in contrast to waste treatment which essentially is an add-on at the end of the process. Minimization of waste generation must be considered by process engineers at the early stages of process development. Once a process has undergone significant development, it is difficult and costly to make major changes. Furthermore, regulatory constraints that are unique to the pharmaceutical industry restrict process modifications once clinical efficacy of the drug is established.

Process simulators enable the user to evaluate the impact of alternative technologies on the type and amount of waste generated from a process. For instance, the impact of alternative extraction solvents or the selection of alternative chromatography elution buffers can readily be assessed. Certain simulators also facilitate waste stream characterization by calculating common environmental stream properties, such as BOD, COD, TKN, TP, TSS, etc. Other simulators allow users to simulate end-of-pipe treatment processes and predict the fate of any waste component during treatment.

End of Part I. Part II of this paper features an example of the power of batch analysis, as applied to a fermentation example.

For more information: Demetri Petrides, President, Intelligen, Inc., 2326 Morse Ave., Scotch Plains, NJ 07076. Tel: 908-654-0088. Fax: 908-654-3866. Email: dpetrides@intelligen.com. Also: Pericles T. Lagonikos, Schering-Plough Research Institute, Chemical Process Technologies, 1011 Morris Avenue, U-1-1-1525, Union, NJ 07083-7197. Tel: 908-820-6572. Fax: 908-820-6675. Email: pericles.lagonikos@spcorp.com.