1
Introduction

Within the span of a half-century, the state of automation in batch facilities has progressed from total manual operation to quite high degrees of automation, even approaching fully automated (or “look ma, no hands”).

Prior to 1970, the instrumentation and controls in a batch facility consisted of the following:

1. Measurement devices.  sight glasses for level, pressure gauges using Bourdon tubes, manometers for differential pressure, temperature gauges using bimetallic strips, and so on.
2. Final control elements.  hand valves, start/stop buttons for motors, and so on.
3. Controller.  process operator and possibly a couple of simple loops for temperature, pressure, and so on.

Printed documents called standard operating procedures, working instructions, or similar were provided to the operator for each batch of product to be manufactured. All data were collected manually, often recorded on these printed documents.

Within the industry, batch was commonly disparaged, and not only from those in continuous segments such as oil refining. Those in batch commonly made derogatory comments about themselves such as “We make a batch of product and then figure out who we can sell it to”: This attests to the high batch-to-batch variability. But as most customers were using these products in processes with similar degrees of automation, the situation was tolerated. “The only reason we have valves is that stopcocks are not manufactured in the size we require”: This reflects the influence of chemists in the decision processes within batch facilities in the chemical industry. Often the plant equipment was merely a larger-size version of the equipment used in the laboratory. The author has visited a batch facility that used wooden vessels for reactors, and not that many years ago!

However, one must be careful with such disparaging remarks. The production volumes might be small, but the profit margins are high. Energy costs are usually a relatively minor part of the production costs. The energy crisis in the 1970s devastated the commodity segment of the chemical industry but had only a minor effect on the batch segment. And corporate management took notice.

Realistically, automation of batch processes only became feasible with the advent of digital control technology. Prior to 1970, the control system suppliers largely ignored batch facilities, basically selling them products developed for other markets. This has changed dramatically. All control system suppliers now offer products with features developed specifically for automating batch processes.

Whether intentional or not, the marketing hype could be understood as “just install our equipment and all will be well.” Unfortunately, one can spend a million dollars to automate a hunk of junk (a few data points are already available; no more are needed). Install a turbocharger on a Model-T and what do you have? A Model-T. Spending money on controls must go hand in hand with modernizing the production equipment.

This book takes a balanced approach to automating batch production facilities that includes the following topics:

1. Process measurements.  The basis of every automation system is quality data from the process measurements. Chapter 2 focuses on the issues that commonly arise in batch facilities.
2. Control issues.  All batch processes operate over a range of conditions, which imposes requirements on the controls to do likewise. Chapter 3 examines such issues.
3. Discrete devices.  In most batch facilities, the number of discrete values greatly exceeds the number of analog (actually digital) values. Chapter 4 examines the various aspects of discrete measurements and final control elements.
4. Material transfers.  The material transfer systems are generally capable of either transferring one of several raw materials to a destination or transferring a given raw material to one of several destinations. Chapter 5 examines various equipment configurations for material transfers.
5. Structured logic.  Structured programming and other concepts from computer science are applicable to batch processes, the key being to separate as much as possible equipment-specific logic from product-specific logic. Chapter 6 presents these concepts.
6. Batch or process unit.  Properly defining batch or process units is crucial in any batch automation endeavor. Chapter 7 discusses these issues.
7. Sequence logic.  Batch processing involves conducting the appropriate sequence of activities in the process equipment required to manufacture the desired product. Chapter 8 examines various approaches to implementing sequence logic.
8. Batches and recipes.  In a batch facility in which a variety of products are manufactured in the same production equipment, the product recipe must be the controlling document for manufacturing each product batch. Chapter 9 examines these and related topics.

1.1.  Categories of Processes

Processes are generally classified as follows:

1. Continuous.  Product flows out of the process on a continuous basis. The conditions within the production equipment are ideally constant, but in practice, minor adjustments to operating conditions are occasionally appropriate.
2. Batch.  Product is manufactured in discrete quantities called batches. The conditions within the production equipment change as the product progresses through the manufacturing process.
3. Semi-batch.  Part of the process is continuous, but other parts are batch. Why not call these semi-continuous?

1.1.1.  Continuous Processes

Continuous processes are normally appropriate for manufacturing a large quantity of a given product, such products being referred to as commodity products. The production facility must manufacture the product in the most efficient manner possible, meaning the lowest manufacturing cost.

But there is a downside. Flexibility is sacrificed to gain efficiency. The facility can do what it is designed to do very, very well. Maintaining the efficiency of the production process is essential—commodity markets are competitive, and even those with deep pockets cannot manufacture a commodity product at a loss for very long. But changes in the nature of the raw materials, the specifications for the final product, and so on, that were not anticipated during the process design can lead to significant and expensive changes to the production facility.

1.1.2.  Batch Processes

Batch processes are most commonly used to manufacture specialty products such as adhesives, cosmetics, and so on. The lower production volumes do not permit a continuous process to be designed and constructed to manufacture a given product. Instead, several “related” products are manufactured by a given suite of production equipment. In this context, “related” means that the manufacture of these products has something in common, one possibility being that the major raw materials are the same for all of these products.

A few commodity processes are batch, two examples being the polyvinyl chloride (PVC) manufacturing processes and the older style of pulp digesters. But to improve production efficiency, efforts are normally expended to develop continuous processes for commodity products. For example, the Kamyr digester is a continuous digester that has replaced many of the older batch pulp digesters.

Using ethylene as an example of a commodity product and adhesives as an example of a specialty product, there is another important distinction. The ethylene being manufactured today is largely the same as the ethylene manufactured 50 years ago. Perhaps the purity of the ethylene produced today is higher, but basically ethylene is ethylene.

However, the adhesive being manufactured today is very different from the adhesive manufactured 50 years ago. Such products are constantly being tailored to their application, with changes to make it set faster, provide a stronger bond, and so on. Sometimes, tailored products are developed to address a subset of the applications, such as providing a better bond between two specific materials. The number of different adhesives available today is much greater than 50 years ago.

The flexibility of the production facility for a specialty product is more important than its production efficiency. Specialty products tend to evolve, thanks to a development group whose mission is to do exactly that. Many specialty products must perform in a certain manner—adhesives must bond two materials. If an adhesive can be modified to shorten the time required to set, customers view this in a very favorable light, preferring this product over that from competitors and possibly willing to pay a higher price for the product (a shorter setup time makes the customer's production process more efficient). Development groups are tasked with coming up with product enhancements, producing sufficient quantities of the enhanced product for customers to test, and then moving the enhanced product to the production facilities. In many specialty production facilities, predicting what products will be manufactured a year from now is difficult, but rarely is it the same as being manufactured today.

1.1.3.  Semi-Batch Processes

A semi-batch production facility uses...