Process Technology: An Introduction - Haan A.B. 2015

16 Development and engineering
16.1 Introduction

Process development can be defined as the study of development of a process from a laboratory to a commercial scale. It is not an independent field in chemical engineering, but contains and integrates all fields in process technology such as the basic fields physical transport phenomena, kinetics, thermodynamics, and the applied fields chemical reaction engineering, physical/chemical separations, and equipment design. The starting point generally consists of laboratory results that concern a chemical reaction whose translation into a commercial plant appears viable. To go directly from a laboratory scale to the industrial scale is rarely feasible. As a rule, one or more additional experiments are necessary to reproduce the laboratory results on a larger scale. The main goal of process development is to find out which steps in the process may be expected to present difficulties and which additional research needs to be performed to solve these problems at minimum cost and as quickly as possible. It is here that the methodology of process development, and hence of scale-up, becomes decisive for the success of the operation. Because most operations are scale-dependent, scale modification is an essential part of the work of the process engineer. By studying processes, the so-called scaling rules can be defined. Scaling rules allow us to apply knowledge and experience from small laboratory equipment to a larger industrial unit.

16.1.1 General aspects of scaling up

The necessity for scaling up in the process industry lies in the growth of the consumer market and in the competition between producers. On enlargement of the capacity, it is financially more favorable to build one large apparatus instead of several parallel units. In the latter case the cost of investment I increases proportionally to the production capacity C, while in the first case the so-called six-tenth rule applies for rough estimates on process installations:

(16.1)

The exponent m is called the degression coefficient. It varies from case to case. Larger installations are advantageous as long as m is smaller than unity. From a process viewpoint, anything is allowed during the designing and construction of an apparatus and installations. However, there are constructional limitations. These often determine the maximum size of fitting, such as castings, stopcocks etc. If equipment becomes so large that transportation is not feasible, it has to be constructed on spot. Duplication of installations has the advantage of a more rapid design and a shorter delivery time.

For the salary costs in the process industry a similar relation as between I and C can be used:

(16.2)

So scaling up reduces the salary costs per unit product, but is disadvantageous for employment.

A different aspect of scaling up is the vulnerability of the production. If one large installation and an identical installation built from parallel units are compared, large installations will sooner give losses when not running at 100% capacity, since the fixed costs are related to the investments. In case of a breakdown or maintenance of one large installation, everything will stop. In a multiple unit installation, a part can still continue. The organization of the maintenance and construction of large installations should be tight, as delay and retardation can result in important financial losses.

Raw materials can often be managed more economically in large installations as opposed to multiple-installations. Furthermore energy and material losses tend to be proportional to the surface area, which decreases relative to the volume when the geometrical dimensions increase. The larger the volume of the streams in a plant the more worthwhile recycling and recovery of energy and materials becomes. On the other hand more security measures are necessary because of the higher risk of severe consequences of a calamity.

16.1.2 Ways of scaling up

There are three common ways of scaling up. These are used both for scaling up of separate pieces of equipment and for complete processes. One can enlarge an apparatus or installation by adding one or more identical units in parallel to the existing system. This is called a multiple train installation, for which the total capacity C_N is described by the following simple formula:

(16.3)

where N is the number of identical installations, and C₀ is the capacity of a single unit. Examples are the furnaces in the naphtha crackers and the polymerization of polypropylene. Another option is to enlarge the equipment geometrically. Here the shape remains the same but the absolute dimensions are increased. A complete factory can be enlarged by enlargement of the individual different parts. The capacity usually increases with the scale ratio to the third power:

(16.4)

Examples where this has been applied are the oxidation of cyclohexane, ammonia production, high-pressure polyethylene polymerization, the caprolactam process, and an aspartame plant. The third way of scaling up is by simply increasing the throughput of an apparatus or installation. This is usually possible, because there is a certain design margin in a plant, such as in many older polypropylene and caprolactam plants. The word debottlenecking is often used in this context.