Most chemical engineers work in the chemical process industries. These include the plants that manufacture such things as food products, plastics, paper, fertilizers, petroleum products (gasoline, kerosene, fuel oil), synthetic (manmade) fibres such as nylon, and the basic chemicals used by 3 many industries, such as acids, alkalis, and dyes. These are all industries in which raw materials are separated or changed into useful products.
Almost all chemical engineers are college-trained in mathematics and physics, with particular emphasis on chemistry. However, the basis of chemical engineering is the study of unit operations.
Before the First World War (1914 to 1918), all chemical process plants were designed and operated by chemists. (Even today, in some countries such as Germany, this work is done by specially trained chemists.) However, shortly after the war, three American college professors – Walker, Lewis, and McAdams – published a book based on principles common to all chemical process plants. They noted that in all plants materials were mixed together, heated, cooled, moved from place to place, and wanted materials were separated from wastes. Each of these steps was termed a unit operation, and the student was taught both the engineering principles that underlie each one, and the procedures used to design or select equipment for each operation.
Every chemical process consists of a number of sequential unit operations (Fig. 1.1). Much of this book consists of descriptions of unit operations, so they will not be further discussed at this time.
How does a chemical process plant come into being? It starts with an idea – an idea for a completely new product, for improvement of an existing product, or for a way of producing an existing product at a lower cost. Ideas for completely new products usually come from a company’s research laboratories but improvements on existing products may occur to almost anyone.
Once the executives of a company have become interested in the idea of building a new plant, their first step is usually to call for a feasibility study. Such a study involves estimating production costs for the product as well as its potential market. Since essential engineering information is usually lacking, these estimates may contain major uncertainties.
If it appears that the plant will make a reasonable profit, the next step is to develop the engineering data that will be needed in designing it. This is the job of the research and development engineer. The R&D engineers who work for a CPI company are generally chemical engineers, although in large companies some mechanical, electrical, and civil engineers may also be employed.
R&D engineers do part of their work in the library with books and articles. They often work with other specialists, most often chemists, who are expert in some aspect of the problem. And they may do or direct some laboratory work themselves. But there is a great deal of difference between making a product in a laboratory and making it in a chemical plant. For example, penicillin was developed by growing a mould on a nutrient solution in a flask. The first commercial production was the result of doing the same thing in thousands of flasks.
When chemical engineers were called in to work on the problem, they devised a method of growing the mould in thousand-gallon tanks. Large quantities of sterile air were bubbled through the tanks to provide the oxygen the mould needed for growth. (The air had to be sterile so that no bacteria would grow in the solution.) In the flasks, the mould grew only on the surface where it could get oxygen from the air. But in the tanks, there was sufficient air so that the mould could grow beneath the surface as well. Within a short time production was so high – and the price so low – that the drug was widely available.5
Because commercial production can be different from the laboratory process, the R&D engineer will often build and operate a model of the proposed plant in order to find out what kinds of problems may develop and how to solve them. When the research and development work is completed, enough information is available so that the original cost estimate can be refined to a fairly exact figure. Again, the company management has to decide whether to go ahead with the plant or to cancel the project.
If the company decides to go ahead, the next step is process design. In this stage, the chemical engineer decides what kinds of equipment will be needed for each unit operation and calculates the size of each item. He or she must also select the material that each equipment item is to be made of – usually metal, plastic, or glass – and contact various equipment manufacturers about prices.
One of the tools with which the process design engineer organizes all this information is the flow sheet (Fig. 1.2). This is a diagram that shows what happens from the time a raw material comes into the plant to the time it emerges as the desired product. The R&D engineer will probably have made a simple flow sheet to help him or her understand the process, but the one made by the process design engineer will be much more complete. It will show all the pieces of equipment in the plant and how they are connected. The flow sheet will indicate the temperatures, pressures, and flows at each step of the process, and other things as well. One of these other things is the instrumentation that will be needed for operating the plant. Most processes in a CPI plant take place inside the equipment and it is only by using instruments that the operators can tell what is happening. If something might lead to a dangerous condition, the instrumentation designer will generally provide flashing lights or ringing bells to call the operators’ attention to the developing problem. If it cannot be solved immediately, the entire plant may have to be shut down.
In many plants, instruments not only indicate what is happening but also run the process automatically. A person walking through a modern chemical process plant for the first time is often surprised at how few people are working there. It is possible to run a very large plant with only a 6
few operators and maintenance people because the instrumentation does so much of the work.
After the process design has been completed, the design engineer often supervises the building of the plant. Chemical process plant is usually built by specialized construction companies accustomed to working closely with process design engineers. When the plant is finished, chemical engineers are placed in charge of it to ensure its proper operation. They are known as plant operation engineers and are usually executives who spend most of their time at desks. Many people work under them: operators who run the plant on a day-to-day basis; maintenance personnel who keep the equipment operating; material handling personnel who move materials from place to place in the plant; cleaners, clerks, and others. Supervising equipment maintenance is an important part of the plant engineers’ work.
They must be sure that both spare parts and trained maintenance personnel are always available to prevent shutdown of the plant. The main priority is to anticipate and prevent machinery problems. Repairing equipment before it breaks down is called preventive maintenance. Sometimes instruments are installed to indicate if equipment is running hot or vibrating excessively. But more often, elaborate records are kept on critically important machines – records that show how long each part is expected to last so that new parts can be installed before the previous ones fail. Another responsibility of the plant operation engineer is to keep a supply of raw materials on hand, although some raw materials, like natural gas, are brought into the plant by pipeline and are always available. Another term for raw material is feedstock.
It is not uncommon for a CPI plant to run continuously for a year without a break. But some pieces of equipment, such as high speed pump, cannot be expected to run that long without being stopped for maintenance. In such cases, a spare piece of equipment will be installed permanently, with piping arranged so that either one can be used. Then, if the item must be repaired, the spare can be put into service to handle the load.
Although most chemical engineers work either in R&D, process design, or plant operation, some follow other careers. They may become college teachers to train new engineers or they may become salesmen of chemical-process equipment. An engineer who has become well known as an expert in some phase of chemical engineering may work as a consultant, charging high fees for solving problems too difficult for the average engineer. (College professors often earn extra money by working as consultants.) And recently, more and more engineers are taking jobs with government. These usually involve enforcing laws that relate to health of safety.
Special Terms
Chemical Process Industries (CPI): A large group of industries that use chemical and engineering principles to separate or change materials into salable products.
Raw Material: The material that comes into a plant, where it is processed to produce a salable product. Petroleum is the raw material for the manufacture of gasoline. Sulphur is the raw material for the manufacture of sulphuric acid (H2SO4).
Unit Operation: One of the processing steps that materials undergo in a chemical process plant. Mixing and drying are examples of unit operations.
Feasibility Study: An analysis of a project to see if can be carried out successfully. This is a common preliminary step in the planning of a new plant.
Research and Development (R&D): The gathering of the basic information needed for the design of a plant. Some of the information may be found in the library or learned from experts; the rest must be discovered in the laboratory.
Process Design: Making the decision on equipment to be used and developing all the information needed for building a chemical process plant.
Flow sheet: A diagram that shows the equipment used and the steps by which a raw material is changed into a finished product. All process design is based on the flow sheet.
Instrumentation: The devices used for measuring or controlling a property such as temperature or pressure. The individual devices are called instruments. In an automobile, for example, the instrumentation includes the speedometer, the gasoline-level indicator, and the water-temperature indicator.
Plant Operation Engineer: The engineer in charge of a process plant after it is built. He or she may be the plant manager or may report directly to the plant manager.
Preventive Maintenance: The job of maintaining equipment in working order before it breaks down. In a chemical process plant the breakdown of an important piece of equipment may force the entire plant to be shut down, which can be very expensive in terms of lost production.
Consultant: An expert in some field who sells specialized knowledge to persons who need it.