Process Design Tasks

I[. Main Process Design Tasks ILl Feasibility study... 

The synthesis of the correct structure and the optimization of parameters in the design of the reaction and separation systems are often the single most important tasks of process design. Usually there are many options, and it is impossible to fully evaluate them unless a complete design is furnished for the outer layers of the onion. For example, it is not possible to assess which is better. 

After the estimates are made, process design has taken shape, and the funds to build the plant are authorized, it s time to buy the equipment. To the uninitiated, the task can appear quite formidable. Even to the veteran, the chore is great with a multitude of details to remember. Note, the title of the chapter is not how to, because that is left to each engineer and his client or company to decide. What will be presented here can be more nearly described as a checklist and outline to help the purchaser recall all the details and avoid as many surprises as possible. 

We could go through that same kind of problem analysis for many different structures, and, in the process, design requirements could be expressed for each of them. The most common (Jesign requirements would be expressed in some manner in terms of strength, stiffness, and life, but there are many other issues as well. Whether the material will corrode, for example. Whether it will provide the proper insulation or just the opposite, sufficient conductivity, and so on. There are many, many different types of tasks that we ask a structure to perform. 

Signal-flow graphs are particularly useful in two respects. First, they make the process designer examine in considerable detail the dynamic structure and fimctioning of the process. Second, the nature of the interface between person and machine can be seen more clearly. The variables that are displayed in a system are, of course, available for study, but workers frequently respond to derivative functions of variables or "hidden" variables that must be deduced. Given that the process variables to be displayed will influence the worker s control strategy and that the number of deductions to be made will affect the mental workload involved, a process designer can select the type and amoimt of process information which will enhance performance of the task. 

It is doubtful if any design is entirely novel. The antecedence of most designs can usually be easily traced. The first motor cars were clearly horse-drawn carriages without the horse and the development of the design of the modern car can be traced step by step from these early prototypes. In the chemical industry, modem distillation processes have developed from the ancient stills used for rectification of spirits and the packed columns used for gas absorption have developed from primitive, brushwood-packed towers. So, it is not often that a process designer is faced with the task of producing a design for a completely novel process or piece of equipment. 

The basic adsorption process design. Sub-tasks within that include the adsorbent selection, made in view of aU of the requirements imposed on the dehydration process. The adsorption step time, regeneration and cooHng step times all need to be settled and these in view of mechanical details. The overall vessel configuration, for example, the vessel ID and length, which quantities are typically sized based on pressure drop. Finally we need to make some estimate of the expected service Hfetime for the adsorbent product. 

Flowsheet optimization is also regarded as a key task in the structural optimization of a flowsheet. As a described in the introduction, structural optimization for process design can be formulated as a mixed integer nonlinear program (MINLP). This then allows for addition or replacement of existing units, and consideration of a number of design options simultaneously. In these formulations individual units are turned on and off over the course of the optimization, as suggested by the MINLP master problem. 

Ail answers to process design questions cannot be put into a book. Even at this late date in the development of the chemical industry, it is common to hear authorities on most kinds of equipment say that their equipment can be properly fitted to a particular task only on the basis of some direct laboratory and pilot plant work. Nevertheless, much guidance and reassurance are obtainable from general experience and specific examples of successful applications, which this book attempts to provide. Much of the information is supplied in numerous tables and figures, which often deserve careful study quite apart from the text. 

The preceding tasks involve process design and development, which are familiar to chemical engineers for the chemical processes. Similar techniques which have been working successfully in chemical processes can be employed with modifications. The basic questions which need to be asked for the process development and design are as follows ... 

Factors relating to the safety of a chemical plant have so far been ignored. This is mainly because it is impossible to discuss all the aspects of plant design initially in a set of notes. The designer will obviously design items of equipment, and perform other tasks in process design, in... 

Another important area of chemical engineering applications is in the selection of solvents, co-solvents, or entrainers—generally summarized as solvents—in the early phase of a process design, development, or optimization project. For such tasks, beyond the fixed essential starting materials and products of the process, a large number of compounds has to be considered as potential... 

---