Design process Routing

Once the process route has been chosen, it may be possible to synthesize flowsheets that do not require large inventories of materials in the process. The design of the reaction and separation system is particularly important in this respect, but heat transfer, storage, and pressure relief systems are also important. 

Briefly describe the processing route which you would specify for making the pressure hull of Problem 27.2 from each of the materials listed in the table. Comment on any particular problems which might be encountered. [You may assume that the detailed design will call for a number of apertures in the wall of the pressure hull.]... 

Is selection of a specific process route, or other design option, more appropriate on safety grounds ... 

The process designer must be aware of costs as reflected in the (1) selection of a basic process route (2) the equipment to be used in the process and (3) the details incorporated into the equipment. The designer must not arbitrarily select equipment, specify details or set pressure levels for design wdthout recognizing the relative effect on the specific cost of an item as well as associated equipment such as relieving devices, instruments, etc. 

Identify the major safety hazards and eliminate them, if possible Locate critical areas on the flow diagrams and layout drawings Is selection of a specific process route, or other design option, more appropriate on safety grounds ... 

Clearly, step counting methods can only, at best, give a very approximate idea of the probable cost of a plant. They are useful in the conceptual stage of process design, when comparisons between alternative process routes are being made. 

The chemical process industries are competitive, and the information that is published on commercial processes is restricted. The articles on particular processes published in the technical literature and in textbooks invariably give only a superficial account of the chemistry and unit operations used. They lack the detailed information needed on reaction kinetics, process conditions, equipment parameters, and physical properties needed for process design. The information that can be found in the general literature is, however, useful in the early stages of a project, when searching for possible process routes. It is often sufficient for a flow-sheet of the process to be drawn up and a rough estimate of the capital and production costs made. 

It is worthwhile estimating the F El index at an early stage in the process design, as it will indicate whether alternative, less hazardous, process routes should be considered. 

The book is directed to those persons involved in research, process development, pilot plant scale-up, process design, and commercial plant operations. It is important for technical people considering alternative process routes to know the potential hazards from the main reactions and from the unwanted side reactions in each case so that the hazards of reactivity are included in the factors reviewed in developing and selecting the final process route. 

Another widely used safety analysis method in process industry is the Hazard and Operability Analysis, better known as Hazop (Kletz, 1992). The conventional Hazop is developed to identify probable process disturbances when complete process and instrumentation diagrams are available. Therefore it is not very applicable to conceptual process design. Kletz has also mentioned a Hazop of a flowsheet, which can be used in preliminary process design, but it is not widely used. More usable method in preliminary process design is PIIS (Edwards and Lawrence, 1993), which has been developed to select safe process routes. 

Edwards and Lawrence (1993) have developed a Prototype Index of Inherent Safety (PIIS) for process design. The inherent safety index is intended for analysing the choice of process route i.e. the raw materials used and the sequence of the reaction steps. This method is very reaction oriented and does not consider properly the other parts of the process even they usually represent the majority of equipment. 

The inherent safety is the pursuit of designing hazards out of a process, as opposed to using engineering or procedural controls to mitigate risk. Therefore inherent safety strives to avoid and remove hazards rather than to control them by added-on systems. The inherent safety is best considered in the initial stages of design, when the choice of process route and concept is made. 

Some years ago a hydrometallurgical route, called the AmMAR concept, was presented [24], The concept (Fig. 14.15) represented a general processing route for a wide variety of feed materials. The detailed design of each specific process involved a specific number of known chemical operations, combined in unique ways. The main thread was the ammonia ammonium chloride or carbonate leach solution and its extraordinary chemical flexibility. 

Work-up becomes a major consideration in designing processes for preparation of all phases of drug development after Phase 1, as 60 to 80% of both capital expenditures and operating costs go to separations. (Eckert, 2000) Work-up conditions can limit the selection of reagents and routes. Simple work-ups with a minimal number of transfers decrease the number of opportunities for physical losses and contamination. 

THE FEASIBILITY study for a chemical process design investigates both the technical and economic feasibility of the proposed project. This feasibility study is only an introductory assessment, at this stage the process route has not yet been finalised although a preferred route may be apparent. Part of the work in preparing the feasibility study is to obtain information regarding the alternative process routes, and to provide an assessment of the suitability of these routes for the particular project. 

The economic feasibility of the process should be established at this stage. Again, this is only an introductory assessment performed more to establish that the plant is not definitely a loss-maker, rather than deciding that it is a particularly attractive proposition. A full and detailed economic evaluation of the plant and process is performed later in the design study (see Chapter 6) after the process route has been finalised, a detailed equipment listing prepared, and preliminary equipment designs have been performed. The following steps need to be performed to establish the economic feasibility of the process ... 

FOLLOWING THE feasibility study, the next stage in the design project is the evaluation and comparison of the alternative process routes for manufacture of the chemical. The selection of an appropriate process is an important decision, all the subsequent work depends upon this choice. Although the selection can be changed or modified at a later stage, at least before the plant is built, such a decision results in a serious waste of time and money. However, probably not such a waste as building an uneconomic or unsafe plant ... 

As with many aspects of design work, the final choice will usually depend upon a compromise between various features of different processes. It is unlikely that one process will possess all the advantages and no disadvantages. Sometimes there will be one overriding factor that influences the selection of a particular process, e.g. availability of a particular raw material, minimum cooling water requirements, etc. However, whichever process route is finally selected, it must fulfill all the criteria established in the project brief (Section 1.1). 

Although not mentioned so far in this section, the process control and instrumentation requirements (see Section 8.2.2) must be considered when selecting the process route. The ability to provide suitable operational control over the process and the availability and cost of necessary instrumentation are major considerations. If these aspects are ignored at this stage, it could well be that the detailed plant design is at... 

There is no doubt that the ultimate development of process intensification leads to the novel field of microreaction technology (Figure 1) (7-9). Because of the small characteristic dimensions of microreaction devices, mass and heat transfer processes can be strongly enhanced, and, consequently, initial and boundary conditions as well as residence times can be precisely adjusted for optimizing yield and selectivity. Microreaction devices are evidently superior, due to their short response time, which simplifies the control of operation. In connection with the extremely small material holdup, nearly inherently safe plant concepts can be realized. Moreover, microreaction technology offers access to advanced approaches in plant design, like the concept of numbering-up instead of scale-up and, in particular, the possibility to utilize novel process routes not accessible with macroscopic devices. 

---