Preliminary processes

Preliminary process optimization is greatly simplified, as will be seen in the next chapter. 

In preliminary process design, the primary consideration is contact by inhalation. This happens either through accidental release of toxic material to the atmosphere or the fugitive emissions caused by slow leakage from pipe flanges, valve glands, and pump and compressor seals. Tank filling causes emissions when the rise in liquid level causes vapor in the tank to be released to the atmosphere. 

Process economics have three basic roles in preliminary process... 

Preliminary process optimization. Dominant process variables such as reactor conversion can have a major influence on the design. Preliminary optimization of these dominant variables is often required. 

To evaluate design options and carry out preliminary process optimization, simple economic criteria are required. What happens to the revenue from product sales after the process has been commissioned The sales revenue first pays for fixed costs which are independent of the rate of production. Variable costs, which do depend on the rate of production, also must be met. After this, taxes are deducted to leave the net profit. 

Two simple economic criteria are useful in preliminary process design ... 

In preliminary process design it mig] it be necessary to use marginal costs for steam, but the designer should keep in mind the dangers of using such costs. 

The review and approval process for procedures for the toll needs to be clearly understood between the toller and their client. This can be addressed in the management of change system. A preliminary process or batch procedure may be written or proposed as part of the toller selection process or may be included in the initial... 

Preliminary Process Flowsheet. This will show major equipment and lines, preliminary equipment details (vessel diameter, number of trays, pump flow and driver horsepower, etc.), major instrumentation, and, it is hoped, have a material balance at the bottom of each drawing with flows keyed to a numbering system on the diagram. The process flowsheets should cover both the process and utility sides of the plant. 

Preliminary process capability studies are those based on measurements collected from one operating run to establish that the process is in statistical control and hence no special causes are present. Studies of unpredictable processes and the determination of associated capability indices have little value. Preliminary studies should show acceptable results for special characteristics before production approval can be given. These studies and associated indices only apply to the measurement of variables and not to attributes (see below). 

In the feed pretreatment section oil and water are removed from the recovered or converted CCI2F2. The reactor type will be a multi-tubular fixed bed reactor because of the exothermic reaction (standard heat of reaction -150 kJ/mol). After the reactor the acids are selectively removed and collected as products of the reaction. In the light removal section the CFCs are condensed and the excess hydrogen is separated and recycled. The product CH2F2 is separated from the waste such as other CFCs produced and unconverted CCI2F2. The waste will be catalytically converted or incinerated. A preliminary process design has shown that such a CFC-destruction process would be both technically and economically feasible. 

Acknowledgement R. Artan, R.A.M. Avontuur, B.J. Bezemer, J.H.A. van den Hoogen, N.B.G. Nijhuis, Y. Verbeek, and C.P. Luteijn are greatfully acknowledged for their work in the preliminary process design. 

Selection of processing equipment and materials Production procedure investigation Development and organization of processing facilities Preliminary processing Preliminary drying... 

The shortcomings of capital cost estimates using installation factors are less serious in preliminary process design if used to compare options on a common basis. If used to compare options, the errors will tend to be less serious as the errors will tend to be consistent across the options. 

This book applies a systems philosophy to the preliminary process design and cost estimation of a plant. In doing so, it tries to keep in perspective all aspects of the design. There is always a tendency on the part of designers to get involved in specific details, and forget that their job is to produce a product of the desired quality and quantity, at the lowest price, in a safe facility. What is not needed is a technological masterpiece that is difficult to operate or costly to build. 

The initial goal of the preliminary process study is to obtain an economic evaluation of the process, with the minimum expenditure of time and money. During this stage, all information necessary to obtain a reasonably accurate cost estimate for building and operating the plant is determined. It is expected that these costs will be within 10% of the actual costs. 

The U. S. government has ruled that none of the costs involved in the planning, construction, and testing of a plant can be deducted from the company s income as a business expense. These are known as capitalized costs. Depreciation is the only means by which these costs can be used to reduce income taxes. All the costs that occur from the time the preliminary process design is begun until the plant begins production fall into this category. They include nearly all the items listed in Table 9-7. 

Systems that require two-dimensional treatment are sensitive to the parameters in the model, and, as a result, the transport coefficients (ke and De) must be well known. Consequently, a one-dimensional model is usually used for preliminary process design,... 

In this thesis an Inherent Safety Index for conceptual chemical process design is presented. This is required, since inherent safety should be considered in the early phases of design when the major decisions on the chemical process are made. The presented methodology allows such a consideration since the index is based on the knowledge available in the preliminary process design stage. 

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. 

Process R D literature review patent review research reports bench scale pilot reports sketch of flow sheet chemicals and their characteristics chemical reactions and interactions thermodynamics physical properties preliminary process concept Laboratory screening and testing for chemicals (toxicity, instability, explosibility) for reactions (explosibility) for impurities Pilot plant tests... 

Change Early means identification of hazards as early as possible in the process design. The payback for early hazard identification can make or break the capital budget of a new process. This can be achieved by dedicated safety evaluation methodologies which are designed for preliminary process design purposes. 

There seems to be plenty of evaluation methods for inherent safety. Unfortunatelly they are not directly suitable safety analysis tools to be used with novel design systems in preliminary process design. Most existing safety analysis methods need detailed process information and are not directly applicable in early design stages. On the other hand all methods are not suitable for computerized use with optimization and simulation tools. 

Factors Selected to Represent the Inherent Safety in Preliminary Process... 

As mentioned earlier the lack of information complicates the safety considerations in preliminary process plant design. Therefore it is important to utilize all the available details that may affect the inherent safety of the plant. To make this possible the inherent safety characteristics should be evaluated systematically during the process design. For this purpose a dedicated methodology for estimating inherent safety by using the scarce information available is required. 

In the Inherent Safety Index (TSI) the process pressure is determined on the basis of the maximum pressure in the process area under normal operation. In the preliminary process design estimates of pressure levels are available. The pressure limits in Table 17 are based on the Dow E F Index (Dow, 1987). 

Many different alternative process configurations can be created for a process in the conceptual design phase. In choosing the most feasible alternative safety should be one of the major evaluation criterias. Therefore information on the safety features of alternative process structures are needed on preliminary process design. 

The conceptual design phase is the most critical when designing inherently safer plants, since the alternative process concepts are created and analyzed in this phase. This emphasizes the need to introduce safety evaluation tools into the preliminary process design. Time and money is saved when fewer expensive safety modifications are needed during the later stages of design. 

The safety evaluation has to be closely integrated into existing preliminary process design environments to make it readily available during design. The safety tools also benefit from the existing databanks and simulation programs since they can be used for physical property and phase conditions calculations. 

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