Flow-sheeting calculations

General Properties of Computerized Physical Property System. Flow-sheeting calculations tend to have voracious appetites for physical property estimations. To model a distillation column one may request estimates for chemical potential (or fugacity) and for enthalpies 10,000 or more times. Depending on the complexity of the property methods used, these calculations could represent 80% or more of the computer time requited to do a simulation. The design of the physical property estimation system must therefore be done with extreme care. 

The total stream and individual component flows do not normally need to be shown to a high precision on the process flow-sheet at most one decimal place is all that is usually justified by the accuracy of the flow-sheet calculations, and is sufficient. The flows should, however, balance to within the precision shown. If a stream or component flow is so small that it is less than the precision used for the larger flows, it can be shown to a greater number of places, if its accuracy justifies this and the information is required. Imprecise small flows are best shown as TRACE . If the composition of a trace component is specified as a process constraint, as, say, for an effluent stream or product quality specification, it can be shown in parts per million, ppm. 

It is good practice to show on the flow-sheet the basis used for the flow-sheet calculations. This would include the operating hours per year the reaction and physical yields and the datum temperature used for energy balances. It is also helpful to include a list of the principal assumptions used in the calculations. This alerts the user to any limitations that may have to be placed on the flow-sheet information. 

Basis for the flow-sheet calculations Time basis... 

It is usually easiest to carry out the sequence of flow-sheet calculations in the same order as the process steps starting with the raw-material feeds and progressing stage by stage, where possible, through the process to the final product flow. The required production rate will usually be specified in terms of the product, not the raw-material feeds, so it will be necessary to select an arbitrary basis for the calculations, say 100 kmol/h of the principal raw material. The actual flows required can then be calculated by multiplying each flow by a scaling factor determined from the actual production rate required. 

A main executive program which controls and keeps track of the flow-sheet calculations and the flow of information to and from the sub-routines. 

Developing treatment flow sheets, calculation of chemical addition ratios (and methods of addition), temperatures and retention times. 

CEA s flow sheet calculations (Leybros, 2009) indicate no heat requirements in Bunsen section (the Bunsen reaction is actually quite exothermal), and a small electricity requirement of 4 kj/mol for S02/02 separation through compression. Due to the lack of adequate thermodynamic models, the effect of the presence of impurities in the acid phases is not taken into account. However, CEA, together with the University of Toulouse, has undertaken to build a model capable of describing hydroiodic and sulphuric acid mixtures (Hadj-Kali, 2009b), which will serve as the basis for future evaluations. 

The Bunsen reaction proceeds at 120-130°C, under a pressure of 3 to 6 bars. For flow sheet calculations, product flows were supposed pure (i.e. no iodine trace in sulphuric acid, no sulphur trace in hydroiodic acid). Thermal balance of this section is summarised in Table 1, with values in kj/mol H2. 

In the initial stages of the process design and evaluation, when only a rough, approximate, material balance is required, the use of a full simulation program is often not justified. Simpler programs, which calculate only the material balance, have been developed and these can be used as an aid to manual flow-sheeting calculations. They will be particularly useful if the process involves several recycle streams. 

Some of the full simulation flow-sheeting packages can also be used to calculate the material balance without simultaneous solution of the energy balance, or use of the equipment design routines. They should be used in this mode for the initial, scouting, flow-sheet calculations, to economise on computing costs. 

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