Flowsheeting results

Fig. 29. Flowsheet resulting from use of recycle mixing to improve separations.

For the design study of a particular separation system, we typically start by using the Aspen built-in parameters of a suitable physical property model. The phase equilibrium behavior predicted by the Aspen built-in parameters should be compared with experimental data for validation purpose. It is obvious that an inaccurate description of the phase equilibrium behavior of a separation system will give flowsheet results that do not match the results of the true system. The worst case may be a failure of the separation task in the proposed design flowsheet. Thus, the validation stage is important before doing any design study. The experimental data that can typically be found in hterature include the Txy and yx data, binary and ternary LLE data, VLLE data, and azeotropic information. 

The above design flowsheet results in a large flow circulated between the heterogeneous azeotropic column and the recovery column. The main reason (as can be seen in... 

Figure 9.20 provides the flowsheet results from the Aspen Plus simulation, which are in good agreement with the results of Al-Arfaj and Luyben. Figures 9.21 and 9.22 give composition and temperature profiles. 

Figure 17.8 gives the process flowsheet, and Table 17.3 gives parameter values. This flowsheet results in a TAC of 1,576,000 for the reactive distillation column alone, which is 620% of that of the type 1 system. The reason is that, in order to achieve high conversion, the reflux to feed ratio in the reactive distillation is extremely high (20.5) and the vapor boilup to feed ratio is 19.1. This corresponds to a column of 30... 

Where possible, introducing extraneous materials into the process should be avoided, and a material already present in the process should be used. Figure 4.6h illustrates use of the product as the heat carrier. This simplifies the recycle structure of the flowsheet and removes the need for one of the separators (see Fig. 4.66). Use of the product as a heat carrier is obviously restricted to situations where the product does not undergo secondary reactions to unwanted byproducts. Note that the unconverted feed which is recycled also acts as a heat carrier itself. Thus, rather than relying on recycled product to limit the temperature rise (or fall), simply opt for a low conversion, a high recycle of feed, and a resulting small temperature change. 

When synthesizing a flowsheet, the designer should consider carefully the problems associated with operation under extreme conditions. Attenuation will result in a safer plant, providing the attenuation does not increase the inventory of hazardous materials. 

The choice of the appropriate azeotropic distillation method and the resulting flowsheet for the separation of a particular mixture are strong functions of the separation objective. For example, it may be desirable to recover all constituents of the original feed mixture as pure components, or only some as pure components and some as azeotropic mixtures suitable for recycle. Not every objective may be obtainable by azeotropic distillation for a given mixture and portfolio of candidate entrainers. 

Size reduction uses either hammer mills or blade cutters (shredders). Hammer mills are likely to break glass into finer sizes maldng it hard to separate. Better results may be obtained in a flowsheet where size reduction follows separation (Savage, Seminar on the Apphcation... 

The result of process synthesis is a flowsheet which represents the configuration of the various pieces of equipment and their interconnection. Next, it is necessary to analyze the performance of this flowsheet. 

The moment-based method has been tested on eomplieated partieulate proeess flowsheets and eompared with sophistieated diseretization methods to reveal that moment based method gives aeeeptably aeeurate results with eonsiderably less eomputer time. 

Chemical engineering performance design for specific items of equipment required for a flowsheet, and mechanical interpretation of this to a practical and reasonable specification. Here the process requirements are converted into hardware details to accomplish the process end results at each step in the product production process. 

The flowsheet shown in the introduction and that used in connection with a simulation (Section 1.4) provide insights into the pervasiveness of errors at the source, random errors are experienced as an inherent feature of every measurement process. The standard deviation is commonly substituted for a more detailed description of the error distribution (see also Section 1.2), as this suffices in most cases. Systematic errors due to interference or faulty interpretation cannot be detected by statistical methods alone control experiments are necessary. One or more such primary results must usually be inserted into a more or less complex system of equations to obtain the final result (for examples, see Refs. 23, 91-94, 104, 105, 142. The question that imposes itself at this point is how reliable is the final result Two different mechanisms of action must be discussed ... 

In this example the flowsheet of the EDTA process has been modified by removing the following valves vlO, vll, vl3, and v20. The result of the modifications is shown in Fig. 17. 

When the flowsheet is complex and involves numerous process steps, a low-energy efficiency will result. The metals titanium and magnesium are difficult to reduce, and their production involves chloride intermediates which are produced from the oxide raw materials. Titanium requires magnesium or sodium as the reducing agent, and these metals are themselves obtained by electrolytic processes which are energy-intensive. Another feature which may add to the complexity of the process flowsheet is the need to separate impurities and by-products using special processes this is the case with copper, lead, and nickel. 

The separation in Figure 12.21 can be carried out in an alternative sequence. Sketch the mass balance for the alternative sequence in a triangular diagram and the resulting flowsheet. 

The use of excess reactants, diluents or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes the recycling of unwanted byproduct to the reactor can inhibit its formation at source. If this can be achieved, it improves the overall usage of raw materials and reduces effluent disposal problems. However, the recycling results in an increase of some costs. 

A flowsheet for the Wellman-Lord process is shown in Figure 25.26. Again the gas stream with S02 enters a scrubber into which is sprayed a sodium sulfite solution. This then goes to an evaporator/crystallizer to crystallize out the resulting sodium bisulfite, which converts the sodium bisulfite back to sodium sulfate, releasing the S02. The crystals are dissolved in water and recycled to the scrubber. The effect of the Wellman-Lord process is to produce a concentrated S02 stream from a dilute S02 stream. The resulting concentrated S02 still needs to be treated. 

Secondly, the minimum amount of intermediate storage is determined with and without the PIS operational philosophy. In both cases the production goal was set to that which was achieved when the model was solved with infinite intermediate storage. In the illustrative example a 20% reduction in the amount of intermediate storage is achieved. The design model is an MINLP model due to the non-linear capital cost objective function. This model is applied to an illustrative problem and results in the flowsheet as well as determining the capacities of the required units. 

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