Basic Process Design

In this chapter some fundamental equations are given that allow a first design of a one-stage membrane separation unit. Questions to be answered are what is the maximum enrichment that can be achieved with a membrane of a given selectivity How is the separation performance influenced by feed and permeate pressure It will be explained why for some applications a high selective membrane will be outperformed by a membrane with, a lower selectivity. 

For the following we look at a simple gas-separation unit with two components, which is illustrated in Fig. 7.10. 

One parameter, which does of course determine the gas enrichment, is the membrane selectivity a, which is a membrane property and defined here as a = P2jP with 2 and as permeability coefficients for gas 2 and 1. 

Equally important are two process parameters the stage cut 6 and the pressure ratio The stage cut is defined as the ratio permeate flow/feed flow and the pressure ratio is the ratio of total feed pressure to total permeate pressure. 

For the sake of simplicity we start with a stage cut close to zero, i.e. there is no concentration difference between feed and retentate. The maximum enrichment of the faster component 2 can now be determined easily. For the maximum enrichment the maximum driving force is needed, i.e. the permeate pressure can be neglected when compared to the feed pressure. The flux of component 1 is proportional to its volume fraction on the feed side, for component 2 we have as an additional factor the membrane selectivity. 

For a laboratory experiment the goal is to achieve an extract containing the required substances. With a more technical approach, the goal is to determine the optimum process parameters, the size of the extracting equipment, and the quantity of solvent needed Tor a certain extraction result. As the technical approach includes the necessary knowledge of laboratory experiments, the technical approach is now considered. This approach is partly empirical. Rigorous models, based on physico-chemical properties, thermodynamics, and related basic physical laws, are fltted to experimental results by introducing coefficients as adjustable parameters. 

An investigation of the extraction is carried out, determining the major influences of the most important parameters on the product specifications. These parameters include the process temperature T, process pressure p, extraction time t, amount of solvent ms, solvent ratio (amount of solvent per unit of time and amount of solid) S, conditions of extract removal (precipitation) from solvent 7p, pp, and eventual pretreatment of solid feed material. In addition, the solubility of the extracted compounds in the solvent as a function of pressure and temperature may be investigated. 

The course of the extraction is an unsteady process for the solid as well as for the solvent. The course of the process can be followed by determining the amount of extract against time of extraction. From these data, more information on the process can be deduced, as discussed below. 

In the second part of the extraction, two effects cause a declining medium concentration of extract in the outflowing solvent  

The extract in the solid substrate near the solid-gas interface is depleted for most of the solid substrate. Transport of the extract within the solid to the interface then adds an additional transport resistance. 

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A Conceptual Design is the starting point for a basic process design (Phase 0) and can be used to prepare both order-of-magnitude and conceptual estimates to support initial business planning decisions. 

These estimates are prepared when the basic process design is essentially complete and the scope of the offsite and waste treatment facilities has been established but not completely defined. Preliminary estimates are normally used to prepare firm project execution plans, to complete the project economics, and, frequently, for appropriation purposes. The Phase 1 design package, described in Chapter 6. contains the necessary information for the preparation of a preliminary estimate. In fact, a thorough Phase 0 package could also be used for this purpose. Appendix L illustrates a preliminary estimate. 

Occupational exposure to lead and other toxic substances can be problematic at the multiple stages of the recycling process. Although much attention has been paid to exposure reduction via ventilation controls or PPE, basic process design is also important. Process modifications may be feasible to eliminate or reduce the generation of emissions, for example ... 

Tab. 7.1 shows the established applications in the field of membrane gas separation. One of the new and currently small appHcations shown in Tab. 7.1 is natural gas dehydration. Problems related to this separation wiU be discussed in the last part of this chapter (basic process design considerations). 

Challenges for Model-Based Life Cycle Inventories and Impact Assessment in Early to Basic Process Design Stages... 

LCI ASPECTS IN EARLY TO BASIC PROCESS DESIGN STAGES... 

The basic process design work is developed, culminating in process flowsheets and process data sheets for plant equipment items. 

The basic process design - essentially the process technology package as previously discussed, plus follow-through activities to assist and confirm its implementation - may be carried out by the client, or may be obtained from a third party. 

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