Continuous separations, process design cycles

The growth in both variety and scale of gas-pliase adsorption separation processes, particularly since 1970, is due in part to continuing discoveries of new, porous, high-surface-area adsorbent materials (particularly molecular sieve zeolites) and, especially, to improvements in the design and modification of adsorbents. These advances have encouraged parallel inventions of new process concepts. Increasingly, the development of new applications requires close cooperation in adsorbent design and process cycle development and optimization. 

To provide cost-effective safety engineering, the system and safety analysis and design process needs to consider the humans in systems—including those that are not directly controlling the physical processes—not separately or after the fact but starting at concept development and continuing throughout the life cycle of the system. 

Reliability takes into account the on-stream factors which can cause unscheduled shut-downs. Membrane systems are extremely reliable with respect to the on-stream factor. The membrane separation process is continuous and few control components can cause a shut-down. Typically, the response to unscheduled shut-downs is rapid for GS whereas PSA systems are moderately reliable owing to the numerous valves associated with the process which can cause unexpected shut-downs. The new PSA are designed with alternative modes of operation, in which 100% of design capacity can be achieved while by-passing any failed valve or instrument, with only a slight recovery loss. Failures are automatically detected and by-passed by the microprocessor-based control system. Flowever, stronger and periodic control cycles are required. The cryogenic process is considered by refiners to be less reliable than PSA or membrane... 

TSA and PSA processes are, by virtue of the distinct adsorption and regeneration components of the cycle, not continuous processes, although a continuous flow of product may be achieved by careful design and bed utilization. Moving bed and simulated moving bed processes are, however, by their very nature truly continuous. Examples of these are given in Chapter 7, but here it suffices to say that a number of continuous commercial processes for the separation of aromatic mixtures, the separation of n-parafhns from branched and cycloalkanes, the production of olefins from olefin and paraffin mixtures and the isolation of fructose from corn syrup, have been in operation since the early 1980s. 

---