Impediments, identifying

From a computational viewpoint, the presence of recycle streams is one of the impediments in the sequential solution of a flowsheeting problem. Without recycle streams, the flow of information would proceed in a forward direction, and the cal-culational sequence for the modules could easily be determined from the precedence order analysis outlined earlier. With recycle streams present, large groups of modules have to be solved simultaneously, defeating the concept of a sequential solution module by module. For example, in Figure 15.8, you cannot make a material balance on the reactor without knowing the information in stream S6, but you have to carry out the computations for the cooler module first to evaluate S6, which in turn depends on the separator module, which in turn depends on the reactor module. Partitioning identifies those collections of modules that have to be solved simultaneously (termed maximal cyclical subsystems, loops, or irreducible nets). 

In recent years, the search for superlubric materials has become a subject of practical importance, which may be most evident in the development of miniaturized systems, where nanoscopic surfaces slide past one another. In many cases, friction and wear are the main impediments to miniaturizing such devices even further. Identifying or designing superlubric materials may... 

Once a correct sample has been defined, it must be correctly taken. An extraction error occurs if the sample that has been identified cannot be obtained. In other words, a delimitation error may be avoided by defining a correct boundary for the sample, but if it cannot actually be recovered, then an extraction error is incurred. One of the main impediments to extracting the defined sample is the equipment used. It is especially difficult to sample material in three dimensions such as that in a tank, rail car, landfill, or 100 g of powder in the lab. To obtain a random sample, the bottom of a container as weU as the top must be accessible. While sampling from the top is generally easy and from the bottom difficult, getting a theoretically predefined sample of material from the middle is virtually impossible For liquids, dropping a bottle to the desired depth of a large tank or drum is one way to approximate this, but for solids no practical technique that is also theoretically sound has been developed. 

Another aspect of handling combinatorial chemicals versus natural products is tidiness. If one looks at a scatterplot of activity from an assay for combinatorial chemicals, the data look relatively tidy (Figure 1). TTie majority of samples demonstrate no appreciable activity, and it is not difficult to identify Wts. A scatterplot for natural product extracts in the same assay will show a broader range of activity (Figure 2). This breadth in assay response is proportional to the greater breadth of chemical diversity inherent in the natural product mixtures. It seems logical that the greater breadth of activity would be desirable, but this is frequently perceived to be an impediment because the HTS interpretative software is biased toward a one-to-one, sample-to-structure, system. 

During the first five years of the program the effectiveness, safety, economics, reliability, and major environmental impact of reference processes will be evaluated for full scale application of a selected process. If any impediments to full scale application are identified as a result of these studies, they will be evaluated and further experiments performed to find solutions during the process scale-up phase. 

Multidrug resistance is a serious impediment to improved healthcare. Multidrug resistance is most frequently due to active transporters, such as the P-glycoprotein (ABCBl) identified 30 years ago, that pump a broad spectrum of chemically distinct molecules out of the cells, including antibiotics, antimalarials and cancer chemotherapeutics in humans. ... 

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