Transportation design trade-offs

A prominent trade-off in fixed-bed reactor design concerns the catalyst particle size. What is the basis for the choice of a certain particle size When the catalyst performance is to be optimized, the application of the Thiele model helps to provide an answer (Figure 7). The Thiele modulus accounts for the competition between the chemical reaction and the limitation of transport of reactants by diffusion in a porous catalyst particle. It is defined as the square root of the ratio of the characteristic diffusion time fo = L /D and the characteristic reaction time (r. For a... 

Capacity Requirements The operating capacity of a transfer station must be such that collection vehicles do not have to wait long to unload. In most cases, it will not be cost-effective to design the station to handle the ultimate peak number of hourly loads. An economic trade-off analysis should be made between the annual cost for the time spent by the collection vehicles waiting to unload against the incremental annual cost of a larger transfer station and/or the use of more transport equipment. Because of the increased cost of transport equipment, a trade-off analysis must also be made between the capacity of the transfer station and the cost of the transport operation, including both equipment and labor components. 

To maximize the reaction rate in the fixed-bed reactor, rj should be equal to one. In order to do this, smaller particles are necessary (see Chapter 6). As the catalyst particles get smaller, the pressure drop along the reactor increases. (For example, ponder the differences in how difficult it is to have water flow through a column of marbles versus a column of sand.) Thus, a major consideration in the design of a fixed-bed reactor is the trade-off between pressure drop and transport limitations of the rate (illustrated in Figure 10.1.2). A practical design typically involves 17, 1. Methodologies used to describe fixed-bed reactors given this situation are outlined in the next section. 

The catalyst layers need to be designed to generate high rates of the desired reactions and minimize the amount of catalyst necessary for reaching the required levels of power output. An ideal catalyst layer should maximize the active surface area per unit mass of the electrocatalyst, and minimize the obstacles for reactant transport to the catalyst, for proton transport to exact positions, and for product removal from the cell these requirements entail an extension of the three-phase boundary. In general, individual property specifications should be a compromise between conflicting requirements. The catalyst layer structure should be optimized with respect to the interactions between components, with trade-offs between several effects. 

A manager must consider many trade-offs during network design. For example, building many facilities to serve local markets reduces transportation cost and provides a fast response time, but it increases the facility and inventory costs incurred by the firm. 

Identify trade-offs that shippers need to consider when designing a transportation network. 

The trade-off between transportation and inventory costs is significant when designing a supply chain network. Two fundamental supply chain decisions involving this trade-off are... 

Identify trade-offs that shippers need to consider when designing a transportation network. When designing transportation networks, shippers need to consider the trade-offs among transportation cost, inventory cost, operating cost, and customer responsiveness. The supply chain goal is to minimize the total cost while providing the desired level of responsiveness to customers. 

Design Constraints Using Extremes to Illustrate Trade-offs Often in engineering systems the basic frade-offs can be best illustrated by consideration of extreme cases. Consider the generic fuel cell channel design with typical geometric parameters shown in Figure 6.34. The lands are an obstruction for mass transport that can potentially cause dead zones where reactant is unavailable. In the extreme case, the entire flow channel would be land, and no reaction could take place. 

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