Specification product flow

The focus of the examples given in this chapter is clearly on micro reactors for chemical processing in contrast to p-TAS or Lab-Chip systems for bioanalytical applications. In the latter microfluidic systems, the fluidic requirements are somehow different from those in micro reactors. Typically, throughput plays only a minor role in p-TAS systems, in contrast to micro reactors, where often the goal is to achieve a maximum molar flux per unit volume of a specific product. Moreover, flow control plays a much greater role in p-TAS systems than in micro reactors. In... 

The amount of solvent needed for the purification of a unit amount of target compound is conveniently described by the term specific production. Thus, the minimum solvent consumption can be determined for a given purification. The amount of solvent pumped through the column during one cycle is proportional to the mobile phase flow rate and the cycle time. The amount of purified product made in one cycle is the product of the amount injected and the recovery yield. Thus, the specific production can be written as [43]... 

In Eq. (8.11), Sf is the substrate feed rate, and FSf consequently is the molar flow into the fermenter (there is no flow out of the fermenter). Yx/s and YpyS are the two empirical yield coefficients of cell X or product P on substrate S [g (g substrate)-1] and qp is the specific production formation rate [g product (g cells)-1 h-1] (qp-X = rp, the product formation rate [g product h 11). 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

Flow rate ratio of atomizing air to flue gas Specific productivity of the reactor Hydraulic resistance of the reactor Desulfurization efficiency, %... 

Inspecting Equation (5.29), we notice that three of the state variables (namely, Mr, My, and Ml) are material holdups, which act as integrators and render the system open-loop unstable. Our initial focus will therefore be a pseudo-open loop analysis consisting of simulating the model in Equation (5.29) after the holdup of the reactor, and the vapor and liquid holdup in the condenser, have been stabilized. This task is accomplished by defining the reactor effluent, recycle, and liquid-product flow rates as functions of Mr, My, and Ml via appropriate control laws (specifically, via the proportional controllers (5.42) and (5.48), as discussed later in this section). With this primary control structure in place, we carried out a simulation using initial conditions that were slightly perturbed from the steady-state values in Table 5.1. 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

Only a few modifications of the algorithm were required to make it applicable to absorption and reboiled absorption. The changes were mainly in the handling of the enthalpy and total mass balance equations to accommodate different specification combinations involving the reflux, heat duties, and top and bottom product flow rates. The results of two example problems, one each for absorption and reboiled absorption, are shown in Table II. 

The treatment of conflicting specifications leading to convergence problems has been developed elsewhere [7]. For example, this situation arrives when the distillation columns are specified by fixed product flow rates. These specifications, correct for standalone columns, lead to nonconvergence when the units are placed in recycles. The explanation is that during the iterative solution it is impossible to... 

Composition specification. If one recovery or one product flow is specified, the concentration of one component either in the distillate or in the bottom (but not both) can be specified. If neither a recovery nor a product rate is specified, the concentration of one component in the distillate and one component in the bottom can be specified. 

Heat addition or removal. For each point of heat addition or removal, an additional specification is required. This specification is usually a heat duty or an internal product flow. 

Fixing S is equivalent to setting a composition specification. At a fixed S, each D/F in Fig. 3.1a specifies a recovery and a composition. These specifications are sufficient for setting all product flows and purities (Sec. 3.1.1). Since D/F fixes distillate and bottoms flows and purities, each D/F also fixes the total product value. The total (distillate plus bottom product) values can therefore be calculated and plotted against D/F at fixed S (Fig. 3.2). 

Section 3.1.1 states that in a process design, a separation is specified in terms of purities and product flows. For a simple column, two specifications are made and at least one must be a purity. Section 3.1.1 also states that the purity specification can be substituted by a physical property which is a function of the purity or composition, while a product flow can be substituted by a recovery specification. 

For a column with side products, the number of specified variables increases with each side product. In most methods, the product flow rate is specified for each side product, but sometimes it is possible to specify tbe purity of a side product. For columns with interreboilers or inteitandensers, the number of specified variables increases by the number of these exchangers. Usually, the interreboilers or intercon-denser duties are specified, but in seme methods, these duties may be allowed to vary to meet a product specification.. With these complex columns, inconsistent specifications are a m jor pitfall and simpler specifications are preferred. 

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