Purging flow rate

The air process has similar purity requirements to the oxygen process. The ethane content of ethylene is no longer a concern, due to the high cycle purge flow rate. Air purification schemes have been used to remove potential catalyst poisons or other unwanted impurities ia the feed. 

The concept of local age and local purging flow rate was introduced in refs. 39 and 41. These parameters were first studied in connection with CFD in refs. 32 and 42. Local age at a point is understood to mean the time that... 

TABLE 11.3 The Purging Flow Rate for the Six Regions Defined in Fig. 11.14 ... 

Davidson, L., Olsson, E. Calculation of age and local purging flow rate in rooms. Bldg. Enin-ron., vo). 22, pp. 111-127, 1987. 

Peng, S. H., Davidson, L, Towards the determination of regional purging flow rate. Build. Environ., vol. 32, pp. 513-525, 1997. 

Sandberg, M, Ventilation effectiveness and purging flow rate—A review. In Int. Symp. on Room Air Convection and Ventilation Effectiveness, Tokyo, 1992. 

Itoh (1987) These results arc achieved at high feed/purging flow rate ratios. 

In this study the ratio of the particle sizes was set to two based on the average value for the two samples. As a result, if the diffusion is entirely controlled by secondary pore structure (interparticle diffusion) the ratio of the effective diffusion time constants (Defl/R2) will be four. In contrast, if the mass transport process is entirely controlled by intraparticle (platelet) diffusion, the ratio will become equal to unity (diffusion independent of the composite particle size). For transient situations (in which both resistances are important) the values of the ratio will be in the one to four range. Diffusional time constants for different sorbates in the Si-MCM-41 sample were obtained from experimental ZLC response curves according to the analysis discussed in the experimental section. Experiments using different purge flow rates, as well as different purge gases... 

Diagrams showing the pressures, feed, product and purge flow rates for each column during a cycle are presented by Weaver and Hamrin (7J. 

In practical applications, for economic and operational reasons, the flow rate of the purge stream is very small compared with the throughput of the process. Hence, we can assume that the ratio of the purge flow rate to the feed flow rate under steady-state conditions is very small, i.e., Ps/FotS = e 1. We will also consider that the mole fraction of the impurity in the feed (and, consequently, the rate at which the impurity enters the system) is very small, or yio = /3ie, where fa is an 0(1) quantity. 

Let Fq denote the feed flow rate to the first unit, Fio the rate at which the impurity is input to the system,1 Fj, j = 1,..., N, the outlet flow rate from the jth unit, Fr the recycle flow rate, and Fp the purge flow rate. Also, let W, i = 1,..., C — 1 and N denote the net rates at which the th component and, respectively, the impurity, are separated from the recycle loop. 

This is consistent with the fact that these constraints correspond to the limit as the purge flow rate and the inflow of the impurity become zero. In this limit, the number of moles of the impurity leaving the reactor is identical to that leaving the condenser, hence the redundant constraint. Note also that, in the fast time scale, only the flow rates F, R, and L affect the dynamics and can be used for addressing control objectives such as stabilization of holdups, production rate, and product quality. The purge flow rate has, of course, no effect on the dynamics in this fast time scale. 

Figure 4.11 Evolution of (a) the condenser liquid holdup and (b) the purge flow rate for a 10% increase in the production rate at t = 0.

Also, the inlet flow rate Fio,s of the impurity is comparable in magnitude to the purge flow rate ... 

Equation (5.12) effectively corresponds to the dynamics of the individual process units that are part of the recycle loop. The description of the fast dynamics (5.12) involves only the large flow rates u1 of the recycle-loop streams, and does not involve the small feed/product flow rates us or the purge flow rate up. As shown in Chapter 3, it is easy to verify that the large flow rates u1 of the internal streams do not affect the total holdup of any of the components 1,..., C — 1 (which is influenced only by the small flow rates us), or the total holdup of I (which is influenced exclusively by the inflow Fjo, the transfer rate Af in the separator, and the purge stream up). By way of consequence, the differential equations in (5.12) are not independent. Equivalently, the quasi-steady-state condition 0 = G (x)u corresponding to the dynamical system (5.12) does not specify a set of isolated equilibrium points, but, rather, a low-dimensional equilibrium manifold. 

We proceed with the model reduction using the method developed in Chapter 4, by considering the limiting case of the purge flow rate and the impurity feed being set to zero, i.e., 2 —> 0. In this limit, we obtain a description of the dynamics in the intermediate time scale, that is, the time scale immediately succeeding the fast boundary layer ... 

Each of the reduced-order models derived for the fast, intermediate, and slow dynamics (Equations (5.12), (5.21), and (5.28)) involves only one group of manipulated inputs, namely the large internal flow rates u1, the small flow rates us, and the purge flow rate up, respectively. Thus, control objectives in each of the... 

Notice that the above model is still stiff, due to the presence of the parameter 2. Considering the limit 2 —> 0, corresponding to the absence of the inert component from the feed and a zero purge flow rate, we obtain the following description of the intermediate (process-level) dynamics ... 

Slow Inert levels in the reactor, j/13 Purge flow rate P Nonlinear, model-based... 

Figure 5.18 Closed-loop evolution of (a) the condenser vapor holdup setpoint and (b) the purge flow rate for a 15% increase in the production rate occurring at t = 0, under plant-model parameter mismatch. The reaction rate and the mass-transfer coefficient /Cb in the controller model are assumed to be overestimated by 10% compared with their values in the plant.

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