Define a Base Case

The first step for tower performance evaluation is to simulate the original tower design, because it is uneommon that the original tower datasheets are unavailable or 

Energy and Process Optimization for the Process Industries, First Edition. Frank (Xin X.) Zhu. 

Once the process simulation is developed, it is desirable to verify the simulation fidelity using different process conditions. The predicted product rates and purity and compositions as well as key operating parameters such as reflux rate and reboiling/condensing duties can then be compared with measurement. In some cases, performance tests are required to gather key data to compare with simulation for the accuracy and reliability of the simulation. To do this, performance tests must be conducted under steady and smooth conditions to mimic steady state operations. 

If the simulation fidelity is proven to be sufficient enough, it is ready to move to the next task, which is evaluation of the tower performance, because the purpose of reproducing the original design data is to understand the tower hydraulic and thermal performances of the base case and use the well defined base case to conduct what if analysis in order to identify improvements. 

TABLE 13.1. Major Data Set for a Heat Pumped C3 Splitter 

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Defining a base case is to determine the base case operation of the tower of interest. This requires extracting two kinds of data. One kind is process data in terms of feed and product conditions, such as flows and compositions, while the other is tower operating data including temperature, pressure, and reflux rate. The former defines the mass and composition balances and the latter sets the heat balance around the tower with Table 13.2 giving such an example of the C2 splitter column. 

The basic approach then consists of defining a base case, usually the design case, to firm up feed-tray location (number of trays above and below the feed tray), reflux ratio, and boilup ratio. With these in hand, we may calculate the effect of changing reflux ratio, and so forth. 

The numerical investigation by Dukovic [63] centered on a base case of copper electrodeposition into a 10-micron cavity defined in a 5-micron resist wall. The following conditions were assumed a current density of 50 mA/cm, a diffusion boundary layer extending 40 microns beyond the resist layer, a Cu2+ concentration of 100 mM in the plating bath, a Cu " diffusion coefficient of 5.2 X 10cm /s, an exchange current density of 1 mA/cm, transfer coefficients of = 0.5 and Oj = 1.5, and a bath conductivity of 0.5 2 cm . 

In this case ethylate ions are formed instead of hydroxyl ions. Evidently we limit ourselves to water as a solvent when we define a base as a substance which yields hydroxyl ions in solution. Furthermore we should need a separate definition for each solvent. This difficulty is eliminated by the terminology of BrOnsted, for his method of expression is more general. Thus the dissociation of ammonia in water is the analogue of the process in alcohol, as is shown by... 

An example of this process is illustrated in Figure 7.2. In this example, an operation was evaluated with a base case risk level of I, or a higher risk that required consideration of risk reduction options. Three risk mitigation options were developed and evaluated. As can be seen, one option reduced the consequence (Option 1), one option reduced the likelihood (Option 2), and one option reduced both consequence and likelihood (Option 3). Option 3 resulted in the greatest risk reduction benefit. While the options that reduce only consequence or likelihood did reduce risk (movement across or down the matrix), the resulting level of risk, as defined on the risk matrix, did not change from risk level I. Therefore, purely on a benefit basis, Option 3 provided the greatest risk reduction. 

The chapter is organized as follows first a base case is defined and the costs per ton of CO2 emissions avoided are calculated. Subsequently, a sensitivity analysis of some key process parameters is discussed. Finally, the results are compared to the results of economic studies on CO2 capture via amine scrubbing [23] and membrane technology [20]. 

An important aspect of defining the base case is gathering aU the important data for the material and heat balances in one single sheet for a tower of interest. It would be very informative to have important mass flows, temperature, pressure, and composition data in one table so that a snap shot of the tower performance can be seen at a glance. Such an example is a heat pumped C3 splitter shown in Figure 13.1 and Table 13.1. In building such a table, it is a good practice to include the tag number of the instrument for each parameter so that the data can be retrieved readily from the historian to produce the table with snapshots of different... 

Because the goal of optimization is to improve the process, it is essential that one start from a defined process, that is, a base case. 

Using a base-case ratio often reduces the need for knowing actual values of physical and transport properties (physical properties refer to thermodynamic and transport properties of fluids), equipment, and equipment characteristics. The values identified in the ratios fall into three major groups. They are defined below and applied in Example 17.2. 

Unit for temperature The SI unit system defines a base unit for temperature kelvin with the dimension ( ). The physical origin of this definition is the thermodynamic temperature scale where the temperature unit is defined as being proportional to the kinetic mean energy of the molecules in a system of matter. Therefore, for dimensional analysis it is admissible to use the same unit for temperature quantities as for energy, if this is expedient. In this case, the dimension for temperature is (L MT ). 

Based on this, it is normal to define a mass transfer factor consistent with this case, analogous with Eq. (4.245) ... 

The composition of a production fluid is usually not well defined. In most cases, only a specific gravity is known. Compositions are important to the prediction of physical properties of the fluid as it undergoes phase changes. Estimations can be made based only upon specific gravity, however, for good reliability, molecular compositions should be used wlien available. 

It, therefore, appears that the equilibrium approximation is a special case of the steady-state approximation, namely, the case i > 2- This may be, but it is possible for the equilibrium approximation to be valid when the steady-state approximation is not. Consider the extreme but real example of an acid-base preequilibrium, which on the time scale of the following slow step is practically instantaneous. Suppose some kind of forcing function were to be applied to c, causing it to undergo large and sudden variations then Cb would follow Ca almost immediately, according to Eq. (3-153). The equilibrium description would be veiy accurate, but the wide variations in Cb would vitiate the steady-state description. There appear to be three classes of practical behavior, as defined by these conditions ... 

Figure 9-13 shows the Ynryn P ° obtained for the first case where four regions are defined a region of complete separation, two regions where only one outlet stream is 100 % pure and a last region where neither of them is 100 % pure. The closed circles are numerical results based on the equivalence between the TMB and the SMB the thick lines connect those results. The thin line in Fig. 9-13 has two branches. The diagonal 7 -7 corresponds to zero feed flow rate therefore, 7 must be higher than Yn- The horizontal branch Ym corresponds to zero raffinate flow rate in this case, the extract flow rate is 25.09 mL min k... 

Ritchie and Sager (124) distinguish three types of reaction series according to whether the Hammett equation or the isokinetic relationship is obeyed, or both. The result that the former can be commonly valid without the latter seems to be based on previous incorrect statistical methods and contradicts the theoretical conclusions. Probably both equations are much more frequently valid together than was anticipated. The last case, when the isokinetic relationship holds and the Hammett equation does not, may be quite common, of course, and has a clear meaning. Such a series meets the condition for an extrathermo-dynamic treatment when enough experimental material accumulates, it is only necessary to define a new kind of substituent constant. 

Membrane reactors are defined here based on their membrane function and catalytic activity in a structured way, predominantly following Sanchez and Tsotsis [2]. The acronym used to define the type of membrane reactor applied at the reactor level can be set up as shown in Figure 10.4. The membrane reactor is abbreviated as MR and is placed at the end of the acronym. Because the word membrane suggests that it is permselective, an N is included in the acronym in case it is nonpermselective. When the membrane is inherently catalytically active, or a thin catalytic film is deposited on top of the membrane, a C (catalytic) is included. When catalytic activity is present besides the membrane, additional letters can be included to indicate the appearance of the catalyst, for example, packed bed (PB) or fluidized bed (FB). In the case of an inert and nonpermselective... 

The competition model and solvent interaction model were at one time heatedly debated but current thinking maintains that under defined r iitions the two theories are equivalent, however, it is impossible to distinguish between then on the basis of experimental retention data alone [231,249]. Based on the measurement of solute and solvent activity coefficients it was concluded that both models operate alternately. At higher solvent B concentrations, the competition effect diminishes, since under these conditions the solute molecule can enter the Interfacial layer without displacing solvent molecules. The competition model, in its expanded form, is more general, and can be used to derive the principal results of the solvent interaction model as a special case. In essence, it seems that the end result is the same, only the tenet that surface adsorption or solvent association are the dominant retention interactions remain at variance. 

This electrophile/nucleophile dichotomy can be looked upon as a special case of the acid/base idea. The classical definition of acids and bases is that the former are proton donors, and the latter proton acceptors. This was made more general by Lewis, who defined acids as compounds prepared to accept electron pairs, and bases as substances that could provide such pairs. This would include a number of compounds not previously thought of as acids and bases, e.g. boron trifluoride (39),... 

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