Necessary

The accuracy of our calculations is strongly dependent on the accuracy of the experimental data used to obtain the necessary parameters. While we cannot make any general quantitative statement about the accuracy of our calculations for multicomponent vapor-liquid equilibria, our experience leads us to believe that the calculated results for ternary or quarternary mixtures have an accuracy only slightly less than that of the binary data upon which the calculations are based. For multicomponent liquid-liquid equilibria, the accuracy of prediction is dependent not only upon the accuracy of the binary data, but also on the method used to obtain binary parameters. While there are always exceptions, in typical cases the technique used for binary-data reduction is of some, but not major, importance for vapor-liquid equilibria. However, for liquid-liquid equilibria, the method of data reduction plays a crucial role, as discussed in Chapters 4 and 6. 

The data base contains provisions for a simple augmentation by up to eight additional compounds or substitution of other compounds for those included. Binary interaction parameters necessary for calculation of fugacities in liquid mixtures are presently available for 180 pairs. 

If we vary the composition of a liquid mixture over all possible composition values at constant temperature, the equilibrium pressure does not remain constant. Therefore, if integrated forms of the Gibbs-Duhem equation [Equation (16)] are used to correlate isothermal activity coefficient data, it is necessary that all activity coefficients be evaluated at the same pressure. Unfortunately, however, experimentally obtained isothermal activity coefficients are not all at the same pressure and therefore they must be corrected from the experimental total pressure P to the same (arbitrary) reference pressure designated P. This may be done by the rigorous thermodynamic relation at constant temperature and composition ... 

In the calculation of vapor-liquid equilibria, it is necessary to calculate separately the fugacity of each component in each of the two phases. The liquid and vapor phases require different techniques in this chapter we consider calculations for the vapor phase. 

Witn the advent of electronic computers, it is no longer necessary to make drastic simplifying assumptions to reduce the... 

To use Equation (13), it is first necessary to calculate the true fugacity coefficient (ft. This calculation is achieved by utilizing the Lewis fugacity rule... 

Since parameters for many fluids of interest are not given in this monograph, it may be necessary to estimate the required parameters T, P, R, y, and n. ... 

When no experimental data at all are available, activity coefficients can sometimes be estimated using the UNIFAC method (Fredenslund et al., 1977a, b). However, for many real engineering problems it is often necessary to obtain new experimental data. 

In some cases, the temperature of the system may be larger than the critical temperature of one (or more) of the components, i.e., system temperature T may exceed T. . In that event, component i is a supercritical component, one that cannot exist as a pure liquid at temperature T. For this component, it is still possible to use symmetric normalization of the activity coefficient (y - 1 as x - 1) provided that some method of extrapolation is used to evaluate the standard-state fugacity which, in this case, is the fugacity of pure liquid i at system temperature T. For highly supercritical components (T Tj,.), such extrapolation is extremely arbitrary as a result, we have no assurance that when experimental data are reduced, the activity coefficient tends to obey the necessary boundary condition 1... 

In typical situations, we do not have the necessary experimental data to find constants b... To obtain these constants, we need experimental vapor-liquid equilibria (i.e. activity coefficients) as a function of temperature. 

In many process-design calculations it is not necessary to fit the data to within the experimental uncertainty. Here, economics dictates that a minimum number of adjustable parameters be fitted to scarce data with the best accuracy possible. This compromise between "goodness of fit" and number of parameters requires some method of discriminating between models. One way is to compare the uncertainties in the calculated parameters. An alternative method consists of examination of the residuals for trends and excessive errors when plotted versus other system variables (Draper and Smith, 1966). A more useful quantity for comparison is obtained from the sum of the weighted squared residuals given by Equation (1). 

If the reaction involves more than one feed, it is not necessary to operate with the same low conversion on all the feeds. Using an excess of one of the feeds enables operation with a relatively high conversion of other feed material, and still inhibits series reactions. Consider again the series reaction system from Example 2.3 ... 

The solid particles are fluidized by air and fuel, which are fed to the bed and burnt to produce the high temperatures necessary for the reaction. 

Having made an initial specification for the reactor, attention is turned to separation of the reactor effluent. In addition, it might be necessary to carry out separation before the reactor to purify the feed. Whether before or after the reactor, the overall separation task normally must be broken down into a number of intermediate separation tasks. The first consideration is the choice of separator for the intermediate separation tasks. Later we shall consider how these separation tasks should be connected to the reactor. As with reactors, we shall concentrate on the choice of separator and not its detailed sizing. 

If the solvent is volatile, there will be some loss with the vapor. This should be avoided if the solvent is expensive and/or environmentally harmful by using a condenser (refrigerated if necessary) on the vapor leaving the absorber. 

The recycling of material is an essential feature of most chemical processes. Therefore, it is necessary to consider the main factors which dictate the recycle structure of a process. We shall start by considering the function of process recycles and restrict consideration to continuous processes. Later the scope will be extended to include batch processes. 

To calculate the vapor load for a single column of a sequence, start by assuming a feed condition such that q can be fixed. Initially assume saturated liquid feed (i.e., q = 1). Equation (5.1) can be written for all NC components of the feed and solved for the necessary values of 0. There are (JVC - 1) real positive values of 0 which satisfy Eq. (5.1), and each lies between the a values of the... 

The final restriction of simple columns stated earlier was that they should have a reboiler and a total condenser. It is possible to use materials fiow to provide some of the necessary heat transfer by direct contact. This transfer of heat via direct contact is known as thermal coupling. 

Analogous effects are caused by the inappropriate use of utilities. Utilities are appropriate if they are necessary to satisfy the enthalpy imbalance in that part of the process. Above the pinch in Fig. 6.7a, steam is needed to satisfy the enthalpy imbalance. Figure 6.86 illustrates what happens if inappropriate use of utilities is made and some cooling water is used to cool hot streams above the pinch, say, XP. To satisfy the enthalpy imbalance above the pinch, an import of (Q mjj,+XP) is needed from steam. Overall, (Qcmin+AP) of cooling water is used. ... 

These rules are both necessary and sufficient to ensure that the target is achieved, providing the initialization rule is adhered to that no individual heat exchanger should have a temperature difference smaller than... 

FIgur 7.4 If film transfer coefficients difier significantly, then nonvertical h t transfer is necessary to achieve the minimum area. (Reprinted from Linnhoff and Ahmad, Cost Optimum Heat Exchanger Networks I. Minimum Energy and Capital Using Simple Models for Capital Cost," Computers Chem. Engg., 7 729, 1990 with permission from Elsevier Science, Ltd.)... 

Increasing the chosen value of process energy consumption also increases all temperature differences available for heat recovery and hence decreases the necessary heat exchanger surface area (see Fig. 6.6). The network area can be distributed over the targeted number of units or shells to obtain a capital cost using Eq. (7.21). This capital cost can be annualized as detailed in App. A. The annualized capital cost can be traded off against the annual utility cost as shown in Fig. 6.6. The total cost shows a minimum at the optimal energy consumption. 

When synthesizing a fiowsheet, the designer should consider carefully the problems associated with operation under extreme conditions. Attenuation will result in a safer plant, providing the attenuation does not increase the inventory of hazardous materials. If the inventory does not increase, then attenuation not only will make the process safer but also will make it cheaper, since cheaper materials of construction and thinner vessel walls can be used and it is not necessary to add on so much protective equipment. 

As the design progresses, it is necessary to carry out hazard and operability studies. These are generally only meaningful when the design has been progressed as far as the preparation of detailed flowsheets and are outside the scope of this text. 

In both batch and continuous processes, it may be necessary to clean equipment to prevent contamination of new product. Materials used for equipment cleaning often cannot be recycled, leading to waste. 

When the bed is saturated, regeneration of the adsorbent is necessary. Carbon beds are typically regenerated with steam, hot air, or a combination of vacuum and hot gas. 

Incineration. Incinerators were discussed in Sec. 11.1. When incinerators are used to treat gaseous pollutants in relatively low concentration, auxiliary firing from fuel or other waste material normally will be necessary. The capital and operating costs may be high. In addition, long duct lines are often necessary. 

If steam is used as stripping agent, either live steam or a reboiler can be used. The use of live steam increases the effluent volume. The volatile organics are taken overhead, condensed, and recycled to the process, if possible. If recycling is not possible, then further treatment or disposal is necessary. 

If air is used as stripping agent, further treatment of the stripped material will be necessary. The gas might be fed to an incinerator or some attempt made to recover material by use of adsorption. 

The following steps are necessary in establishing a heat integrated distillation sequence... 

Figure 16.4a shows the grid diagram with a CP table for design above the pinch. Cold utility must not be used above the pinch, which means that hot streams must be cooled to pinch temperature by recovery. Hot utility can be used, if necessary, on the cold streams above the pinch. Thus it is essential to match hot streams above the pinch with a cold partner. In addition, if the hot stream is at pinch conditions, the cold stream it is to be matched with must also be at... 

Given a network structure, it is possible to identify loops and paths for it, as discussed in Chap. 7. Within the context of optimization, it is only necessary to consider those paths which connect two different utilities. This could be a path from steam to cooling water or a path from high-pressure steam used as a hot utility to low-pressure steam also used as a hot utility. These paths between two different utilities will be designated utility paths. Loops and utility paths both provide degrees of freedom in the optimization. ... 

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