Low product concentrations

When concentrations of the more volatile component of either product is very low the steps on the McCabe-Thiele diagram become very small and difficult to plot. This problem can be overcome by replotting the top or bottom sections to a larger scale, or on log-log paper. In a log plot the operating line will not be straight and must be drawn by plotting points calculated using equations 11.21 and 11.22. This technique is described by Alieva (1962) and is illustrated in Example 11.2. 

If the operating and equilibrium lines are straight, and they usually can be taken as such when the concentrations are small, the number of stages required can be calculated using the equations given by Robinson and Gilliland (1950). 

Acetone is to be recovered from an aqueous waste stream by continuous distillation. The feed will contain 10 per cent w/w acetone. Acetone of at least 98 per cent purity is wanted, and the aqueous effluent must not contain more than 50 ppm acetone. The feed will be at 20°C. Estimate the number of ideal stages required. 

There is no point in operating this column at other than atmospheric pressure. The equilibrium data available for the acetone-water system were discussed in Chapter 8, Section 8.4. 

Bubble point of feed (interpolated) = 83°C Latent heats, water 41,360, acetone 28,410 J/mol 

Xr = mol fraction of least volatile component at reference point, 

K = equilibrium constant for the least volatile component, s = slope of top operating line. 

---

The third major Hmitation of bioprocesses is the quite low product concentration compared with chemical processes, resulting in high downstream processing costs. This is mainly caused by product inhibition of cell growth and biosynthesis. Physiological improvements in cell growth and product formation only have a limited impact on this aspect. Chemical or directed mutagenesis may provide better chances for improvement. Unfortunately, the molecular mechanisms of product inhibition are not well understood. 

The reaction will proceed toward oxaloacetate formation in the cell if low product concentration is maintained. Oxidation of NADH by the mitochondrial electron transport system and utilization of oxaloacetate in the formation of citrate shifts the malate-oxaloacetate reaction toward oxaloacetate production. 

The productivity of batch cultures in general is limited by the low cell concentration and, consequently, the low product concentration at the end of the process. This is related to the fact that the initial concentration of substrates must be relatively low to avoid cell death due to osmotic effects (Morbach and Kramer, 2003), as well as to the fact that the cells are submitted to environmental conditions that keep changing during the whole process. 

P) atoms with 1-alkynes and from the reactions of CH2 with 0(3P), 02 and C02. These reactions can not be readily studied with other techniques such as chemiluminescence (due to low product concentrations as well as the difficulty in producing radical species in flow experiments). With the aid of simple statistical models, the dynamics and branching ratios of these reactions can be reasonably interpreted and crudely estimated. More detailed discussion of the application of this technique to many other examples including energy transfer reactions has recently been reviewed elsewhere... 

Thus, as so frequently is the case, the chemical engineer must choose between alternatives should high product recoveries with low energy expenditures and low product concentrations, or modest product recoveries with high energy inputs to achieve high product purities be chosen, or is some intermediate case best Such questions can only be answered on an ad hoc basis. 

Explain why it is important to record fluorescence spectra at low product concentrations. 

Explain why it is important to record fluorescence spectra at low product concentrations. R3. A high concentration may increase the optical density at the excitation and/or the emission wavelengths. This will distort the fluorescence emission spectrum by decreasing the real fluorescence intensity and by shifting the emission peak. 

This means that when the reaction between A and B is at equilibrium (i.e., when the reaction has come to completion and when neither A or B is being lost nor C or D is being formed), the ratio of the concentrations of the products times each other divided by the concentrations of the reactants times each other is constant. If K is very small, then there are relatively low product concentrations compared to the reactant concentrations.1 In fact, most interesting K values are usually small hence, we use the pK notation. Also remember for pure water, the reaction... 

Exposure to zinc oxide can occur through inhalation, ingestion, and eye or skin contact. Ingestion of zinc oxide ointments is most common in household settings and is generally considered nontoxic due to relatively low product concentrations. Inhalation of zinc oxide in industrial areas, as particulate matter or fumes, may lead to potentially toxic exposures. 

For the whole process attention was paid to safety, efficiency, and economics. The biotransformation was carried out in aqueous solution (which is often the case with biotransformations). Large volumes of aqueous solution containing relatively low product concentrations are difficult to handle in chemical plants (com-... 

The basic process as outlined above is very flexible, and modifications and variations can be easily incorporated into it to further improve the overall efficiency and/or to make it more suitable for specific types of feeds. Thus, external fuel, recycle gas, or liquid fuels can be easily introduced into the burner in the case of lean tar sands. By providing for a purge gas stream off the top of the combustion bed, one can adjust the flow rate of fluidizing gas to the pyrolysis bed. If desired, after recovery, gas produced in the pyrolysis bed can be recycled back to that bed and used instead of combustion gases to fluidize it. This is very important for lean tar sands which would otherwise have very low product concentration in the combined exit gas stream, making product recovery difficult. 

If the rate coefficient of the forward reaction is noticeably higher than that of the backward reaction, meaning k+ k-, then Eq. (17.10) transforms into the familiar Eq. (16.11) for first-order reactions. In this case, the equilibrium reaction can be described by a simple rate law. Moreover, at the beginning of the equilibrium reaction, when it is far away from equilibrium, the reaction can be treated as onesided process because the rate of the backward reaction is very small due to the very low product concentration. 

---