Mass separation processes

A TOF-mass analyzer is by far the instrument of choice for SALI applications. Figure 4 illustrates the mass separation process. Electrostatic acceleration is the simplest way to distinguish between ions of different m/z in a given bunch. A potential difference V (typically 3kV) between the sample and extraction electrode (on a distance of 2mm) gives all ions the same Ekin and, hence, an m/z-dependent velocity v ... 

The second class of distillation operation using an extraneous mass-separating agent is extractive distillation. Here, the extraneous mass-separating agent is relatively involatile and is known as a solvent. This operation is quite different from azeotropic distillation in that the solvent is withdrawn from the column bottoms and does not form an azeotrope with any of the components. A typical extractive distillation process is shown in Fig. 3.11. ... 

In principle, extractive distillation is more useful than azeotropic distillation because the process does not depend on the accident of azeotrope formation, and thus a greater choice of mass-separating agent is, in principle, possible. In general, the solvent should have a chemical structure similar to that of the less volatile of the two components. It will then tend to form a near-ideal mixture with the less volatile component and a nonideal mixture with the more volatile component. This has the effect of increasing the volatility of the more volatile component. 

The ion current resulting from collection of the mass-separated ions provides a measure of the numbers of ions at each m/z value (the ion abundances). Note that for this ionization method, all ions have only a single positive charge, z = 1, so that m/z = m, which means that masses are obtained directly from the measured m/z values. Thus, after the thermal ionization process, m/z values and abundances of ions are measured. The accurate measurement of relative ion abundances provides highly accurate isotope ratios. This aspect is developed more fully below. 

Whereas Hquid separation method selection is clearly biased toward simple distillation, no such dominant method exists for gas separation. Several methods can often compete favorably. Moreover, the appropriateness of a given method depends to a large extent on specific process requirements, such as the quantity and extent of the desired separation. The situation contrasts markedly with Hquid mixtures in which the appHcabiHty of the predominant distiHation-based separation methods is relatively insensitive to scale or purity requirements. The lack of convenient problem representation techniques is another complication. Many of the gas—vapor separation methods ate kinetically controUed and do not lend themselves to graphical-phase equiHbrium representations. In addition, many of these methods require the use of some type of mass separation agent and performance varies widely depending on the particular MSA chosen. 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

Kirwan, Mass Transfer Principles in Rousseau, Handbook of Separation Process Technology, Wiley, 1987. 

Transfer of material between phases is important in most separation processes in which two phases are involved. When one phase is pure, mass transfer in the pure phase is not involved. For example, when a pure liqmd is being evaporated into a gas, only the gas-phase mass transfer need be calculated. Occasionally, mass transfer in one of the two phases may be neglec ted even though pure components are not involved. This will be the case when the resistance to mass transfer is much larger in one phase than in the other. Understanding the nature and magnitudes of these resistances is one of the keys to performing reliable mass transfer. In this section, mass transfer between gas and liquid phases will be discussed. The principles are easily applied to the other phases. 

Historically, measurements have classified ambient hydrocarbons in two classes methane (CH4) and all other nonmethane volatile organic compounds (NMVOCs). Analyzing hydrocarbons in the atmosphere involves a three-step process collection, separation, and quantification. Collection involves obtaining an aliquot of air, e.g., with an evacuated canister. The principal separation process is gas chromatography (GC), and the principal quantification technique is wdth a calibrated flame ionization detector (FID). Mass spectroscopy (MS) is used along with GC to identify individual hydrocarbon compounds. 

Calculations. To check a design for possible fogging, a procedure is presented that rightly considers mass transfer and heat transfer as two separate processes. 

The atom flux sputtered from a solid surface under energetic ion bombardment provides a representative sampling of the solid. Sputtered neutral mass spectrometry has been developed as method to quantitatively measure the composition of this atom flux and thus the composition of the sputtered material. The measurement of ionized sputtered neutrals has been a significant improvement over the use of sputtered ions as a measure of flux composition (the process called SIMS), since sputtered ion yields are seriously affected by matrix composition. Neutral panicles are ionized by a separate process after sputter atomization, and SNMS quantitation is thus independent of the matrix. Also, since the sputtering and ionization processes are separate, an ionization process can be selected that provides relatively uniform yields for essentially all elements. 

The spark source is an energetic ionization process, producing a rich spectrum of multiply charged species (Af/2, Af/3, Af/4, etc.). These masses, falling at halves, thirds, and fourths of the unit mass separation can aid in the positive identification of elements. In Figure 2, species like Au and are labeled. The most abundant... 

In addition to the fixed capital investment needed to purchase and install process equipment and auxiliaries, there is a continuous expenditure referred to as operating cost, which is needed to operate the process. The operating cost (or manufacturing cost or production cost) includes raw materials, mass-separating agents, utilities (fuel, electricity, steam, water, refrigerants, air, etc.), catalysts, additives, labor, and maintenance. The total annualized cost of a process is defined as follows ... 

Distillation is probably Ihe most widely used separation (mass transfer) process in the chemical and allied industries. Its applications range from the rectification of alcohol, which has been practiced since antiquity, lo the fractionation of crude oil. The separation of liquid mixtures by distillation is based on differences in volatility between the components. The greater the... 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

In this chapter, consideration will be given to the basic principles underlying mass transfer both with and without chemical reaction, and to the models which have been proposed to enable the rates of transfer to be calculated. The applications of mass transfer to the design and operation of separation processes are discussed in Volume 2, and ihe design of reactors is dealt with in Volume 3. 

The principal applications of mass transfer are in the fields of distillation, gas absorption and the other separation processes involving mass transfer which are discussed in Volume 2, In particular, mass transfer coefficients and heights of transfer units in distillation, and in gas absorption are discussed in Volume 2,. In this section an account is given of some of the experimental studies of mass transfer in equipment of simple geometry, in order to provide a historical perspective. 

If color reactions occur these serve to help characterize the substance. They can only ever act as a pointer to the presence of a substance, but never as proof even when accompanied by a separation process. Unequivocal identification requires a mosaic of many pieces of information (h/ f values, color reactions UV/VIS, IR, Raman, mass spectra etc). 

As mentioned at the beginning of Section 3.2.3 the separation process can be modeled by mass balances. Two mass balances have to be made as the liquid appears in two phases dispersed in the gas phase (the droplets) and continuous in the accumulated state. Figure 3.2.12 describes the process and the conditions in addition to what was explained at the very beginning in Section 3.2.1. 

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