Liquid streams

In an equilibrium separation, a feed stream containing m components at given composition, pressure, and enthalpy (or temperature if in a single phase) is split into two streams in equilibrium, here taken to be a vapor and a liquid. The flow rates of the feed, vapor, and liquid streams are, respectively,... 

Phase equilibrium between the vapor and liquid streams re-... 

The reactor effluent is thus likely to contain hydrogen, methane, benzene, toluene, and diphenyl. Because of the large differences in volatility of these components, it seems likely that partial condensation will allow the effluent to be split into a vapor stream containing predominantly hydrogen and methane and a liquid stream containing predominantly benzene, toluene, and diphenyl. 

The liquid stream can be separated readily into pure components by distillation, the benzene taken ofif as product, the diphenyl as an unwanted byproduct, and the toluene recycled. It is possible to recycle the diphenyl to improve selectivity, but we will assume that is not done here. 

As a first stage, the stream of liquid from an HPLC eluant is passed through a narrow tube toward the LINC interface. Near the end of the tube, the liquid stream is injected with helium gas so that it leaves the end of the tube as a high-velocity spray of small drops of liquid mixed with helium. From there, the mixture enters an evacuation chamber (Figure 12.1). The formation of spray (nebulizing) is very similar to that occurring in the action of aerosol spray cans (see Chapter 19). 

The particle-beam interface (LINC) works by separating unwanted solvent molecules from wanted solute molecules in a liquid stream that has been broken down into droplets. Differential evaporation of solvent leaves a beam of solute molecules that is directed into an ion source. 

Nebulizers can be divided into several main types. The pneumatic forms work on the principle of breaking up a stream of liquid into droplets by mechanical means the liquid stream is forced through a fine nozzle and breaks up into droplets. There may be a concentric stream of gas to aid the formation of small droplets. The liquid stream can be directed from a fine nozzle at a solid target so that, on impact, the narrow diameter stream of liquid is broken into many tiny droplets. There are variants on this approach, described in the chapter devoted to nebulizers (Chapter 19). 

Using Poiseuille s formula, the calculation shows that for concentric-tube nebulizers, with dimension.s similar to those in use for ICP/MS, the reduced pressure arising from the relative linear velocity of gas and liquid causes the sample solution to be pulled from the end of the inner capillary tube. It can be estimated that the rate at which a sample passes through the inner capillary will be about 0.7 ml/min. For cross-flow nebulizers, the flows are similar once the gas and liquid stream intersection has been optimized. 

The fast-flowing narrow liquid stream has a high relative linear velocity with respect to the slower flow of the argon gas stream. This leads to breaking up the liquid stream into fast-moving droplets, which strike the impactor bead and form much smaller droplets. 

A sample solution is drawn or pumped into a V-shaped groove cut into the end of a capillary tube. The crossed gas and liquid streams form an aerosol. An impactor bead can be used to provide an even smaller droplet size. 

Finally, in yet another variant, the sample liquid stream and the gas flow are brought together at a shaped nozzle into which the liquid flows (parallel-path nebulizer). Again, the intersection of liquid film and gas flow leads to the formation of an aerosol. Obstruction of the sample flow by formation of deposits is not a problem, and the devices are easily constructed from plastics, making them robust and cheap. 

Nebulizers are used to introduce analyte solutions as an aerosol spray into a mass spectrometer. For use with plasma torches, it is necessary to produce a fine spray and to remove as much solvent as possible before the aerosol reaches the flame of the torch. Various designs of nebulizer are available, but most work on the principle of interacting gas and liquid streams or the use of ultrasonic devices to cause droplet formation. For nebulization applications in thermospray, APCI, and electrospray, see Chapters 8 and 11. 

The particle-beam interface is used to remove solvent from a liquid stream without, at the same time, removing the solute (or substrate). 

Gas chromatography (GC) deals with volatile substances that can be vaporized into a gas stream. Liquid chromatography (LC) concerns mostly nonvolatile substances dissolved in a liquid stream. 

High-pressure liquid chromatography (HPLC) is simply a variant on LC in which the moving liquid stream is forced along under high pressure to obtain greater efficiency of separation. 

Removal of trace impurities from gases or liquid streams Bulk separations (gas or liquid)... 

A hypothetical moving-bed system and a Hquid-phase composition profile are shown in Figure 7. The adsorbent circulates continuously as a dense bed in a closed cycle and moves up the adsorbent chamber from bottom to top. Liquid streams flow down through the bed countercurrently to the soHd. The feed is assumed to be a binary mixture of A and B, with component A being adsorbed selectively. Feed is introduced to the bed as shown. 

Sample Withdrawal from Process A number of considerations are involved in the design of sample-withdrawal devices that wih provide representative samples. For example, in a horizontal pipe that conveys process fluid, a sample point on the bottom of the pipe wih collect a maximum amount of rust, scale, or other solid materials being carried along by the process fluid. In a gas stream, such a location will also collect a maximum amount of liquid contaminants. A sample point on the top side of a pipe will, for liquid streams, collec t a... 

Note that the lower operating hne is used until the fourth stage is passed, at which time the constnic tion switches to the upper operating hne. This is necessaiy because the vapor and liquid streams passing each other between the fourth and fifth stages must fall on the upper line. 

The actual liquid-to-gas ratio (solvent-circulation rate) normally will be greater than the minimum by as much as 25 to 100 percent and may be arrived at by economic considerations as well as by judgment and experience. For example, in some packed-tower applications involving veiy soluble gases or vacuum operation, the minimum quantity of solvent needed to dissolve the solute may be insufficient to keep the packing surface thoroughly wet, leading to poor distribution of the liquid stream. 

This equation gives the relation between the bulk compositions of the gas and liquid streams at each level in the tower for conditions in which the operating curve can be approximated by a straight hne. 

Stripping Equations Stripping, or desorption, involves the removal of a volatile component from the hquid stream by contact with an inert gas such as nitrogen or steam. In this case the change in concentration of the liquid stream is of prime importance, and it... 

Classical Adiabatic Design Method The classical adiabatic method assumes that the heat of solution serves only to heat up the liquid stream and that there is no vaporization of solvent. This assumption makes it feasible to relate increases in the hquid-phase temperature to the solute concentration x by a simple eutnalpy balance. The equihbrium curve can then be adjusted to account For the corresponding temperature rise on an xy diagram. The adjusted equilibrium curve will become more concave upward as the concentration increases, tending to decrease the driving forces near the bottom of the tower, as illustrated in Fig. 14-8 in Example 6. 

The orifice-riser distributor is designed to lay the hquid carefully onto the bed, with a minimum of contact with gas during the process. It can be designed to provide a large number of liquid streams, with the limit of sufficient liquid head to provide uniform liquid flow through the orifices. The gas risers must oe designed to accommodate the expected variations in flow rate, often with a minimum of pressure drop. For veiy distribution-sensitive packings, it is necessaiy to include pour points in the vicinity of the column wall (to within 25 mm). 

The power input from a liquid stream injected with a hydraulic spray nozzle may usually be taken as approximately equal to the product of the nozzle feed pressure pf and the volumetric hquid rate. The liquid-phase contacting power Pe may then be calculated from the following formulas ... 

Solid phase is formed internally thus, only liquid streams enter and exit the column. Solid phase is formed in external equipment and fed as slurry into the purifier. 

CiystaUization is the preferred method of forming many final prod-uc ts because veiy high purification is possible. High purity antibiotic ciystals can be produced from colored, rather impure solutions if the filter cake is uniform and amenable to good washingto remove the mother hquor. When a sterile pharmaceutical produc t is desired, ciystals are formed from liquid streams that have been sterihzed by filtration. 

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Eric Jenett/ M.S.Ch.E./ Manager, Proce.ss Engineering, Brown Root, Inc. as.sociate member, AlChE, Project Management Institute Registered Professional Engineer (Texas). (Power Recoveiy from Liquid Streams)... 

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