Carry-over problems

Type 2 autosamplers behave in a very similar way to type I except partial loop volume can be filled and delivered. Additionally, this allows the injection of low volume with high precision compared to type I autosamplers. Both types of autosamplers suffer from carry over problems if appropriate wash cycle and wash solvents are not used between each sampling from the vials. 

The coil length depends primarily on the speed of reaction and the sample plus reagent flow-rates, and varies considerably in different applications. However, extremely long reaction coils create excessive impedance, as well as carry-over problems in the... 

In another type of membrane extraction devices, porous polypropylene hollow fibers are used, often in a disposable way, which minimizes carry-over problems and reduces costs. On the other hand, manual manipulations are needed, limiting the possibility for automation. With these devices, the extraction can be carried out in a static mode, either in large sample volumes, where the extraction is not intended to be complete, or in small volumes aiming for complete extraction. Usually stirring is applied to increase the speed of mass... 

This carry-over problem has been shown to be due to foaming. Whenever hydrogen is being flashed from a cool, heavy stream (distillate, gas oil, decanted oil), foaming should be suspected as the cause of carry-over. 

After checking the interface level, consider the mix valve as a possible cause of the carry-over problem. To get good contacting between the hydrocarbon and wash liquid, a pressure drop of 10 psi to 25 psi is about right. Use the same pressure gauge to check pressure up and downstream of the mix valve. Try reducing the pressure drop to 10 psi and see if the carry-over problem is diminished. Remember that setting it too low can decrease hydrocarbon cleanup efficiency. 

When troubleshooting a carry-over problem for a wash tower, first calculate the vertical velocity of the discontinuous phase, propane in this case. Divide the volume of propane in cubic feet per minute by the tower cross-sectional area. For velocities of less than 1.0 ft/min, entrainment should be negligible. For 1.2-1.6 ft/min, entrainment may be marginal. For more than 1.8 ft/min, excessive entrainment can be expected. 

A persistent carry-over problem, as indicated by a constant accumulation of liquid in downstream equipment, is likely due to entrainment of small droplets of liquid in the gas stream. For example, the small amount of liquid that is withdrawn daily from the case drain of certain compressors is due to entrainment. 

If a carry-over problem has been traced to entrainment, the use of a demister will probably help. The qualitative effect of a demister on entrainment is also shown in Table 21-1. A demister is a pad placed in front of the KO drum outlet nozzle, as indicated in Figure 21-2. The droplets of liquid impinge and are coalesced on the Brillo-like fibers of the demister pad. 

Demister separators are employed where liquid carry over is a problem. The recovery... 

The problem with removing large amounts of formic acid by distillation is that it takes a long time to do so. Really big batches can take an entire day to distill. So a second option [10] after removal of the acetone would be to cool the formic acid solution then extract the whole thing with ether. The black ether layer is then washed with an ice cold 5% sodium carbonate (Na2C03) solution to neutralize any formic acid that was carried over, then washed... 

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

Whereas addition of hydrogen to feedwater helps solve the O2 or ECP problem, other complications develop. An increase in shutdown radiation levels and up to a fivefold increase in operating steam plant radiation levels result from the increased volatiUty of the short-Hved radioactive product nitrogen-16, N, (7.1 s half-life) formed from the coolant passing through the core. Without H2 addition, the in the fluid leaving the reactor core is in the form of nitric acid, HNO with H2 addition, the forms ammonia, NH, which is more volatile than HNO, and thus is carried over with the steam going to the turbine. 

The relatively low pX values seen for the benzoyl acetanilides, especiaHy as two-equivalent couplers, minimize concerns over slow ionization rates and contribute to the couplers overaH reactivity. But this same property often results in slow reprotonation in the acidic bleach, where developer carried over from the previous step can be oxidized and react with the stiH ionized coupler to produce unwanted dye in a nonimage related fashion. This problem can be eliminated by an acidic stop bath between the developer and the bleach steps or minimized by careful choice of coupling-off group, coupler solvent, or dispersion additives. 

We have been discussing a class of penetration problems that are accurately modeled by two-dimensional calculations. There are many three-dimensional problems, however, that can be well approximated by two-dimensional analyses, and the greatly reduced computer memory and time requirements for such calculations make them attractive alternatives for scoping studies, or for parameter sensitivity studies. Although good quantitative predictions may not be obtained with such approximations, the calculations can be expected to reveal trends and qualitative results that will carry over to the full three-dimensional problem. 

Fuel systems can cause many problems, and fuel nozzles are especially susceptible to trouble. A gaseous fuel system consists of fuel filters, regulators, and gauges. Fuel is injected at a pressure of about 60 psi (4 Bar) above the compressor discharge pressure for which a gas compression system is needed. Knockout drums or centrifuges are recommended, and should be implemented to ensure no liquid carry-overs in the gaseous system. 

Problems with blade fatigue are common. When interstage cooling is used, water from the cooler is carried over to the blades. These water particles... 

Problem An amine absorber was carrying over due to foaming. 

Lattice models have the advantage that a number of very clever Monte Carlo moves have been developed for lattice polymers, which do not always carry over to continuum models very easily. For example, Nelson et al. use an algorithm which attempts to move vacancies rather than monomers [120], and thus allows one to simulate the dense cores of micelles very efficiently. This concept cannot be applied to off-lattice models in a straightforward way. On the other hand, a number of problems cannot be treated adequately on a lattice, especially those related to molecular orientations and nematic order. For this reason, chain models in continuous space are attracting growing interest. 

Except for this modification, all the procedures and steps discussed in Section V carry over to the solution of multiobjective problems. 

Note that the second loss corresponds to 3.92 mol of water per mol of reactant, whereas the 3rd loss is 1.08 mol. This illustrates a serious problem that can be encountered in dynamic TGA, If the rate of heating is too fiemt and not enough time occurs during programming to achieve true equilibrium between successive solid state reactions, then the loss of water firom one reaction carries over into the next succeeding reaction. 

Historical instrumental problems (plugging with larger samples, carry-over effects)... 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

The first issue, a setup state carry-over between adjacent periods, is depicted in Figure 11.3. On a continuous time scale a sequence of five products (A to E) can be modeled straightforwardly. However, if the same is to be done in a bucket-oriented setting some difficulties arise. Production within a bucket is coupled with a corresponding setup operation which causes a lot of problems. 

Dow/Chirotech [35, 36], Topcro Pharma [37] as well as by Solvias [38] using Rh-DuPhos catalysts (Fig. 37.6). Besides these successful examples, a process using Rh-DuPhos was abandoned because of reproducibility problems due to impurities carried over from the preceding step, and because of concerns about the toxicity of 2-nitropropane, even though ee-values of 99% were achieved [39]. 

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