Special Problems

A number of issues routinely arise during withdrawal and are worth addressing. Of course, these are not the only special problems that come up, but they are among the more salient ones. 

When the patient has been prescribed multiple drugs at once, it is usually easier and safer to taper one drug at a time. 

Removing more than one drug at a time can increase the hazards of withdrawal. In addition, it makes it difficult or impossible to determine which drug is causing problems during withdrawal. 

In the absence of an emergency or a special reason to the contrary, it is usually easiest and safest to begin by tapering the drug that has been most recently started. Drugs that have been taken for a relatively shorter period of time are generally easier to withdraw from. 

It is generally preferable to remove sleeping aids last. 

Many situations exist in which conventional approaches to design and operation of separation processes must be modified or changed altogether to meet processing olqeciives. Usi ly, these cases involve systems that are characterized by certain constraints that limit the potential range of operating conditions or scope of separation alternatives. Sometimes, even completely new separation methods rmrst be developed. 

Some examples of situations that require special processing con deration ate discussed below. 

To be able to bring the advances in biotechnology to the marketplace, the products of marry Inottattsfiir-mations somehow must be recovered from the com dex reaction tttedia in whidt they ate formed. Becaitse of the nature of biological products, conventional separation methods (e.g., distillation) often ate not appropriate, and completely new approaches often have been devdoped.  

For any intracellular product, the cells first must be harvested from die fermentation broth and lysed, or broken open, to release their contents. The liquid fraction, which contains the product, must then be separated from the cells. Cellular debris can be removed 1 either filtration or centrifugation, or a com-binafion of both. Crossflow filtration, using microporous media or irhtafilttation membranes, has been shown to be extremely effective for this step, but problems with membrane blockage have been reported. 

For extracellular products, one is faced with the problem of removing the liquid phase, which contains the product, from the biomass. This can be accomplished using techniques similar to those used for 

Results from laboratory tests and field jobs show that iron presents a significant and complex problem in stimulation operations [1653]. The problem 

Iron can be controlled with certain complexing agents, in particular glucono-5-lactone, citric acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, hydroxyethylethylene diaminetriacetic acid, hydroxyethyliminodiacetic acid, and the salts from the aforementioned compounds. These compounds must be added together with nitrogen-containing compounds such as hydroxylamine salts or hydrazine salts [486,643,1815]. 

In general, chelating agents possess some unique chemical characteristics. The most significant attribute of these chemicals is the high solubility of the free acids in aqueous solutions. Linear core flood tests were used to study the formation of wormholes. Both hydroxyethylethylene diaminetriacetic acid and hydroxyethyliminodiacetic acid produced wormholes in limestone cores when tested at 150° F. However, the efficiency and capacities differ. Because these chemicals have high solubility in the acidic pH range, it was possible to test acidic (pH less than 3.5) formulations [644]. 

To control the iron in an aqueous fracturing fluid having a pH below 7.5, a thioalkyl acid may be added [243]. This is a reducing agent for the ferric ion, contrary to the complexants described in the previous paragraph. 

During the initial fracturing process, a degradation, which results in a decrease of viscosity, is undesirable. The polymer in fracturing fluids will degrade at elevated temperatures. 

The approach taken to recover biological products depends on the nature of the product. Biological products can be classified as either high-vatue-low-vdume or low-value-high-vrdume. In the case of the former, many of the techniques used in analytical applications have been adapted to larger scale to handle product purification. For low-value-high-volume products, such as ethanol, ttxrre conventional procedures can be u, but these often require some modification. 

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Identification of normal paraffins by chromatography presents no special problems with the exception of biodegraded crudes, they are clearly distinguished. The problem encountered is to quantify, as shown in Figure 3.14, the normal paraffin peaks that are superimposed on a background representing other hydrocarbons. 

This poses a special problem because the source of the electromagnetic field may lie outside the defined boundaries of the system. A detailed discussion of this is outside the scope of this section, but the basic features can be briefly sunnnarized. 

The fifth and final chapter, on Parallel Force Field Evaluation, takes account of the fact that the bulk of CPU time spent in MD simulations is required for evaluation of the force field. In the first paper, BOARD and his coworkers present a comparison of the performance of various parallel implementations of Ewald and multipole summations together with recommendations for their application. The second paper, by Phillips et AL., addresses the special problems associated with the design of parallel MD programs. Conflicting issues that shape the design of such codes are identified and the use of features such as multiple threads and message-driven execution is described. The final paper, by Okunbor Murty, compares three force decomposition techniques (the checkerboard partitioning method. 

In general, we know bond lengths to within an uncertainty of 0.00.5 A — 0.5 pm. Bond angles are reliably known only to one or twx) degrees, and there arc many instances of more serious angle enxirs. Tn addition to experimental uncertainties and inaccuracies due to the model (lack of coincidence between model and molecule), some models present special problems unique to their geometry. For example, some force fields calculate the ammonia molecule. Nlln to be planar when there is abundant ex p er i m en ta I evidence th at N H is a 11 i g o n a I pyramid. 

Heterocycles with two heteroatoms present no special problems and there are often seyeral ways to do the first disconnection. One guide is to look for a small recognisable fragment containing the two heteroatoms. Try this one ... 

In view of the synthetic importance of dicarbonvl compounds surprisingly little has been done, apart from carotene synthesis, on dialkenylation with Wittig reagents. However, from the few examples reported one may conclude, that no special problems are involved. Benzocyclobutanedione was converted by two equivalents of methoxycarbonylmethylidenetri-phenylphosphorane to the corresponding diene in 85% yield (M. P. Cava, 1960). 

A special problem arises in the preparation of secondary amines. These compounds are highly nucleophilic, and alkylation of an amine with alkyl halides cannot be expected to stop at any specifle stage. Secondary amides, however, can be monoalkylated and lydrolyzed or be reduced to secondary amines (p. 11 If.). In the elegant synthesis of phenyl- phrine an intermediate -hydroxy isocyanate (from a hydrazide and nitrous acid) cyclizes to pve an oxazolidinone which is monomethylated. Treatment with strong acid cleaves the cyclic irethan. 

Chiral separations present special problems for vaUdation. Typically, in the absence of spectroscopic confirmation (eg, mass spectral or infrared data), conventional separations are vaUdated by analysing "pure" samples under identical chromatographic conditions. Often, two or more chromatographic stationary phases, which are known to interact with the analyte through different retention mechanisms, are used. If the pure sample and the unknown have identical retention times under each set of conditions, the identity of the unknown is assumed to be the same as the pure sample. However, often the chiral separation that is obtained with one type of column may not be achievable with any other type of chiral stationary phase. In addition, "pure" enantiomers are generally not available. 

The special problems for vaUdation presented by chiral separations can be even more burdensome for gc because most methods of detection (eg, flame ionization detection or electron capture detection) in gc destroy the sample. Even when nondestmctive detection (eg, thermal conductivity) is used, individual peak collection is generally more difficult than in Ic or tic. Thus, off-line chiroptical analysis is not usually an option. Eortunately, gc can be readily coupled to a mass spectrometer and is routinely used to vaUdate a chiral separation. 

Materials that have been buried underwater cause a special problem. Waterlogged woods and leathers (139), although quite stable under such burial conditions, are ia danger of irreversible damage through drying out upon recovery. Indeed, after excavations from bogs or upon recovery from underwater sites, these items need to be stored underwater until laboratory treatment. 

Flocculating agents differ from other materials used in the chemical process industries in that their effect not only depends on the amount added, but also on the concentration of the solution and the point at which it is added. The process streams to which flocculants are added often vary in composition over relatively short time periods. This presents special problems in process control. 

The nomenclature of biochemical compounds is in large measure a part of organic nomenclature. However, it has its own special problems, arising partiy from the fact that many biochemical compounds must be given names before their chemical stmctures have been fully determined, and partiy from the interest in grouping them according to biological function as much as to chemical class. 

The in situ process is simpler because it requires less material handling (35) however, this process has been used only for resole resins. When phenol is used, the reaction system is initially one-phase alkylated phenols and bisphenol A present special problems. As the reaction with formaldehyde progresses at 80—100°C, the resin becomes water-insoluble and phase separation takes place. Catalysts such as hexa produce an early phase separation, whereas NaOH-based resins retain water solubiUty to a higher molecular weight. If the reaction medium contains a protective coUoid at phase separation, a resin-in-water dispersion forms. Alternatively, the protective coUoid can be added later in the reaction sequence, in which case the reaction mass may temporarily be a water-in-resin dispersion. The protective coUoid serves to assist particle formation and stabUizes the final particles against coalescence. Some examples of protective coUoids are poly(vinyl alcohol), gum arabic, and hydroxyethjlceUulose. 

Nonaqueous phase Hquids (NAPLs) present special problems for soil and ground water cleanup. Contaminant transport through ground water depends in part on the water solubiHty of the compound. Because NAPLs cling to subsurface particles and are slow to dissolve in ground water, they hinder cleanups and prolong cleanup times. Dense nonaqueous phase Hquids (DNAPLs) migrate downward in the aquifer and can coUect in pools or pockets of the substmcture. Examples of DNAPLs are the common solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) which were used extensively at many faciHties before the extent of subsurface contamination problems was realized. 

Environmental Concerns. Dyes, because they are intensely colored, present special problems in effluent discharge even a very small amount is noticeable. However, the effect is more aesthetically displeasing rather than ha2ardous, eg, red dyes discharged into rivers and oceans. Of more concern is the discharge of toxic heavy metals such as mercury and chromium. 

Sealing tlie Centi-ifiig Chemical Pump Altliougb detailed treatment of. shaft seals is presented iu tbe subsection Sealing of Rotating Shafts, it is appropiiate to mention here tlie special problems... 

Difficult Separations Some binary separations may pose special problems because of extreme purity requirements for one or both products or because of a relative volatihty close to 1. The y-x diagram... 

Jobs which raise special problems. Such jobs might include entry to vessels and other confined spaces, hot work, and responsibilities of contractors. 

Simple mixing of raw materials and intermediates may present special problems when processing significantly larger quantities in comparison to the pilot process or laboratory bench amounts. All aspects of the toll should be considered while performing the PHA to identify potential problems caused by the scale-up. 

The design is viable only if it can be produced economically. The choice of production and fabrication method is largely determined by the choice of material. But the production route will also be influenced by the size of the production run, and how the component will be finished and joined to other components each class of material has its own special problems here they were discussed in Chapters 14, 19, 24 and 25. The choice of material and production route will, ultimately, determine the price of the product, so a second major iteration may be required if the costing shows the price to be too high. Then a new choice of material or component design, allowing an alternative production path, may have to be considered. 

Special Problems in Cathodic Protection Near Railways 11.5.1 General Comments... 

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