Rules-of-thumb

The common types of control loops are level, flow, temperature, and pressure. The type of controller and the settings used for any one type are sometimes pretty much the same from one application to another. For example, most flow control loops use PI controllers with wide proportional band and fast integral action. 

Some heuristics are given below. They are not to be taken as gospel. They merely indicate common practice and they work in most applications. 

A- FLOW LOOPS. PI controllers are used in most flow loops. A wide proportional band setting (PB — ISO) or low gain is used to reduce the effect of the noisy flow signal due to flow turbulence. A low value of integral or reset time (t, - 0.1 minute per repeat) is used to get fast, snappy setpoint tracking. 

The dynamics of the process are usually very fast. The sensor sees the change in flow almost immediately. The control valve dynamics are the slowest element in the loop. So a small reset time can be used. 

One of the most common errors in laying out a control structure for a plant with multiple units in series is the use of PI level controllers. If P controllers are used, the process flows rise or fall slowly down the train of units with no overshoot of flow rates. Liquid levels rise if flows increase and fall if flows decrease. Levels are not maintained at setpoints. See Fig. 7.14. 

In the preceding chapters, details were presented for estimating the physical properties of acid gas, and procedures were presented for designing the equipment required. Note that in this section some of the conversions are not exact because of the approximate nature of the information presented. Although some of the rules of thumb presented in this section are general in nature, the reader is wise to apply them only to acid gas systems. 

The following are some rules of thumb for the physical properties of compressed acid gas  

Using these rules of thumb, the standard flow rates can be converted in to actual flow rates. For example, a flow rate of 28 x 103 Sm3/d (1 MMSCFD) of acid gas is equivalent to about 40 L[act]/min (10 USgal[act]/min) at 10°C and 7 MPa (50°F and 1000 psia). 

The water content diagram of an acid gas mixture has two characteristic shapes 1. A minimum in the water content, and 2. Liquefaction of the acid gas - the liquefied acid gas holds more water than the gas at similar pressures. Both can be used to reduce the water content of an acid gas stream using compression and cooling alone. 

The presence of a liquid water phase leads to concerns about corrosion. Acid gas without free water is not corrosive to common carbon steels. 

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Porter, K. E., and Momoh, S. O., Finding the Optimum Sequence of Distillation Columns—An Equation to Replace the Rules of Thumb (Heuristics), Chem. Engg. J., 46 97, 1991. 

The maximum temperature cross which can be tolerated is normally set by rules of thumb, e.g., FrSQ,75 °. It is important to ensure that Ft > 0.75, since any violation of the simplifying assumptions used in the approach tends to have a particularly significant effect in areas of the Ft chart where slopes are particularly steep. Any uncertainties or inaccuracies in design data also have a more significant effect when slopes are steep. Consequently, to be confident in a design, those parts of the Ft chart where slopes are steep should be avoided, irrespective of Ft 0.75. 

The increased exposure to the reservo/r results from the long horizontal sections which can be attained (between 500m and 1000m horizontal section is now common). Because the productivity index is a function of the length of reservoir drained by a well, horizontal wells can give higher productivities in laterally extensive reservoirs. As an initial estimate of the potential benefit of horizontal wells, one can use a rough rule of thumb, the... 

The effects of TIP also appear in figure B 1,11.3 and figure B 1.11.4. In the NMR spectrum, all the resonances of the sp carbons lie above 100 ppm (a usefiil general rule of thumb) because A is smaller for multiple bonds. The highest shifts are for the carbonyl C at 169 ppm and the ring C attached to oxygen at 155... 

As a qualitative rule of thumb, the larger the geometry differenee between the initial and final state potentials, the broader will be the Franek-Condon profile (as shown above) and the larger the vf value for whieh this profile peaks. Differenees in harmonie frequeneies between the two states ean also broaden the Franek-Condon profile, although not as signifieantly as do geometry differenees. 

For many projects, a basis set cannot be chosen based purely on the general rules of thumb listed above. There are a number of places to obtain a much more quantitative comparison of basis sets. The paper in which a basis set is published often contains the results of test calculations that give an indication of the accuracy of results. Several books, listed in the references below, contain extensive tabulations of results for various methods and basis sets. Every year, a bibliography of all computational chemistry papers published in the previous... 

Different motions of a molecule will have different frequencies. As a general rule of thumb, bond stretches are the highest energy vibrations. Bond bends are somewhat lower energy vibrations and torsional motions are even lower. The lowest frequencies are usually torsions between substantial pieces of large molecules and breathing modes in very large molecules. 

A transition structure is the molecular species that corresponds to the top of the potential energy curve in a simple, one-dimensional, reaction coordinate diagram. The energy of this species is needed in order to determine the energy barrier to reaction and thus the reaction rate. A general rule of thumb is that reactions with a barrier of 21 kcal/mol or less will proceed readily at room temperature. The geometry of a transition structure is also an important piece of information for describing the reaction mechanism. 

Transition structures are more dihicult to describe than equilibrium geometries. As such, lower levels of theory such as semiempirical methods, DFT using a local density approximation (LDA), and ah initio methods with small basis sets do not generally describe transition structures as accurately as they describe equilibrium geometries. There are, of course, exceptions to this, but they must be identihed on a case-by-case basis. As a general rule of thumb, methods that are empirically dehned, such as semiempirical methods or the G1 and G2 methods, describe transition structures more poorly than completely ah initio methods do. 

Side chain generation is often a source of error. It will be most reliable if certain rules of thumb are obeyed. Start with structurally conserved side chains and hold them fixed. Then look at the energy and entropy of rotamers for the remaining side chains. Conventional conformation search techniques are often used to place each side chain. 

The choice of where to locate the boundary between regions of the system is important. A number of studies have shown that very poor end results will be obtained if this is chosen improperly. There is no rigorous way to choose the best partitioning, but some general rules of thumb can be stated ... 

As a check for the presence of spin contamination, most ah initio programs will print out the expectation value of the total spin <(A >. If there is no spin contamination, this should equal. v(.v + 1), where s equals times the number of unpaired electrons. One rule of thumb, which was derived from experience with... 

The simplest case of structure-property relationships are qualitative rules of thumb. For example, the statement that branched polymers are generally more biodegradable than straight-chain polymers is a qualitative structure-property relationship. 

In general, the computation of absolute chemical shifts is a very difficult task. Computing shifts relative to a standard, such as TMS, can be done more accurately. With some of the more approximate methods, it is sometimes more reliable to compare the shifts relative to the other shifts in the compound, rather than relative to a standard compound. It is always advisable to verify at least one representative compound against the experimental spectra when choosing a method. The following rules of thumb can be drawn from a review of the literature ... 

A rule of thumb is that a C=C substituent stabilizes a carbocation about as well as two methyl groups Al though allyl cation (H2C=CHCH2 ) is a primary carbocation it is about as stable as a typical secondary carbocation such as isopropyl cation (CH3)2CH-"... 

SpartanView displays calculated vibrations and frequencies for selected models Calculated frequencies are listed m wavenumbers (cm ) and are consistently larger than observed frequencies (observed frequency = 0 9 X calculated frequency is a good rule of thumb)... 

As a rule of thumb, one can say that the efficiency of separation of mixtures and the simplicity of operating and maintaining apparatus are much greater for GC than for LC. Hence, other things being equal, GC is most often the technique of first choice and can be used with a very wide variety of compound types. However, for nonvolatile or thermally labile substances like peptides, proteins, nucleotides, sugars, carbohydrates, and many organometallics, GC may be ruled out completely... 

When multicharged ions are formed, the simple rule of thumb used widely in mass spectrometry that m/z = m because, usually, z = 1 no longer applies for z > 1 then m/z < m, and the apparent mass of an ion is much smaller than its true mass. Accurate mass measurement is much easier at low mass than at high, and the small m/z values, corresponding to high mass with multiple charges, yield accurate values for the high mass. 

Requirements for energy, protein, carbohydrates, Hpids, vitamins and minerals have been determined for the species commonly cultured (9). As a rule of thumb, trout and salmon diets will, if consumed, support growth and survival in virtually any aquaculture species. Such diets often serve as the control against which experimental diets are compared. 

A comparison of the characteristics associated with propellant burning, explosive detonation, and the performance of conventional fuels (see Coal Gas, NATURAL Petroleum) is shown ia Table 1. The most notable difference is the rate at which energy is evolved. The energy Hberated by explosives and propellants depends on the thermochemical properties of the reactants. As a rough rule of thumb, these materials yield about 1000 cm of gas and 4.2 kj (1000 cal) of heat per gram of material. 

Conversion of hardness data to some measure of strength is also possible and has been done for several common materials (Eig. 1). Rules of thumb have also been developed relating tensile strength to hardness for steel, eg,... 

As appHed to hydrocarbon resins, dsc is mainly used for the determination of glass-transition temperatures (7p. Information can also be gained as to the physical state of a material, ie, amorphous vs crystalline. As a general rule of thumb, the T of a hydrocarbon resin is approximately 50°C below the softening point. Oxidative induction times, which are also deterrnined by dsc, are used to predict the relative oxidative stabiHty of a hydrocarbon resin. 

Every iadustry has its owa sease of what a reasoaable royalty might be. Whereas earned royalties are usually based oa aet sales, a very rough rule of thumb is that an earned royalty reflects approximately 25% of profits earned on the particular technology. Minimum royalties are often based on one-fourth to one-thind of a conservative projection of sales. 

Process plant design has come a long way from the early 1930s when process designers used the rule-of-thumb that a process faciUty could not be scaled-up more than 10-fold (2). American Oil s Ultracracking unit (Texas City, Texas) for example, was designed from data from a small pilot plant with a scale-up factor of 80,000 (3). 

Selection of pump for a given appHcation is not a trivial task. Often more than one pump type can accomplish the required job. Thus a final choice on a pump type is often a result of personal experience and usage history. As a rule of thumb, the choice of a kinetic, such as centrifugal, or a positive displacement pump is made on the basis of the specific speed. Whereas specific speed is appHcable primarily for centrifugal but not positive displacement pumps, the US value can be used as a guide. Generally, for calculated values of specific speed, eg, nS > 10 [NS > 500), kinetic-type pumps are usually selected. For nS < 10 [NS < 500), positive displacement pumps are typically appHed. 

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