Bottleneck

De-bottlenecking is particularly important when the producing field is on plateau production, because it provides a means of earlier recovery (acceleration) of hydrocarbons, which improves the project cashflow and NPV. 

Enhancements to the process may be required due to sub-optimal initial design of the equipment, or to implement new technology, or because an idea for improving the production system has emerged. De-bottlenecking would be an example of an... 

As introduced in Section 14.2, bottlenecks in the process facilities can occur at many stages in a producing field life cycle. A process facility bottleneck is caused when any piece of equipment becomes overloaded and restricts throughput. In the early years of a development, production will often be restricted by the capacity of the processing facility to treat hydrocarbons. If the reservoir is performing better than expected it may pay to increase plant capacity. If, however, it is just a temporary production peak such a modification may not be worthwhile. 

As a field matures, bottlenecks may appear in other areas, such as water treatment or gas compression processes, and become factors limiting oil or gas production. These issues can often be addressed both by surface and subsurface options, though the underlying justification remains the same the NPV of a debottlenecking exercise (net cost of action versus the increase in net revenue) must be positive. 

This seems obvious, but it is not always easy to predict how a change in one part of a processing chain will affect the process as a whole (there will always be a bottleneck... 

The types of facilities bottleneck which appear late in field life depend upon the reservoir, development scheme and facilities in place. Two of the most common capacity constraints affecting production include ... 

The existence of the polyad number as a bottleneck to energy flow on short time scales is potentially important for efforts to control molecnlar reactivity rising advanced laser techniqnes, discussed below in section Al.2.20. Efforts at control seek to intervene in the molecnlar dynamics to prevent the effects of widespread vibrational energy flow, the presence of which is one of the key assumptions of Rice-Ramsperger-Kassel-Marcns (RRKM) and other theories of reaction dynamics [6]. 

As discussed in section A 1.2.17. the existence of the approximate poly ad numbers, corresponding to short-time bottlenecks to energy flow, could be very important in efforts for laser control, apart from the separate question of bifiircation phenomena. 

A situation that arises from the intramolecular dynamics of A and completely distinct from apparent non-RRKM behaviour is intrinsic non-RRKM behaviour [9], By this, it is meant that A has a non-random P(t) even if the internal vibrational states of A are prepared randomly. This situation arises when transitions between individual molecular vibrational/rotational states are slower than transitions leading to products. As a result, the vibrational states do not have equal dissociation probabilities. In tenns of classical phase space dynamics, slow transitions between the states occur when the reactant phase space is metrically decomposable [13,14] on the timescale of the imimolecular reaction and there is at least one bottleneck [9] in the molecular phase space other than the one defining the transition state. An intrinsic non-RRKM molecule decays non-exponentially with a time-dependent unimolecular rate constant or exponentially with a rate constant different from that of RRKM theory. 

As a result of possible recrossings of the transition state, the classical RRKM lc(E) is an upper bound to the correct classical microcanonical rate constant. The transition state should serve as a bottleneck between reactants and products, and in variational RRKM theory [22] the position of the transition state along q is varied to minimize k E). This minimum k E) is expected to be the closest to the truth. The quantity actually minimized is N (E - E ) in equation (A3.12.15). so the operational equation in variational RRKM theory is... 

In the above discussion it was assumed that the barriers are low for transitions between the different confonnations of the fluxional molecule, as depicted in figure A3.12.5 and therefore the transitions occur on a timescale much shorter than the RRKM lifetime. This is the rapid IVR assumption of RRKM theory discussed in section A3.12.2. Accordingly, an initial microcanonical ensemble over all the confonnations decays exponentially. However, for some fluxional molecules, transitions between the different confonnations may be slower than the RRKM rate, giving rise to bottlenecks in the unimolecular dissociation [4, ]. The ensuing lifetime distribution, equation (A3.12.7), will be non-exponential, as is the case for intrinsic non-RRKM dynamics, for an mitial microcanonical ensemble of molecular states. 

As discussed in section A3.12.2. intrinsic non-RRKM behaviour occurs when there is at least one bottleneck for transitions between the reactant molecule s vibrational states, so drat IVR is slow and a microcanonical ensemble over the reactant s phase space is not maintained during the unimolecular reaction. The above discussion of mode-specific decomposition illustrates that there are unimolecular reactions which are intrinsically non-RRKM. Many van der Waals molecules behave in this maimer [4,82]. For example, in an initial microcanonical ensemble for the ( 211 )2 van der Waals molecule both the C2H4—C2H4 intennolecular modes and C2H4 intramolecular modes are excited with equal probabilities. However, this microcanonical ensemble is not maintained as the dimer dissociates. States with energy in the intermolecular modes react more rapidly than do those with the C2H4 intramolecular modes excited [85]. 

MD, one needs to use multiple time step methods to ensure proper handling of the sprmg vibrations, and there is a possible physical bottleneck in the transfer of energy between the spring system and the other degrees of freedom which must be handled properly [199]. In MC, one needs to use special methods to sample configuration space efficiently [200, 201]. 

Direct dynamics attempts to break this bottleneck in the study of MD, retaining the accuracy of the full electronic PES without the need for an analytic fit of data. The first studies in this field used semiclassical methods with semiempirical [66,67] or simple Hartree-Fock [68] wave functions to heat the electrons. These first studies used what is called BO dynamics, evaluating the PES at each step from the elech onic wave function obtained by solution of the electronic structure problem. An alternative, the Ehrenfest dynamics method, is to propagate the electronic wave function at the same time as the nuclei. Although early direct dynamics studies using this method [69-71] restricted themselves to adiabatic problems, the method can incorporate non-adiabatic effects directly in the electionic wave function. 

The main difference between the force-bias and the smart Monte Carlo methods is that the latter does not impose any limit on the displacement that m atom may undergo. The displacement in the force-bias method is limited to a cube of the appropriate size centred on the atom. However, in practice the two methods are very similar and there is often little to choose between them. In suitable cases they can be much more efficient at covering phase space and are better able to avoid bottlenecks in phase space than the conventional Metropolis Monte Carlo algorithm. The methods significantly enhance the acceptance rate of trial moves, thereby enabling Icirger moves to be made as well as simultaneous moves of more than one particle. However, the need to calculate the forces makes the methods much more elaborate, and comparable in complexity to molecular dynamics. 

Since computation time is the most important bottleneck to conformation searching, the following list starts with the methods most amenable to the largest molecular systems ... 

Figure 2.6 Model of a particle P, a hole H, and the bottleneck between them in a liquid of spherical molecules.

Documentation abstracts, useful as they are, may also prove inadequate for final decisions, which would make it necessary for the searcher to obtain and examine copies of full patent specifications. Although the increasing availabihty of sets of full specifications on CD-ROM at modest cost is making it easier for searchers to have in their collections copies of those specifications they might need to consult, the necessity to go beyond computer output, whether to abstracts or full specifications, is stiU one of the bottlenecks of computer searching, and therefore an area in which significant new developments are hoped for. 

Wort separation and beer filtration are two common bottlenecks ia the brewiag process. Poor lauteriag aot only causes a loss ia productioa capacity, but can also lead to losses ia extract yield. Furthermore, a slow lauteriag aegatively affects the quaHty of the wort, which may give beer filtratioa problems and problems with the flavor and stabiHty of the beer. 

Demethanizer Overhead Expander and Multifeed Fractionation. Incorporation of an expander into the conventional high pressure demethanizer system eliminates bottlenecks in the refrigeration system, the demethanizer condenser, and charge gas compressor. It reduces the cost by lowering the refrigeration power. Multiple feed deethanization and ethylene fractionation debottlenecks the deethanizer, ethylene fractionator, and the refrigeration systems, thereby reducing power consumption. 

Problem Selection. To select the problem correcdy, the criteria discussed earHer should be carefully appHed before launching a project the existence of a knowledge bottleneck, the inappHcabiHty of exact numerical methods, the existence of either an expert or a theory for the task, the narrowness of the domain, and the business issues of payout and cost. If needed, the various criteria can be quantified and weighted based on their... 

Where pipelines and high-voltage cables run parallel to each other, spacing of at least 0.4 m should be observed to ensure adequate working space. In bottlenecks, the spacing should not be less than 0.2 m [2]. 

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