Limiting Operating Conditions Research Articles

The product composition regions of single-feed azeotropic distillation columns

We show how the limiting operating conditions, total reflux and thermodynamically optimum operation, can be used to determine the feasibility of a desired separation by continous distillation. To assess feasibility, so-called pinch point trajectories are established as the limits of separations achievable in each column section. These trajectories may be determined graphically using residue curve maps. The feasibility criterion is generalized to result in a method to establish the ranges of top and bottom product compositions achievable by a single-feed distillation column for a given ternary feed. One particularly interesting application is to reveal where and to what extent distillation boundaries for azeotropic mixtures, derived for total reflux, can be surpassed in columns operated at finite reflux ratios. We present a criterion to estimate the maximum reflux for such separations and illustrate process schemes that exploit the possibility of crossing of total reflux boundaries to separate azeotrope-forming mixtures. Finally, we demonstrate how intermediate heat exchangers can be used to improve separations of azeotropic mixtures across total reflux boundaries

Calibration of the Hund tyndallometer for measuring environmental tobacco smoke in a test duct

The Hund tyndallometer (Type TM Digital μP) has recently been used to study the air quality in SAS aircraft. This publication contains an evaluation of its performance in measuring environmental tobacco smoke (ETS), comparing its output with the gravimetric method which was used for measuring respirable dust concentration (van der Wal 1988). The influence of several parameters, such as the age of ETS, the type of smoke (main stream or side stream), and the relative humidity was investigated. The Hund measurements of ETS generated by a smoking machine (ISO 7210) in a test duct were calibrated against gravimetric measurements of respirable dust trapped on teflon membrane filters. An accurate calibration was, however, not possible for outputs of less than 1 mV. This is 0.1% of the electrical output range (IV) and corresponds to 100μg/m 3 of respirable dust. For times of up to 7 min and relative humidity (RH) levels between 8 and 72%, the ageing of ETS did not appear to influence the output of the Hund. RH levels in this range also did not clearly influence the instrument response factors. Higher RH levels did, however, affect the response factor, probably due to condsenation of water vapour on the particles. The average response factors for three types of cigarettes were respectively 177, 173, and 138 mg m −3 V −1. The average response factor of the Hund for measurements taken during SAS flights was 131 mg m −3 V −1, with a relative standard deviation of 24%. As some of the measurements did not meet the Hund preconditions (output <1 m V, gravimetric concentration <100 μg m −3), they meet excluded from this evaluation. The Hund tyndallometer can be a useful instrument to collect additional information of real-time peak concentrations under limiting operating conditions.

Limiting operating conditions for continuous thickeners

Parameters encountered in idealized thickeners include underflow concentration (ε su ), flow rate of solids in the underflow ( q s ), and the sediment height ( L). Commonly used design procedures based upon the Coe and Clevenger (CC) (1916 Trans. Am. Inst. Min. Engrs 55, 356–384) method only involve the solids rate q s and ε su and do not include L. Although valid for freely settling suspensions, the CC method fails when a compression zone is formed. The permeability, porosity, and unbuoyed sediment weight must then be considered. Equations relating q s and L to ε su are developed in this paper on the basis of Darcy's law. For a given underflow concentration, it is shown that there is a minimum required sediment height and a maximum underflow solids rate that cannot be exceeded. At the maximum flow rate, a saddle point is encountered in the relation between the solids concentration (ε s ) and the height ( x). Placing dε s /d x=0 and d 2ε s /d x 2=0 yields two simultaneous equations which lead to analytical equations relating q smax to ε su . The resulting equations bear a similarity to the standard CC formulas. Examples are presented for a number of constitutive equations appearing in the literature.

The pseudoadiabatic regime for catalytic fixed bed reactors: The limiting operating conditions

The pseudoadiabatic operation of fixed bed catalytic reactors was examined further. This regime provides a continuously increasing temperature profile in the axial direction and there is significant heat exchange with the coolant fluid. This coolant fluid is circulated fully cocurrently with respect to the reactants. An a priori equation was derived in the present study to define the limiting conditions between the pseudoadiabatic operation for all temperatures inside the reactor and the non-pseudoadiabatic temperature profiles for the reactor centerline. The analytical expression developed was applied to the modelling of the catalytic conversion of methanol into hydrocarbons (gasoline range).

The application of reliability-based techniques to Technical Specifications

As required by 10 CRF 50.36, plant Technical Specifications (TS) for power reactors are to include: (1) safety limits and limiting safety system settings, (2) limiting conditions of operation (LCO), (3) surveillance requirements (SR), (4) design features, and (5) administrative controls. Purportedly to make the Technical Specifications process (which operating license applicants are required to utilize) more effective and efficient, the NRC developed and required, on a forward-fit basis, the use of Standard Technical Specifications (STS) since 1975. In part, and in attempting to comply with the elements contained within TS, engineering judgment, codes and standards requirements, and manufacturer's recommendations are used as the basis for establishing testing and surveillance policies specified in the SR element of TS. However, situations have arisen which indicate that test and surveillance intervals that are either too short or too long could, through different mechanisms, be adverse to safety. On August 3, 1983, an interoffice and interdisciplinary Executive Directors Office (EDO) Task Group of the US Nuclear Regulatory Commission was formed to address some of these concerns. Their findings, documented in NUREG-1024, resulted in a directive wherein Brookhaven National Laboratory was charted to conduct the needed research to firmly anchor future TSs, TS changes, and reviews of TSs to a probabilistic risk basis. To scope out the project several issues were identified to evaluate the safety implications of technical specifications. In increasing order of importance, they are: • - What are the possible approaches for evaluating technical specifications? • - What are the possible measures of technical specification performance? • - What technical specifications aspects (attributes) need to be evaluated? • - What are the possible objectives of technical specifications? This paper describes the development of the program and some of its early findings.

Operation theory of pyroelectric detectors

A new physical model of the pyroelectric detectors of radiation is presented and the expressions for the voltage response are derived in a unitary manner under limiting operating conditions, for every modulation function of the radiation. It is found that the complete set of operation modes is described by transfer functions which are either linear, or contain an integral or a differential of the input function. The operation mode is completely determined by three parameters: the duration of the irradiation sequence, and the thermal and electrical time constants of the detector.

Theoretical calculations on the production of water from the atmosphere by absorption with subsequent recovery in a solar still

A proposed cycle for an absorbent in the production of palatable water from the air is presented. A Composition-Psychrometric chart is constructed and is then employed to combine atmospheric conditions, conditions in a solar-still recovery unit, and equilibrium properties of the absorbent such that theoretical limiting operating conditions can be established. The use of the charts is extended to show how water recovered can be changed with change in temperature-humidity in various parts of the process. An example is given to illustrate how the CP charts can be used to obtain comparisons between various potential absorbers. No attempt is made to size any equipment nor is attention given to energy requirements.

A gas-filled glow discharge character display tube

The construction of a gas-filled glow discharge display tube and its limiting operating conditions are discussed. A graphical method is described for determining the correct working point in a typical simple circuit. A number of circuits are described which show the application of the tube to typical counting systems with counting speeds ranging from a few cycles/second to 2 Mc/s. It is shown that, provided suitable precautions are taken, end of life can be predicted and failure in service is unlikely.

Optimum and limiting operating conditions for downward diffusion cloud chambers

Measurements on a diffusion cloud chamber operated with fillings up to pressures of 11 atm are interpreted according to a theoretical treatment by Shutt. The results can be used to determine temperature conditions required for good operation with any permanent gas at all useful pressures and for any given mean radiation intensity. Factors which interfere with normal operating conditions are considered.