L. T. Fan

Fault-Tree Analysis by Fuzzy Probability

In conventional fault-tree analysis, the failure probabilities of components of a system are treated as exact values in estimating the failure probability of the top event. For many systems, it is often difficult to evaluate the failure probabilities of components from past occurrences because the environments of the systems change. Furthermore, it might be necessary to consider possible failure of components even if they have never failed before. We, therefore, propose to employ the possibility of failure, viz. a fuzzy set defined in probability space. The notion of the possibility of failure is more predictive than that of the probability of failure; the latter is a limiting case of the former. In the present approach based on a fuzzy fault-tree model, the maximum possibility of system failure is determined from the possibility of failure of each component within the system according to the extension principle. In calculating the possibility of system failure, some approximation is made for simplicity.

Activated carbons prepared from phosphoric acid activation of grain sorghum

The production of activated carbons from grain sorghum with phosphoric acid activation has been studied by means of two processes, i.e., one-stage and two-stage. The former comprises simultaneous carbonization and activation after impregnation; the latter, the carbonization of the precursor at 300 degrees C for 15 min, followed by the activation of the resultant char after impregnation with phosphoric acid. The preparation conditions, e.g., activation duration, phosphoric acid concentration, and activation temperature, have been varied to determine the optimal processing conditions. The optimal activation conditions for the highest surface areas have been determined to be 600 and 500 degrees C with a phosphoric acid concentration of 35% for the one-stage and two-stage processes, respectively. The two-stage process has been found to greatly enhance the porosity development, especially the microporosity.

Fluidized-bed steam gasification of rice hull

Steam gasification of grain by-products can be a significant biomass conversion technology because of the need to utilize agricultural waste for non-food applications including energy resources. The most obvious beneficiary will be the developing countries whose economies are often tied to agricultural produce and are lacking in conventional fuels. One agricultural by-product that shows promise is the rice hull; it is found in abundance in the rice mills of producer countries and is considered as a waste material. Although gasification of rice hull has been proposed as a potential waste disposal and energy recovery method, little has been done to fully realize this proposition. In the present work, data were obtained for steam gasification of rice hull in a bench-scale fluidized-bed gasifier, a technology which has proven to be feasible for other grain by-products. The produced gas, which is rich in hydrogen, has been found to have a heating value ranging between 12.1 and 11.1 MJ m−3 at the respective reactor temperatures of 700 and 800°C; energy recovery varies between 35 and 59%.

Systems Reliability Subject to Multiple Nonlinear Constraints

A simple computational procedure has been developed for maximizing reliability of multistage parallel systems subject to multiple nonlinear constraints. It appears that the procedure can be applied to a variety of optimization problems with separable objective and multiple constraint functions.

Optimization of System Reliability by the Sequential Unconstrained Minimization Technique

The reliability of a complex system was optimized by a new approach for implementing the sequential unconstrained minimization technique (SUMT) with the aid of Hooke and Jeeves pattern search and heuristic programming. Two optimization problems were considered. In the first, the system reliability was maximized subject to a nonlinear weight constraint. In the second, the weight of the system was minimized without violating the requirements of the minimal system reliability and the minimum reliability for each component. The sensitivities of the system reliability and that of the system weight to the reliability of each component were determined under optimal conditions.

Grain-based activated carbons for natural gas storage.

Natural gas has emerged as a potential alternative to gasoline due to the increase in global energy demand and environmental concerns. An investigation was undertaken to explore the technical feasibility of implementing the adsorbed natural gas (ANG) storage in the fuel tanks of motor vehicles with activated carbons from biomass, e.g., sorghum and wheat. The grain-based activated carbons were prepared by chemical activation; the experimental parameters were varied to identify the optimum conditions. The porosity of the resultant activated carbons was evaluated through nitrogen adsorption; and the storage capacity, through methane adsorption. A comparative study was also carried out with commercial activated carbons from charcoal. The highest storage factor attained was 89 for compacted grain-based activated carbons from grain sorghum with a bulk density of 0.65 g/cm(3), and the highest storage factor attained is 106 for compacted commercial activated carbons (Calgon) with a bulk density of 0.70 g/cm(3). The storage factor was found to increase approximately linearly with increasing bulk density and to be independent of the extent of compaction. This implies that the grain-based activated carbons are the ideal candidates for the ANG storage.

FUZZY LINEAR REGRESSION ANALYSIS OF CELLULOSE HYDROLYSIS

The dependence of enzymatic hydrolysis rate on the structural features of lignocellulosics, such as specific surface area, crystallinity index, and lignin content, has been examined. A model for this relationship has been proposed and linear regression based on fuzzy set theory, or the so-called fuzzy linear regression has been employed for estimation of model parameters. The resultant fuzzy model appears well suited for representing complex or ill-conceived processes, e.g., biological processes. This fuzzy model is essentially a quantitative translation of the human perception of the process.

Wood Chip Gasification in a Commercial Downdraft Gasifier

Fixed bed and moving bed gasifiers for the production of low Btu gas from wood and charcoal were widely used in Sweden and other countries during the World War II era. The Swedish experience was compiled by the Swedish Academy of Engineering, and this work was recently translated by Reed and Jantzen.1 After the war, the need for gasifiers dwindled although the Swedes continued their development efforts.

Liquid Radial Dispersion in Liquid-solid Circulating Fluidized Beds with Viscous Liquid Medium

Abstract Liquid dispersion in the radial direction was investigated in the riser of a viscous liquid-solid fluidized bed 0.102 m in diameter and 3.5 m in height. Pressure fluctuations in the riser were also measured and analyzed to examine the behavior of fluidized particles. Effects of liquid velocity (0.15–0.45 m/s), solid circulation rate (2–8 kg/m2s), particle size (1–3 mm), and liquid viscosity (0.96–38 mPas) on pressure fluctuations and the liquid radial dispersion coefficient were determined. The infinite space model was employed to obtain the radial dispersion coefficient from the radial concentration profiles of the tracer. The pressure fluctuations were analyzed by means of autocorrelation coefficient as well as power spectral density function. The dominant frequency obtained from the autocorrelation coefficient or power spectral density function of pressure fluctuations decreases with increasing liquid viscosity or liquid velocity, but it increases with increasing particle size. The liquid radial dispersion coefficient decreases with increasing liquid velocity or viscosity, but it increases as the solid circulation rate or particle size increases. The liquid radial dispersion coefficient is related closely to the resultant behavior of fluidized particles. The radial dispersion coefficient has been well correlated with operating variables in terms of dimensionless groups.

Optimization of Life Support Systems Reliability by an Integer Programming Method

Abstract The problem considered is to minimize the weight of the subsystems of a life support system subject to several separable nonlinear constraints while maintaining an acceptable level of reliability of the system. Zero-one integer programming is used to solve this problem. The subsystems of the life support system designed by the Space Division of the North American Rockwell Corporation are studied examples. They are the refrigerant circuit, the cryogenic oxygen circuit, the carbonization cell, and the water reclamation unit. The results obtained are compared with the original design proposed by the North American Rockwell Corporation. The comparison indicates that the use of the integer programming for determining the optimization reliability would result in an improved arrangement of the standby units.