C.L. Hwang

Analysis and optimization of a reverse osmosis water purification system Part I. Process analysis and simulation

Abstract A mathematical model of a reverse osmosis water purification system that can be used in process optimization studies is developed. A boundary layer flow model is used to relate water production rate to the operating pressure, Reynolds number, and membrane area. Cost equations that relate the capital and operating costs to the design variables are also developed. These relations are then used in economic analysis of several reverse osmosis systems. The results of computer simulation of single stage processes are presented. The model is believed to be accurate enough to describe the process and yet simple enough to be used in the simulation, design, and optimization of the process.

Analysis and optimization of a reverse osmosis water purification system—part II. Optimization

Abstract A mathematical model of a reverse osmosis water purification system that can be used in process optimization studies and cost equations that relate the capital and operating costs to the design variables is developed in Part I of this work. In this part, the model is used to determine design and operating variables of the system, which minimize the cost of water production. Several three multistage reverse osmosis systems are considered. They are a multistage operation without the use of a flow-work exchanger and with a variable membrane area at each stage (System 1), that with equal membrane area at each stage (System 2), and a multistage operation with the use of a flow-work exchanger and with variable membrane area at each stage (System 3). In the optimization study, the recirculation rate in each stage, the brine composition leaving each stage, the ratio of membrane area to feed at each stage, and the operating pressure in each stage are controlled to arrive at a minimum water production cost.

Simultaneous control of temperature and humidity in a confined space: Part 1—methmatical modeling of the Dynamic behavior of the temperature and humidity in a confined space

Abstract A pair of nonlinear differential equations which describe the transient behavior of temperature and humidity in a confined space have been derived from simultaneous material balances of dry air and water along with the enthalpy balance of moist air. The equations are sufficiently general to take into account external heat loads, and internal heat and moisture loads within the confined space. Since temperature-humidity control systems allow usually only small deviations of temperature and humidity from a desired operating point, a linearization of the above equations within the bounds of small deviations is justified. Hence the nonlinear equations are linearized around a desired steady state operating point. On using available relations for the specific volume and enthalpy of moist air, the linearized equations further result in a pair of linear uncoupled differential equations. The response from the linear equations is found to compare very favorably with that from the original nonlinear equations.

Heat transfer on magnetohydrodynamic flow in the entrance region of a flat duct

ZusammenfassungUnter Voraussetzung konstanter Wärmestromdichte an der Wand werden die Temperaturprofile und Wärmeübergangsparameter einer laminaren MHD-Strömung in der Einlaufpartie eines flachen Kanals berechnet. Geschwindigkeit und Temperatur werden im Eintrittsquerschnitt als konstant vorausgesetzt. Eine Kennzahl β wird eingeführt, die ein Kriterium für den Einfluss der Zähigkeitskräfte ist und ähnlich definiert ist wie die Eckertsche Kennzahl. Numerische Ergebnisse werden angegeben für den Wert 1 der Prandtlzahl und die Werte 4 und 10 der Hartmannzahl. Die genannte Kennzahl β wird dabei zwischen −1 und + 1, eine das elektrische Feld kennzeichnende Grösse zwischen 0,5 und 1,0 variiert.

Analysis and optimization of a multieffect multistage flash distillation system—Part I. Process analysis

Abstract A mathematical model of a multieffect multistage flash distillation system that can be used in process optimization studies is developed. The model enables the heat input to the brine heater, the recycle flow rate in each effect, the concentration and temperature of the flashing brine leaving each effect, and the number of stages in each effect to be treated as variables in an optimization study. The model is believed to be accurate enough to describe the process yet simple enough to be used in the simulation, design, and optimization of the process.

Heat transfer to magnetohydrodynamic flow in a flat duct

ZusammenfassungDie Untersuchung befasst sich mit der Wärmeübertragung von den Wänden eines flachwandigen Kanals auf eine elektrisch leitende Flüssigkeit bei erzwungener Laminarströmung und in Gegenwart eines quergerichteten Magnetfeldes. Betrachtet wird der Fall konstanter Wandtemperatur mit variierender innerer Wärmeentwicklung durch viskose und elektrische Energiedissipation. Die massgebende Differentialgleichung wird durch eine Differenzengleichung ersetzt und mit der elektronischen Rechenmaschine gelöst. Als Resultat wird die Nusseltzahl angegeben, für die Prandtlzahl 1, die Hartmannzahlen 0, 4, 10 und die Graetzzahlen von 10 bis 10 000, wobei die Kennzahlen für Zähigkeit und elektrische Feldstärke als Parameter auftreten.

Simultaneous control of temperature and humidity in a confined space: Part 2 — feedback control synthesis via classical control theory

Abstract Classical control theory is used to synthesize feedback controllers for the simultaneous control of temperature and humidity in a confined space. The decoupling or diagonalization approach is employed to reduce the multivariable control system to a pair of noninteracting control loops and the Ziegler-Nichols settings are then determined for each loop. The temperature-humidity control system is then compared with conventional temperature-only control systems.

The optimal design of desalination systems

Abstract Process analysis and optimization studies have been made for a multi-effect multi-stage flash distillation system and a multistage reverse osmosis system. In the distillation system, the heat supplied to the brine heater, the brine reject concentration, the recirculation rates within each effect, and the allocation of brine temperature and composition among the effects are controlled to arrive at an optimal policy. In the reverse osmosis process, multistage operation is compared with single stage operation. The recirculation rates in each stage, the allocation of brine composition among the stages and the operating pressures in each stage are controlled to arrive at the optimal operating condition. A flow-work exchanger is incorporated into the reverse osmosis system and its quantitative advantages are examined under optimal conditions. A discrete form of the maximum principle in combination with other optimization techniques has been used for most of the optimization studies.

Analysis and optimization of a multieffect multistage flash distillation system — part II. Optimization

Abstract A mathematical model of a multieffect, multistage flash distillation system which can be used in process optimization investigations has been developed in Part I of this work. In the present part, the model is used together with an economic model of the system to minimize the cost of water production. The heat input to the brine heater per unit of feed, the recycle flow rate in each effect, and the concentration and temperature of the flashing brine leaving each effect are chosen so that the water production cost is a minimum.

Systems analysis of dual-purpose nuclear power and desalting plants Part I. Optimization

Abstract The optimal process design of a dual purpose plant for producing power and water is investigated. A nuclear reactor and steam turbine power generator for steam and power production is coupled with two water plants, a multi-stage flash plant and a reverse osmosis plant. The total system cost for producing given levels of power and water is minimized. Optimal designs are presented for several combinations of water and external power demands ranging from 25 mgd to 150 mgd and from 50 MWe to 200 MWe.