The efficiency of a separation process using membranes is often hindered by membrane fouling and concentration polarization. As a consequence, membrane cleaning costs increase, membrane life is shortened, and/or more capital is spent on purchasing more membrane area. The current practices to minimize the effect of these factors on membrane performance are classified into four general groups based on their mechanisms and are briefly reviewed in this paper. One of the approaches, the external fields approach, utilizes dc electric field and its electrokinetics as one of the means to control membrane fouling. The electric field approach is the major focus of this paper. During the electric field application, the phenomena of electrophoresis and electroosmosis occur as a result of electrokinetics. Besides these phenomena, electrolysis, Joule's heating, ion migration, etc., also occur. One or more of these mechanisms simultaneously act on the applied membrane system and improve its performance. The electric field's application, its mechanism, and examples are discussed in this paper.

The p o t e n t i a l o f u l t r a s o n i c energy t o dry heat-sensitive materials was proven by studies conducted a number o f years ago. Ultrasonic i r r a d i a t i o n improved the r a t e s o f drying and the temperatures seldom exceeded 1 C above ambient. Recent developments i n energy related areas of technology have caused a new search f o r more economical methods o f drying. This paper reviews both past and present l i t e r a t u r e on acoustic dewatering and drying and discusses the current s t a t e o f the technology. The studies reviewed represent a broad spectrum including e f f e c t s a t very low sonic t o u l t r a s o n i c frequencies. The fundamental aspects and applications o f the technique t o a v a r i e t y o f materials are presented. INTROOUCTION Suggestions t o use u l t r a s o n i c energy f o r drying materials were made many years ago. Ultrasonics was especially appealing as a means o f drying heat-sensitive materials, such as pharmaceuticals, where rapid drying could prevent spoilage, i f damaging heat levels could be avoided. Ultrasonic drying proved t o meet both c r i t e r i a --d r y i n g was r a p i d and temperatures seldom exceeded 1 C above ambient. Although the u l t r a s o n i c method was e f f e c t i v e , i t was also expensive and i t s usefulness was l i m i t e d t o these more exotic applications. Copyrisht O 1985 by Marcel Dekker, Inc. Recent developments i n energy r e l a t e d areas o f technology have caused a new search f o r more e f f e c t i v e and more economical methods o f drying. This search has produced a renewed i n t e r e s t not o n l y i n the possible use o f ultrasound i n drying but a l s o i n the r e l a t e d area o f dewatering. Ultrasonics shows considerable promise as a p o t e n t i a l l y economical means o f removing water from c e r t a i n types o f products t o low levels, especially when i t i s used i n combination w i t h other techniques. This paper reviews both past and recent l i t e r a t u r e on acoustic drying and dewatering and discusses the current status of the technology. Included i n the review are the references not only t o u l t r a s o n i c and high sonic frequencies, but also t o some r e l a t e d drying l i t e r a t u r e i n the low sonic range. The fundamental aspects and applications o f the drying techniques t o a v a r i e t y o f materials are presented. DRYING AND DEWATERING--DEFINITIONS Drying i n the present context r e f e r s t o removing moisture from a product by vaporization, i.e., the water changes from l i q u i d phase t o vapor (or steam). Therefore, a s i g n i f i c a n t amount o f the energy f o r drying i s consumed i n supplying the heat o f vaporization. Theoretically, the amount o f energy required t o evaporate a pound o f water i s approximately 1.055 x lo6 Joules. Dewatering r e f e r s t o removal o f water from a product without producing a phase change i n the water. I t can be considered a " p o s t -f i l t r a t i o n u ( l ) process. During the dewatering phe...

The kinetics of the coal char-C02 reaction have been studied in the absence of effects of particle size, bed size, and gas film diffusion. The reaction rate is shown by multiple regression to fit an expression involving intrinsic carbon activity and dominant terms involving the weight percent calcium oxide (from ash) and residual oxygen in the char. There is a cross-product interaction term between the weight percent CaO and oxygen which indicates an apparent catalytic mechanism. A correlation coefficient of 0.978 was obtained for the proposed expression. Good agreement was obtained with previous published results on reactivity. If the activation energy for the intrinsic carbon reactivity is taken at 70 kcal/mol, a value of 55 kcal/mol for the "catalytic" reactivity is found.

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