Specific surface area, heat exchangers

Micro heat exchangers and also any kind of micro channel devices, heated or cooled externally, offer considerably improved heat transfer owing to their large internal specific surface areas. Hence they offer unique possibilities to steer oxidations to increased selectivity of the partial-oxidation products. 

The determination of surface properties of particles is an important key to understanding interactions of trace elements and organic compounds between particulate and dissolved phases in estuarine and coastal systems. Specific surface area (SSA), cationic exchange capacity (CEC) and heat of immersion (AH) have been measured on native and treated suspended sediment and after oxidation with 15% H202- SSA and A H have also been measured on samples leached with NaOH and Na-dithionite in order to remove amorphous aluminosilicates. 

In the present study, amorphous silica-alumina nanomaterials with controlled mesoporous distribution have been synthesized by two templateless approaches (1) vacuum-sol process, and (2) ultrasonic-sol process. It is found that the preparation method affects the precursor sol properties and the specific surface area and pore volume of the final materials. Ultrasonic-sol method favors the formation of monodispersed sol particles with narrow size distribution. Because of several base-exchange cycles and absence of drying process prior to heat treatment, the gel derived fiom ultrasonic-sol method may have enough stiffiiess to protect the network fiom pore collapse by capillary force, thus, leading to produce the materials with... 

Table 3.3 Overview of heat exchanger types and their specific surface area. (Adapted from Ref [22].)...

The surface to volume ratio for microdevices can be as high as 50 000 m m [4]. For comparison, the specific surface area of typical laboratory and production vessels seldom exceed 100 m m . Moreover, because of the laminar flow regime within microcapillaries, the internal heat transfer coefficient is inversely proportional to the channel diameter. Therefore, overall heat transfer coefficients up to 25000 W m" can be obtained, exceeding those of conventional heat exchangers by at least 1 order of magnitude [5]. Indeed, conventional heat exchangers have overall heat transfer coefficients of less than 2000 W m [6]. 

Because of the small thermal diffusion paths and the high specific surface area, micro heat exchangers are used for rapid heat transfer between hot and cold fluids. Compared to conventional heat exchangers, the size can be considerably... 

The exchanges with and Cs" from NaY lead to a decrease of the surface areas and micropore volumes (Table 1) for HY and CsY and also to a decrease of the intensities in the XRD diffractograms (Fig. 1). For HY, the dealumination process which occurs in the protonated zeolites may lead to a partial breakdown of the lattiee. For CsY, Cs cation is bigger than Na and the exchange is rather difficult [6]. Therefore, several exchanges are needed and as the sample is heated many times, a partial breakdown of the zeolites could occur. The HY and CsY samples thus contain a certain amount of amorphous material of which the adsorptive properties are not known. The proportion of amorphous material may be estimated from the specific surface area ratio considering the NaY sample as the reference. This approximation leads to a proportion of amorphous material equal to 25.1% for HY and to... 

Microstructured reactors (microreactors) represent a new type of reaction equipment for applications in chemistry. Small dimensions of microchannels provide a short diffusion time, better temperature and pressure control, large specific surface area, high heat exchanging efficiency, and a higher level of safety [1]. 

When heat-exchange surface is being provided in the design of an absorber, the isothermal design procedure can be rendered valid by virtue of the exchanger design specifications. With ample surface area and a close approach, isotherm operation can be guaranteed. 

Before eonsidering the effects of water injection in an EGT type plant, it is worthwhile to refer to the earlier studies on the performanee of some dry recuperative cycles. Fig. 6.6 shows the T..s diagram of a [CBT i X r cyele, with a heat exchanger effectiveness of unity. It is implied that the surface area for heat transfer is very large, so that the outlet temperature on the cold side is the same as the inlet temperature on the hot side. However, due to the higher specific heat of the hot gas, its outlet temperature is higher than the inlet temperature of the cold air. 

Fig. 4.25 represents a steady-state, single-pass, shell-and-tube heat exchanger. For this problem W is the mass flow rate (kg/s), T is the temperature (K), Cp is the specific heat capacity (kJ/m s), A (= 7i D Z) is the heat transfer surface area (m ), and U is the overall heat transfer coefficient (kJ/m s K). Subscripts c and h refer to the cold and hot fluids, respectively. 

Objective functions that allow only discrete values of the independent variable ) occur frequently in process design because the process variables assume only specific values rather than continuous ones. Examples are the cost per unit diameter of pipe, the cost per unit area for heat exchanger surface, or the insulation cost considered in Example 1.1. For a pipe, we might represent the installed cost as a function of the pipe diameter as shown in Figure 4.2 [see also Noltie (1978)]. For... 

Taking the specific net power output (net output power per unit total heat exchanger surface area) as the design objective function, optimize the warm-side (heater or high-temperature side heat exchanger) and cold-side (cooler or low-temperature side heat exchanger) working fluid temperatures. 

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