Movement, cocurrent-countercurrent

This section presents a classification of conversion concepts (read section 0), described in the literature on PBC, for example, fuel-bed mode (batch or continuous) and fuel-bed configuration (cocurrent, countercurrent, crosscurrent). Some new conversion concepts, namely fuel-bed movement (fixed, moving and mixed) and fuel-bed composition (homogeneous, heterogeneous), are introduced by these authors, which makes this an extended classification compared with earlier classifications in literature [8,9,10,11], The classification is made in the context of the three-step model. The three-step model, see Figure 14 below, is a system theory, which places the theory of thermochemical conversion of solid fuels into the context of PBC. 

The fuel-bed system may also be characterised with respect to what is here called fuel-bed configuration. Fuel-bed configuration may be defined as the relative movement between air flow (interstitial gas phase) and solid phase flow [19,38], Three basic configurations arise as a consequence of the possible 90°-combinations and are commonly referred to as cocurrent, countercurrent and crosscurrent., see Figure 27. 

The classification is based on the different conversion concepts cited in the literature, such as fuel-bed mode (batch and continuous), fuel-bed configuration (cocurrent, countercurrent, and crosscurrent), and some new concepts presented by the author, such as fuel-bed movement (fixed, moving, and mixed) and fuel-bed composition (homogeneous and heterogeneuos). The classification resulted in 18 types of updraft conversion systems, according to Figure 32. Some of them are more or less hypothetical, while others are found in practice, see section B 3.4 below. 

According to the three-step model, proposed by the authors, a PBCS can be divided into three subsystems, namely a conversion system, combustion system, and boiler system. It is in the conversion system that the thermochemical conversion of the solid fuel takes place. The conversion system can be designed according to several conversion concepts. The conversion concept can be classified with respect to fuel-bed mode (batch and continuous), fuel-bed configuration (countercurrent, cocurrent and crosscurrent), fuel-bed composition (homogeneous and heterogeneous), and fuel-bed movement (fixed, moving and mixed). 

Gas Flcnc Pattern in Dryer Where there is a significant gas flow, it may be in cross-flow, cocurrent, or countercurrent flow compared with the direction of solids movement. 

Movement of fuel countercurrent flow vortex cocurrent flow cocurrent flow... 

Mixed flow Particle movement through the drying chamber experiences both cocurrent and countercurrent phases. 

Moving-bed reactors are preferred when there is a need for continuous catalyst regeneration. In this operation, fresh catalyst is fed from the top of the reactor, and it moves in the downflow direction by gravitational forces. Spent catalyst leaving the reactor at the bottom is usually replaced in the continuous mode. While the catalyst movement is downward, reactive mixture flow can be cocurrent or countercurrent to that of the catalyst flow. 

The term moving bed arises from the mode in which the spent catalyst is replaced. The catalyst bed is displaced periodically downward by gravitational forces. The fresh catalyst enters at the top of the reactor, and the deactivated catalyst leaves the reactor through the bottom. Liquid flow can be supplied either cocurrently or countercurrently with respect to the movement of the bed. The rate of deactivation determines how frequently the catalyst is replaced. Commonly, catalyst replacement is a batch operation and is done once or twice a week [68]. 

Spray droplet movement is classified according to the dryer chamber layout and can be designated as cocurrent, countercurrent, or mixed flow although these designations are not complete representations of the actual conditions. 

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