Solids Holdup and Pressure Drop in Gas−Flowing Solids−Fixed Bed Contactors

Abstract

Fundamentally based model for pressure drop in gas-flowing solids-fixed bed contactors is presented, together with a phenomenological semiempirical model for prediction of dynamic holdup. These simple models do not require any parameters that need to be determined by measurements in the actual system of interest. The predictions are compared with all available data and give good agreement for a wide range of experimental conditions, different constructions, types, and dimensions of packing and for a variety of flowing solids properties.Countercurrent flow of gas and fine solids through packed beds was patented as an idea in 1948, 1 and the first recorded industrial use occurred in 1965 (Compagnie de Saint Gobain) 2 in heat-transfer applications. The fluid dynamics of such systems received considerable attention over the years, 3-14 and this included heat-and mass-transfer studies. 8,[15][16][17] The interest in exploiting the unique features of the countercurrent gas-fine solids systems was enhanced by the studies of Westerterp and colleagues, 18,19 who proposed the use of fine solids as a regenerative adsorbent flowing through the bed of catalyst for methanol synthesis. Additional reactor-oriented studies included catalytic oxidation of hydrogensulfide 20 and regenerative desulfurization of flue gases. 17,21 The application of this type of gas-flowing solidsfixed bed contactors would be enhanced if the fluid dynamics in these systems could be fully quantified, at least in the macroscopic engineering sense. Reliable prediction of pressure drop, flowing solids holdup, residence time distribution, back mixing, and so forth, are some of the quantities needed when assessing the applicability of the "flowing" or "trickling solids" systems in a variety of processes.Previous studies resulted in a semiquantitative description of the fluid dynamics of the system and empirical correlations for determination of some quantities of interest. Three flow regimes were observed, similar to gas-liquid systems: preloading, loading, and flooding. Gas-flowing solids interaction increases with the increase in gas superficial velocity. When the terminal velocity of gas relative to flowing particles is approached, a sudden increase in pressure drop and fine solids holdup occurs, together with accumulation of solids at the top of the bed and unstable operation, which is characteristic for flooding.The complexity of the fluid dynamics of these systems did not permit, so far, a unique pressure drop equation to emerge without empirical constants. The problem is further complicated by the lack of consensus in accounting for the effects of particle shape, size, roughness, bed porosity distribution, and so forth. The models presented were often developed by fitting the data of a few studies and were not extensively tested, thus lacking in predicting ability. Moreover, the approach used was such that data on the system of interest were always needed to complete the correlation (i.e., one had to have data to predict them!). 6,10 In our earlier s...

Adsorptive systems for post-combustion CO2 capture : design, experimental validation and evaluation of a supported amine based process

“…Regarding the support material, porous carbons [19], zeolites [20], polymers such as poly(methyl methacrylate) [21] and polystyrene [22], and silica's [23][24][25] have all been considered as support candidates. A porous support will provide an open, accessible backbone resulting in a large contact area for the contact between amine and CO2.…”

“…This seems trivial but this implies that (1) the sorbent can be regenerated while producing a high purity CO2 gas, (2) the sorbent can be saturated with CO2 within a reasonable time frame (no more than a few minutes) and (3) the sorbent is suitable for application in a gas-solid contactor which places restrictions on, for instance, the sorbent particles size. 24 Cutting the cost of carbon capture Like CO2, H2O is a combustion product. Consequently, flue gas emitted by power plants also contains water.…”

“…the CO2 capacity does not degrade in presence of H2O ( Figure 3). In many cases H2O was even found to promote the CO2 capacity [5,18,24,42,43]. This phenomenon is quite common for supported amine sorbents and is usually attributed to the interference of H2O in the adsorption mechanism.…”

“…Still, these sorbent materials also capture significant amounts of H2O under conditions relevant for post-combustion CO2 capture. Franchi et al [18] reported an adsorption capacities for DEA on pore expanded MCM-41 of 5.37 mol/kg at 28% relative humidity (Rh), and Xu et al [24] measured the adsorption capacity for PEI on MCM-41 to be 2.63 mol/kg and 3.24 mol/kg at 26% Rh and 31% Rh respectively. Serna-Guerrero et al reported capacities up to 7.5 mol/kg for aminopropylgrafted pore-expanded MCM-41 silica at 74% Rh [44].…”

“…In summary, the role of water in this process is complex as the H2O present in flue gas (1) interferes with the CO2 adsorption mechanism [18,24] and affects (2) the sorbent stability [42,[46][47][48][49] as well as (3) the process energy demand. However, the studies on the H2O adsorption by supported amine sorbents are limited.…”