Solid Bed Dehydration

Solid bed dehydration systems work on the principle of adsorption. Adsorption involves a form of adhesion between the surface of the solid desiccant and the water vapor in the gas. The water forms an extremely thin film that is held to the desiccant surface by forces of attraction, but there is no chemical reaction. The desiccant is a solid, granulated drying or dehydrating medium with an extremely large effective surface area per unit weight because of a multitude of microscopic pores and capillary 

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Solid-Bed Dehydration. Sihca gel, bauxite, activated alurnina, or molecular sieves can be used for removing dissolved water to meet propane specifications. The soHd-bed dehydrators are used in a cycHc adsorption process. After an adsorption cycle has completed, the bed is heated with a purge gas or a vaporized Hquid-product stream for regeneration. If the latter is used, the Hquid product is condensed, separated from the free water, and returned to the process. After the beds are regenerated, they are cooled and returned to the adsorption cycle. 

This chapter discusses the design of liquid glycol and solid bed dehydration systems that are the most common methods of dehydration used... 

Figure 8-21. Simplified flow diagram of a solid bed dehydrator.

The detailed design of solid bed dehydrators. should be left to experts. The general rules of thumb presented in this chapter can be used for preliminary design as shown in the following example ... 

Nielsen and Bucklin compared the cost of a methanol injection plant for 600 psig natural gas to costs of solid-bed dehydration. They concluded that the methanol injection plant would have lower capital and operating costs than an activated alumina system for low water content gas (4 Ib/MMscO and a lower capital cost, but about the same operating cost as a molecular sieve system for high water content feed gas (23 Ib/MMscO. 

A summary of the factors which are known to influence ethanol production from glucose in a gas-solid fluidized bed fermenter, or which may have an influence based on observations with submerged fermentations, is shown in Figure 6.1. In anaerobic beds, the key factors are the fermentation temperature and ethanol inhibition, both of which have a dramatic effect on the specific rafe of ethanol production. Bed dehydration and its influence on yeast pellet moisture content is also important, since a failure of fermentation may occur if the pellets become too dry (Bauer, 1986). 

Fixed-bed dehydration is the most widely-used method even though it is not the most economical process since it can be employed to achieve very low concentration of water (< 1 ppm) in the exit gases. Silica gel also can be used as a dehydration agent in a solid-bed operation. Further reading material relevant to this topic are listed in references [67—72],... 

In the design of solid-desiccant dehydration plants, it is important that the pressure drop through the bed be estimated as accurately as possible, because the work required to overcome this pressure drop can represent a major operating cost. 

In its simplest form, an adsorber is normally a cylindrical tower filled with a solid desiccant. The depth of the desiccant may vary from a few feet to 30 ft or more. The vessel diameter may be from a few inches to 10 or 15 ft. A bed height to diameter (L/D) ratio of higher than 2.5 is desirable. Ratios as low as 1 1 are sometimes used however, poor gas dehydration, caused by non-uniform flow, channeling and an inadequate contact time between the wet gas and the desiccant sometimes result. 

Hajek et al. [173] have reported a detailed kinetic study of the solid phase decomposition of the ammonium salts of terephthalic and iso-phthalic acids in an inert-gas fluidized bed (373—473 K). Simultaneous release of both NH3 molecules occurred in the diammonium salts, without dehydration or amide formation. Reactant crystallites maintained their external shape and size during decomposition, the rate obeying the contracting volume equation [eqn. (7), n = 3]. For reaction at 423 K of material having particle sizes 0.25—0.40 mm, the rate coefficients for decompositions of diammonium terephthalate, monoammonium tere-phthalate and diammonium isophthalate were in the ratio 7.4 1.0 134 and values of E (in the same sequence) were 87,108 and 99 kJ mole-1. 

The sorbitol solution produced from hydrogenation is purified in two steps [4]. The first involves passing the solution through an ion-exchange resin bed to remove gluconate and other ions. In the second step, the solution is treated with activated carbon to remove trace organic impurities. The commercial 70% sorbitol solution is obtained by evaporation of the water under vacuum. The solid is prepared by dehydration until a water-free melt is obtained which is cooled and seeded. The crystals are removed continuously from the surface (melt crystallization). The solid is sold as flakes, granules, pellet, and powder forms in a variety of particle size distributions. 

Selective tertiary-huimoX (tBA) dehydration to isobutylene has been demonstrated using a pressurized reactive distillation unit under mild conditions, wherein the reactive distillation section includes a bed of formed solid acid catalyst. Quantitative tBA conversion levels (>99%) have been achieved at significantly lower temperatures (50-120°C) than are normally necessary using vapor-phase, fixed-bed, reactors (ca. 300°C) or CSTR configurations. Substantially anhydrous isobutylene is thereby separated from the aqueous co-product, as a light distillation fraction. Even when employing crude tBA feedstocks, the isobutylene product is recovered in ca. 94% purity and 95 mole% selectivity. 

The specific volume of calcine will be about 40 liters/MT of heavy metal for combined HLW and MLW, corresponding to that to be expected from the AGNS plant. For final disposal, the product from the fluidized-bed calcination will have to be consolidated by melting with a glass flux. If it is to be stored for extended periods directly in sealed canisters, the calcined solid will have to be stabilized (denitrated, dehydrated) at approximately 900 C. 

A series of condensation reactions were performed in the gas phase over fixed-bed solid base catalysts. The organic feeds tested were acetone, butanone, n- and /50-butanal. The solid base catalysts were alkali doped silica materials. A reaction scheme common to all of the organic feeds is shown in Fig. 2. The first step in the reaction is the base catalysed formation of the aldol intermediate . It should be noted that from an asymmetric ketone (i.e. butanone) two possible aldol intermediates can be formed depending on which side of the carbonyl group reacts. Under our reaction conditions rapid dehydration of the aldol... 

D. N. Miller and R. S. Kirk [AIChE J., 8, 183 (1962)] studied the kinetics of the catalytic dehydration of primary alcohols to produce the respective olefins. These investigators employed a TCC silica-alumina catalyst in a fixed bed reactor operating at 1 atm and temperatures from 400 to 700° R The catalyst is characterized by a specific surface area of 350 rc /g and a porosity of ca. 0.5. Within the bed the apparent density of the catalyst is 1.15 g/cm. The density of the nonporous bulk solid silica-alumina is 2.30 g/cm. The catalyst received from the vendor was sieved to obtain five sets of particles with apparent particle diameters equal to 0.40, 2.30, 3.05, 4.06, and 5.11 mm. 

In a dynamic process, the mixture is mixed different methods of mixing can be used. Movement of food particles in a stationary solution, mixing of the whole suspension, and the flow of the osmoactive substance through the stationary layer of food pieces are the commonly used designs of the dynamic process. If crystals of the osmoactive substance are used, the fluidized bed is the solution for the dynamic process. It has been shown that the rate of motion has little effect on the rate of osmotic dehydration [71,133]. It is just sufficient to induce motion of particles or solution in the system to have increased mass transfer rates. Moreover, it was shown that the motion of osmotic solution in a turbulent region affected water flux but no difference in solids gain occurred in comparison with laminar flow [134]. 

It must also be noted that multiple PAT analyzers can be used on a single unit operation to understand the process. For example, Aaltonen et used in-line Raman spectroscopy and in-line NIRS to monitor the dehydration of MT in a lab-scale fluid bed dryer. A PLS model was generated to quantify the solid-state transformation using Raman spectra while water removal was monitored using NIRS. This proved to be an effective combination because physical and chemical information were monitored and extracted by way of NIRS and Raman spectroscopy, respectively. Such combinations of process analyzers offer enhanced capabilities to improve process understanding and control. 

The catalyst can be deactivated by the modification of the acid components if it does not contain the proper amount of water.Excessive water causes the catalyst to get soft and the pellet to collapse, increasing the pressure drop in the bed. Too little water dehydrates the phosphoric acid to inactive polyphosphoric aeids therefore small quantities of water are added to the reaction mixture to maintain the optimum degree of hydration. 350 — 400 ppm H2O is the range for 473 — 593 K. The solid phosphoric acid catalysts are not easily regenerable. 

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