Gas-fed slurry reactors

These reactors contain suspended solid particles. A discontinuous gas phase is sparged into the reactor. Coal liquefaction is an example where the solid is consumed by the reaction. The three phases are hydrogen, a hydrocarbon-solvent/ product mixture, and solid coal. Microbial cells immobilized on a particulate substrate are an example of a three-phase system where the slurried phase is catalytic. The liquid phase is water that contains the organic substrate. The gas phase supplies oxygen and removes carbon dioxide. The solid phase consists of microbial cells grown on the surface of a nonconsumable solid such as activated carbon. 

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Usually, the typology of batch reactors also includes the semi-batch gas-liquid reactors, in which a gaseous phase is fed continuously in order to provide one of the reactants. A typical example is given by the reactors used both in different oxidative industrial processes and in the active sludge processes for the treatment of wastewater. It is possible to distinguish between the bubble columns (Fig. 7.1(c)), in which the gas rises undisturbed in the liquid phase, and the bubble stirred reactor, in which a mechanical mixer is added. Finally, the slurry reactors can be considered, in which the liquid phase contains a finely dispersed solid phase as well, which can act as a reactant or as a heterogeneous catalyst these reactors assume in general the features of Fig. 7.1(d). 

An aqueous solution containing 7.00 wt% sodium carbonate and a gas stream containing 70.0 mole% CO2 and the balance air are fed to the reactor. All of the sodium carbonate and some of the carbon dioxide in the feed react. The gas leaving the reactor, which contains the air and unreacted CO2, is saturated with water vapor at the reactor conditions. A liquid-solid slurry of sodium bicarbonate crystals in a saturated aqueous solution containing 2.4 wt% dissolved sodium bicarbonate and no dissolved CO2 leaves the reactor and is pumped to a filter. The wet filler cake contains 86 wi% sodium bicarbonate crystals and the balance saturated solution, and the filtrate is also saturated solution. The production rate of solid crystals is 500 kg/h. 

There are mainly two types of F-T reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurized boiling water. The other process uses a slurry reactor in which preheated synthesis gas is fed to the bottom of the reactor and distributed into the slurry consisting of liquid and catalyst particles. As the gas bubbles upwards through the slurry, it is diffused and converted into liquid hydrocarbons by the F-T reaction. The heat generated is removed through the reactor s cooling coils where steam is generated for use in the process. 

Figure P4.20 shows the Amoco Model 4 lluidizcd-bed catalytic cracking unit. Several hydrocarbon feeds (gas oil, slurry recycle, etc.) are fed into the reactor alotig with hot solid catalyst from the regenerator. The endothermic cracking reactions cool the catalyst and deposit coke on it. The catalyst from the reactor is circulated back to the regenerator, where air is added to bum off the coke. The heat from the combustion reaction heats the catalyst. 

A slurry of nickel (catalyst) powder in oil is fed into the slurry reactor and the gas containing H2 is bubbled through the slurry. Slurry containing the nickel and hydrogenated... 

The particular characteristics of three-phase fluidized bed reactors have been covered in several recent reviews by Ostergaard [1], Wild [2], Epstein [3], Baker [4] and Muroyama and Fan [5]. Epstein [3] distinguished in particular the difference between three-phase fluidized beds and slurry reactors. In slurry reactors the size of the solid particles is normally smaller than 0.1 mm while in three-phase fluidization the particle diameter is bigger than 0.2 mm. The volumetric solid fraction is another significant difference, being 10% or below for slurry reactors and between 20-40% in three-phase fluidized bed units. In three-phase fluidized beds the particles are supported by the liquid and/or the gas while in slurry reactors the solid particles are suspended by the momentum transferred from the gas bubbles to the liquid and from the liquid to the solids. In slurry reactors the solid particles are normally carried into and out of the unit by the liquid stream. In three-phase fluidized beds the solids are not transported out of the unit by the liquid stream, they are fed and withdrawn independently of the liquid stream [3]. Epstein [3] introduced an interesting classification for three-phase reactors and particularly proposed four modes of operation for three-phase fluidized beds Mode I Cocurrent upflow circulation of gas and liquid with the liquid as a continuous phase. Mode II cocurrent upflow circulation of gas and liquid with the gas as a continuous phase. Mode III Countercurrent circulation of gas (upflow) and liquid (downflow) with the liquid as the continuous phase. Mode IV Countercurrent circulation of gas (upflow) and liquid (downflow) with the gas as the continuous phase. 

In the process (Figure 8-4), the feedstock is mixed with coal powder in a slurry mixing vessel, heated in the furnace and fed to the reactor where the feedstock undergoes thermal cracking reactions for several hours at a temperature higher than 400°C (750°F) and under pressure. Gas and distillate from the reactor... 

From the lower section of the reactor, bottom slurry oil (SLO) which contains catalyst, uncracked residuum and a small amount of vacuum gas oil fraction are withdrawn. Vacuum gas oil is recovered in the slurry separation section, and the remaining catalyst and coke are fed to the regenerator. 

One of the drawbacks associated especially with slurry and solution CSTR processes is the necessity of removing the solvent or diluent in a post-production step. In a gas phase reactor the polymerisation takes place in a fluidised bed of polymer particles. Inert gas or gas mixture is used for fluidisation. The gas flow is circulated through the polymer bed and a heat-ex-changer in order to remove the polymerisation heat. Gaseous ethylene and comonomer are fed into the fluidisation gas line of the reactor, and a supported catalyst is added directly to the fluidised bed (Fig. 7). Polymerisation occurs at a pressure of about 20-25 bar and a temperature of about 75-110 °C. The polymer is recovered as a solid powder which is, however, usually pelletised. Due to the limited cooling capacity of the fluidising gas, reactor... 

Pure SO2 is injected to achieve the desired concentration in the flue gas. Regeneration of spent scrubbing liquor is performed in the four-tank-in-series reactor train with a total residence time of 80 minutes. Limestone is fed to the first reactor as 45% slurry. 

Polymerization reactors contain one or more phases. There are examples using solvents in which the reactants and products are in the liquid phase, the reactants are fed as a liquid (gas) but the products are solid, or the reactants are a slurry and the products are soluble. Phase transformations can occur, and polymers that form from the liquid phase may remain dissolved in the remaining monomer or solvent, or they may precipitate. Sometimes beads are... 

The similar, older slurry process uses a less active catalyst. The monomer is dissolved in isooctane, the titanium catalyst and aluminium cocatalyst are added and this mixture is fed to the reactor which is maintained at 70°C. The inorganic corrosive (Cl) residues are removed in a washing step with alcohols. The atactic material is removed by extraction. A third process employs propene as the liquid in combination with a high activity catalyst. The Himont Spheripol process, which uses spherical catalyst particles, gives spherical polymer beads of millimetre size that need no extrusion for certain purposes. A more recent development is the gas-phase polymerization using an agitated bed. All processes are continuous processes, where the product is continuously removed from the reactor. Over the years we have seen a reduction of the number of process steps. The process costs are very low nowadays, propene feed costs amounting to more than 60% of the total cost. 

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