Liquid-entrained reactor

Several types of reactors have been proposed such as the liquid entrained reactor(l) and the Trickle bed reactor(2). The authors have been studying a liquid-phase methanol synthesis process in order to develop a new technology as an alternative for a gas-phase process, and reported that a new process employing liquid-liquid separation of the products from the solvent has several advantages in practical methanol synthesis(3). 

Vijayaraghavan, P. Kulik, C.J. Lee, S. Modelling of a liquid entrained reactor for liquid phase methanol synthesis process. Fuel Sci. Technol. Int. 1992, 10 (9), 1501-1521. 

An important relatively recent achievement is the development of the so-called liquid entrained reactor (LER) for liquid phase methanol synthesis. To assess the performance of these reactors, much simultaneous work has been done on MARs, comparing the performance of the two types of reactors for this reaction. A good discussion of this can be found in a paper by Vijayaraghavan et a. (1993). An equation developed for the overall gas-liquid mass transfer coefficient (see Ko, 1987 Lee et al., 1988 Parameswaran et al., 1991) is... 

Multiphase reaction engineering has developed into a very active field of research, with international symposia held at frequent intervals. We have only considered a few common reactor types currently in use. A few others of potential importance in relatively large volume intermediates production are the gas-liquid-solid fluidized beds, liquid entrained reactors, and rotating drum reactors. 

Pressure drop Fixed-, moving-bed, and BFB G/S reactors Liquid-phase reactors Generally all deep beds Entrained-bed reactors... 

New catalysts have helped increase the conversion and yields. The older, high-pressure processes used zinc-chromium catalysts, but the low-pressure units use highly active copper catalysts. Liquid-entrained micrometer-sized catalysts have been developed that can convert as much as 25 percent per pass. Contact of the synthesis gases with hot iron catalyzes competing reactions and also forms volatile iron carbonyl that fouls the copper catalyst. Some reactors are lined with copper. 

A comparison between a liquid entrained slurry reactor and a mechanically agitated slurry reactor for methanol synthesis was made by Vijayaraghavan et al. [11]. 

During the last few years, interest has increased in reactors in which high mass transfer is realized by means of liquid jet or injection devices providing gas or liquid entrainment jet reactors (N3, B30), hydrocyclones (Bll), venturis (S16, J2), and other high-velocity gas-liquid contactors. 

In 1979 Chem Systems initiated a program to develop a liquid-entrained catalyst reactor which would provide improved contacting of syngas with the catalyst in a three phase system (ref. 38). This reactor system uses much finer catalyst particles than the fluidized bed reactor, and the catalyst-liquid slurry circulates through the reactor. The syngas can be contacted with the catalyst-liquid slurry either counter currently or co-currently. It appears that this process is more efficient than the original fluidized bed process. However, a major problem with this type of three phase system will no doubt be the development of a suitable catalyst since it is unlikely that conventional co-precipitated Cu-ZnO-A Oj catalysts will have the desired characteristics, particularly mechanical strength. 

In case of processes operating with catalyst systems where pulverized catalyst is circulated with the aid of a carrier liquid (liquid entrained system) or small-lump catalyst maintained in suspension in a liquid (liquid fluidized system), it is possible to transfer the reaction heat inside the reactor to the carrier liquid and to remove it outside of the reactor. The various means of temperature control inside and outside the catalyst chamber are schematically shown in Fig. 3.3. 

Fig.3.3a-d. Various types of methanol synthesis reactors, (a) Cold gas quench (b) cooling by evaporation - multistage, adiabatic (c) cooling by evaporation - tubular, near isothermal (d) liquid entrained system using heat carrier liquid... 

Chem Systems operate with methanol catalyst attainable on the market but do, however, carry out their own development work also. This is mainly directed at the application of the methanol produced as combustion fuel (not as motor fuel) as this application is definitely in the foreground of the Chem Systems endeavors regarding process development. Crude methanol produced in the reactor with the liquid-entrained catalyst is particularly little suited to manufacturing pure methanol due to its high percentage of byproducts as treatment of this methanol is technically difficult and costly. 

Evans GM, Jameson GJ. (1991) Prediction of the gas film entrainment rate for a plunging liquid jet reactor. In Proceedings of AICHE Symposium on Multiphase Reactors, Houston, TX, USA. 

The increased compactness of the next generation of liquid metal reactors is accompanied by greater complexity of the hydraulic and thermal problems, and a need to decrease uncertainties, particularly for gas entrainment phenomena, temperature fluctuations for thermal striping, and modelling of the core outlet area. 

Moderate intensity kLa of order 0.05 s for fast reactions with other slower steps where particle suspension and/or heat transfer require enhancement. Agitated vessels are useful here, and indeed are often selected where the intensity needs are uncertain, or may vary widely (as in general-purpose reactors). The larger top surface area per unit volume than can be achieved with bubble columns allows higher exit gas flow rates without liquid entrainment and carryover. 

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

Reactor stripper. Operational and hardware changes to the stripper improve its performance by removing the entrained and adsorbed hydrocarbons. The benefits are lower delta coke and more liquid products. 

Since much of the vented material will be liquid, separators such as knockout pots or tangential entry separators can provide disengagement and possible recovery. Figure 5 is a typical vapor-liquid separator design found to be effective for these applications. Inlet design superficial vapor velocity is about 100 ft/sec, with sufficient volume provided to accumulate the entire reactor liquid contents. The lip on the outlet vapor line and the horizontal plate to separate the accumulated liquid are important features to prevent re-entrainment. 

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