Jet loop

In the electrolysis plant of Akzo Nobel in Rotterdam a hypochlorite production unit is in operation. This unit has two functions handling chlorine-containing waste gases from the plant and production of hypochlorite. The reaction is carried out in a two-step apparatus in which a liquid jet-loop reactor and a packed column are in series. In this way chlorine is converted to hypochlorite and emissions of chlorine to the atmosphere are avoided. 

Production of hypochlorite takes place in a two-step absorption unit in which 23% caustic solution is fed counter-currently to the chlorine feed-stream. In the first step -the liquid jet-loop reactor - about 90% of the chlorine is converted to hypochlorite. In step two - a packed column - a very efficient absorption [1-3] is carried out in which the chlorine concentration in the off-gas is reduced to <1 ppm. The operating window of this apparatus with respect to chlorine load is quite large and varies from 100 to 6000 kg h-1 of chlorine. This high capacity is necessary for the consumption of peak loads from the electrolysis plant during short time periods. During start-up or shutdown of one electrolyser the total chlorine peak load can vary from 100 to 300 kg in just a few minutes. 

In the first step the chlorine from the tail gas and chlorine feed reacts with the caustic in the jet-loop reactor. The advantage of the jet-loop reactor is that it also acts as a suction device for the gas stream. The residence time of the liquid in step one is dependent on the capacity of the hypochlorite production and liquid level in the tank and varies between 1 and 4 h. A heat exchanger in the loop controls the temperatures in steps one and two. The amount of caustic in the feed-tank of step two is the back-up for failure of chlorine liquefaction. 

The inert flow in the system is about 5000 m3 h-1. Taking into account the flow of liquid in step one, the gas-liquid ratio is about 35 which is far from normal for jet-loop reactors as such reactors normally operate at a gas-liquid ratio of 0.5-3 [4]. 

When the capacity is increased under the same process conditions the caustic concentration is increased on higher dosing. As the quantity of free caustic in the end-product and in the liquid flow of the jet-loop reactor is the same, the percentage of caustic reacting with chlorine increases by roughly 25-60% when the production capacity is increased. Depletion of caustic at the liquid-gas interface can then occur more easily. 

When there is a peak flow of chlorine in the feed-stream the redox potential decreases and the amount of caustic that is dosed increases automatically. Caustic is fed to the jet-loop and its flow is regulated by the measurement of the redox potential. When the lead time is long there is a shortage of caustic, which results in a lower level of free caustic that leads to a higher chlorine concentration. This chlorine subsequently reacts with the chlorite in the hypochlorite solution to form chlorine dioxide. The absorption of chlorine dioxide by the caustic is then very limited in step one and is hardly absorbed at all in the caustic present in step two of the production and is emitted with the inert gas stream. 

On the one hand, the differential reactor with recycle permits kinetic measurements of high accuracy. On the other hand, a transfer equipment is required to recycle a fraction of the reaction mixture. This can be difficult when the pressure is high. For this purpose, a jet loop reactor was developed which is equipped with an ejector to recycle the fluid. The design of the jet loop reactor is described in Chapter 4.3.4. 

The advantage of the jet-loop reactor for kinetic measurements is demonstrated in the investigation of the synthesis of methanol from hydrogen and carbon monoxide at pressures of 2 - 8 MPa and temperatures of 225 - 265°C. A copper catalyst in the form of cylindrical pellets, with both diameter and length of 5 mm, was used. 

Loop air-lift pressure cycle propeller loop jet loop... 

A loop fermenter is a tank or column fermenter with a liquid circulation loop, which can be a central draft tube or external loop. Depending on how the liquid circulation is induced, it can be classified into three different types air-lift, stirred loop, and jet loop (Figure 6.22). 

Probably inspired by the fact that the Jet Loop Reactor has successfully been applied in industry, Gaddis and Vogelpohl [111] proposed an impinging stream loop reactor, as shown in Fig. 7.4. It seems that their main purpose is to lengthen residence times in the reaction vessel. The principles of the reactor s operation are somewhat similar to those of the Air Left Reactor (ALR). The only difference lies in the fact that... 

The most suitable driving force in BI is the reduction of the diffusion path that already operates in transport processes across biological bilayers. Consequently, biocatalyst membranes and specially designed bioreactors, such as jet loop and membrane reactors, are available to intensify biochemical reactions. " " Supported biocatalysts are often employed to enhance catalytic activity and stability and to protect enzymes/ microorganisms from mechanical degradation and deactivation.f Immobilization of the cells is one of the techniques employed to improve the productivity of bioreactors. 

Keskinler, B. Akay, G. Pekdemir, T. Yildiz, E. Nuhoglu, A. Process infensificafion in wasfewafer treatment oxygen transfer characteristics of a jet loop reactor for aerobic biological wastewater treatment. Int. J. Environ. Technol. Manag. 

Whereas Mersmann and Einenkel [112] correlated their experimental data on the basis of energy equations and Liepe and Joschek [334] set the power needed for particle suspension proportional to the stirrer power, Zehner [603] made use of the laws of momentum transfer. This appeared more reasonable to him, because in this case the efficiency of mechanical energy transfer did not have to be introduced. In his first fundamental investigations he utilized the jet loop, which due to the directional flow in the draught tube and in the annular space enabled easier balancing. 

Kouakou E, Salmon T, Toye D, Marchot P, Crine M. Gas-liquid mass transfer in a circulating jet-loop nitrifying MBR. Chem Eng Sci 2005 60 6346-6353. 

Single-stage and multistage stirred-tank reactors, self-inducing reactor, jet-loop reactor, plunging- jet reactor, surface aerator... 

In fine-chemicals production with heterogeneous catalysis two main types of cylindrical reactor are in common use-stirred tank reactors with a small so-called aspect ratio (length-to-diameter ratio), and column reactors (e. g., jet-loop reactor, bubble-column reactor, trickle-bed reactor) with a relatively large aspect ratio. 

The stirred-tank, bubble-column, and jet-loop reactors (Figure 2) are all suspension reactors in which very fine catalyst particles (1-200 pm) are distributed throughout the volume of the liquid. There are many variations of each reactor type-for instance, stirred tanks can have different types of agitator, cooling jacket, or cooling coils. Bubble columns can be empty, packed, or fitted with trays. 

In bubble columns, agitation of the liquid phase, and hence suspension of the catalyst is effected by the gas flow. The gas is often recycled to cause more turbulence and thus better mixing. Circulation of the liquid is often required to obtain a more uniform suspension. This can either be induced by the gas flow (airlift loop reactor) or by use of an external pump. In the latter instance it is possible to return the slurry to the reactor at a high flow rate through an ejector (Venturi tube). The local under-pressure causes the gas to be drawn into the passing stream, thus affording very efficient mixing. This type of reactor is called a Jet-loop or Venturi reactor. 

Jet-loop reactors tend to replace stirred-tank reactors in recently built equipment for fine-chemical hydrogenation. The external heat exchanger on the liquid circu-... 

The rate of mass transfer between gas and liquid, determined by the product of the gas-liquid interfacial area, a, and the mass transfer coefficient, kj, is an important parameter many heterogeneously catalyzed gas-liquid reactions are limited by mass transfer of the gaseous reactant. The greater the product a k, the faster is mass transfer, and therefore, the observed rate of reaction for reactions in which mass transfer is the controlling step, i. e. for intrinsically fast reactions. The largest can be achieved in stirred-tank reactors and jet-loop reactors, so... 

Reactor Stirred tank Bubble column Jet-loop 3-Phase fluid bed Trickle-bed 3-Phase Monolith... 

For exothermic reactions, heat transfer is usually also an important factor, for reasons of temperature control and energy costs. In this respect, slurry reactors are superior to fixed-bed reactors. In particular, the jet-loop reactor with its external heat exchanger provides excellent temperature control. 

---