Liquid phase reactors

Product removal during reaction. Sometimes the equilibrium conversion can be increased by removing the product (or one of the products) continuously from the reactor as the reaction progresses, e.g., by allowing it to vaporize from a liquid-phase reactor. Another way is to carry out the reaction in stages with intermediate separation of the products. As an example of intermediate separation, consider the production of sulfuric acid as illustrated in Fig. 2.4. Sulfur dioxide is oxidized to sulfur trioxide ... 

Clearly, in the liquid phase much higher concentrations of Cfeed (kmol m ) can be maintained than in the gas phase. This makes liquid-phase reactions in general more rapid and hence leads to smaller reactor volumes for liquid-phase reactors. 

Figure 10.5 The direct chlorination step of the vinyl chloride process using a liquid phase reactor. (From McNaughton, Chem. Engg., December 12, 1983, pp. 54-58 reproduced by permission.)...

Vapour-phase reactor Liquid-phase reactor... 

Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

Figure 10-11. The Institut Francais du Petiole process for the hydrogenation of benzene to cyclohexane " (1) liquid-phase reactor, (2) heat exchanger, (3) catalytic pot (acts as a finishing reactor when conversion of the main reactor drops below the required level), (4) high-pressure separator, (5) stabilizer.

The feed is charged all at once to a batch reactor, and the products are removed together, with the mass in the system being held constant during the reaction step. Such reactors usually operate at nearly constant volume. The reason for this is that most batch reactors are liquid-phase reactors, and liquid densities tend to be insensitive to composition. The ideal batch reactor considered so far is perfectly mixed, isothermal, and operates at constant density. We now relax the assumption of constant density but retain the other simplifying assumptions of perfect mixing and isothermal operation. 

An integral form of Equation (3.15) was used to derive the pressure ratio for scaleup in series of a turbulent liquid-phase reactor, Equation (3.34). The integration apparently requires ji to be constant. Consider the case where ii varies down the length of the reactor. Define an average viscosity... 

Jensen gives several examples for his present highly integrated chip systems [101], including a gas-phase reactor, a liquid-phase reactor, a catalyst-testing reactor, and a packed-bed multi-phase reactor. In addition, he provides the vision of a multiple micro-reactor test station (see Section 1.5.5.2). 

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

Processes involving oxygen and nitrogen oxides as catalysts have been operated commercially using either vapor- or liquid-phase reactors. The vapor-phase reactors require particularly close control because of the wide explosive limit of dimethyl sulfide in oxygen (1—83.5 vol %) plants in operation use liquid-phase reactions. Figure 2 is a schematic diagram for the liquid-phase process. The product stream from the reactor is neutralized with aqueous caustic and is vacuum-evaporated, and the DMSO is dried in a distillation column to obtain the product. 

Reactors can be operated either in a batch or continuous-flow mode. The combination, batch with respect to the liquid and continuous-flow with respect to the gas, is called semibatch. Often this fine distinction is ignored and it is commonly referred to as batch. The majority of ozonation experiments reported in the literature have been performed in one-stage semi-batch heterogeneous systems, with liquid phase reactor volumes in the range VL = 1-10 L. Most full-scale applications are operated in continuous-flow for both phases. 

In the process (Figure 9-37), the residue feed is slurried with a small amount of finely powdered additive and mixed with hydrogen and recycle gas prior to preheating. The feed mixture is routed to the liquid phase reactors. The reactors are operated in an up-flow mode and arranged in series. In a once through operation conversion rates of >95% are achieved. Typically the reaction takes place at temperatures between 440 and 480°C and pressures between 150 and 250 bar. Substantial conversion of asphaltenes, desulfurization and denitrogenation takes place at high levels of residue conversion. Temperature is controlled by a recycle gas quench system. 

Pressure is more directly connected to the concept of explosion nevertheless, it is less directly connected to the reactor status, since, for liquid-phase reactors, pressure nonlinearly depends on temperature (trough the vapor pressure relationship) and concentration (through the activity coefficients in liquid phase). Moreover, since pressure measurements are usually less accurate than temperature measurements, they are to be considered in particular for gassy reactions, i.e., when the runaway produces small temperature effects but large amounts of incondensable products in gaseous phase. 

In this chapter we study the steady-state design of perfectly mixed, continuously operating, liquid-phase reactors. The effects of a wide variety of reaction types, kinetics, design parameters, and heat removal schemes are explored. The important elfects of design conversion and design temperature on heat transfer area and other process parameters are quantitatively studied. 

A flashing flammable chemical (that possessed flammability properties similar to butane) had been manufactured in this new unit for just a few years. The liquid phase reactor had a normal liquid inventory of 13,000 gallons (59,000 L).The vertical reactor was about 9 ft. (2.74 m) in diameter and about 35 ft. (10.7 m) high and operated at about 120 psig (826 kPa gage). 

Classification by Phase Despite the generic classification by operating mode, reactors are designed to accommodate the reactant phases and provide optimal conditions for reaction. Reactants may be fluid(s) or solid(s), and as such, several reactor types have been developed. Singlephase reactors are typically gas- (or plasma- ) or liquid-phase reactors. Two-phase reactors may be gas-liquid, liquid-liquid, gas-solid, or liquid-solid reactors. Multiphase reactors typically have more than two phases present. The most common type of multiphase reactor is a gas-liquid-solid reactor however, liquid-liquid-solid reactors are also used. The classification by phases will be used to develop the contents of this section. 

In what follows, we begin by introducing two examples of process systems with recycle and purge. First, we analyze the case of a reactor with gas effluent connected via a gas recycle stream to a condenser, and a purge stream used to remove the light impurity present in the feed. In the second case, the products of a liquid-phase reactor are separated by a distillation column. The bottoms of the column are recycled to the reactor, and the trace heavy impurity present in the feed stream is removed via a liquid purge stream. We show that, in both cases, the dynamics of the system is modeled by a system of stiff ODEs that can, potentially, exhibit a two-time-scale behavior. 

In conclusion, compared with the previous design the flowsheet based on reactive distillation appears to be more economical as hardware and more efficient from an energy-integration viewpoint. However, the premise of feasibility is the availability of a catalyst with superior properties in terms of activity and selectivity compared with those used in a liquid-phase reactor. 

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. 

Because of the small mass storage capacity compared to liquid-phase reactors, the danger of a sudden re-... 

Description Polymer-grade ethylene is oligomerized in the liquid-phase reactor (1) with a catalyst/solvent system designed for high activity and selectivity. Liquid effluent and spent catalyst are then separated (2) the liquid is distilled (3) for recycling unreacted ethylene to the reactor, then fractionated (4) into high-purity alpha-olefins. Spent catalyst is treated to remove volatile hydrocarbons and recovered. The table below illustrates the superior purities attainable (wt%) with the Alpha-Select process ... 

Description Benzene is alkylated to cumene over a zeolite catalyst in a fixed-bed, liquid-phase reactor. Fresh benzene is combined with recycle benzene and fed to the alkylation reactor (1). The benzene feed flows in series through the beds, while fresh propylene feed is distributed equally between the beds. This reaction is highly exothermic, and heat is removed by recycling a portion of reactor effluent to the reactor inlet and injecting cooled reactor effluent between the beds. 

Description Butenes enter the Dimersol-X process, which comprises three sections. In the reactor section, dimerization takes place in multiple liquid-phase reactors (1) using homogeneous catalysis and an efficient recycle mixing system. The catalyst is generated in situ by the reaction of components injected in the recycle loop. The catalyst in the reactor effluent is deactivated in the neutralization section and separated (2). The stabilization section (3) separates unreacted olefin monomer and saturates from product dimers while the second column (4) separates the octenes. A third column can be added to separate dodecenes. 

The methyl-acetylene and propadiene in the C3 cut are hydrogenated to propylene in a liquid-phase reactor. Polymer-grade propylene is separated from propane in a C3 splitter (16). The residual propane is either recycled for further cracking, or exported. C4s and light gasoline are separated in a debutanizer (17). 

This example has a liquid-phase reactor, where volume can potentially be varied. If the reactor were vapor phase, reactor volume would be fixed. However, we now have an additional degree of freedom and can vary reactor pressure to affect reaction rate. 

We can draw a very useful general conclusion from this simple binary system that is applicable to more complex processes changes in production rate can be achieved only by changing conditions in the reactor. This means something that affects reaction rate in the reactor must vary holdup in liquid-phase reactors, pressure in gas-phase reactors, temperature, concentrations of reactants (and products in reversible reactions), and catalyst activity or initiator addition rate. Some of these variables affect the conditions in the reactor more than others. Variables with a large effect are called dominant. By controlling the dominant variables in a process, we achieve what is called partial control. The term partial control arises because we typically have fewer available manipulators than variables we would like to control. The setpoints of the partial control loops are then manipulated to hold the important economic objectives in the desired ranges. 

---