Fixedbed Catalytic Distillation

1.1. FCD advantages

1.1.1. Introduction

Reactive separation processes such as reactive distillation, sorption-enhanced reaction absorption, reactive extraction and reaction crystallization combine the essential tasks of reaction and separation in a single vessel. The most important example of reactive separation processes (RSP) is reactive distillation (RD). RD, the combination of chemical reaction and distillation in a single column, is one of the most important industrial applications of the multifunctional reactor concept. Since the 1980s, the research on RD is very flourishing and RD becomes a research hotspot. Up to date, there are many articles about it published every year in the international journals [1-48].

RD processes can be divided into homogeneous ones either auto-catalysed or homogeneously catalyzed, and heterogeneous processes often referred as catalytic distillation (CD) in which the reaction is catalyzed by a solid catalyst. The equipment used for homogeneous reactive distillation processes consists of bubble-cap or sieve trays with high weirs that provide the necessary liquid holdup and residence time needed for reaction. Heterogeneous reactive distillation processes use a solid catalyst, where most of the reactions take place within the catalyst particle. The equipment consists primarily of catalyst containing packing which allows for simultaneous reaction and mass transfer between vapor and liquid phases.

1.1.2. Advantages

RD has many advantages over sequential processes [12, 13], for instance, a fixed-bed reactor followed by a fractionating column in which the distillate or bottom of the reaction mixture is recycled to the reactor inlet. The most important advantage in use of RD for equilibrium-controlled reactions is the elimination of conversion limitations by continuous removal of products from the reaction zone. Apart from increased conversion, the following benefits can be obtained:

(1) An important benefit of RD technology is a reduction in capital investment because two process steps can be carried out in the same device. Such an integration leads to lower costs in pumps, piping and instrumentation.

(2) If RD is applied to exothermic reaction, the reaction heat can be used for vaporization of liquid. This leads to savings of energy costs by the reduction of reboiler duties.

(3) The maximum temperature in the reaction zone is limited to the boiling point of the reaction mixture so that the danger of hot spot formation on the catalyst is reduced significantly. A simple and reliable temperature control can be achieved.

(4) Product selectivities can be improved due to a fast removal of reactants or products from the reaction zone. Thus, the probability of consecutive reactions, which may occur in the sequential operation mode, is lowered.

But for catalytic distillation (CD), it has more extra advantages in comparison with homogeneous reactive distillation:

(1) An optimum configuration of the reaction and separation zones is permitted in a RD column whereas expensive recovery of liquid catalysts may be avoided.

(2) If the reaction zone in the CD column can be placed above the feed point, poisoning of the catalyst (especially ion exchange resins) by metal ions can be avoided. This leads to longer catalyst lifetime compared to conventional systems.

(3) Many catalysts used in CD column are environment-friendly such as supported molecule sieves, ion exchange resins and so on.

That is why catalytic distillation is more desirable than homogeneous reactive distillation. Table 1 lists some application of RD [13]. It can be seen that the technology of RD is applied only for etherification, esterification and alkylation (synthesis of ethylbenzene or cumene) on an industrial scale. Furthermore, the cases of catalytic distillation are larger than those of homogeneous reactive distillation. Many researchers believe that RD, especially catalytic distillation, is very promising and the potential of this technique should go far beyond today's application.

In this chapter we only pay more attention to catalytic distillation since it is used more often than homogeneous reactive distillation and similar results may hold for homogeneous reactive distillation. Catalytic distillation (CD) can be divided into two types: fixed-bed catalytic distillation (FCD) and suspension catalytic distillation (SCD). FCD is relatively conventional, but SCD is a recent development. In order to clarify the characteristics of CD, let us begin with one example of FCD.

Fig. 1 [17] shows the flow scheme of a common industrial process for the production of the fuel ethers tert-amyl-methylether (TAME) or methyl-tert-butylether (MTBE) in comparison with a possible process in which a catalytic distillation technique is employed. In the original process (Fig. la), in the first fixed-bed reactor (enveloped with dashed line) the main part of the overall conversion of reactive olefins is attained. This reactor is filled with an acid ion exchange resin as catalyst. For the further increase of the olefin conversion a second reactor is necessary in the common industrial process. After this, the product of TAME or MTBE is isolated via distillation from hydrocarbons and methanol. It is evident that the whole process is somewhat complicated.

Methanol Recycle Methanol

Methanol Recycle Methanol

Mtbe Process Flow Diagram
Methanol Recycle Methanol
Mtbe Product Flow Diagram
Fig. I. The original and FCD flow scheme for the production of fuel ethers TAME or MTBE; adapted from the source [17].

It is known that the synthesis of TAME or MTBE is a chemical equilibrium-limited reaction, and thus the conversion of reactive olefins isn't complete even in case of two fixed-bed reactions are set. However, the process (in Fig. lb) which uses a fixed-bed catalytic distillation (FCD) combines three process steps of the common process, in addition to significant saving of energy and investment costs, it is possible to achieve high purity of inert hydrocarbons and TAME or MTBE,

Note that in Fig. lb the FCD column is composed of two zones: reaction zone in the upper part and stripping zone in the lower part, not including rectifying zone. However, another kind of FCD column is also found for producing TAME or MTBE in the references [6-10] which is composed of three zones: rectifying zone in the upper part, reaction zone in the middle part and stripping zone in the lower part (sec Fig.2). The merit of this kind of FCD

column is that methanol with relative high boiling point and C4/C5 with relative low boiling point is in count-current contact. The mixing condition is favorable, which possibly results in reduced catalyst amount and high conversion. But catalyst contamination from metal ions brought out by methanol can be produced, which leads to deactivation of catalyst due to ion exchange over a long time. However, it doesn't occur in Fig. lb where the reaction zone is located above the inlet. This arrangement prolongs the catalyst lifetime. So the suitable arrangement should be selected depending on the economic consideration in industry.

Extractive Distillation Amyl Alcohol

Nonreacted C4/C5

Rectifying zone

Reaction zone

Stripping zone

TAME(MTRB)

Nonreacted C4/C5

Rectifying zone

Reaction zone

Stripping zone

TAME(MTRB)

Fig. 2, The FCD column composed of rectifying zone, reaction zone and stripping zone.

1.2. Hardware structure l"he outstanding characteristics of catalytic distillation is that solid catalyst is placed in the reaction zone of the distillation column, which is different from homogeneous reaction distillation and conventional distillation. Undoubtedly, how to design the hardware containing catalyst is crucial for catalytic distillation.

Towler and Frey [1, 74] have highlighted some of the practical issues in implementing a large-scale application of catalytic distillation. These are discussed below: (1) Installation, containment and removal of the catalyst

It is important to allow easy installation and removal of the RD equipment and catalyst, if the catalyst undergoes deactivation, the regeneration is most conveniently done ex situ. So there must be provision for easy removal and installation of catalyst particles. Reactive distillation is often passed over as a processing option bccause the catalyst life would require frequent shut down. An RD device that allowed on-stream removal of catalyst would answer this concern. But for the fixed-bed catalytic distillation column, it is difficult to realize loading and downloading catalyst from time to time. From the viewpoint, the fixed-bed catalytic distillation technique is more challenging on the condition that the life of catalyst is enough long, preferably over one to three years or even longer.

(2) Efficient contacting of liquid with catalyst particles

The hardware design must ensure that the following "wish-list" is met:

(a) Good liquid distribution and avoidance of channeling. Liquid misdistribution can be expected to have a more sever effect in RD than in conventional distillation.

(b) Good radical dispersion of liquid through the catalyst bed. This is required to avoid reactor hot-spots and runaways and allow even catalyst ageing. The requirement of good radical mixing has an impact on the choice of the packing configuration and geometry.

(3) Good vapor liquid contacting in the reaction zone

If the reaction rate is fast and the reaction is equilibrium-limited, then the required size of the reactive zone is strongly influenced by the effectiveness of the vapor-liquid contacting. Vapor-liquid contacting becomes less important for slower reactions. Commonly used devices for good vapor-liquid contacting are the same as for conventional distillation and include structured packing, random packing and distillation trays.

(4) Low pressure drop through the catalytically packed reaction section

This problem arises because of the need to use small catalyst particles in the 1-3 mm range in order to avoid intra-partiele diffusional limitations. The smaller are the catalyst particles, the larger pressure drop but the lower intra-particle diffusional resistance. A compromise between these two contradictory factors must be achieved. Counter-current operation in catalyst beds packed with such small-sized particles has to be specially configured in order to avoid problems of excessive pressure drop and flooding.

(5) Sufficient liquid hold-up in the reactive section

The liquid hold-up, mean residence time, and liquid residence time distribution (RTD) are all important in determining the conversion and selectivity of RD. This is in sharp contrast with conventional distillation where liquid hold-up and RTD are often irrelevant as the vapor-liquid mass transfer is usually controlled by the vapor-side resistance. For RD tray columns the preferred regime of operation would be the froth regime whereas for conventional distillation we usually adopt the spray regime.

(6) Designing for catalyst deactivation

Even though it is desirable to allow on-line catalyst removal and regeneration, such devices haven't been commercialized as yet. Catalyst deactivation is therefore accounted for in the design stage by use of excess catalyst. Besides adding excess catalyst, the reaction severity can be increased by (a) increasing reflux, leading to increased residence time and (b) increasing reaction temperature (by increase of column pressure).

Anyway, three different approaches to locate catalyst particles have ever been tried: (1) Place beds of catalyst in a tray column, either in downcomers or above the trays

Fig. 3 shows catalyst bales above the sieve tray. Details of this catalyst bales configuration are available in [29].

Applications of RD processes (homogeneously catalyzed (bom.); heterogeneousIy catalyzed (het.)); adapted from the source [13] with some eplenishment

Reaction type

Synthesis

Catalyst

References

isieri Ileal Ion

Methy! acetate from methanol and acetic acid

hom.

[49]

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