Catalytic distillation benefits

As mentioned earlier, a major cause of high costs in fine chemicals manufacturing is the complexity of the processes. Hence, the key to more economical processes is reduction of the number of unit operations by judicious process integration. This pertains to the successful integration of, for example, chemical and biocatalytic steps, or of reaction steps with (catalyst) separations. A recurring problem in the batch-wise production of fine chemicals is the (perceived) necessity for solvent switches from one reaction step to another or from the reaction to the product separation. Process simplification, e.g. by integration of reaction and separation steps into a single unit operation, will provide obvious economic and environmental benefits. Examples include catalytic distillation, and the use of (catalytic) membranes to facilitate separation of products from catalysts. 

The term reactive distillation (RD) refers to both catalyzed and uncatalyzed reaction systems. Catalytic distillation systems may use a homogenous or heterogenous catalyst to accelerate the reaction. Reactive distillation is a well-known example of reactive separation process, and is used commercially. The first patent and early journal articles deal mainly with homogenously catalyzed reactions such as esterifications, transesterifications, and hydrolysis.f Heterogenous catalysis with RD is a more recent development. The key advantages for a properly designed RD colunm are complete conversion of reactants and attainment of high selectivity. An example of the benefits of RD is the acid catalyzed production of methyl acetate by... 

The oligomerization of olefins is an exothermic consecutive reaction, which benefits from the application of CD for enhanced selectivity to intermediate products. Catalytic distillation plays a particularly important role in enhancing the catalyst lifetime because in situ separation reduces the undesirable high-molecular-weight oligomers or polymers, which will form coke and deactivate the catalyst. The use of reaction heat for distillation also reduces the formation of hot spots and catalyst deactivation due to sintering. 

Housing the reaction and separation in the same unit could lead to significant savings in both capital and operation costs. Reactive (catalytic) distillation has received an increased interest in the recent years. Several industrial applications demonstrate its benefits, as shown in Chapter 7. 

A more recent innovation in MTBE synthesis is the use of catalytic distillation in which the reactor and MTBE fractionator are combined in one vessel [130,131]. The reactive distillation unit is basically a tray distillation column with catalyst held in a proprietary packing placed on the trays. In this way the heat of reaction is recovered and used for the distillation and recovery of the MTBE. Among the major benefits of this design are efficient conversion of the isobutylene [130] and lower operating and capital costs [131], Many processes based on this technolo have been established recently [126,132]. 

The benefits of using biodiesel as renewable fuel and the difficulties associated with its manufacturing are outlined. The synthesis via fatty acid esterification using solid acid catalysts is investigated. The major challenge is finding a suitable catalyst that is active, selective, water-tolerant and stable under the process conditions. The most promising candidates are sulfated metal oxides that can be used to develop a sustainable esterification process based on continuous catalytic reactive distillation. 

Biodiesel can be produced by a sustainable continuous process based on catalytic reactive distillation. The integrated design ensures the removal of water byproduct that shifts the chemical equilibrium to completion and preserves the catalyst activity. The novel alternative proposed here replaces the liquid catalysts with solid acids, thus dramatically improving the economics of current biodiesel synthesis and reducing the number of downstream steps. The key benefits of this approach are ... 

Most studies of catalysis in ionic liquids have focused on issues of increased selectivity and particularly the easy separation of product from the catalyst and catalyst recycling via use of a biphase. In some cases, the reaction may occur in a biphase in others, the biphase is only used for product separation. In some special cases, the second phase is exclusively product, due to insolubility of the organic products in the ionic liquid, and is easily separated by decantation, allowing the recovered ionic catalytic solution to be reused. Of course, use of an organic solvent for extraction does reduce some of the potential green benefits of the ionic liquid approach. More recently, SCCO2 has been used to extract the products. Alternatively, volatile products can be separated from the ionic liquid and catalyst by distillation. 

In hybrid systems different processes are coupled, for example, reaction and separation by membranes, adsorption, or distillation. This could lead to a reduction of the investment costs as two different functions are combined in one vessel, and one process step is eliminated. For example, a reactor with a catalyst and a membrane may be used or a distillation column with a catalytic packing, which could also lead to an optimal heat integration. Other benefits depend on the specific reaction. For example, equilibrium-limited reactions would benefit if a product is continuously removed in situ, which leads to an enhanced yield beyond the equilibrium. ... 

In refineries, the largest benefits of model-predictive control come from crude distillation units and gasoline blenders, for which the throughput is high, and from fluid catalytic cracking (FCC) and other conversion units, for which the difference in value between feeds and products usually is high. 

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