Hybrid Unit Operations

The first case deals with multifunctional equipment that couples or uncouples elementary processes (transfer-reaction-separation) to increase productivity and/or selectivity with respect to the desired product and to facilitate the separation of undesired by-products. Numerous reactive separation processes involving unit operation hybridization exist. 

The concept of reactive or catalytic distillation has been commercialized successfully, both in petroleum processing, where packed bed catalytic distillation columns 

The benefits of using reactive distillation are clearly proven in the production of fuel components (ethers) such as ferf-amyl methyl ether (TAME), methyl tert-butyl ether (MTBE) and methyl acetate. The latter is synthesized from acetic acid and methanol with a reversible liquid-phase reaction  

Other industrial processes that have taken advantage of the process intensification deriving from the introduction of reactive (catalytic) distillation are (i) production of high purity isobutene, for aromatic alkylation (ii) production of isopropyl alcohol by hydration of propylene (iii) selective production of ethylene glycol, which involves a great number of competitive reactions and (iv) selective desulfurization of fluid catalytic cracker gasoline fractions as well as various selective hydrogenations. Extraction distillation is also used for the production of anhydrous ethanol. 

An alternative reaction-separation unit is the chromatographic reactor. It uses differences in the adsorptivity of the different components involved rather than 

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Plants may operate by batch production, in which the plant processes a quantum of feed per cycle, and stops at the end of each cycle for removal of the product and replacement of the feed. Alternatively, plants may operate continuously, 24 h per day, without stopping and there are hybrid plants, or hybrid unit operations within plants, which are described as semi-continuous, in that the internal operation is cyclical but the cycles follow continuously, one after the other, with little operator intervention. 

The Aromax process was developed in the early 1970s by Toray Industries, Inc. in Japan (95—98). The adsorption column consists of a horizontal series of independent chambers containing fixed beds of adsorbent. Instead of a rotary valve, a sequence of specially designed on—off valves under computer control is used to move inlet and withdrawal ports around the bed. Adsorption is carried out in the Hquid phase at 140°C, 785—980 kPA, and 5—13 L/h. PX yields per pass is reported to exceed 90% with a typical purity of 99.5%. The first Aromax unit was installed at Toray s Kawasaki plant in March 1973. In 1994, IFP introduced the Eluxyl adsorption process (59,99). The proprietary adsorbent used is designated SPX 3000. Individual on-off valves controlled by a microprocessor are used. Raman spectroscopy to used to measure concentration profiles in the column. A 10,000 t/yr demonstration plant was started and successfully operated at Chevron s Pascagoula plant from 1995—96. IFP has Hcensed two hybrid units. 

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

Naturally, there exist a variety of membrane separation processes depending on the particular separation task [1]. The successful introduction of a membrane process into the production line therefore relies on understanding the basic separation principles as well as on the knowledge of the application limits. As is the case with any other unit operation, the optimum configuration needs to be found in view of the overall production process, and combination with other separation techniques (hybrid processes) often proves advantageous for large-scale applications. 

Novel processing methods, such as integration of reaction and one or more unit operations in so-called multifunctional reactors and integration of two or more separation techniques in hybrid separations Use of alternative forms and sources of energy for chemical processing Novel methods of process/plant development and operation... 

Reactive distillation, as the name implies, refers to a distillation process that incorporates a reaction and a separation step within a distillation column. The technique offers a key opportunity for improving the structure of a process. - It is a so-called hybrid process, i.e. it merges two different unit operations in a single apparatus, namely reaction and distillation. But the combination of distillation and reactions is possible only if the conditions of both unit operations can be combined. This means that the reactions have to show reasonable data for conversions at pressure and temperature levels that are compatible with distillation conditions. Because of the limited hold-up in distillation column, those reactions having a conversion half-time of 10-30 min are preferred. So, the judicious use of the chemical equilibrium constant is the basis for the design of reactive distillation processes. 

Hybrid Extraction Processes Hybrid processes employ an extraction operation in close association with another unit operation. In these processes, the individual unit operations may not be able to achieve all the separation goals, or the use of one or the other operation alone may not be as economical as the hybrid process. Common examples include the following. 

Concurrently to these new unit operations based on the use of a local function defined on the thin layer itself, we can also recall many other achievements involving more complex functions attained by coupling membrane materials with other external unit operations of chemical engineering this is the so-called concept of hybridization. ... 

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