Process 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. 

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. 

Distillation is still the most common unit operation to separate liquid mixtures in chemical and petroleum industry because the treatment of large product streams and high purities with a simple process design is possible. Despite of this the separation of azeotropic mixtures into pure components requires complex distillation steps and/or the use of an entrainer. Industrial applied processes are azeotropic, extractive or pressure swing distillation (Stichlmair and Fair, 1998). Another sophisticated method for the separation of binary or multicomponent azeotropic mixtures is the hybrid membrane process, consisting of a distillation column and a membrane unit. 

For a fundamental understanding of the hybrid process it is necessary to describe the interactions between two different unit operations with appropriate models. Making basic parameter studies the equilibrium stage model for distillation and a short-cut model for membrane separation is sufficient. The models are well established and the model parameters are quite accessible. This combination gives an first survey on the influence of structural and operational parameters on the concentration profiles in the column and on the maximum amount of water, which can be removed. 

Hybrid processes in the separation techniques are described by the combination of two unit operations. Each unit procedure itself is considered to be a separation process, but the combination results in remarkable advantages. In the case of membrane hybrid technology, these advantages are mainly increased product purity and process simplification. This leads to a noticeable reduction in investment costs, discharge costs and energy costs. 

Historically, the basis of gas permeation module design was first proposed by Weller and Steiner in 1950. Nowadays, modem computation techniques provide numerical solutions to the problems thanks to dedicated routines. Orthogonal collocation methods or perturbation methods " are reported to be particularly attractive when a minimum resolution time and computational efforts are required. Several of these routines have been implemented in commercial process simulation software, where advantage can be taken of thermodynamics or unit operation design packages in order to simulate hybrid or multi-stage operations with gas separation membranes. Nevertheless, much effort has been devoted to... 

A membrane separation system may sometimes be coupled with another gas separation unit operation (such as adsorption, absorption or cryogenic distillation) to obtain an economically optimum hybrid process. 

Because a membrane system operates best with higher CO2 concentration feed and an amine system is best suited to produce pipeline spec gases. For high volume units where two or more trains may be required, hybrid process can reduce the system to a single, less expensive train. Membranes can also be very useful in debottlenecking the existing units. There are many hybrid systems operating in the world today. 

Multistep and multistage membrane systems of the type shown in Figure 21.5 have been used to treat a number of vapor-gas streams. However, hybrid processes in which membrane separation is combined with another separation process, are attractive because they may enable each unit operation to operate in its preferred range, improving overall process efficiency. The most important of these hybrid processes is the combination of condensation under pressure with membrane separation, illustrated in Figure 21.6. In this process, a vapor-permeable membrane unit is combined with a vapor condensation-flash unit (Baker and Wijmans, 1994 Baker et al., 1998 Wijmans, 1993/1992). 

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