**2.4 Membrane assisted reactive distillation**

Membrane not only plays the role of a separator, but also used in the reaction itself. Membrane assisted reactive distillation has emerged as a novel technique of hybrid process intensification to achieve higher efficiency and yield in the production of bulk chemicals. In several cases, non-ideal aqueous-organic mixtures are formed which tend to form azeotropes. They can be overcome using membrane

separations like pervaporation and vapor permeation since they are very selective and not limited by vapor-liquid equilibrium Rautenbach [57]. The choice of membrane type to be used in membrane reactor depends on parameters such as the productivity, separation selectivity, membrane life time, mechanical and chemical integrity at the operating conditions and, particularly, the cost. Consequently, a hybrid process consisting of membrane-assisted reactive distillation contributes to sustainable process improvement due to arising synergy effects and allows for reduction of investment and operational costs. A review of hybrid processes combining pervaporation with one or more other separation technologies was given by Lipnitzki et al. [58]. The analysis of hybrid separation processes combining membrane separation with conventional distillation was described in Kreis and Gorak [59]. They have presented various configurations corresponding to this hybridization which offers significant advantages like reduced energy requirement, lower production cost, etc.

Membrane hybrid processes can achieve separations which are impractical to achieve with either conventional process. An example for the investigation of a reactive hybrid process concept is the transesterification of methyl acetate and butanol to butyl acetate and methanol by the combination of reactive distillation and pervaporation, as studied by Steinigeweg and Gmehling [60]. They carried out experiment using various catalytic packing is like Katapak S, Katapak SP and Sulzer. However, experiment was not presented for the hybrid technique but only simulation results have been reported. The industrially operated hybrid process for the continuous production of fatty acid esters by reactive distillation and pervaporation was presented by Scala et al. [61]. Ozdemir et al. [62] presented an overview of the commercial polymers used as membranes as well as of other polymers having high potentially for application as a membrane material. However, many industrial processes involve operations at high temperatures. Luo et al. [63] showed that integration of a membrane unit for a side stream withdrawal from the section of reactive distillation where the azeoptrope is liable to form which further improves the product yield.

Polymeric membranes are not generally useful in hybrid reactor and therefore inorganic membranes are preferred. Gorak et al. [64] have shown higher efficiency and capacity of membrane assisted reactive distillation with special focus on Pervaporation unit. Author has deeply identified the current challenges and future predicted trends for implementation of this hybrid technique in the field of chemical and biochemical industry. Holtbruegge et al. [65] represented synthesis of dimethyl carbonate using this hybrid technique to overcome the limitation of chemical equilibrium and azeotrope formation. Replacing membrane at various location in reactive distillation yield different efficiency, which is rigorously studied by Bida et al. [66]. They proved that placing pervaporation membrane at the bottom shows remarkably improved performance with effective economy and energy efficiency. Thus, we can say that combination of pervaporation and reactive distillation exploits the advantages of minimization of cost by reducing energy expenditures and making higher degree of separation.

#### **2.5 Biodiesel in reactive divided wall column: design and control**

Due to gradual depletion of world petroleum reserves and impact of environment pollution there is a need for alternative fuels for use in diesel engine. Biodiesel has emerged as a promising alternative because it is renewable and environment friendly and leads to reduction of exhaust emission. Masjuki et al. [67] first researched on the various aspects of use of biodiesel as future fuel by considering the rising cost and increased pollution from conventional carbon

#### *A Review on AI Control of Reactive Distillation for Various Applications DOI: http://dx.doi.org/10.5772/intechopen.94023*

containing fuels. They proposed that biodiesel can be manufactured from easy to available raw material like animal oil or used vegetable oil which is generally discarded as a waste. This mishandling also serves as a matter for pollution in water or soil. Thus, biodiesel also minimizes the waste in one way or other. We [68] worked on the effect of molecular weight of fatty acid on the octane rating of biodiesel. There are many processes to convert vegetable oil into biodiesel, but transesterification reaction was found to be most viable process of oil modification. Biodiesel can be produced from animal as well vegetable oil, which is reported by Cr et al. [69]. Today economic factors along with environmental concerns are playing a key role in increase in thermal efficiency. Studies show that biodiesel is much better fuel than fossil fuel-based diesel in term of engine performance, emission reduction, lubricity, and environmental benefits. The production of biodiesel by transesterification in existing conventional processes requires excess alcohol. This excess alcohol must be recovered and purified for reusing by rectification and distillation, which involves additional capital and operating cost. Kiss et al. [70] first reported reactive divided wall distillation column consisting of one condenser, one reboiler, reactive zone a pre fractionators and main column in a single shell leads to process integration and intensification, leading to cost saving and increased purity of final product and side streams product. Later on, several research works have been carried out in the field of reactive divided wall distillation column for biodiesel production. Kiss et al. [71] described the increase in purity and energy reduction of 30% in a divided wall column in which at the bottom of diving wall section, the vapor flow was split proportionally to the cross sectional area of each side. They focused on enhanced methanol recovery from the DWC unit. Delgado et al. [72] recommended the use of petluyk distillation column when the molar fraction of middle component is low. **Figure 4** shows divided wall distillation column.

**Figure 4.** *Divided wall distillation column.*

Since reactive divided wall distillation column is a case of process intensification, there is a complex interaction between vapor liquid equilibrium, vapor liquid mass transfer, intra catalyst diffusion and chemical kinetics. Such interactions and strong nonlinearity lead to multi steady states and complex dynamics. Bravo et al. [73] verified this nonlinearity of divided wall in laboratories as well as small pilot plants. The work has been carried out to study the various control system for reactive divided wall distillation column which can lead to process optimization. R-DWC was designed for a quaternary reactive system – two reactants (one in excess) and two products – more difficulties concerning the process control may be expected due to consideration of the high degree of integration of the process. Study was carried to tackle the optimal design, dynamics, and control of such an integrated unit and proposes an efficient control structure for a biodiesel process based on reactive DWC technology. Chongkhong et al. [74] proposed the novel distillation technologies for enhanced bioethanol dehydration, by extending the use of dividingwall columns (DWC) to energy efficient Extractive Distillation (ED) and Azeotropic Distillation (AD). This technology is beneficial because the industrial production of anhydrous bioethanol requires energy demanding distillation steps to overcome the azeotropic behavior of the ethanol-water mixtures. A recently proposed process by Gomez- Castro et al. [75] depicts control of divided wall technology involving the use of short chain alcohols at supercritical condition that avoids the use of catalyst and this condition was applied to a reactive petlyuk column that results in thermal coupling and more of vapor liquid interactions. Aspen Plus and Aspen Dynamics were used as computer aided process engineering (CAPE) tools to perform the rigorous steady-state and dynamic simulations, as well as the optimization of the new R-DWC based biodiesel process. These control structure described the excellent performance of RDWC for biodiesel production.

#### **2.6 Membrane assisted reactive divided wall column**

The production of biodiesel by transesterification in existing conventional processes requires excess alcohol. This excess alcohol must be recovered and purified for reusing by rectification and distillation, which involves additional capital and operating costs. For production of biodiesel various techniques have been proposed such as Reactive Divided Wall. However, a major question remains that which material should be used to make that wall in middle of the column. For this Atadashi et al. [76] have reported membrane biodiesel production technique to provide high quality biodiesel fuel. In this technique the membrane system exploits the characteristic of high selectivity, high surface area and their potential for controlling mixing between two phases. The author has taken canola oil as the base oil for biodiesel synthesis and has studied the effect of membrane pore size and catalyst on the performance of membranes. The results show that membrane reactor restrict the passage of unreacted oils to the biodiesel product mixture and the use of alkaline catalyst result into soap formation while acid catalyst avoids the same.

There are various types of membrane separation processes available like ultrafiltration, microfiltration, pervaporation, etc. Vapor permeation along with RD is a novel technique in which the volatile components are separated by nonporous membrane. One possible process alternative using vapor permeation was suggested by Buchaly et al. [77] for n-propyl propionate synthesis, in which Amberlyst 46 was used to compete the side product formation. The author has used online data reconciliation by satisfying mass, component, and reaction rates as boundary conditions. In another work by Buchaly et al. [78] on same case, a comparison of most common modeling depths such as Maxwell Stefan's equation, equilibrium model with and without considering reaction kinetics was presented. Thermal coupling

*A Review on AI Control of Reactive Distillation for Various Applications DOI: http://dx.doi.org/10.5772/intechopen.94023*

**Figure 5.** *Membrane reactive distillation column.*

between two columns in a sequence has proven to be very successful in providing energy savings with respect to conventional trades. The control of thermally coupled membrane RDWC was presented by Wang et al. [79]. It was reported that controlled stage temperature shows no multiplicity and proper temperature control can maintain reactant inventory. These thermally coupled systems show higher thermodynamic efficiency. Some improvement for dividing wall distillation column likes catalytic packing and non-welded walls have also been proposed by Aspiron et al. [80]. A rigorous study of ASTM standards for biodiesel was carried out by researchers and reported that Membrane biodiesel separation processes provides high-quality biodiesel fuel. He et al. [81] and Saleh et al. [82] have shown that a refining step is necessary to be accompanied with transesterification of biodiesel. Also, the membrane separation processes for biodiesel were carried out under moderate temperature and pressure conditions and their scale-up process found less cumbersome. Sarmento et al. [83] critically examined the production and refining of biodiesel using membrane technology.

Some application and future aspects of dividing wall column with bioethanol production was investigated by Delgado [84]. He has also reviewed practical application of dividing wall column by performing simulation studies. The major emphasis was given on environmental impact of fuel burning and prices of oil and biofuels and the results obtained were compared and summarized by taking the difference between them. The schematic diagram of membrane reactive distillation is presented in **Figure 5**.
