Recent Developments in Distillation Processes.

          Distillation is often described as a mature technology that is well understood and established, no longer requiring research and development. This thinking is flawed, as distillation has come a long way in the past three decades and has even more room to grow. 

         Defining a technology as mature should not be based on an understanding of the principles that govern its operation. A technology’s maturity should be based on how much further progress can be made and what remains to be learned. Distillation, by this definition, is not mature and still has plenty of room to advance.

In the past three decades, distillation has advanced in:

• thermodynamics and thermodynamic efficiency
• process configurations
• process simulations
• process control
• column internal design
• column peripherals (e.g., heat transfer)
• reliability and availability
• operation in harsh environments
• diagnostics.

 Innovations in related fields such as fluid flow have also played a role in advancing distillation. For example, computational fluid dynamics (CFD) is now used extensively to design and troubleshoot distillation columns. Even small improvements in distillation capacity, efficiency, availability, and cost will have a major impact. 

Need for development

While distillation has come a long way, the field needs to continue to advance. Progress would be welcome in areas such as modelling and simulation, improved reliability, improved capacity and efficiency, diagnostic tools, and hybrid processes that combine distillation with other separation techniques.

Other distillation configurations and hardware concepts have proven useful in the lab, but encounter significant hurdles at larger scales, including short-path distillation for heat-sensitive materials and microchannel distillation for performance improvement at close to minimum reflux.

Distillation is extremely effective at large scales, so displacing it from industrial use is rarely a successful economic endeavour. Nevertheless, alternatives to distillation exist in some instances, even at larger and medium scales, such as the use of adsorption processes for medium-scale gas separations and reverse osmosis membranes for water purification.

Now let's discuss some emerging, newly developed distillation techniques, such as Membrane Distillation, Cyclic Distillation, Reactive Distillation, Heat Integrated Distillation Column etc. 

Membrane Distillation

Membrane Distillation technology as an emerging separation process has become competitive with other separation techniques in recent times. MD process can be used in a wide variety of applications such as desalination and wastewater treatment. Generally, MD is a process in which water is a main component of the feed solution and only water vapor can pass through a hydrophobic membrane pores. 

The driving force for MD is the vapor pressure difference across a hydrophobic membrane resulting in transfer of water vapor from hot to cold side. 

In recent years, many experiments have been carried out to find well-suited membrane type and module. Also, applying solar or waste heat as heat source and the capability of coupling with other processes like forward osmosis and osmotic distillation distinguish MD process from other membrane processes. 

Various attempts to modify the membrane surfaces:

Base Polymer	Modification applied	Objective
PVDF	Immobilization of detonation nanodiamonds.	To avoid wetting
PVDF	Grafting of polyethylene glycol followed by deposition of TiO2 particles .	Incorporation of anti-oil fouling properties
PVDF	Hydrophobic modified CaCO3 nanoparticles.	Improvement in pore size distribution, surface roughness and porosity
PVDF	Deposition of TiO2 nanoparticles on microporous membrane followed by flourosilanization.	Improvement in hydrophobic character
CNT bucky-paper membranes	Thin layer coating of PTFE.	Improvement in hydrophobicity and mechanical strength
PVDF	Grafting of CCF4 through plasma technique.	Incorporation of super hydrophobic character
Polyetherimide	Blending followed by surface segregation of surface modifying macromolecules.	Fabrication of hydrophobic or hydrophilic membrane

Cyclic Distillation :

       Cyclic Distillation emerged as another important trend for improving distillation performance: enhancing the separation efficiency through pseudo-steady-state operation based on separate phase movement (SPM) and providing up to 50% energy savings. Basically, cyclic operation can be achieved by controlled cycling, stepwise periodic operation, a combination of these two, or by stage switching. 

      Cyclic distillation can bring new life to old distillation columns, by simply changing the internals and operating mode, and therefore providing key benefits, such as: increased column throughput, lower energy requirements, as well as better separation performance. Moreover, the separate phase movement of the vapor and liquid phases throughout the column provides more degrees of freedom which contribute to excellent process control and trouble-free operation.

          

                                 

       

          At production scale, cyclic distillation is already used in the food industry (in Ukraine) for concentrating alcohol, from about 8 %wt to 27-45 %wt ethanol. MaletaCD implemented several cyclic distillation columns of 15 stages, 0.5 m diameter and 20 m3 /day capacity.  Other potential applications include: biofuels (bioethanol, biodiesel, 20 biobutanol), organic synthesis, specialty chemicals, gas processing, petrochemicals. 

Dividing-wall Column :

            In a typical DWC, the feed is introduced into the prefractionator side facing the partition wall. Deflected by the wall, the lightest component flows upward and exits the column as top distillate, while the heaviest component drops down and is withdrawn as bottom product. 

            DWC technology found great appeal in the chemical industry – with Montz and BASF as the leading companies – because it offers some major benefits as compared to classic distillation design: high thermodynamic efficiency due to reduced remixing effects, 25-40% lower energy requirements, high purity for all product streams, reduced maintenance costs, small footprint and up to 30% lower investment costs due to the reduced number of equipment units.

           Nonetheless, despite its many benefits, one should keep in mind that DWC has some limitations as well: operation at a single pressure, larger size (diameter and height) as compared to any single column of a conventional separation sequence, and larger temperature span across the column which requires more expensive utilities and limits the potential application of heat pumps.

Reactive Distillation :

         In a RD process, the reaction and distillation take place in the same piece of equipment, the reactants being converted with the simultaneous separation of the products and recycle of unused reactants. 

        Note that a small number of industrial applications of RD have been around for several decades, but even today the RD crown is still carried by the Eastman process that reportedly replaced a methyl acetate production plant with a single RD column using 80% less energy at only 20% of the investment costs. Nowadays, the application with the largest number of installations remains the methyl tertiary butyl ether (MTBE) that is used in gasoline blending.

                                            

        Reactive distillation can be further integrated with membrane separation thus combining the benefits of reactive and hybrid separation processes in a highly intensified process unit. Albeit reactive distillation and membrane separations are established technologies, the combination of them is relatively new with only several publications reported so far.

        More recently, the use of microwaves to enhance a reactive distillation process was also reported as a remarkable process intensification improvement. 

Heat Integrated Distillation Column : 

        Heat integrated distillation column (HIDC) is the most radical approach of heat pump design, making use of internal heat-integration. 

       The research and development on the HIDC configurations was so far primarily confined to the separations of binary mixtures. Considering that multi-component mixture separations represent a major application of distillation columns, the development of a corresponding HIDiC technology is an important and extremely challenging topic for future work – especially if HIDiC becomes associated with other technologies – such as dividing-wall column or reactive distillation. 

      Crude fractionation is a high‐energy demand process, requiring approximately 3% of the total energy used in the United States. Currently, there are incentives to reduce energy usage due to rising costs of energy. A recent development in distillation technology that has shown potential savings of up to 60% is the heat‐integrated distillation column (HIDC). HIDC columns save energy by recovering excess heat from the rectifying section for usage in the stripping section. However, this technology has only been applied to specific hydrocarbon systems. This study investigates the application of some HIDC concepts in crude fractionation.  

Conclusion:

Therefore, although considered a mature technology, distillation is still young and full of breakthrough  opportunities, as significant cost reductions and high energy efficiency can be achieved by  employing various approaches and by developing new techniques.