Hybrid Separation Techniques used in Industries

Industrial separation processes are technical procedures which are used in industry to separate a product from impurities or other products. The original mixture may either be a natural resource (like ore, oil or sugar cane) or the product of a chemical reaction (like a drug or an organic solvent).

Separation processes are of great economic importance as they are accounting for 40 – 90% of capital and operating costs in industry. The separation processes of mixtures are including besides others washing, extraction, pressing, drying, clarification, evaporation, crystallization and filtration.

The study of hybrid separation technology is gaining momentum in developing more sustainable and economical processes.  

Separation procedures account for 40 to 70% of a wide range of industries' capital and operating costs. Energy consumption, production profits, and product costs are all heavily influenced by separation activities. Distillation is the chemical and petroleum industries' workhorse for manufacturing high-purity chemicals and recovering organic solvents from waste streams. Unfortunately, distillation requires a lot of energy. It uses a lot of energy, whether it's steam, cooling tower water, chilled water, or refrigerated brine. Distillation is going to enjoy its supremacy over other separation operations, until and unless the new ventures warrant the use of new and economically viable alternative separation techniques. 

The chemical sector consumes the most energy of any manufacturing business. It accounts for 6% of total residential energy consumption and 24% of overall manufacturing energy consumption in the United States. Petroleum refining is the second largest energy consumer, accounting for approximately 10% of total manufacturing energy use in the United States. Separations account for over 60% of in-plant energy consumption in these two industries.

 

With 40,000 distillation columns working in over 200 different processes, distillation operations account for 95% of total separation energy utilized in the refining and chemical processing industries. This high utilisation rate is primarily due to distillation's flexibility, low capital investment in comparison to other separation technologies, and low operational risk. Unfortunately, a commercial distillation column has a low energy efficiency of less than 10 percent being typical.

                  

Fig 1. Schematic for Hybrid Separation Process

 Hybrid Separation Process

A hybrid system can be defined as a process system consisting of different unit operations that are interlinked and optimized to achieve a predefined task. Therefore, if distillation is the basic separation system, a hybrid distillation system consists of a distillation column that is interlinked with another unit operation to achieve a better (cheaper, easier, enhanced) separation. 

Hybrid separation processes combine different separation principles and constitute a promising design option for the separation of complex mixtures. Particularly, the integration of distillation with other unit operations can significantly improve the separation of close-boiling or azeotropic mixtures. Chemical production and petroleum refining are mature, highly capital-intensive, and competitive industries. These factors, and the long economic life of existing facilities, are deterrents to the implementation of economic, large-scale, non-conventional, energy-saving separation technologies. Technology development planning must incorporate the economic realities of replacing existing equipment. To overcome the market acceptance and economic barriers described above, new and advanced separation materials must be developed with superior performance (higher selectivity, higher flux, etc.). 

In addition, the separating materials and the equipment housings in which the process operates must be more tolerant of harsh environments. However, improvements in the separating agents as well as in the materials of construction for the processing equipment are likely to increase the costs of the alternative processes, making economics unfavorable. Therefore, new equipment design will be required to make new technology implementation economically feasible.

Enabling tools that will allow evaluations at the plant-specific level are also critical in the development of improved distillation technologies.

Extraction and Absorption Hybrid Systems 

Hybrid extraction-distillation processes generally involve transferring a component of a liquid mixture into a solvent followed by an easy distillation to separate the solvent and transferring component. 

For hybrid processes involving solvent extraction and absorption to be economically competitive with distillation, efficient solvents will be needed. Next-generation solvents must have greater resistance to chemical and thermal degradation and improved solubility and emulsification characteristics. These solvents must be able to be “tuned” for high selectivity for specific separation applications and must also provide for rapid mass transfer and phase separation. In addition to solvent development requirements, the lack of an adequate solvent screening procedure results in costly experimental evaluations. A rapid solvent screening tool would significantly reduce the cost of developing new solvents. 

Application Areas

• Olefin/paraffin separations (gas and liquid mixtures) 

• Phosphoric Acid 

• Cumene/phenol 

• p-Xylene 

• Organic / water mixtures 

• Acids in water 

• N-paraffin / iso-paraffin 

Solvent extraction and absorption have traditionally been characterized by relatively large equipment sizes per unit throughput. In some cases, the potential energy savings from technological innovations may allow the economic use of new relatively expensive solvents, such as ionic liquids, in these systems. Depending on the thermodynamic driving force, processes may require such large solvent inventories that they would not be economically competitive with distillation. In these cases, new equipment designs that minimize solvent requirements may be critical to implementation.

Reactive Distillation with Membrane Separation

Over the years, the focus of the chemical and process industry has shifted towards the development and application of integrated processes combining the mechanism of reaction and separation in one single unit. This trend is motivated by benefits such as a reduction in equipment and plant size and improvement of process efficiency and hence, a better process economy. Reactive distillation is an important example of a reactive separation process.

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 vapour permeation since they are very selective and not limited by vapour–liquid equilibrium.

Heterogeneously catalysed n-propyl propionate synthesis from 1-propanol and propionic acid. The membrane module is located in the distillate stream of the reactive distillation column in order to selectively remove the produced water without use of entrainers. Key aspects for the theoretical description of reactive distillation processes are discussed. For the stand-alone reactive separation process. Consequently, a hybrid process consisting of membrane Assisted reactive distillation contributes to sustainable process improvement due to arising synergy effects and allows reduction of investment and operational costs.

Application Areas 

 • Azetrope breaking (CO2 / C2H6 separation) 

 • Pre-concentrator for distillation 

 • Bulk gas separations for low-temperature streams where existing polymers could be applicable 

 • Vent gas recovery for refining and olefin/paraffin separations 

 • Desalination/RO for phosphoric acid and caustic applications 

In vapour permeation, volatile components are separated by a non-porous membrane due to different sorption and diffusion properties. Consequently, the separation is very selective and not limited by the vapour–liquid equilibrium. The driving force is generated by lowering the partial pressure of the favourable permeating component on the permeate side by applying a vacuum.

In vapour permeation, the feed is supplied as vapour, whereas in pervaporation, the feed stream enters the membrane module as liquid. For the coupling of a membrane separation unit with a reactive distillation column, the membrane is operated in vapour permeation mode to avoid polarisation effects in the boundary layer adjacent to the membrane surface at low feed streams. 

Hybrid Separation Process for Acid Production

Hybrid separations such as extraction followed by distillation and reactive distillation can often be used to reduce the energy costs of conventional distillation alone.

The hybrid separation schemes studied in this work consist of liquid-liquid extraction followed by two distillations an acetic acid recovery column and a solvent recovery column with and without solvent recycle. See Figure 3. Hybrid separation is one way that acetic acid and water are separated in industry and thus represents current practice. The primary purpose of extraction is to first remove large amounts of water by phase separation. Moreover, the solvent is usually chosen so that the relative volatility of solvent-acetic acid is much higher than that of water-acetic acid so the internal flows in the subsequent distillations are smaller.

For a proposed hybrid separation scheme, some of the important synthesis and design questions include the following: 

(1) How many stages are required for the extraction column?

(2) What is the number of stages for the subsequent distillations? 

(3) How much extraction should be performed so that the subsequent distillations use a                        minimum amount of energy and still produce the desired acetic acid composition? 

In reality, these questions are strongly interrelated. Moreover, the synthesis and design of the distillations require comparisons of columns that have different feeds because they depend on the separation performed by the extraction column. Both energy efficiency and overall process economics can be strongly influenced by sensible heat effects, material recycling, and heat integration. For the illustrative examples, the sensible heat effects that come from subcooled feeds to the acetic acid recovery and solvent columns have been determined to be small compared to the latent heat effects of boiling and condensing. Thus, their impact on the minimum energy required.

SUMMARY

Hybrid distillation systems can be used for different purposes. The best known is the use of membranes to break thermodynamic limitations like azeotropes. While most advances in distillation equipment design have been incremental, the potential exists for step changes in mass transfer and heat transfer equipment. Even small improvements in distillation column operation can produce major energy savings. However, despite all these uncertainties, the economic potential of the system shows that in most cases the hybrid system outperforms the original distillation column.

For Existing Distillation unit it is quite difficult to start with hybrid separation technology because of high capital investment for smaller units, but for new plants it is more advantageous to go for this technology. We can also use this technology for system forming azeotrope we remove product and azeotrope recycle azeotrope to membrane layer and separate it. Without doing any alteration in the existing column though also we can improve purity of product with Hybrid technology.