Distillation plays a key role in many process chemical and refining applications. Although the principles behind distillation are simple, the huge distillation units and tall distillation columns that do the job in refineries and chemical plants are very inefficient.
The efficiency of conventional distillation technologies is just 24-40 per cent. This, in turn, bumps up energy consumption, with all that that implies in terms of cost and environmental footprint. Bearing in mind that separations account for 60 per cent of the energy used in the chemical and petroleum refining industries and of that 95 per cent, or nearly 6 per cent of total US energy consumption, is used for distillation, finding ways improving the efficiency of distillation processes is a very worthwhile goal.
Microchannel technology is one promising way forward. This developing field of chemical processing, which involves reducing the dimensions of reactor systems, is a form of process intensification that works by minimising heat and mass transfer limitations. This, in turn, results in more rapid production rates.
Devices using microchannel technology are characterised by parallel arrays of microchannels with diameters in the range of 0.1-5.0mm. In microchannel devices processes are accelerated 10- to 1000-fold by reducing heat and mass transfer distances and decreasing transfer resistances between process fluids and channel walls. This process intensification makes it possible to optimise capital, energy, environmental and safety benefits, and to significantly decrease equipment size/production-capacity ratio, energy consumption and waste production. Like the microelectronics that revolutionised the computer industry, microchannel technology shrinks the hardware while at the same time improving performance.
When applied to distillation, microchannel technology offers the potential for great improvements in efficiency for a number of distillation applications, and enables the efficient, cost-effective distributive distillation at industrial scales. Its environmental credentials are also impressive. Microchannel distillation can significantly reduce energy consumption and greenhouse gas emissions from both the petroleum (7.174 exajoules (EJ); 305 MMTCO2) and the chemicals (6.86 EJ; 316 MMTCO2) industries - the two most energy-intensive industries now operating.
In conventional distillation towers bulk quantities of liquids and gases come into contact on trays or in packing. In contrast, in microchannel distillation the liquid and vapour phases flow in a counter-current within a single very thin vertical channel.
"Instead of flowing over the surface of packing, we have a very thin layer of liquid flowing down the wall of the microchannel, and a very thin layer of vapour flowing in a counter current direction in contact with the liquid," explains Jan Lerou, chief technology officer at the Oxford Catalysts Group.
Mass transfer properties
The microchannel distillation technology under development by the Plain City, Ohio-based company Velocys, part of the Oxford Catalysts Group, incorporates a radical new modular design approach to provide very efficient mass transfer properties in a very small package.
Compared with other continuous distillation methods, the size of the distillation channels are greatly reduced in microchannel distillation. As a result the distance to reach the equilibrium state between liquid and vapour phases is much smaller. Given that distillation efficiency is increased when the equilibrium state is reached as quickly as possible and over the shortest possible distance, size really matters. In terms of size, microchannel distillation offers obvious advantages over conventional distillation columns.
In the Velocys design (Fig. 1) microchannel slabs, rather than cylinders, are used. The channels are formed by altering the distances between two rectangular slabs. The distance between slabs can be very small. Depending on the application, single microchannel channel diameters range from 0.1 mm up to 10 mm. Microchannels adjacent to process channels can be used as integrated heat exchangers, doing away with the need for separate pieces of equipment. Instead temperature can be controlled and optimised for particular applications simply by flowing warm or cool liquids through the heat exchange channels.
Individual modules can incorporate varying numbers of channels. For example, modules for small speciality chemical applications may contain a single process channel, while modules for large scale commodity plants might be made up of thousands of process channels incorporated into a unit the size of a 0.6 m cube.
Separation takes place when a downward flowing liquid film comes into intimate contact with an upward flowing vapour stream. These flows are controlled by a combination of capillary and gravitation forces.
Since the mass transfer through the vapour and liquid is controlled by diffusion, the reduced mass and heat transfer distances in microchannel distillation leads to greatly enhanced mass and heat transfer rates compared to conventional distillation technologies. Where a conventional column may require 60cm to achieve a stage of separation, the same separation could take place over a distance of less than one cm in a microchannel distillation unit. This leads to significant benefits in terms of lower capital costs and reduced energy consumption.
The reduction in capital costs is down to the fact that microdistillation units integrate all heat transfer and fractionation requirements into a single device, and the highly efficient heat and mass transfer rates they offer greatly reduce the overall size and cost of the devices.
In addition, like other types of microchannel devices, such as microchannel Fischer-Tropsch (FT) reactors, microchannel distillation devices are modular. As a result, plant size can be increased to allow for incremental changes in capacity simply by 'numbering up', or linking individual modules together, rather than by building additional distillation towers.
The reduction in energy consumption - and energy costs - results because microchannel technology facilitates heat control in the distillation process, making it possible to tailor heat loads to match individual distillation stage requirements. It also reduces lost work and the overall energy required for distillation.
Microchannel distillation offers other advantages too, including new opportunities to optimise plant performance by, for example, designing new configuration and flow sheet conditions. Other benefits are lower capital costs, decreased energy cost and the ability to incorporate incremental changes in capacity. The devices also feature a compact footprint and very low hold-up time, which minimises exposure of chemicals to the temperatures required for distillation.
Although microchannel distillation is not suitable for every distillation process - even its greatest champions acknowledge that oil refineries are unlikely to replace their tall distillation columns with microchannel equipment - this technology shows great promise for a wide range of applications. It could, for example, prove to be particularly advantageous for separation of close-boiling compounds, such as light gases or for separating alkanes from alkenes, as well as in processes used to produce liquified natural gas (LNG).
Microchannel distillation could offer advantages too in the pharmaceutical and fine chemicals industries, and in the recovery of precious compounds or other useful materials that could be reused. It would also be a useful lab scale device, and could be used for smaller scale distillations for pilot systems, for testing systems or in distributed production systems.
Microchannel distillation technology has been demonstrated at Velocys in laboratory proof-of-principle experiments, using units made up of slabs around 35cm long with a single channel on the order of a centimetre wide. It is now seeking opportunities to allow it to further develop the technology.
Laura Silva is director, IP and Licensing, at the Oxford Catalysts Group. She is based at Velocys Inc, in Plain City, Ohio, USA. www.velocys.com or www.oxfordcatalysts.com