Advantages of Process Intensification

Advantages of Process Intensification:

Although [1] named the reduction of the cost of a production system as the primary incentive for process intensification (PI) there are other advantages. They are:

• Improved quality
• More efficient use of raw material
• Reduced energy consumption
• Improved product quality
• Greater reliability
• Reduced waste
• Easier scale-up
• Distributed plant

These advantages will now be discussed separately.

Improved Safety:

Significantly smaller plants have smaller contained volumes. If all other risks remain equal, a reduced plant inventory must increase the safety of a plant. The major safety, health and environment related incidents such as Flixborough, Bhopal and Seveso all occurred when a large volume of material escaped. It has been mentioned in [6] that

“What is not there cannot leak, explode or otherwise endanger life or the environment”.

It is possible that the contained energy of smaller plants will be small enough that an explosion or reaction run away can be contained within the vessel (removing the need for relief valves etc.). If the process unit is considered a consumable and sealed permanently, then flanges can be minimized or eliminated, removing the hazards associated with vessel entry and leakages from flanges [6].

The increased safety of smaller plants may provide the opportunity for the improvement of processes. Some hazardous processes using conventional equipment, such as nuclear reprocessing, are carried out at uneconomically low concentrations for safety reasons [1]. If very compact equipment were to replace the conventional equipment then it may be possible for higher concentrations to be used without compromising the reliability of the plant.

More Efficient Use of Raw Materials:

Improved production methods generated from new equipment and processing ideas can convert more of the raw material into the final product - increasing productivity and purity and therefore reducing downstream processing.

In certain processes the equipment and methods used limit the outcomes. If mass transfer and mixing rates were produced by reactors and crystallizers which matched kinetic rates then when the kinetics are fast the residence time can be decreased, or the volume can be reduced. In many reactions short residence times result in higher selectivities or reduced product degradation. An example of a situation where selectivity can be increased is when a process involves two competing reactions, one giving the desired product (R1) and the other giving a product that is not desired (R2). If the intrinsic rate of R1 is faster than that of R2, but the observed rate of R1 is limited by poor mixing, then the ratio of the apparent rates of R1 to R2 could be increased by using a reaction vessel that provides more intense mixing [5].

Reduced Energy Consumption:

Higher heat-transfer coefficients can be generated in intensified units compared with conventional equipment. It is possible for micro channel heat exchangers (containing micro machined channels 20 - 1000 µm in diameter) to have heat transfer coefficients of more than 10000 W/m2K [3]. Large heat transfer coefficients can be exploited to reduce the temperature driving forces needed to operate heat exchangers and energy transformers such as heat pumps, increasing thermodynamic and energy efficiency.

In addition, energy consumption reductions follow from other advantages of PI, such as the more efficient use of raw materials.

Improved Product Quality:

PI can be used to improve product quality in two ways. Product quality can be made more constant, or the characteristics of the product can be made more desirable.

Product quality can be made more constant by converting from batch processes to continuous processes. Batch processes are usually used for the manufacture of fine chemicals as the level of production needed is low (50 - 5000 tonnes per annum) [7]. If PI is used to decrease processing time, the conversion of batch processes to continuous operation is possible and therefore increased product quality is also possible.

It is possible to use PI to increase the level of control of product characteristics. An example is the use of a spinning disc reactor (SDR) for the production of polymers. A SDR has a disc at its center, which rotates, often hundreds of times per second. As the disc rotates, reactants wash over it and a highly sheared liquid film is produced. Grooves in the surface of the disc create extra disturbance in the reagents, improving mixing. Polymers produced using a SDR have a consistent narrow molecular weight distribution, or low polydispersity whereas conventional processes result in polymers with high polydispersity. Polydispersity is a measure of quality (lower values are better) and so a SDR can be used to improve polymer quality. In addition, the use of a SDR allows polymer properties to be further controlled as the degree of branching of the polymer can be altered by changing the rotational speed of the disc [8].

Greater Reliability:

It is possible that small-scale equipment will have fewer and simpler moving parts, reducing the risk of problems.

Reduced Waste:

Reduced waste follows from the more efficient operation of processes and better use of raw materials.

Easier Scale-up:

There is a strong pressure to reduce the time needed to bring new products to the market as patents have a 20-year life. Traditionally a new recipe is developed in a 9 small flask (around one litre). The process is then modified so it can be done in a pilot plant. Finally the process is transferred to full-scale production. At each stage the characteristics of the reaction vessel are different and so the results experienced at the laboratory scale are not always the same as those at the other stages. If a process is developed such that the laboratory scale is full scale then the scale-up process is not needed. This is especially important in the pharmaceutical industry where regulatory authority approval for the full-scale process is needed. If a larger production rate is required, instead of being scaled-up, the process can be “scaled out” or “numbered up” by the use of a number of small units.

Distributed Plants:

If plants can be made smaller and cheaper it may become feasible to economically manufacture chemicals where they are needed and avoid the need for transport of hazardous chemicals. The use of distributed manufacture will also reduce risks other than those associated with transport. If a chemical is produced in distributed plants then a disaster at one plant will not greatly affect the total production capacity. This is in addition to the impact on the surroundings due to a disaster at a small plant being much less than those of a large plant.

However, there are some issues that need to be addressed, the main one being that for decades the process industries have believed in economies of scale. Physics leads to the “seven tenths power law of cost versus size” [6], which is applied to plant items. In addition, large plants grouped together can share utilities and effluent treatment facilities as well as interchanging feedstocks, products and energy. Rather than challenging the economy of scale, it can be applied differently. Instead of producing small numbers of large plants, large numbers of small plants can be produced (the production capacity of plants is large rather than the capacity of an individual plant). This spreads the design and engineering costs over a large number of plants. The use of these small plants means that areas where demand does not merit a large plant can supply their needs and do not have to rely on imports and gives the user more control over the supply.

References:

1. Gerberich, H. R., “Formaldehyde” in Encyclopaedia of Chemical Technology, volume 11, Kroschwitz, J. I., New York, John Wiley and Sons, 1994, 929 – 951.
2. Ramshaw, C., “Process Intensification: a game for n players” The Chemical Engineer, volume 416, 30 – 33, 1985.
3.  Stankiewicz, A. I. and Moulijn J. A., “Process Intensification: Transforming Chemical Engineering”, Chemical Engineering Progress, volume 96(1), 22 – 34, 2000.       
4. Cross, W. T. and Ramshaw, C., “Process Intensification: Laminar Flow Heat Transfer”, Chemical Engineering Research and Design, volume 64(4), 293 – 301, 1986.     
5.  Green, A., Johnson, B. and Arwyn, J., “Process Intensification Magnifies Profits”, Chemical Engineering, volume 106(13), 66 -73, 1999.  
6.  Benson, R. S. and Ponton, J.W., “Process Miniaturisation - A Route to Total Environmental Acceptability?” Chemical Engineering Research and Design, volume 71(A2),160 – 168, 1993.
7. Ramshaw, C., “Process Intensification and Green Chemistry”, Green Chemistry 1(1), G15 - G17, 1999. 
8. O'Driscoll, “Industry in a spin”, Chemistry in Britain, March, 34 – 36, 2002.