Process Intensification

The aim of intensification is to optimize capital, energy, environmental and safety benefits by radical reduction in the physical size of the plant.

Process intensification in the chemical process industry is very recent subject in the field of chemical engineering. The concept is hardly a decade old. It is a highly innovative concept in the design of chemical process plants. The aim of intensification is to optimize capital, energy, environmental and safety benefits by radical reduction in the physical size of the plant. Thus the concept is intimately connected with the physical nature of the plant and not some thing which will take a long time to move from theory to final commercial application using existing and available hardware.

In other words process intensification consists of the development of novel apparatuses and techniques, as compared to the present state-of-art, to bring dramatic improvements in manufacturing and processing, substantially decreasing equipment size/production-capacity ratio, energy consumption, or waste production. Perhaps a simpler definition could be; any chemical engineering development that leads to a substantially smaller, cleaner, and more energy-efficient technology is process intensification.

There is a caveat to the definition: development of a new chemical route or a change in composition of a catalyst, no matter how dramatic the improvements they bring to existing technology, do not qualify as process intensification.

This concept has started a revolution and already 5-6 international conferences have been held. It could change the way chemical engineering curriculum iscurrently structured in the universities. Already some universities in the west have started offering courses.

There is some discussion as to the meaning of dramatic improvement, Prof. Colin Ramshaw, of University of New Castle, a pioneer of this concept, talks of 100 times reduction in equipmentvolume, however many would be happy with volume decrease by half or more. Prof Ramshaw's assertion is not some thing airy-fairy but a very achievable target at least for some operations.

What is in the chemical engineers arsenal to attack the problem of process intensification? Broadly they could be divided into hardware and methodologies.

It should be noted that many of the equipments (hardware)are of type never known before but there are quite few equipments which have been available to the chemical engineer but their potential was never fully exploited. Examples are compact heat exchangers, structured packed columns, static mixers, etc. Examples of new developments are the HIGEE column, spinning disc reactor, oscillating flow reactor, loop reactors, spinning tube in tube reactor, Heat exchange reactor, supersonic gas liquid reactor, static mixing catalysts, microchannel reactors, microchannel heat exchangers, etc.

The methodologies which are available are, reactive distillation, reactive extraction, membrane separations, oscillating flows in reactors, membrane reactions, fuel cells, etc. Use of ultrasound, microwave, centrifugal fields, etc. supercritical fluids, etc

Continuous reactors, including simple plug-flow pipe units, tubular models containing static or other mixing devices, and various jet devices have been used to efficiently produce toxic materials for immediate consumption in downstream processing with little or no inventory.

Given here are examples of process intensification from the industry indicating vast gains in capital and operating expenses.

Organic Nitration

Many years ago, nitrated products such as nitroglycerine were manufactured in large batch reactors. Now, modern nitration plants use either small continuous stirred tank reactors that provide intense mixing and large heat transfer areas, or jet reactors to deliver the intense mixing and rapid contacting of reactants.

Similarly small intensely stirred reactors are used to produce TNT in a inherently safe mode. A Canadian company has built jet impingement reactors for producing nitrobenzene, with a ten fold increase in reaction rate.

The Buss loop reactor known for quite some time has been successfully applied to hydrogenation, amination and sulphonation.

Phosphoric acid

Large capacities are the order of day for phosphoric acid manufacture. For a plant of 2000 tons P₂O₅/day, a US based company reduced the volume by half, power consumption was reduced by 1/3, number of equipments reduced from 30 to 9 and the number of motors from 15 to 3. The other advantages claimed are,

1. Lower Environmental Emissions

The system is simpler to operate and control than conventional installations.

Obnoxious fluorine gases produced in the reactions of the wet process are condensed by, and removed with, the condenser water. The usual fluorine scrubber system with its ductwork, dampers and fans, normally required to prevent atmospheric pollution, are unnecessary.

2. Process Advantages of the Isothermal Reactor Process

Simplicity of isothermal reactor crystallizer cooler operation. Higher P₂O₅ recovery efficiency, superior sulfate control, high operating factor. The P₂O₅ content of gypsum is 0.7%, phosphoric acid concentration 28%.

This gigantic single vessel, combining, reactor, crystallizer and cooler, (12 meter dia, 1300 M3 volume) occupies less space, requires fewer moving parts and is substantially less expensive to build, operate, clean and maintain than conventional installations, thereby substantially reducing capital and operating costs.

Packed Catalytic Reactor

Packed bed three phase catalytic reactor. The problem was that the catalyst was getting fouled up and required opening up of the column every 2-3 weeks. There were local hot spots in the reactor. If production had to be increased a new plant costing $5 million would be required. The other solution was to increase productivity. A $ 20,000 retrofit of static mixer for the gas-liquid feed with some other changes increased the productivity by 42%. This saved the company nearly $0.3 million per year and the investment of $ 5 million was avoided.

Phosphorus oxychloride

In conventional method phosphorus trichloride is reacted with oxygen or air in batch reactors. About 500 tons per month can be produced in 3 reactors aggregating to 34 M³ volume. In a continuous process with a radical change inreactor design the reacting volume was only 0.5 M³ and it is possible to produce 700 tons per month. The productivity goes up by a factor of 95. It also results in only 5% excess oxygen consumption compared about 15-25% excess by batch process.

There are other benefits like more uniform loading of utilities such as chilled water, cooling water, etc. The size of utility plants become smaller as they do not have to meet peak loads.

Monobromo-benzaldehyde

Monobromo-benzaldehyde is required for the manufacture of meta-phenoxy benzaldehyde, a pesticide intermediate, typically it is produced in batch reactors. There are also side reactions and therefore the conversion has to be limited. The batch process has productivity of about 15.5 kg/M³/hr. The same process if made continuous has a productivity about 34.5 kg/M³/hr. A large amount of infrastructural facilities get downsized.

Methyl Acetate

Eastman Chemicals successfully changed the methyl acetate process. The process involves the esterification of methanol with acetic acid in presence of catalyst, removal of water of reaction, distillation of product and recovery and recycle of excess reactants. There are as many as six distillation columns that have been replaced by single multifunctional distillation column. Imagine the reduction of number of reboilers, condensers, pumps, etc. The heat input and rejection is practically only at two points.

Hydrogen Peroxide

Sulzer has similarly changed the process of hydrogen peroxide distillation. A Norwegian Company has intensified the process of manufacturing hydrogen peroxide which uses static mixers extensively to combine oxidation and extraction, etc. The company feels that it may be possible to set up on site plants to meet local consumer demand.

Heat Exchangers

The developments, by a Spanish company, in workhorse of chemical industry, the shell-tube heat exchangers have led to considerable savings examples are reduction in heat transfer area by 55% in sugar cane juice heater, the volume of the HE was reduced to 16% of the shell-tube HE volume. In another case the area was reduced by half and the volume of HE to 6.5% of the shell-tube HE volume. There are similar reductions specially where viscous liquids are involved.

Chlorination

Chlorination using Thionyl chloride has been mainly practiced as batch process. The process can be made continuous using a loop type reactor with a heat exchanger in the loop. The productivity would vary with the organic product being chlorinated however for a tonnage product the productivity is 340 kg/hr/M³ of reactor volume. This is compared to the productivity of about 10 kg/hr/M³ in a batch reactor. 18M³ ofglass lined reactor capacity could be replaced by a reactor ofabout 0.5M³ a volume a reduction of 36 times.The equipment can be literally fixed to a side wall. A lab. bench model would have throughput of about 10 kg /hr sufficient to produce 5-6 tons of product per month.

Caro's Acid

Caro's acid, used in metal processing is made by reacting concentrated sulphuric acid with hydrogen peroxide. This acid is a powerful oxidizing agent and decomposes readily. A process was developed to manufacture 1000 kg/day of Caro's acid in a tubular reactor with a volume of only 20 ml and a residence time of less than 1 sec, with a product being mixed immediately with the solution to be treated.

Filteration/Centrifuging

Process intensification can take the route of combining two or more operation s in one equipment. An example is the filtering centrifuge-cum-dryer. The centrifuge combines these operations for a pesticide/herbicide/pharmaceutical product with recycle of the solvent used for crystallization. This saves on floor area, operators, conveying, drying equipment, etc.

Styrene-Butadiene Rubber

Elastomers like Styrene-Butadiene Rubber are produced by coagulation of latex, washing, extrusion, dewatering and drying to give crumbs. Usually all these operations are carried out in separate equipments, however these operation have been combined in a single equipment, by a US based company. A typical 2000 kg/hr main equipment occupies a floor area of 25 sq meters, comparedto a building ofabout 400 sq. meters or more that such plants occupy. The plant has been used for ABS, SBR, NBR and CR to date. The other advantages are less water usage, less waste treatment, recycle of solids and water, monomer recovery, lower energy, utilities, manpower, environment friendliness. Plants have been built with capacities of 100 kg/hr to 7000 kg/hr

The HIGEE Reactor

HIGEE packed columns have been there since its invention in 1980's, however the most successful development has been done very recently in China in deareation of flooding waters for oil wells, and another by Dow Chemicals of stripping of hypochlorous acid from brines.

The HIGEEreactor replaces towers up to 50-60 ft tall and can process up to 250 tons of water per hour. The size of the equipment is about 6 ft tall. They are small enough to be located on oil well platforms. The deoxygenated water is required for oil well injection to enhance oil well production. This could also be used for boiler water deaeration.

Phosgene on Demand Technology

Phosgene on demand technology: Phosgene a very toxic chemical yet a very useful chemical presented the problem of large quantities to be stored for keeping the continuity of production. A continuous tubular reactor was developed to make this chemical for immediate consumption by a group of batch processing vessels. One plant using the new design contains 70 kg of gaseous phosgene, compared to an inventory of 25,000 kg of the liquid in equipment and storage in the old facility.

Avoiding A Bhopal

Methyl isocyanate (MIC), the infamous chemical that was released at Bhopal, can be generated and immediately converted to final product (a pesticide) in a process that contains a total inventory of less than 10 kg of MIC.

Vermiculite

The expansion and exfoliation of crude vermiculite ore produces the basic material used in the manufacture of fire protection and industrial insulation products. The usual process is to have rotary furnaces for heating and reaction. A UK based company replaced set of 3 rotary furnaces of 1.5 tons/hr capacity with a single Toroidal fluidized bed (Torbed) furnace of 1 meter dia. with a capacity of 2 tons/hr.

It reduces overall energy consumption and vermiculite wastage. Other benefits included lower maintenance costs and much improved environmental conditions. The savings achieved by this project resulted in a pay back period of 16 months. Now 11 plants are operational in Europe

This Torbed is being commercialized to produce silica from rice husk. A task more easily performed than by conventional methods. There are other areas like food processing, roasting of sulphide ores, etc.

Milling and Grinding

Grinding is another area where the traditional ball mill, roll mill etc. in case of fine grinding some times combined with dispersion has been replaced by dispersed media mills. The mills operate in liquid media with small size steel, ceramic or glass balls (beads) which are circulated in a vessel having rotating members inside. There is vast reduction in power consumption and civil infrastructure. These have been applied to paint and ink industry, red phosphorus, ferrite powder, pigments, etc.

Solid- Solid Mixing

An interesting example in batch solid-solid mixing is an invention by a Norwegian company that gives an almost perfect mix with in a matter of minutes. A 250 - 500 kg batch is mixed in 3-5 minutes. Compare this with a traditional ribbon or cone blender which can take hours. There are numerous installations.

Micro-Reactor systems

Micro-reactor systems are coming of age as illustrated by CYTOS micro reactor system for 6 stage synthesis of Ciprofloxacin . The reactors are of the size of book 100 mm x 150 mm. These reactors are now commercially available. Reactions which are difficult to carry out in ordinary reactors have shown to be safe and feasible such as (a) Nitration of toluene with highly explosive acetyl nitrate,

(b) Nitration of pyridine-N-oxide at high temperature(c) Nitration of 2-methylindole , emphasizing safe handling and the very short contact time. Potential of such reaction systems could perhaps be realized in the possibility of obtaining kilogram quantities of new molecules for testing purposes in a very short time.

A miniplant customized by it, is able to handle the suspension formed in the synthesis of a dye. With the equivalent of six CYTOS microreactors running in parallel, the mini-plant has a capacity of 30 tons per year on a footprint no larger than that of an office table.

Micro-heat-exchanger, absorption systems, evaporators, etc are now moving away from laboratory curiosities to commercial evaluation for such diverse applications as absorption refrigeration systems, man cooling systems, etc.

Silane-blocking agents are employed to derivatize and protect various substrates during synthetic sequences. This has been done in continuous reaction system also. Continuous synthesis reaction for ethylalkoxysilanes has been done.

Reduction in Capital Cost

One international chemical giant proposed a $ 6 million plant for a product. The strategy of process intensification cut the capital cost down to $ 2.5 million.

PET

A process developed by Hitachi for production of PET from EG and PTA by esterification and polycondensation accomplishes it in three reactors compared to requirement two reactors and four mixers for the esterification reaction process and three reactors and three mixers for the polycondensation reaction process. In the Hitachi process compared to conventional systems, the cost per unit power of the main reactor is approximately one-sixth, and there are only one-seventh as many places requiring maintenance. This is a highly energy-efficient process.

Safety

While cost reduction was the original target for process intensification, it quickly became apparent that there were other important benefits, particularly in respect of improved intrinsic safety, reduced environmental impact and energy consumption. Given the anticipated plant volume reductions, the toxic and flammable inventories of intensified plant are

correspondingly reduced, thereby making a major contribution to intrinsic safety. It can be inferred that the Dow hazard index and Dow toxicity index could be drastically reduced.

The above are some examples of successful process intensification projects in operation taken from literature as well as personal knowledge. However it is difficult to come by examples as in most cases the benefits are so large that a high level of secrecy is kept in many cases the product name is not revealed.

The examples indicate application of PI to a wide variety chemistry's, processes and across many industries. There are very large number of laboratory initiatives at present. Some of them are like the use of Spin disc reactors(SDR), Oscillating flow reactor(OFR), Heat exchange reactor(HEX), which are being actively being considered for various processes.

Some other equipmentssuch Sonochemical reactor, Microwave reactors, ceramic cross flow heat exchangers and reactors, gas lift reactors, membrane reactors, are also available.

There are many chemical processes where these methods could be used. However the strategies have to be carefully crafted, because in most cases the existing information may not be suitable. The actual kinetic data for reaction could be camouflaged by flow, mass and heat transfer effects. The intensity of mixing could be more than magnitude or two over conventional mixing. It would be necessary to run bench scale and pilot plant to understand the operations and gather information for design and scale up. Initially it may appear that benefits would be their if the reactions are fast, slow reactions may not give the desired benefits. However innovations such as oscillating flow reactor could be a pointer.

Some chemical process industries or chemical processes where process intensification could give large benefits are mentioned below. The basic idea is the possible use of well proven technologies that already exist that can be used in an intensification strategy as also some of the newer equipment which are coming on stream.

1. Red phosphorus - changing from batch to continuous process. The problems associated with the change over are known and require fine tuning using the latest developments in process intensification.
2. Production of precipitated calcium carbonate by reaction of CO₂ with milk of lime to produce very small particle size.
3. Production of HEDP- (hydroxy ethylene diphosphonic acid) this product has a simple chemistry but there are problems of recycle and recovery and therefore quite a few variations in processing are practiced. Continuous production with innovative reactor strategies can lead to vast improvements in productivity and reduction in energy and improved recycling.
4. Emulsion polymerizationor co-polymerization of acrylates, styrene, butadiene, vinyl acetate, etc.
5. Low temp. chlorination - a pesticide intermediate is prepared by chlorination at -40^o C. Considerable heat is evolved. The process could be carried out in novel types of mixers heat exchanger and coolant system to make it more efficient, with excellent temp. controls and other benefits.
6. Continuous production of trioxane and the polymer polyacetal. The polymerization is almost instantaneous with explosive violence.
7. Organic nitration: Process intensification methodologies can be used to produce anddinitration products of xylenes in small sized equipments. The high heat transfer rates would possibly eliminate the use of inert diluent and thus simplifying post processing. As a corollary one could think of dinitration of derivatives of xylidines, (intermediate for Pendimethalin) using these methods. It is likely the by-product formation would be reduced.
8. Another area where it can be applied is the dilution of acids and alkali such as sulphuric acid, caustic soda, etc Combinations of static mixers and heat exchangers can make equipment with very small foot print, even for capacities of many tons per hour.
9. Another potential area of these methods for reactions with high heats of reaction such as neutralization, nitration, sulphonation, etc.
10. These methods could be extensively applied to dye chemistry, reducing by-product formations, reducing utility costs,reducing solvent requirements, etc.
11. Production of aromatic isocyanates by continuous reaction.
12. Catalytic nitration of aromatics without the use of mixed acid.
13. Dehydration of organic solvents more specifically solvent mixtures by membrane processes in contrast to azeotropic and extractive distillation.
14. Continuous hydrolysis of nitriles to amides.
15. Surface catalytic reactions combined with heat transfer such as in KATAPAC.
16. Continuous production of phosphorus penta sulphide - this reaction has a very high heat of reaction, reacts with almost explosive violence. Many methods have been devised earlier to control it.
17. Reaction of alkali phenate with aromatic acetal is normally carried out batch wise with an inert diluent. This has a very high heat of reaction and it appears that heat transfer may be the limiting factor, therefore process intensification strategies may turn out to be very effective.
18. Hydrolysis of phenoxy acetal can use process intensification methods to reduce reactor volumes as well as to produce more concentrated recycle streams.
19. Roasting of minerals in reducing or oxidizing atmosphere.

This is a very limited illustrative list, there could be hundreds of other potential applications of process intensification.

A word of caution

Having read about the benefits one may be tempted to leap into process intensification, however there are certain cautionary signals. In words of Prof. Ramshaw,

"A strategy of process intensification requires a step change in the philosophy of plant and process design. If effectively implemented it will lead to major improvements in environmental acceptability, energy efficiency, intrinsic safety and capital cost. A major cultural change is required on behalf of chemists, engineers and managers and it is this, rather than technical difficulty which represents the main obstacle to progress".

To paraphrase Prof Ramshaw's observations, in general it is perceived "that the biggest obstacle to the adoption of process intensification technology will be business process issues rather than technology. In particular, chemists involved in process development have both a lack of awareness of concept of process intensification and a fear of `mechanical` innovations."

This attitude is not only about chemists but most chemical engineers brought up upon a diet of conventional batch processing. What could also be called the 5 liter flask syndrome.

"The strategy would be to use continuous systems at the outset because once beakers and flasks areused in the initial development process it is very difficult to gain the acceptance of the chemist for a continuous operation and we will end up with the same old pots and pans."