In this article we will discuss about the environmental care undertaken during the manufacture of chemicals.
Two approaches are possible for any chemical manufacturing or chemical utilising units in order that its wastes be taken care of:
1. Adaptation of otherwise traditional forms of controlling and managing wastes to new, more stringent ecological requirements.
2. Striving to overhaul the processes and plant equipment in such a manner that it will result in cutting down, doing away or doing without wastes.
Making unavoidable production less detrimental is another consideration and is also an acceptable one.
From the viewpoint of environment control, a perfect or near perfect achievement of this goal is possible only when the process steps have been studied with respect to many factors:
1. Possible production of detrimental effluents at any stage of the process.
2. Likely sources that pave the way for wastage of materials in any form.
3. Applicable remedial measures for the above aspects including recovery of any loss of energy or materials at an early stage of its production or detection in the system; or utilisation of these produce as secondary materials at some stage or the other of the process on hand, etc.
Although it is easier said than done, it is not to be brushed aside as impossible. Nonstop experimentation, study, and deeper introspection are required. A major portion of such a study is required to be completed in the design stage itself as once the plant is on stream, any alterations comes with production loss and related hurdles.
Few routes to achieve this are:
1. Design of tonnage plants on mathematical methods.
2. Techniques for process optimisation.
3. Syntheses of closed material systems.
Proper selection of material of construction of equipment in the designing of a process has a long lasting contribution in the manufacture of a commodity. It saves energy and labour costs for not only fabrication, but also operation.
Efficiency in the usage of utilities will rise to optimum levels and the corrosion characteristics of the material will induce longevity.
Low pressure systems, medium pressure systems and high-pressure systems applicable to a given process step must be studied carefully and the right one selected for implementation. The adoption of the right pressures, with or without catalysing or accelerating agents, boosts the performance of the total set-up to a desired efficiency and avoids the otherwise possible environmental deterioration, in the near or distant future. With application of various pressure systems, there is total possibility of using different types of equipment and different forms of the materials in deriving a given product.
The selection of the pressures or for that fact, any other parameters of operation, must be viewed with respect to all these connected factors too. In case of exothermic reactions, it is generally easier to obtain a high degree of conversion by keeping down the temperature and maintaining isothermal conditions. As it is highly difficult to maintain a temperature profile that is isothermal, at least in fixed-bed reactors, multi-section reactors with intermediate cooling arrangements are being used in modern designs.
It is a generally adopted rule to have one of the reactants in excess of the stoichiometric proportion, although it leads to marked increase in capital costs and the size of the plant. Instead, reactions can be carried out in two stages, continually withdrawing one of the product streams. Wherever the reaction is followed by a drop in volume of the feed components, raising the pressure in the system can be a useful alternative to get rid of certain operational hassles.
Zero discharge technologies are possible in many process systems, thanks to modern science and technology. These involve utility cycles that are specially designed. Closed configurations are capable of permitting all-round utilisation of total materials and utilities, and ensure total improvement in the process efficiency, and cost reduction via minimised usage of energy and utilities.
Production of chemicals is generally categorised as a selection or mixture of operations that are either parallel or series in nature. This is done to shorten the line of production to the minimum possible. Combinations of parallel/series operations or flow patterns are nothing but individual plants that are of self-contained nature. The hidden aim of such designs is to envisage minimal in-process losses, of either materials or utilities that have something to do with that stage of process. Long lines of production tend to give material loss as well as possible problems of maintenance of equipment, besides increasing the probability of pollution to an uncalled-for magnitude.
With a decision to employ the best possible technology, it is always better to go in for a plant of higher capacity. This brings in savings via usage of lesser manpower, reduced maintenance of utilities, etc. and easy procedures for abatement of pollution. Higher capacity plants have been proven to use considerably fewer quantities of utilities, compared to their smaller counterparts. Increase in plant capacity need not be necessarily via scaling up the physical dimensions of the equipment.
This approach generally brings with it a number of hurdles, viz. elaborate designs, huge requirement of space, utility consumption and disposal problems associated with the utility generators, increase in cost for provision of storage facilities for raw materials, piping, increase in production costs, etc.
The other methods that need to be studied with respect to the type and characteristics of the product are:
1. Increase in feedstock concentration.
2. Usage of higher process pressures.
3. Usage of higher process temperatures.
4. Usage of better catalysts.
5. Usage of better reactants.
6. Better flow sheets and processes.
Explorers in quest of small equipment for a given process have ultimately come out with techniques of combining several unit operations in an equipment, although such ideas have posed stricter norms for fabrication and erection.
It has to be added here that these developments and challenges have ended up with success and have resulted in innovative outcomes like efficient recovery systems for outlet streams, improving ecological and safety aspects.
Possibility of recovery of energy released within the process, as input at some stage or other of the same process, is more in large capacity plants. Energy output from the stages of a process is fed to systems designed for generation of utilities for manufacture of many chemicals. Cooling by adsorption is considered beneficial in some designs, compared to indirect heat transfer systems, as they depend on low potential heat requirements.
Application of inert gas atmosphere in reaction systems has been proved to be efficient, safe and economical in many designs of chemical manufacture. Catalysts and accelerators are of importance for speedy processing. Methods of recovery of catalysts must be designed well. Stepping up the reactivity of catalysts is a development of interest in this direction.
Removal of by-product streams may facilitate high conversion rates in some cases. Selective sorption techniques may call for intentional introduction of a foreign stream into a system. Here, the recovery of the foreign stream must be thought of as an economic measure, if not compelled by the end product quality demands.
Closed circuits can help in the following respects:
1. Recovery of un-reacted source materials.
2. Saving in usage of heat transfer media.
3. Re-use of heat or other forms of energy used in the system.
4. Maintain effect of process on environment to be under confinable and acceptable limits.
In order to reduce the hassles posed by fine dust or mist in the process, one or more of the following equipment can be made use of:
2. Filter mist eliminators.
3. Fluidised beds.
4. Froth apparatus.
5. Packed towers.
6. Suspended packing units.
7. Venturi scrubbers.
8. Wetted wall spray towers.
9. Condensers (coagulation).
10. Alkaline absorption units.
11. Sorption equipment that envisage the sorption based on the usage of ion-exchange filters.
12. Usage of activated charcoal and/or alumina and/or clay and/or silica gel.
The reduction in intake quantities result in reduction in quantity to be treated, as well as quantity of untreated coolant. It has been practically demonstrated that plants of higher production capacity require comparatively lesser quantities of energy (electrical or otherwise). Pollution of emitted air too is considerably minimised.
Plant integration helps develop multiple products under one facility at the same time, using a given set of equipment. This helps in avoiding repeated shutdowns/start-ups of utility mechanisms, as well as other crucial equipment.
Closed circuit processes often demand the design and efficient usage of some or all of the following systems, in general.
1. Better reactors.
2. Centrifugal compressors.
3. High-speed steam turbines.
4. Sensible heat utilisation in reactors.
5. Usage of cheaper heat transfer agents, etc.
Cleaning of waste gases is mandatory, for the performance of the equipment, as well as reuse of emitted gases elsewhere in the process or even for ensuring a better environment. Irrecoverable loss of low potential energy from various cooling systems is an unavoidable stream in some process flows. Generally, these are harmless in their environment-related aspects. However, these must not be neglected, as they tend to get themselves involved in some fashion or other into larger requirement of energy for the specific needs of the processes. The ultimate attainment of such a flow is addition to loss of heat in systems that produce energy.
Methods of improving combustion will pay off. The organic components of tail gases can be eliminated by burning them in a flare, if not by a catalytic digestion. The amount of escaping gases can be kept under permissible control levels by adopting timely inspection procedures and prompt and immediate attention of any fault. Usage of gas streams as fuels during start-ups is worth considering. Reduction in number of start-ups and shutdowns are important.
Using gas-fired furnaces for production of power steam relieves the burden of electric power stations, but adds to emissions of gaseous materials that demand careful attention and treatment or re-usage before being let off. Irrespective of the extent to which this emission is contaminated with toxic constituents, at least volume-wise, these need proper treatment.
It is an undeniable fact that in many industrial processes, the exit gases generally carry a good amount of heat that can be used somewhere in the process itself, after a thorough study of compatibility and a bit more of additional investment. The investment is worth considering as it will always be cheaper than investment in fresh fuels (Table 9.1).
Bringing down the combustion temperature is another step that can help. Catalytic cleaning units can come in handy in doing away with emission of toxic gaseous streams. These units are designed to operate with the introduction of a reducing gas such as hydrogen or natural gas. Oxygen concentration must be kept low at least during the initial stages of the combustion zone. Here, optimal and efficient utilisation of clean air is emphasised.
Reactions in the high temperature zones of a reactor must be free from any type of resistance whatsoever to the smooth continuation to completion of the reaction in as short a span of time as possible and feasible. Waste gases leaving an absorber must be subjected to a secondary cleaning process, before discharge into the atmosphere.
Combustible gases such as natural gas, hydrogen, carbon monoxide and ammonia can be used for catalytic reduction of the valuable product. Cleaning plants of modern age make use of palladium on a substrate (to reduce the consumption of palladium).
Wherever possible, the present day technologies involve use of enriched gas, instead of the otherwise commonly used natural gas as reducer. This has resulted in avoiding waste heat recovery boiler, etc. and other connected expenses. High temperature catalytic cleaning of exhaust gases will be the basis of designs to come in future. However, the efficiency of operation of such a process depends on the reliability of its components and the ease of start-up/shut-down, in addition to the cost of power. Zeolites make for a great option as absorbing agents.
Chemical Manufacture # Catalysts:
Present day technologies demand the use of catalysts.
Wherever found to be of use for smooth and speedy reactions, catalysts are required to possess some or all of the following properties:
1. High activity at low temperatures.
2. Low pressure drop across the bed.
3. Facilitate low energy consumption.
4. Low sensitivity towards dust.
5. High mechanical strength.
6. Stability at elevated temperatures.
Chemical Manufacture # Prevent Rather than Cure:
One promising way to prevent water bodies from getting polluted is to do away with effluents themselves. It may seem, at first sight that this situation is not achievable in practice, but it will be necessary to forge ahead.
The few possible roads to this destination are:
1. Recycle all the water used for any purpose at any stage of the process.
2. Utilise the effluents, either unpurified or partly purified, within the process.
3. Collect all slurry at one point, neutralise them, allow the neutralised one to settle, allow the clear water to cool and reuse the water while filtering the slurry and remove the cake to a dump pile.
4. Localised recycle loops help reduce quantity of effluents.
5. Use process liquids and not extra water or fluids to wash or absorb gaseous streams or for make-ups.
6. Reduce the contents of impurities in the effluents. Ion exchange techniques come in handy here.
7. Avoid leakage from any container that is in use at any stage of operation, storage, transportation or during breakdowns.
8. Use submersible pumps for handling liquefied gases.
9. Vacuum type transfer facilities help handle vapours in a better fashion.
10. Avoid human element at possible and crucial control points.
Always be conscious about the fact that wash water is not a single entity, but a name given to the collection of varying quantities of starting materials, products, by-products, wash waters, overflows, wastes from the equipment and process stages, leakage, precipitations, oil fractions and carry-overs.
Defects in plant designs, improper running of plants and disorganised manufacture contribute heavily to emissions to environment.
A typical example of what a proper design can do to minimise dangers to atmosphere is given in Table 9.2. The two sets of results enumerated here are those of the analyses of waste-water emanating from a dichloromethane plant before and after treatment by available and appropriate methods, respectively.
Extending the duration between shut-downs can help in minimising the after effects of purging, a process that results in discharge of gases and vapours that remain in the system and flushing, the process that leads to hazardous chemicals being dumped in the sewage treatment area. This is possible only by making use of equipment that is highly reliable for a given performance.