The following discussion proceeds from the most general to the more specific topics on preventing pollution for chemical reactors.
From an environmental perspective, reactors are the most important unit operation in a chemical process. The degree of conversion of feed to desired products influences all subsequent separation processes, recycle structure for reactors, waste treatment options, energy consumption, and ultimately pollutant releases to the environment. Once a chemical reaction pathway has been chosen, the inherent product and by-product (waste) distributions for the process are to a large extent established. However, the synthesis must be carried out on an industrial scale in a particular reactor configuration and under specified conditions of temperature, pressure, reaction media (or solvent), mixing, and other aspects of the reactor operation.
In designing chemical reactors for pollution prevention, there are many important considerations. The raw materials, products, and by-products should have a relatively low environmental and health impact potential. This means that the environmental and toxicological properties of the chemicals involved should indicate that they are relatively benign. In addition, the conversion of reactants to desired products should be high and their conversion toward by-products should be low.
In other words, the reaction yield and selectivity for the desired product should be as high as possible. Finally, energy consumption for the reaction should be low. Another consideration is that the life cycle impacts of reactants, products, and by-products should be relatively low. For example, cumulative emissions and impacts of raw materials should be relatively low, environmental impacts during subsequent use by consumers should be small, and if possible the reaction products should be recyclable. Engineers must balance all of these considerations.
These reactor considerations will be classified as:
1. Material use and selection.
2. Reaction type and reactor choice.
3. Reactor operation.
1. Material Use and Selection for Reactors:
Issues involving the use of materials in a chemical reactor include the choice of feed entering the reactor, the catalyst if one is needed, and solvents or diluents. However, it is important to mention material selection here in light of their influence on the environmental impacts of reactors in chemical processes.
i. Raw Materials and Feedstocks:
Raw materials used in chemical reactions can be highly toxic or can cause undesirable by-products to form. Although some of these raw materials may be converted to relatively benign chemicals through chemical reactions in the process, their presence may be a concern because of the potential for uncontrolled release and exposure to humans in the workplace and also in the environment. An important strategy for environmental risk reduction for chemical processes is to eliminate as many of these toxic raw materials, intermediates, and products as possible.
The elimination of a raw material or the use of a more benign substitution may necessitate the adoption of new process chemistry. For example, phosgene is used in large volumes all over the world in the manufacture of polycarbonates and urethanes. Phosgene –COCl2 is highly toxic and may pose risks for workers at manufacturing facilities and to the surrounding population if large releases occur. In the phosgene process for producing polycarbonates, polycarbonate is produced from bisphenol-A monomer and phosgene in the presence of two solvents, methylene chloride and water.
A new process for polycarbonate synthesis has been demonstrated using solid-state polymerisation in the absence of both phosgene and methylene chloride (also toxic), by including diphenyl carbonate (DPC) and phenol instead. Similarly, alternative phosgene-free routes to urethanes have recently been developed. The ‘Tier 1’ environmental performance tools are very useful for evaluating these and other alternative reactions chemistries.
In the production of fuels for transportation, petroleum refineries are required to remove sulphur from their products. If not removed, SO2 is released to the atmosphere upon combustion of the fuels in automobiles, trucks, or stationary combustion sources. Exposure to SO2-contaminated air causes lung irritation and other more serious health effects, and SO2 emissions contribute to acidification of surface water and ecosystem damage.
Choosing a crude oil raw material with lower sulphur content (sweet crude) reduces the amount of sulphur that needs to be removed and reduces operating costs, but is considerably more expensive to purchase. Another option to consider would be to use, and therefore incur the associated costs with, a hydrodesulphurisation or a hydro treating unit to remove the sulphur. The sulphur can then become a salable product.
In partial oxidation reactions of hydrocarbons to form alcohols or other oxygenated organics, air has traditionally been the source of oxygen in the reaction, and the nitrogen in the air has acted as a heat sink agent (diluent) to help control temperature rise for the exothermic reaction. Some CO2 and H2O are produced, and due to the presence of N2, some NOx is formed. NOx is a precursor in the formation of photochemical smog in urban atmospheres and its emission from industrial facilities is regulated under the Clean Air Act.
One method to reduce or eliminate the formation of NOx in partial oxidation reactions is to use pure oxygen or enriched air as the oxidising agent, thus preventing NOx formation. Carbon dioxide that is recovered and recycled from the reactor effluent or water vapour could be used as the heat sink instead of nitrogen. Another method is to install NOx control equipment on the original process.
An important issue in this case is whether the costs associated with purchasing and operating the CO2 recovery equipment are lower than operating NOx control equipment. Another consideration is whether the additional pollutant releases associated with NOx prevention equipment are lower than the releases in the original process.
Another important class of raw materials used in chemical reactors are solvents. This is especially true in ‘solution’ and ’emulsion’ polymerisation reactions in which the reaction of monomers to create high molecular weight polymers occurs either within the solvent phase or within dispersed droplets of monomer in the solvent phase. In some polymerisations, addition of solvents can enhance precipitation of polymer in solid form, co-solubilise monomer and initiator, and act as a diluting medium to modulate the rate of reaction and rate of heat removal.
The most commonly used polymers are low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), polypropylene, and polystyrene. Solvents used in the production of some of these high volume polymers include xylene, methanol, lubricating oil, hexane, heptane, and water. Solvents are concern due to their high volatility and potential to cause low-level ozone during smog formation reactions in the atmosphere. They may also be a health concern for workers and the general population in the vicinity of the facility.
In addition, there are several on-line resources for evaluating substitute solvents. The solubility, toxicological, cost, and environmental properties of the candidate solvents can be compared with each other and with the original solvent. Supercritical carbon dioxide is being studied as a substitute solvent in many reaction systems. Applications include both homogeneous and dispersed phase polymerisation reactions in which the supercritical CO2 replaces volatile organic compounds and chlorofluorocarbons as traditional solvents in the reaction mixture.
A catalyst is a substance that is added to a chemical reaction mixture in order to accelerate the rate of reaction. Catalysts are either homogeneous, being dissolved in the reaction mixture, or heterogeneous, typically existing as a solid within a reacting fluid mixture. The choice of catalyst has a large impact on the efficiency of the chemical reactor and ultimately upon the environmental impacts of the entire chemical process.
Advances in catalysts can improve the environmental performance of a chemical reactor in several ways. Catalysts can allow the use of more environmentally benign chemicals as raw materials, can increase selectivity toward the desired product and away from unwanted by-products (wastes), can convert waste chemicals to raw materials, and can create more environmentally acceptable products directly from the reactions.
The production of reformulated gasoline (RFG) and diesel fuels from crude oil is a clear example of how improved catalysts can create chemicals that are better for the environment. Because of recent trends in the petroleum refining industry, improved catalysts are being used in several reaction processes within modern refineries.
These trends include:
1. Increased processing of crude oils with lower quality (higher percentages of sulphur, nitrogen, metals, and carbon residues).
2. More demand for lighter fuels and less for heavy oils.
3. Environmental regulations that limit the percentages of sulphur, heavy metals, aromatics, and volatile organic compounds in transportation fuels.
In particular, the inclusion of RFG in the clean air act (CAA) has prompted many changes in catalyst formulation and reactor configurations.
Table 5.2 is a summary of conventional and improved catalytic reaction processes for RFG and diesel production. As seen in the Table, the major emphasis is on catalyst improvements for sulphur and nitrogen removal from heavier crude fractions, reduced aromatics content, and increased production of branched C5-C7 alkanes for octane enhancement.
2. Reaction Type and Reactor Choice:
The details of any chemical reaction mechanism, including the reaction order, whether it has series or parallel reaction pathways, and whether the reaction is reversible or irreversible, influences pollution prevention opportunities and strategies for chemical reactors. These details will determine the optimum reactor temperature, residence time, and mixing. In addition, reactor operation influences the degree of reactant conversion, selectivity, and yield for the desired product, by-product formation, and waste generation.
As a general rule, in designing chemical reactors for pollution prevention; one would like a reaction with a very high conversion of the reactants, high selectivity toward the desired product, and low selectivity toward any by-products. A typical reactor efficiency measure pertaining to reactant conversion is the reaction yield, defined as the ratio of the exiting concentration of product to inlet reactant ([P]/[R]0). Reaction selectivity is defined as the ratio of exiting product concentration to the undesired by-product concentration.
We define a modified selectivity as the ratio of exiting product concentration to the sum of product and by-product (waste) concentrations ([P]/([P]+[W]) = [P]/[Reactant consumed]). This allows us to display both yield and selectivity on the same scale, from 0 to 1. Yields and selectivity values that are very close to unity indicate an efficient reaction, with little waste generation or reactant to separate in downstream unit operations.
Parallel reaction pathways are very common in the chemical industry. An example of an industrial parallel reaction is the partial oxidation of ethylene to ethylene oxide, whereas the parallel reaction converts ethylene to by-products, carbon dioxide, and water.
We will begin our discussion of reaction types and their implications for pollution generation with the simple irreversible first-order parallel reaction mechanism shown below –
R is the reactant, P the product, W a waste by-product, and kp and kw are the first-order reaction rate constants for product formation and waste generation (time–1), respectively. The relative concentrations of products and waste components are significantly affected by the ratio of the first order reaction rate constants, kp/kw.
Figure 5.2 illustrates the dependence of the reactor effluent concentrations of products and waste as a function of reactor residence time for several values of these rate constant ratios. In order to achieve maximum reactor yields, the residence time must be about 5 times the reaction time constant (kp + kw)–1.
The reaction selectivity is constant and independent of reactor residence time for first-order irreversible, isothermal parallel reactions. In a series reaction, selectivity is affected by reactor residence time, and therefore this parameter must also be considered for pollution prevention in chemical reactors.
In a series reaction, the rate of by-product (waste) generation depends on the rate of product formation, as shown by the first-order irreversible series reaction below –
Longer reactor residence times lead to not only more product formation but also more by-product generation. The amount of waste generation for a series reaction depends on the ratio of product formation rate constant (kp) to the by-product generation rate constant (kw) and also on the residence time in the reactor. Figure 5.3 illustrates the effect of reactor residence time on reactant, product, and by-product concentrations for several reaction rate constant ratios (kp/kw).
For each ratio, there is an optimum reactor residence time that maximises the product concentration. Figure 5.4 shows the product yield ([P]/[R]0) and modified selectivity ([P]/([P]+[W]) over a range of reactor residence time for several reaction rate ratios. For irreversible series reactions, the modified selectivity continues to decrease with time. At longer residence times, the rate of waste generation is greater than the rate of product formation.
To minimise waste generation in series reactions, it is important to operate the reactor so that the ratio kp/kw is as large as possible and to control the reaction residence time. Also, if there is a way to remove the reaction product as it is being formed and before its concentration builds up in the reactor, then by-product generation can be minimised.
Reversible reactions are another important category of chemical reactions. Figure 5.5 shows the reactant, product, and by-product concentrations profiles in parallel and series reversible reactions for a wide range of reaction rate constants. It is evident that reversible reactions inhibit full conversion of reactants to products.
Also, the reactor residence time is a key operating parameter for reversible reactions. Selectivity improvements for reversible reactions, operated at equilibrium, can be achieved by utilising the concept of recycle to extinction. As an example of this concept, consider the steam reforming of methane to form synthesis gas (CO + H2) for methanol production.
Both reactions are reversible and at equilibrium. When CO2 is recovered and recycled back to the reactor, it decomposes in the reactor as fast as it forms, and no net conversion of methane to CO2 occurs. This requires additional operating costs, but there is no selectivity loss of reactant, the process is cleaner, and it may be the lowest cost option overall.
Figure 5.6 shows a process flow diagram for a reactor combined with a separator that recycles reactants and by-products back to the reactor. This configuration can be operated such that all reactants fed to the reactor are converted to product with no net waste generation from the process. Selectivity improvements for reversible reactions can also be realised by employing separative reactors.
More complicated chemical reactions, compared to the few simplistic first-order reactions are common in the chemical industry, and their pollution-generating potential must be evaluated on a case-by-case basis. However, the general trends discussed are expected to hold for more complex reaction networks.
The choice of chemical reactor type within which the reaction is carried out is also an important issue for process design and pollution prevention. A continuous-flow stirred-tank reactor (CSTR) is not always the best choice. A plug flow reactor has several advantages in that it can be staged and each stage can be operated at different conditions to minimise waste formation.
In a novel application of a plug flow reactor, DuPont developed a catalytic route for the in situ manufacture of methyl isocyanate (MIC) using a pipeline reactor, resulting in only a few kgs of MIC being inventoried in the process at any one time. This strategy minimises the chance of a catastrophic release of MIC, such as happened at Bhopal, India, in 1984.
When hot spots are a problem for highly exothermic reactions carried out in a fixed-bed catalytic reactor, a fluidised-bed catalytic reactor will likely avoid the unwanted temperature excursions. Good temperature control is critical for reducing by-product formation reactions that are highly temperature- sensitive. An example where a fluidised-bed reactor succeeded in reducing waste formation is in the production of ethylene dichloride, an intermediate in the production of polyvinyl chloride (PVC). The prior fixed-bed design operated with a temperature range of 230°-300°C while the newer fluidised-bed design was able to run at between 220°-235°C.
3. Reactor Operation:
Reaction temperature can influence the degree of conversion of reactants to products, the product yield, and product selectivity. We illustrate the effects of temperature on reaction selectivity by considering the simple irreversible first-order parallel reaction mechanism shown below –
where R is the reactant, P the product, W a waste by-product, and kp and kw are the first-order reaction rate constants for product formation and waste generation (time–1), respectively. The ratio of the reaction rates for product formation to by-product generation is an important indicator of reaction selectivity.
where Ap and Aw are the frequency factors (time–1) and Ep and Ew are the activation energies (kcal/mole) for product and waste respectively, R is the gas constant [ 1.987 x 10–3 kcal/(mole.K)], and T is absolute temperature. Because the reaction rate constants, kp and kw, are functions of temperature, their ratio is also a function of temperature. For the purpose of illustration, we can calculate the change in this ratio [Δ(kp/kw) as the temperature is changed to a new value (T1) above or below a given initial temperature (T0).
Figure 5.7 shows the expected change in the ratio of product/by-product rate constants when temperature is changed (ΔT) above and below T0. When Ep > Ew, the ratio increases with increasing temperature and decreases with decreasing temperature. Therefore, pollution can be prevented in parallel (and also series) reactions by increasing reactor temperature when Ep > Ew. The opposite holds true for when Ep < Ew. Also, as the difference between Ep and Ew increases, temperature has a more pronounced influence on the change in the rate constant ratios.
When a reactant in one inlet stream is added to another reactant that already exists in a well-stirred reactor, the course of complex multiple reactions can be affected by the intensity of mixing in the vessel. For irreversible reactions, the reaction yield and selectivity may be altered compared to the case where the reactants are mixed instantaneously to a molecular level.
This may lead to a greater amount of waste by-product generation. In addition, the rate of reaction can be reduced because of diffusional limitations between segregated elements of the reaction mixture. The complications that arise from imperfect mixing are particularly evident for rapidly reacting systems. In these situations, reactants are significantly converted to products and by-products before mixing is complete.
To illustrate the effects of mixing, it is illustrative to examine the competitive-consecutive reaction carried out in a constant stirred tank reactor (CSTR), using the reaction mechanisms shown below –
This reaction is also sometimes referred to as the series-parallel reaction. This reaction type is a good kinetic representation of the nitration and halogenation of hydrocarbons and saponification of polyesters, among its many industrially-relevant examples. Reactant A is initially charged in the reactor and B is added as a solution through a feed pipe in a continuous manner until a stoichiometric amount of B is added. Species R is the desired product and S is a by-product. If the reactions are first order, mixing will not affect selectivity.
However, if the reactions are second order, the presence of local excess B concentrations can cause overreaction of R to S via the second reaction. This effect of mixing occurs for both homogeneous and heterogeneous reaction systems and for batch or semi-batch reactors (B added to an initial charge of A).
A detailed experimental study of a homogeneous liquid phase second-order competitive-consecutive reaction was conducted to determine the effects of mixing on yield of reactants A and B to product R in a CSTR. The reaction involved the iodisation of L-tyrosine (A) in aqueous solution, as shown in Fig. 5.8.
Smith investigated the effects of reaction temperature, initial concentration of reactant A (A0), rate of addition of B, agitation rate of the vessel impeller, and presence or absence of baffling within the reactor. A correlating equation for all of these parameters was found between the ratio of measured reactor yield to expected yield (Y/Yexp) versus the dimensionless quantity (k1 B0 τ)(A0/B0),
where, k1 = product reaction rate constant (litres/(gmole . sec))
k2 = by-product reaction rate constant (litres/gmole . sec))
A0 = initial concentration of species A in the feed (gmole/litre)
B0 = initial concentration of species B in the feed (gmole/litre)
τ = microtime scale for mixing of eddies of pure B with bulk liquid (sec)
Y = measured yield = R/A0
(k1 B0 τ) = extent of conversion of A and B under conditions of partial segregation
To give an idea of the range of observed yields in the experiments, values of Y/Yexp were measured from 0.66 to 0.98, depending upon mixing intensity and other parameters. A correlation fitting the data is presented in Fig. 5.9. It was found that when the quantity (k1 B0 τ)(A0/B0) was less than or equal to 10–5, Y = Yexp This criterion allows us to ‘set’ the mixing intensity for any second-order competitive- consecutive reaction.
Lf = a characteristic length scale of the vessel (ft)
u’ = fluctuating turbulent velocity (ft/sec.)
v = kinematic viscosity (ft2/sec.)
We can rearrange the equation above for u’ and incorporate a correlation for turbulent fluctuation velocity in an agitated CSTR for feed entering at the impeller (u’ = 0.45 π D N). We arrive at –
Thus, with this equation, we can establish the necessary impeller agitation speed (N, revolutions per second) to ensure that mixing will not adversely affect the yield, given k1, v, Lf, and D, the impeller diameter (ft).
iii. Effect of Reactant Concentration:
The selectivity of series and parallel chemical reactions can be sensitive to the initial concentration, since the rates of product formation and by-product generation are dependent on concentration. For a parallel irreversible reaction, the rates of product formation and waste generation can be expressed as –
If np > nw then increasing the concentration of reactant will increase the reaction selectivity toward the product and away from the waste by-product. Conversely, if np < nw, then increasing reactant concentration will decrease selectivity toward the desired product.