Chemical & Biochemical Reactors

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Questions and Answers

Which two categories are reactors broadly classified in? What are the main differences between the two types (pros/cons)?

Reactors can be broadly classified as chemical or biochemical. Chemical reactors involve traditional chemical processes driven by temperature, pressure, and often catalysis. Biochemical reactors exploit biological processes such as fermentation or enzymatic reactions, often involving microorganisms or enzymes.

Which factors are most important to consider when choosing the reaction path to produce a product?

The reaction paths that use the cheapest raw materials and produce the smallest quantities of byproducts are to be preferred. Reaction paths that produce significant quantities of unwanted byproducts should especially be avoided, since they can create significant environmental problems. However, there are many other factors to be considered in the choice of reaction path. Some are commercial, such as uncertainties regarding future prices of raw materials and byproducts. Others are technical, such as safety and energy consumption.

Which are the six different types of reaction systems? Describe these systems briefly.

Single reactions: just one reaction of the type FEED → PRODUCT 2. Multiple reactions in parallel producing byproducts: a system may involve secondary reactions producing (additional) byproducts in parallel with the primary reaction. 3. Multiple reactions in series producing byproducts. Rather than the primary and secondary reactions being in parallel, they can be in series. 4. Mixed parallel and series reactions producing byproducts: In more complex reaction systems, both parallel and series reactions can occur together 5. Polymerization reactions: monomer molecules are reacted together to produce a high molar mass polymer 6. Biochemical reactions: often referred to as fermentations, can be divided into two broad types. In the first type, the reaction exploits the metabolic pathways in selected microorganisms (especially bacteria, yeasts, molds and algae) to convert feed material (often called substrate in biochemical reactor design) to the required product. In the second group, the reaction is promoted by enzymes.

Which three parameters are used to describe reactor performance? How are they defined?

Conversion = (Reactant consumed in the reactor) / (Reactant fed to the reactor) * 100. Selectivity = (Desired product produced) / (Reactant consumed in the reactor) * Stoichiometric factor. Reactor yield = (Desired product produced) / (Reactant fed to the reactor) * Stoichiometric factor Signup and view all the answers

If there is more than one reactant feed to a reaction, how do you determine which reactant feed the reaction performance should be determined for?

When there is more than one reactant feed in a reaction, the performance of the reactor should generally be determined with respect to the most critical or expensive reactant feed. This choice is driven by the following considerations: 1. Cost Sensitivity: Reactor performance is typically measured with respect to the reactant that has the highest cost or greatest economic impact. For instance, if one reactant is significantly more expensive than others, minimizing its wastage becomes a priority. 2. Stoichiometric Importance: The performance might also be assessed for the limiting reactant if it determines the maximum possible conversion or yield of the desired product. 3. Reaction Pathway Implications: For parallel or series reactions, the selectivity and yield of desired products may depend on specific reactants. Measuring performance with respect to the reactant that strongly influences the selectivity or yield of the primary product ensures a more accurate assessment. Signup and view all the answers

Which parameter is considered most important in describing reactor performance?

In describing reactor performance, selectivity is often a more meaningful parameter than reactor yield. Reactor yield (because it doesn't take into account recirculation) is based on the reactant fed to the reactor rather than on that which is consumed. Signup and view all the answers

Which are the three idealized models used for reactor design? How do they differ?

For the ideal-batch model, the reactants are charged at the beginning of the operation. The contents are subjected to perfect mixing for a certain period, after which the products are discharged. In the second model, the mixed-flow or continuous well-mixed or continuous-stirred-tank reactor (CSTR), feed and product takeoff are both continuous and the reactor contents are assumed to be perfectly mixed. In the third model, the plug-flow model, a steady uniform movement of the reactants is assumed, with no attempt to induce mixing along the direction of flow (so radially mixed but axially unmixed). Signup and view all the answers

Why can you describe a PFR with a series of CSTRS?

Multiple Continuous Stirred Tank Reactors (CSTRs) in series can approximate a Plug Flow Reactor (PFR) because the concentration gradient within the series of CSTRs simulates the gradual decrease in reactant concentration along a PFR's length. As the number of CSTRS increases, the stepwise concentration changes between reactors become smaller, approaching the smooth concentration gradient characteristic of a PFR Signup and view all the answers

Under what conditions is a kinetic model, derived from experimental data, valid?

Kinetic models are only valid for the range of conditions over which they are fitted, at specific molar feed ratios and temperatures. Signup and view all the answers

Which reactor model is best for which reactor system? Explain why.

Single reactions: a mixed-flow reactor requires a greater volume than an ideal-batch or plug-flow reactor. Consequently, for single reactions, an ideal-batch or plug-flow reactor is preferred. Multiple reactions in parallel producing byproducts: If a1>a2 (order of reaction for primary and secondary reactions) the primary reaction to PRODUCT is favored by a high concentration of FEED. Signup and view all the answers

How does the reaction order to product vs byproduct, when you have parallel reactions, affect the choice of reactor model if these orders are not equal?

When dealing with parallel reactions where the reaction orders to product and byproduct are not equal, the choice of reactor model is affected as follows: If the reaction order to the product is higher than the order to the byproduct, a Plug Flow Reactor (PFR) or Batch Reactor is preferred. If the reaction order to the byproduct is higher than the order to the product, a Continuous Stirred Tank Reactor (CSTR) is more suitable. Signup and view all the answers

Which reactor model is the best choice for a reaction system with multiple reactions in series producing byproducts? Explain why.

For a certain reactor conversion, the FEED should have a corresponding residence time in the reactor. In the mixed flow reactor, FEED can leave the instant it enters or remains for an extended period. Similarly, PRODUCT can remain for an extended period or leave immediately. Substantial fractions of both FEED and PRODUCT leave before and after what should be the specific residence time for a given conversion. Thus, the mixed-flow model would be expected to give a poorer selectivity or yield than a batch or plug-flow reactor for a given conversion. A batch or plug-flow reactor should be used for multiple reactions in series. Signup and view all the answers

How does the choice of reactor model affect the product for polymerization reactions?

In a batch or plug-flow reactor, all molecules have the same residence time, and without the effect of termination all will grow to approximately equal lengths, producing a narrow distribution of molar masses. By contrast, a mixed-flow reactor will cause a wide distribution because of the distribution of residence times in the reactor. When polymerization takes place by mechanisms involving free radicals, the life of these actively growing centers may be extremely short because of termination processes such as the union of two free radicals. These termination processes are influenced by free-radical concentration, which in turn is proportional to monomer concentration. In batch or plug-flow reactors, the monomer and free-radical concentrations decline. This produces increasing chain lengths with increasing residence time and thus a broad distribution of molar masses. The mixed-flow reactor maintains a uniform concentration of monomer and thus a constant chain-termination rate. This results in a narrow distribution of molar masses. Because the active life of the polymer is short, the variation in residence time does not have a significant effect. Signup and view all the answers

What determines which reactor model to use for each specific polymerization system?

For polymerization reactors, the main concern is the characteristics of the product that relate to the mechanical properties. The distribution of molar masses in the polymer product, orientation of groups along the chain, cross-linking of the polymer chains, copolymerization with a mixture of monomers, and so on, are the main considerations. Ultimately, the main concern is the mechanical properties of the polymer product. Signup and view all the answers

For a reversible reaction, how is the conversion affected by changes in reaction conditions, such as temperature, pressure and concentration?

Temperature: For exothermic reactions, decreasing temperature increases equilibrium conversion. For endothermic reactions, increasing temperature increases equilibrium conversion. Pressure: (le chatelier) When the reaction involves a decrease in the number of moles, increasing pressure increases conversion. When there is an increase in the number of moles, decreasing pressure increases conversion. Concentration: Using an excess of one reactant shifts the equilibrium toward the products, increasing conversion. Signup and view all the answers

Describe the equilibrium constant and what it defines?

Ka is known as the equilibrium constant. It represents the equilibrium activities for a system under standard conditions and is a constant at constant temperature. Signup and view all the answers

When can the assumption of ideal gas behaviour be made for reactions in a gas phase?

The assumption of ideal gas behavior can be made when the gas is at moderate to high temperature, low pressure, and consists of non-polar molecules. These conditions minimize intermolecular interactions, allowing the gas to behave according to the ideal gas law PV=nRTPV = nRTPV=nRT. homogeneous reactions Signup and view all the answers

What information can you get from the equilibrium constant in terms of reactant and product distribution at equilibrium?

The equilibrium constant indicates the relative concentrations of reactants and products at equilibrium. A large K> 1 suggests that products are favored, while a small K< 1 indicates that reactants are favored. If K is close to 1, there is roughly equal concentration of reactants and products. Signup and view all the answers

Describe Le Chatelier's principle.

A basic principle that allows the qualitative prediction of the effect of changing reactor conditions on any chemical system in equilibrium is Le Châtelier's Principle: “If any change in the conditions of a system in equilibrium causes the equilibrium to be displaced, the displacement will be in such a direction as to oppose the effect of the change.” Le Châtelier's Principle allows changes to be directed to increase equilibrium conversion. Signup and view all the answers

How is the equilibrium constant affected by changes in temperature for an exothermic and an endothermic reaction?

Exothermic: With a higher temperature → decrease in equilibrium constant Endothermic: With a higher temperature → increase in equilibrium constant Signup and view all the answers

How does this affect the equilibrium composition for an exothermic and an endothermic reaction?

For exothermic reactions, decreasing temperature increases equilibrium conversion therefore increasing the amount of products. For endothermic reactions, increasing temperature increases equilibrium conversion therefore increasing the amount of products. Signup and view all the answers

How does the reactor temperature generally affect the rate of reaction?

A qualitative observation is that most reactions go faster as the temperature increases. Signup and view all the answers

Describe how the equilibrium conversion can be increased for an endothermic reaction.

If an endothermic reaction is reversible, then Le Chatelier's Principle dictates that operation at a high temperature increases the maximum conversion. Signup and view all the answers

Which factors affect the highest reactor temperature practically possible?

Safety Considerations. Materials of Construction. Catalyst Life. Reaction Type (Endothermic vs. Exothermic). Reactor Volume and Reaction Rate. Multiple Reactions and Selectivity. Phase Changes. Economics: Higher temperatures raise energy and equipment costs. Signup and view all the answers

How should the reactor pressure be chosen for gas phase equilibrium reactions?

The choice of reactor pressure for gas-phase equilibrium reactions depends on whether the reaction involves a net increase or decrease in the number of moles: 1. Reactions with a Decrease in the Number of Moles. 2. Reactions with an Increase in the Number of Moles. 3. Multiple Reactions Producing Byproducts Signup and view all the answers

How does changes in reactor pressure generally affect the reaction rate?

Increasing the pressure of vapor-phase reactions increases the rate of reaction and hence decreases reactor volume both by decreasing the residence time required for a given reactor conversion and increasing the vapor density. (the particles are more close to each other then) However on liquids and solids it has little to no effect. Signup and view all the answers

Which is the preferred reactor phase of operation? Why?

Given a free choice between gas and liquid-phase reactions, operation in the liquid phase is usually preferred. Clearly, in the liquid phase much higher concentrations of CFEED (kmol/m3) can be maintained than in the gas phase. This makes liquid-phase reactions in general more rapid and hence leads to smaller reactor volumes for liquid-phase reactors. Signup and view all the answers

For what reasons might you use an excess of one of the reactants?

Driving Complete Conversion in Irreversible Reactions. Shifting Equilibrium in Reversible Reactions. Minimizing Byproduct Formation in Multiple Reactions. Safety Considerations. Enhancing Selectivity in Series Reactions Signup and view all the answers

What should be considered when an inert is added to increase the equilibrium conversion for an ideal gas phase reaction?

If inert material is to be added, then ease of separation is an important consideration. Possibly higher reactor volume required, effect on reaction rate and purity due to an increase of moles. Signup and view all the answers

Explain how adding an inert to the reaction A ↔ B + C may increase the equilibrium conversion.

Increasing NT as a result of adding inert material (as a reactant) will increase the ratio of products to reactants. Adding an inert material causes the number of moles per unit volume to be decreased, and the equilibrium will be displaced to oppose this by shifting to a higher conversion. Signup and view all the answers

For what case does the addition of an inert to an ideal gas phase reaction not affect the equilibrium conversion?

If the reaction does not involve any change in the number of moles, inert material has no effect on equilibrium conversion. Signup and view all the answers

Describe ways of minimizing byproduct formation for systems with multiple reactions in series, parallel and mixed series and parallel.

Multiple reactions in series producing byproducts: 1. If the reaction involves more than one feed, it is not necessary to operate with the same low conversion on all the feeds. Using an excess of one of the feeds enables operation with a relatively high conversion of other feed material and still inhibits series reactions. 2. Another way to keep the concentration of byPRODUCT low is to remove the product as the reaction progresses, for example, by intermediate separation followed by further reaction. Multiple reactions in parallel producing byproducts: 1. if inerts are present, increasing the concentration of inert material will decrease byproduct formation. 2. If the secondary reaction is reversible and involves an increase in the number of moles, then, if inert material is present, decreasing the concentration of inert material will decrease byproduct formation. 3. If the secondary reaction has no change in the number of moles, then concentration of inert material does not affect it. Mixed parallel and series reactions producing byproducts: 1. If the byproduct reaction is reversible and inert material is present, then changing the concentration of inert material if there is a change in the number of moles should be considered, as discussed above. 2. Whether or not there is a change in the number of moles, recycling byproducts can in some cases suppress their formation if the byproduct-forming reaction is reversible. Signup and view all the answers

Which factors determine the operating conditions of biochemical reactors?

Temperature, pH, oxygen levels, concentrations of reactants and products and possibly nutrient levels must be carefully controlled for optimum operation. Signup and view all the answers

What effect does a catalyst have on a reaction system?

Catalysts increase the rate of reaction but are ideally unchanged in quantity and chemical composition at the end of the reaction. Signup and view all the answers

Which eight steps are involved in heterogeneous gas-solid reaction on a supported catalyst?

The eight steps are: a) Mass transfer of reactant from the bulk gas phase to the external solid surface. b) Diffusion from the solid surface to the internal active sites. c) Adsorption on solid surface. d) Activation of the adsorbed reactants. e) Chemical reaction. f) Desorption of products. g) Internal diffusion of products to the external solid surface. h) Mass transfer of the products to the bulk gas phase. Signup and view all the answers

Describe three methods for immobilizing enzymes.

Adsorption. Covalent bonding. Entrapment. Signup and view all the answers

Describe ways to control the temperature in a reactor for an exothermic and an endothermic reaction.

Exothermic Reactions (Heat-Generating): 1. Adiabatic Operation 2. Cold Shot Injection 3. Indirect Heat Transfer 4. Heat Carriers 5. Catalyst Profile Adjustments. Endothermic Reactions (Heat-Absorbing): 1. Adiabatic Operation 2. Hot Shot Injection 3. Indirect Heat Transfer 4. Heat Carriers 5. Catalyst Profile Adjustments Signup and view all the answers

Describe the mechanisms for catalyst deactivation.

Loss of catalyst performance can occur in a number of ways: a) Physical loss. b) Surface deposits. c) Sintering. d) Poisoning. e) Chemical change. Signup and view all the answers

How can catalyst degradation be counteracted by control of the reactor?

An operating policy of gradually increasing the temperature of the reactor through time can often be used to compensate for this deterioration in performance. However, significant increases in temperature can degrade selectivity considerably and can often accelerate the mechanisms that cause catalyst degradation. Catalyst regeneration is also used (standby reactors), catalyst profiling. Signup and view all the answers

How does mass-transfer affect reactions taking place in a gas-liquid reactor?

Mass transfer in a gas-liquid reactor affects reaction rates by controlling how quickly reactants move between phases. 1. Resistance in Gas & Liquid Films: Reactants must diffuse through both gas and liquid films before reaction occurs. 2. Control by Solubility: For low-solubility gases (e.g., O2, N2), the liquid film limits the reaction rate, while for high-solubility gases (e.g., CO2, NH3), the gas film is the main resistance. 3. Influence of Reaction Speed: A fast reaction reduces liquid-film resistance, enhancing mass transfer, whereas a slow reaction has little effect. 4. Reactor Design Impact: Choosing packed beds, bubble columns, or agitated tanks affects mixing and interfacial area, influencing mass transfer efficiency. Signup and view all the answers

Which factors are affected by a change in temperature for gas-liquid reactors and in what way?

The influence of temperature on gas-liquid reactions is more complex than homogeneous reactions. As the temperature increases: the rate of reaction increases, solubility of the gas in the liquid decreases, rates of mass transfer increase, due to higher diffusion rates and viscosity of the liquid volatility of liquid phase increases, decreasing the partial pressure of dissolving gas. Signup and view all the answers

Describe ways of improving the mass-transfer rate for gas-liquid and liguid-liquid reactors.

Gas-Liquid Reactors: 1. Increase Interfacial Area: Use packed beds, bubble columns, or spray columns to enhance gas-liquid contact. 2. Improve Mixing: Use agitation or static mixers to reduce film resistance. 3. Operate at Higher Pressures: Increases gas solubility, enhancing transfer. 4. Reduce Film Resistance: Use a fast reaction to consume dissolved gas, lowering liquid-film resistance. 5. Improve Flow Patterns: Use countercurrent flow to maximize concentration gradients. 6. Optimize temperature Liquid-Liquid Reactors: 1. Increase Dispersion: Use agitated tanks, static mixers, or spray columns to break one phase into fine droplets. 2. Optimize Phase Selection: Disperse the smaller volume phase in the larger continuous phase. 3. Control Emulsification: Avoid excessive mixing that may form difficult-to-separate emulsions. Signup and view all the answers

Describe the usual configuration of tubular reactors and when they are most commonly used.

Although tubular reactors often take the actual form of a tube, they can be any reactor in which there is steady movement only in one direction. Because tubular reactors approximate plug flow, they are used if careful control of residence time is important, as is the case where there are multiple reactions in series. A high ratio of heat transfer surface area to volume is possible, which is an advantage if high rates of heat transfer are required. One mechanical advantage tubular devices have is when high pressure is required. Under high-pressure conditions, a small-diameter cylinder requires a thinner wall than a large diameter cylinder. Can be used for multiphase reactions. Signup and view all the answers

Describe the usual configuration of stirred-tank reactors and when they are most commonly used.

Stirred-tank reactors consist simply of an agitated tank and are used for reactions involving a liquid. (ex: homogeneous liquid-phase reactions, heterogeneous gas-liquid reactions, heterogeneous liquid-liquid reactions, heterogeneous solid-liquid reactions, heterogeneous gas-solid-liquid reactions.) In practice, it is often possible with stirred-tank reactors to come close to the idealized mixed-flow model, providing the fluid phase is not too viscous. Signup and view all the answers

Describe the usual configuration of fixed-bed catalytic reactors and when they are most commonly used.

Here, the reactor is packed with particles of solid catalyst. Most designs approximate plug-flow behavior. However, if frequent regeneration is required, then fixed beds are not suitable, and under these circumstances a moving bed or a fluidized bed is preferred; Should not be too high temperatures for catalyst deactivation. Fixed-bed catalytic reactors use a packed solid catalyst, typically following plug-flow behavior. Heat varies through the bed, thus Temperature control is managed via 1. adiabatic beds, 2. intermediate heating/cooling 3. tubular designs. Common in gas-phase reactions like hydrocarbon processing, they risk “hot spots” and require shutdowns for catalyst regeneration. For frequent regeneration, moving or fluidized beds are preferred. Signup and view all the answers

Describe the usual configuration of moving-bed catalytic reactors and when they are most commonly used.

If a solid catalyst degrades in performance, the rate of degradation in a fixed bed might be unacceptable. In this case, a moving-bed reactor can be used. Here, the catalyst is kept in motion by the feed to the reactor and the product. This makes it possible to remove the catalyst continuously for regeneration. ex: refinery hydrocracker reactor Catalysts fed to the system and goes down due to gravity. Signup and view all the answers

Describe the usual configuration of fluidized-bed noncatalytic reactors and when they are most commonly used.

Configuration: Gas-solid fluidization, enhancing heat transfer and reaction rates. Fluidized beds are also suited to gas-solid non catalytic reactions. All the advantages described earlier for gas-solid catalytic reactions apply here (good heat transfer, ever temp profile, catalyst regeneration). As an example, limestone can be heated to produce calcium oxide in a fluidized-bed reactor according to the reaction: heat CaCO3→CaO+CO2 Air and fuel fluidize the solid particles, which are fed to the bed and burnt to produce the high temperatures necessary for the reaction. Signup and view all the answers

Flashcards

What are Chemical Reactors?

Reactors that use traditional chemical processes driven by temperature, pressure and catalysis.

What are Biochemical Reactors?

Reactors that exploit biological processes like fermentation using enzymes and microorganisms.