Equipment III Reactors Course Overview A chemical processing facility converts raw materials into a finished product which meet or exceed customer standards. To accomplish this, the raw materials are moved through processes which cause chemical or physical changes. Here is an example of a common chemical process. Sulfuric acid is typically used in fertilizers and car batteries. However, it is also used to produce a variety of other products such as paints, plastics, and detergents. Controlling Chemical Reactions Chemical reactions are at the heart of chemical manufacturing. In order to contain and control these reactions, a vessel known as a reactor is used. The raw materials, called reagents or reactants, are added to the vessel and the new material produced in the reaction is removed. In reality, controlling a chemical reaction is complicated, requiring highly engineered equipment and operating conditions. A chemical reaction occurs when elements or compounds bond or decompose to form different compounds. Reactions are represented by a chemical equation like this one with the phase of each reactant in parentheses. Reacting sulfur with oxygen in a furnace reactor is an example of a combustion reaction which combines a combustible material with oxygen to form the product and a by-product of heat. Other types of chemical reactions include single replacement, double replacement, synthesis, and decomposition. Chemical bonding occurs when molecules collide but not every time they collide. Temperature, pressure, concentration of reactants and products, contact surface area, and the presence of a catalyst determine the rate of a chemical reaction. Catalytic Reactions A catalyst is a substance that alters the rate of a chemical reaction without being consumed or permanently changed chemically. Catalysts can be solid, liquid, or gas. In addition to improving reaction rates, catalysts are used to break apart long hydrocarbon molecules in a reaction called catalytic cracking. This process is widely used to break heavy petroleum molecules into lighter molecules used as fuels. Catalytic cracking of hydrocarbons restructures straight chain feed hydrocarbons into more complex molecules. Reaction Equilibrium The combustion reaction of sulfur and oxygen to sulfur dioxide is an example of a reaction that goes to completion. That is the reaction stops when all the reactants are consumed. A separate operation in this process is reacting with the clean sulfur dioxide gas with oxygen to form sulfur trioxide. In this reaction, the sulfur trioxide molecules decompose at the same time the reactants are bonding. This is known as a reversible reaction and is written like this. If both reactions occurr at the same rate, the system will reach a state of equilibrium in which sulfur trioxide, sulfur dioxide and oxygen are forming but there is no net gain in product. A system in equilibrium will stay in equilibrium unless a change is made to the system. When a system in equilibrium is disturbed, the reaction shifts to counteract the effect of the disturbance. Depending on the disturbance, the system will either produce more products or more reactants. In chemical processing, reactant concentration, temperature, and pressure are used to drive a reversible reaction toward producing more of the desired product or products rather than more reactants. For example, the reaction of sulfur dioxide and oxygen is exothermic. Since, heat is produced in the reaction. Removing heat will drive this reaction to produce more product, sulfur trioxide. Reactor Models To design large scale systems to produce a desired reaction, engineers use what is known as ideal reactors to build mathematical models. Generally there are two ideal reaction models. The stirred tank or continuous stirred tank reactor model assumes the reactants are well stirred in a vessel so that the concentration of reactants is consistent everywhere in the vessel. Thus, the reactant concentrations in the reactor are equal to their value in the product removed from the reactor. Basically, this model is used to determine what would happen if reactants are fed at a specified rate, temperature, concentration, and pressure as they are mixed with a rotary impeller. The plug flow model simulates adding reactants at one end of a tube. The chemical reaction takes place in a stream that carries the reactants from one place to another so the composition of the content varies from the inlet to the outlet but is completely consistent at any given cross-section. The advantage of the plug flow model is that the reaction rate at the inlet of the tube would be significantly faster than the continuous stirred tank model. This is because more reactions will take place where the reactants are most concentrated. Then as more reactants are consumed, the reaction rate slows. The models help determine key parameters such as the pressure and temperature needed to produce a desired reaction. From the model, a system is designed to emulate the results as closely and economically as possible. This system could be a single reactor or a system that emulates the model in stages. Reactor selection largely depends on the phase or phases of the reactants. Chemical reactions may be single phase, liquid-liquid or gas-gas, or multiple phases. Stirred Tank Reactors For single-phase or homogeneous reactions, an empty tube or a stirred tank is often used. The stirred tank reactor is the workhorse of the reactor vessels. The components of a stirred tank reactor include feed and discharge ports, a rotating shaft, one or more impellers, and a motor and drive system. Baffles along the walls of the vessel prevent the mixture from vortexing at the impellers. Stirred tank reactors may be operated in batch, semi-batch or continuous modes. In a batch operation, the reactants are added or charged in a specific order and concentration, stirred, and allowed to interact for a specified period of time. Then the product and unreacted feed are removed. In a semi-batch operation, continuously adding a reactant or adding it at different times during the reaction process helps control the concentration and temperature in the reactor. In continuous operation, the feed materials are added at the same rate the outlet materials are removed. Most large scale chemical production is done in a continuous operation. Gas-Liquid Reactors Multi-phase or heterogeneous reactions are more common in chemical processing than single-phase reactions and often require more specialized reactor designs. Gas-liquid reactions require contact between a gas reactant and a liquid reactant. The product of the reaction may be a gas, a liquid, or a solid. One step in the production of sulfuric acid for example requires the reaction of sulfur trioxide gas with a liquid sulfuric acid concentration to produce liquid oleum. A bubble column is the simplest example of a gas-liquid reactor design. The gas phase reactant enters the tube at the bottom and bubbles up through the liquid reactant. Reactions take place where the surface of the gas bubble makes contact with the liquid. A fixed or packed bed column contains a layer of packing material over which the reactants flow in a winding path that allows more contact time and surface area for reactions to occur. Column packing comes in many sizes, shapes, and materials. Packing can be evenly or randomly distributed onto one or more support structures in the column. Column packing is designed to give the right amount of phase contact for the given application. Plate and tray columns control the flow of the gas and liquid reactants while creating more areas of contact. The gas passes through perforations in the plate while the liquid is directed over the bubbles of gas. In this venturi reactor, the gas reactant is sprayed by a venturi-type ejector into the liquid reactant, which may or may not contain free-floating catalyst particles. This loop configuration recycles the unreacted mixture back into the ejector where it is mixed with the gas and sprayed back into the vessel. A spray column uses a liquid distribution system to spray the liquid reactant counter-current to the flow of the gas reactant. Spray columns are widely used to remove sulfur dioxide from furnace exhaust by spraying the gas with crushed lime and water. In a trickle bed reactor, the liquid and gas flow counter current. The liquid is sprayed onto a media and flows downward while the gas flows up through the media. In a wetted wall or falling film reactor, gas flows through a tube while a thin film of the liquid flows down the tube walls. The chemical reaction occurs on the surface of the liquid film where the gas and liquid contact. Stirred tanks are also used for gas-liquid phase reactions. Gas-Solid Reactors An example of a gas-solid reaction is the production of gas from coal by combusting the crushed coal with air and steam. This process may be done in a moving bed reactor. In a moving bed reactor, the solid flows downward by gravity counter to the gas in a continuous flow. Another common gas-solid example would be the reaction of gases in a stream with the presence of a solid catalyst. Solid metallic catalytic materials such as platinum, ruthenium, or nickel are mounted on ceramic packing of various shapes and sizes. The reaction occurs as the reactant gases flow over the packing. The shell and tube reactor is a type of fixed bed reactor in which a solid, typically a catalyst, is packed inside a number of tubes. This design maximizes heat transfer surface area for temperature control. This type of reactor can be used to react carbon monoxide with hydrogen gas to produce methanol. In a fluidized bed, solid catalyst material is pushed up by the flow of gas and is constantly moving. In this example the solid and gas flow into a cyclone where the gas is separated from the solids and the solids recycle back to the reactor. In this design large volumes of gas can be processed with a lower pressure drop than a fixed bed design. Liquid-Solid, Liquid-Liquid Reactions A liquid-solid reaction may be a liquid solution reacting in the presence of a solid catalyst, or a liquid reacting with a non-soluble solid. Stirred tanks and fixed bed reactors are commonly used for this type of two-phase reaction. Some liquid-liquid systems are considered multiphase when the two liquids do not mix or mix well. One example would be reacting an ionic solution, such as water, with an organic compound. Continuous stirred tanks and plate columns may be used to increase contact between the liquids. A phase transfer catalyst may also be introduced to increase the reaction rate. Gas-Liquid-Solid Reactions Of the types of three-phase reactions, gas-liquid-solid reactions are the most common. These often exist as a liquid-solid slurry reacted with a gas or a liquid-gas reaction in the presence of a solid catalyst. Trickle beds are widely used for gas-liquid-solid reactions with a gas reactant and liquid reactant flowing countercurrent through a solid catalyst bed. Other Design Considerations As you’ve seen, the type and design of the vessel selected for a reaction varies depending on the phase of the reactants. Other considerations include cost, pressure, temperature, and the corrosiveness or volatility of the materials used. For reactions requiring high pressure or vacuum, the vessel must have a thick reinforced shell and fittings. Applications that require high temperatures to promote a reaction are processed through a furnace or boiler. Complex chemical reactions usually require multiple steps and sometimes multiple reactions. Multiple reactions often occur inside a single reactor as in this gasification reaction. Other reactions require multiple reactors operating either in parallel or in series, like this system where the product of one reactor is a feed stream to the next reactor. Understanding the reaction requirements and the system construction and design is important for controlling and operating a reaction system. A reactor system is designed to optimize a chemical reaction to produce the desired products in the most efficient manner. It is also designed to contain the reactants and products and manage the energy required to control the reaction. Operating parameters for monitoring and controlling a reactor include feed, temperature, pressure, level, and chemical concentration. Material Balance Feed rate or amount is determined by the reaction equation. A balanced equation describes the ratio of reactants required in a reaction. For example, in this balanced equation, two molecules of sulfur dioxide gas are required for every one molecule of oxygen to form two sulfur trioxide molecules. If we look at the weight of each reactant, we see that two sulfur dioxide molecules is 128 grams/mole while one oxygen molecule is 32 grams/mole to create 2 moles of sodium trioxide. However, to help promote the efficiency of a reaction, one or more reactants are fed in excess. For example, in the case of combusting the sulfur dioxide gas, more oxygen is provided to the reactor than is required. The excess oxygen is then removed with the sulfur trioxide gas. Keep in mind that all materials entering and exiting the reactor are conserved. In other words no material is created or destroyed in the reaction. So the mass of the input materials equals the mass of the materials exiting the reactor either in the product stream or the byproduct stream. Energy Balance Temperature sensors monitor the temperature change inside the reactor and may be located at inlets, outlets and/or multiple points inside the reactor. In every chemical reaction, energy in the form of heat is either given off or consumed. As with mass, energy is conserved. The heat energy going into a system and the heat energy generated by the system should equal the energy out. Operators monitor heat input and temperature changes to ensure that heat is not lost due to leaks or equipment failure. Heat from a reactor must be controlled to provide uniformity. Jacketing on a stirred tank reactor is one method of regulating the heat in the reactor. The jacket circulates either warm or cool liquid which transfers heat to or from the vessel. An internal coil serves the same purpose. Internal heat exchangers are used in both stirred tank and tubular reactors. Many systems use external heating or cooling equipment such as heat exchangers, condensers, or fired heaters. Operating Parameters Positive or negative pressure will increase reaction rates especially if one or more of the reactants is a gas. Processes are often reacted in a vacuum to reduce the formation of unwanted by-products. Pressure in a reactor may be monitored remotely and locally. Sensors monitor the concentration of the reactants and product/s. These results are displayed as a means of monitoring the rate and efficiency of the reaction inside the vessel. Remember that the concentration of reactants or product drives a reversible reaction to produce the desired reacted compound. For batch and semi-batch operation, the concentration of product is a function of time. Typically, automatic controls will signal the end of the batch cycle. In some units, operators also monitor catalyst feed as well as catalyst regeneration or recovery operations. Regular sampling of product is done to ensure quality. Sampling data is used to make fine adjustments to operating conditions and to verify that sensors and control equipment are operating correctly. Run-away Reactions Performance issues related to reactor operation include run-away reactions, over pressurization, unwanted side reactions, deactivation of catalysts, leaks and contamination, vessel fouling and corrosion. Exothermic reactions create heat when the reactants bond. If the heat in a reactor is not controlled, the reaction can become uncontrolled. A runaway reaction occurs when the heat generated by the reaction or reactions exceeds the heat that can be removed. This can cause wasted product, release of chemicals, or even serious damage to personnel or equipment. Conditions that can lead to a runaway reaction include inadequate heat control, loss of vacuum, inadequate mixing of reactants, raw material quality or quantity, and human errors. An understanding of the reactor system and the chemistry involved is required to recognize and respond to these conditions. The primary hazard resulting from a runaway reaction is over pressurization. This situation can occur when a gas or vapor is produced as a by-product of a reaction, increasing pressure inside the vessel and potentially causing it to rupture. Relief equipment provides a controlled relief of pressure and released chemicals in cases of a runaway reaction or other hazardous condition. Relief in a reactor system often includes a relief device such as a relief valve and equipment to route and dispose of the relieved material. Emergency shutdown systems and interlocks are hardwired or software safety systems used to prevent or respond to extremely abnormal conditions. These systems are designed specifically to respond to potential hazards associated with a particular reactor system. However, prompt and correct action by the control operator is often critical. Emergency procedures may include adding an inhibitor to stop the reaction, stopping the feed or increasing the take out, or increasing agitation. Both inside and field operators are trained on emergency actions specific to their process. Other Performance Issues A side reaction is a secondary chemical reaction which can occur during a reaction process. These reactions are often unwanted. Side reactions can reduce or contaminate the desired product. In many applications, side reactions are expected, but are kept to a minimum under normal operating conditions. Unexpected side reactions may be caused by the product or reactants reacting with vessel material, contaminates in the stream or by deviations in operating conditions including changes in temperature, pressure, or concentration. Contaminates in the feed stream or vessel can act as catalyst to increase a reaction rate or can be inhibitors, causing the reaction to slow or stop. Equipment and process conditions, especially in batch reactions, should be checked at startup to ensure that water, residual chemicals, or other impurities are purged from the system. Catalyst poisoning occurs when contaminate chemicals bond at the surface of the catalyst but do not release. Catalyst fouling is the accumulation of carbon or impurities on the catalyst or packing. Solid catalyst surface area also degrades in high temperatures. Recovery and replacement of solid catalysts is incorporated in reactor operating procedures. Depending on the process, this may be done continuously or on a scheduled basis. An operator should be aware that condition upsets can cause catalytic metals to deactivate sooner than expected resulting in poor reactor performance. Indications of poisoned or fouled catalysts include reduced product production and change in temperature and pressure due to reduced reaction efficiency. Particularly in packed bed, exothermic systems, hot spots occur where the concentration of reactants is higher. Hot spots can change the conditions in certain locations in a reaction but go undetected and uncontrolled because the temperature sensors are not nearby. As a hot spot temperature increases, the reactions in the area increase, causing the hot spot to grow. In a worst case scenario, the reaction could run away. Good fluid distribution and reactor design reduces the effect of hot spots. A chemical reactor system is often a harsh environment. Solid buildup in pipes and tubes will impact fluid flow and reaction rates. Strong chemicals can cause instruments and automatic controls to fail or produce incorrect readings. Periodic inspection and maintenance help identify or prevent problems due to fouling or corrosion. Inspection and Maintenance To prevent contamination, reactor vessels are cleaned before startup or on scheduled shutdowns. Other maintenance and inspections are performed daily according to specified procedures. Depending on the construction material of the vessel, operators should look for corrosion, cracks, or other signs of wear inside the vessel. Reactor systems are regulated by state and federal safety and environmental standards. These regulations often require certain inspections and documentation to ensure that a process meets these standards. Remember that a reactor vessel may be under pressure and that the vessel and surrounding piping may be extremely hot. Always follow safety procedures for shutting off steam or electrical energy when performing maintenance on the system. Relief and inerting systems are critical safety components and should be in working order at all times. Be aware that exposure to an oxygen-depleted environment is extremely hazardous and can cause asphyxiation. The production of a chemical product such as sulfuric acid often involves several reactions as well as other processes to refine the product