User:SecurityFanBoy/Batch reactor

A batch reactor is a vessel that is used in the process industry for batch operations of chemical reactions. Batch reactors may take many different forms with some having different components, but with all having a vessel to contain the reactants until the product is ready to be retrieved.

Comparison of batch and continuous reactors
Reactors can be divided into two broad categories, batch reactors and continuous reactors.

Batch reactors are tanks sufficiently large to handle the full inventory of ingredients to complete a batch cycle. All of the reactants are charged into the batch reactor at the beginning of the process. At the end of the batch process, the product is removed from the batch reactor.

Continuous reactors react the reactants in a flowing stream through the continuous reactor where the product is created and discharged at the output of the reactor.

Overview
Raw materials, such as reactants are added to a batch reactor at the start of a reaction. The reactants are allowed to react in the reactor usually for a fixed time. In many cases, no additional materials are added or any product withdrawn during the reaction time. The reaction products, along with non-reactants, catalysts, and unreacted materials, are removed from the batch reactor at the end of the time or when the reaction is complete.

In addition to the containment vessel, other components may be used in a batch reactor. These components, such as thermal control, pressure control, and stirring and/or agitation, are designed into a batch reactor by a process or chemical engineer using the principles of chemical kinetics. The engineer will design the process to achieve a goal. In most cases, the goal is to produce as much product as needed with the greatest efficiency. In some cases, the goal may be to optimize costs, such as to deliver specific amounts of product at the least cost.

Batch reactors may be called by a name that reflects the role they perform, such as a crystallizer, a bioreactor, and a fermentor.

Primary Considerations
Many decisions must be made in designing and running a batch reactor. However, the first two considerations are to determine the reaction kinetics and thermodynamics. Once these reaction characteristics are understood, all other design choices may be made.

Reactor Design
Batch reactors may be of any design, but the most common are in the shape of a cylinder or tube with a fixed bottom. Other shapes include spheres and conical sections. Reactors must be configured to withstand the expected pressures, temperatures, and reactants without failing. Guidelines and certifications for the design and operational characteristics of reactors are provided by the American Society of Mechanical Engineers.

In addition to understanding the reaction or process that will occur in a batch reactor, a chemical or process engineer must configure the batch reactor so that raw ingredients and products may be loaded (charged) and unloaded (discharged) from the batch reactor. Solids are usually charged from the top of the reactor using gravity to settle the solids to the bottom of the reactor. Liquids may also be charged from the top of the reactor unless contact with air and/or foaming is to be avoided. In those cases, liquids may be pumped into the reactor from the bottom. If one of the reactants is a gas, it may be charged from the bottom through spargers so that it bubbles through the other fluids in the reactor.

Once the reaction has completed, the product and non-reacted ingredients must be removed from the reactor. Gases may be discharged from the top, while liquids may be drained from the bottom. Solids are usually retrieved through a hatch in a side of the reactor. The hatch may also be used by technicians to clean the reactor after the discharge.

Components
The primary component of a batch reactor is a containment vessel. The most basic batch reactor has been equated to a kitchen cooking pot, where food, spices, and water (the raw ingredients) are put into the cooking pot (the containment vessel) and the contents are heated to make some savory dish (the final product).

The batch reactor may be considered to be a closed system in that no material is added or withdrawn until the reaction is complete.

A batch reactor may vary in size from less than 1 liter to more than 15,000 liter. Batch reactors are usually fabricated from steel, stainless steel, glass-lined steel, glass or alloys. The material of the inner surface of the reactor, along with any components exposed to the reactants, are chosen to ensure that their surfaces are nonreactive to the reactants and the final product.

The advantages of the batch reactor lie with its versatility. A single vessel can carry out a sequence of different operations without the need to break containment. This is particularly useful when processing toxic or highly potent compounds.

Agitation
In many cases, the reactants may be dissolved in a solvent such as water. In other cases, solids may be blended with fluids or with other solids. Agitation, or stirring, of the the ingredients within the vessel ensures that the reactants are thoroughly mixed so that the reaction will proceed as expected.

In smaller cylindrical reactors, where the volume is less than about 4m3 (about 1000 gallons), a motor-driven impeller mounted off-center within the fluid in the reactor may be used. In larger cylindrical reactors, from about 4m3 to about 200m3 (about 50,000 gallons), a coaxially mounted impeller may be used with baffles may be used. Baffles are stationary blades which break up flow caused by the rotating agitator. They may be fixed to the vessel cover or mounted on the interior of the side walls.

In non-cylindrical reactors, side-entering impellers or jet mixers may be used.

Heating and cooling systems
Products within batch reactors usually liberate or absorb heat during processing. In most cases, if the chemical reaction is exothermic (heat is liberated) or endothermic (heat is absorbed), heat will need to be added or removed by a cooling jacket or cooling pipe. If the temperature of the reactants in the vessel is not controlled, the reaction may not proceed in a desired manner. In some cases, having the reactants too hot or too cold may produce unwanted products, or may not produce desired products as quickly as desired. In other cases, the reactants may not react at all. Adiabatic processes may not require any heating or cooling. To maintain a desired temperature for an optimum result, a heat transfer fluid may be passed through heating/cooling coils or external jackets to heat or cool batch reactors.

Within the chemical, pharmaceutical, and food preparation industries, external cooling jackets are generally preferred as they make the vessels easier to clean. The performance of these jackets can be defined by 3 parameters :
 * response time to modify the jacket temperature
 * uniformity of jacket temperature
 * stability of jacket temperature.

Several types of batch reactor cooling systems are described in the table below.

Pressure Regulation
Some reactions require that the reactants be held under pressure. In some cases, pressure greater than atmospheric pressure may be required to keep a reactant that would otherwise be a gas in the solution with other reactants. In that case, the reactants will need to be kept within a pressure vessel until the reaction completes.

In other cases, an undesired gaseous by-product may be removed from the solution by lowering the pressure within the batch reactor. In those cases, a vacuum pump or vacuum ejector may reduce the pressure within a pressure vessel to remove the by-product.

Materials
Batch reactors and their fittings must not react with or corrode from any of the materials that contact it. Many batch reactors are made from glass or stainless steel since both are inert to a wide range of chemical reactants and to corrosion. In larger industrial settings, metal reactors are preferred for their durability.

In some cases, batch reactors may need to be passivated before use.

Monitoring
Sensors may be used to monitor the batch reactor and its contents. Thermometers, pressure sensors, and other devices may be attached to the inside or the outside of the batch reactor, or they may be immersed within the reactants inside of the batch reactor. The sensors may be used to monitor the progress of the reaction.