Control Valve Sourcebook — Chemical Process Overview
ReactorTopic Page
I How it Operates 2
II Where Reactors Are Used 3
III Reactor Application Review 4
Reactor
2
ReactorI. How It OperatesA chemical reactor is a vessel designed to contain reactions in order to produce a desired product. Listed below are the most common types of reactors and methods of operation.
Types of reactors: � Continuous stirred tank � Plug flow reactor � Packaged bed reactor (fixed or fluidized)
Methods of operations: � Batch � Continuous � Semi-continuous
Types of Reactors
Continuous Stirred Tank (CSTR)
The CSTR reactor is primarily used for liquid-phase reactions. As the name suggests, it is stirring to attempt perfect mixing. The conversion of the chemical is dependent on the volume of the reactor. Figure 1 shows a representation of a CSTR.
Each reactor will have a fixed volume which also means it will have a fixed residence time. Knowing these two values feed rate can be controlled accurately.
REACTANT 1
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REACTANT 2
CONTROL VALVES
FLOOR LEVEL
COOLANT
PRODUCT OUTLET
TO VENTSYSTEM
STIRRER
Figure 1. Representation of a CSTR
Plug Flow Reactor (PFR)
This reactor is primarily used for gas-phase reactions. It is modeled and designed after a “plug flow profile.” The conversion of chemicals is also reliant of the reactor volume. This reactor has tubes that the feed (gases and/or liquids) travel through at high velocity. This helps with mixing and causing the feed to take a long path through the reactor. Hence, more time for the feed to be converted into desired products. Refer to Figure 2 for a representation of PFR.
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Figure 2. Representation of a PFR
Packed Bed Reactor (PBR)
A PBR can be either fluidized or fixed bed and is primarily used for gas-phase reactions. The conversion in this reactor is dependent on the mass of the catalyst.
The fixed bed reactors are packed with solid catalyst and as reactant passes through the catalyst, reaction takes place.
The fluidized bed reactor consists of plates with catalyst. These catalyst particles behave like fluid as gas flows through it.
Refer to Figure 3 for a representation of fixed bed and a fluidized bed reactor.
Reactor
3
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DIFFUSER
PRODUCTS ANDUNREACTED MATERIALSTO SEPARATE
REACTANTS
CATALYST ON SUPPORT
FLUID BEDWITH GAS FLOW
GAS
DISTRIBUTOR
GAS
SOLID
SOLID
GAS BUBBLE
SOLID PARTICLE
Figure 3. Fixed bed and fluidized reactor
Methods of Operations
Batch
Batch process is a closed system typically carried out in a CSTR. It operates under unsteady state, which means composition changes with time, however, the composition throughout the reactor is uniform. In a batch process, a defined quantity of input is fed to the reactor where reaction takes place while the process is controlled under certain temperature, pressure, and/or pH. Temperature is typically regulated via the heat exchanger. Refer to Figure 6 for a layout of a batch process
This reactor method is usually used in small-scale production, and/or new process testing. This reactor could also be used when it is difficult to achieve continuous operation conversion. The advantages of this method are high conversion and operation flexibility.
Continuous
Continuous process is typically operated under steady state condition. In this process, reactants are fed to the reactor at constant rate and, at the same, rate desired products and byproducts are exiting the reactor.
This reactor method is typically used in large scale continuous production plants most commonly used in petrochemical, and agrichemical industry. In this method feedstock is constantly moving through a reactor yielding constant flow of product. Similar to the batch reactor controlling pressure, temperature and flow rate will help to optimize the process.
Semi-Continuous
The semi-batch reactor is operated with both continuous and batch inputs and outputs. This process is loaded with a batch that produces a product that must be removed continuously. Controlling pressure, temperature, flow rate, and pH will help to optimize the process.
II. Where Reactors Are UsedReactors are considered the heart of almost all industrial chemical plants. Without these reactors, new products will not be produced. Figure 4 and 5 display a few examples of industries utilizing batch method and continuous method.
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BATCH
SPECIALTYCHEMICALPRODUCT
FOOD& BEVERAGE
ETHANOL
WASTE WATER TREATMENT
ADHESIVES
FINE CHEMICALS
ACRYLATEMONOMERS
FERMENTATION
FERMENTATION
Figure 4. Typical chemical industries using the batch method
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CONTINUOUS
AROMATICREFORMING
AMMONIA
NITRIC ACID
CONVERSION OFPARAFFINIC/NAPHTHENICTO AROMATICS
REACTION BETWEENHYDROGEN AND NITROGENTO FORM AMMONIA
OXIDATION OF AMMONIATO NITROGEN
REACTION OF METHANEAND STEAM TO YIELDHYDROGEN
HyCo
Figure 5. Typical chemical industries using the continuous method
Reactor
4
III. Reactor Application ReviewIn this section, a typical setup of a batch process and one example of a continuous process will be covered.
Batch ProcessFigure 6 shows a layout of the reactor set up under a vacuum.
E1710 REACTOR DISCHARGE
REACTOR JACKET
DRAIN
STEAM / WATER
RAW MATERIAL /REACTANT ADDITION
NITROGEN
RECOVER / KNOCKOUT DRUM
THERMAL OXIDIZER
Figure 6. Set up of Batch Reactor
Batch reactors demand several objectives in addition to continuous reactors such as:
� Fast and accurate charging � Fast heat-up or cooldown to a new set point without oscillation and minimal overshoot
� Stable response to load disturbances
Since batch reactors are used to create a large variety of chemical products, control valve selection is dependent upon the fluid being controlled and process control needs.
Batch Charging Valve
Charging a batch may include the use of a single or multiple lines depending upon batch needs. Faster charging can reduce batch cycle time, but must be done accurately to ensure quality targets are met. Use of a charging valve allows for rate control to ensure accurate batch charging.
Typical valve selection: — Fisher Vee-Ball™ valve, Baumann™ 24000 valve, Fisher GX valve
— Materials of construction: CF8M or high nickel alloy with corresponding trim (300-series stainless or high nickel alloy)
— Class V or VI may be required depending on isolation needs
Temperature Control ValvesBatch temperature is a critical control variable that impacts quality, production rates, and operating costs. Control during initial reaction and cooldown typically requires some additional control parameters and can impact valve selection. Response to an exothermic reaction must be stable to maintain setpoint. Split ranging is a common configuration for temperature control utilizing two control valves. Selection of these two valve is important to minimize dead zones in the split range strategy.
Typical valve selection: — Fisher Vee-Ball valve, Baumann 24000 valve, Fisher GX valve
— Class V shutoff may be utilized to minimize leakage — Materials of construction: Steam application materials or materials compatible with the heating/cooling medium
Vacuum Control Valves
Removal of certain components during a reaction may be required. Therefore, batch reactors may be under a vacuum and maintaining a consistent level of vacuum becomes important. A vacuum control valve can assist to provide a consistent level. If an ejector system is utilized, a load valve may also be needed to decrease vacuum levels.
Typical valve selection — Fisher Control-Disk™ valve, Fisher Vee-Ball valve, Baumann 24000 valve, Fisher GX valve
— Materials of construction are dependent on the process design
Continuous Process
Continuous reactors take several forms, but the common objectives include:
— Holding temperature within a certain margin from the desired set point
— Minimize operator intervention — Minimize consumption of utilities
Feed Control Valve
Due to the high flow rates common to continuous processes, feed control valve selection is important. These valves are responsible for maintaining the right ratio of reactants for high yields. Globe valves are commonly utilized due to precise control needs.
Temperature Control Valve
Temperature control valves are another critical control valve application in continuous reactions. They must adjust quickly to process changes with minimal over/under shoot. A more common application is fuel gas control to furnace burners. Valve selection can depend on burner control needs, but globe valves are commonly selected for precise control.
For additional information on continuous reaction control valve needs, Reference:
Pyrolysis process D352317X012
D352317X012 / MDD23 / Sep17
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