Control Devices

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Control Devices Database

 

               A control device is an equipment item that, when installed in a vent line, has the effect of reducing the compound emission levels of the gas stream.  Example control devices include condensers, scrubbers, and activated carbon units.  The three types of control devices which may be defined and stored in the Control Devices Database include condensers, scrubbers, or general control devices. 

 

Condensers

A condenser is a device which limits the emission level of a process gas stream through a reduction in temperature and reduction in corresponding saturation pressure.

 

· Creating a Condenser in the Controls Database

When creating a condenser for the Control Devices Database it may be initially configured as a process condenser or a control condenser.  While both condensers perform the same vapor calculations, the process condenser is considered to be an integral part of the process and the control condenser is considered to exist primarily for the purpose of controlling emission levels.  Once a condenser has been selected for a specific process model it may be designated as either a process device or a control device depending upon the specific process application at the time. 

 

A zero stage condenser is one in which the exit gas vapor is set at equilibrium with the liquid batch contents composition but using the condenser temperature instead of the batch temperature.  On the other hand, a single stage condenser is one in which the exiting gas stream is calculated to be in equilibrium with the exiting condensate stream at the configured condenser temperature.  The single stage condenser calculate will tend to result in more conservative and accurate results for most operations. 

 

Scrubbers

A scrubber is a control device that reduces the emission level of a gas stream by absorbing individual compounds into a scrubbing liquid.  In many cases, the scrubbing solution contains specific chemicals which can be used to enhance the scrubbers performance by chemically reacting with or altering the volatile nature of the pollutant compound.  A liquid normally used in many scrubbers is a dilute aqueous solution of sodium hydroxide.  For a caustic based scrubber, the program performs the emission calculations on the incoming gas stream based on its list of chemical removal efficiencies and before it corrects for the saturation vapor pressure of water. 

 

· Creating a Scrubber in the Controls Database

1.        Enter the scrubber's equipment tag name.
2.        Enter an equipment identification code.
3.        Enter the scrubber's location.
4.        Define the scrubbing solution to be used in the scrubber.
5.        Specify individual compound removal efficiencies in the Efficiencies Box from the Chemicals Database.

       

General Control Devices

This control device is similar to the scrubber device except that a scrubbing solution is not required.  Emission calculations are performed solely based on the unit's predefined compound removal efficiency list. 

 

· Creating a General Control Device record in the Controls Database

1.Enter an equipment tag name.
2.Enter an identification code.
3.Enter an equipment location.
4.Specify individual compound removal efficiencies in the Efficiencies Box from the Chemicals Database.

 

Vacuum Pump

A vacuum pump can be a liquid ring or a dry vacuum pump.  Emission calculations are performed based on the inlet vent stream from the process operation, specified discharge temperature, discharge pressure, and any working liquid fluid used .  If the vacuum operation

 

· Creating a Vacuum Pump record in the Controls Database

1.Enter an equipment tag name.
2.Enter an identification code if known.
3.Enter an equipment location if known.
4.If the vacuum pump is a liquid ring type and requires a flow of water or other fluid in order to operate then the composition and flow rate of this stream should be entered.  Enter the net throughput rate of the fluid
5.If air, nitrogen, or a different gas is directed into the vacuum system for vacuum level control then this gas stream should be specified in the 'Gas Sweep' section. 

 

Vacuum Steam Jet

Emission calculations for a vacuum steam jet are performed based assumed equilibrium conditions existing between the exit vapor and liquid condensate streams and at the discharge pressure and temperatures.  

 

· Creating a Vacuum Steam Jet record in the Controls Database

1.Enter an equipment tag name.
2.Enter an identification code if known.
3.Enter an equipment location if known.
4.A vacuum steam jet operates with a flow of steam that normally enters a discharge condenser.  In this way, the steam exits the vacuum jet unit in the form of a condensed liquid stream.  Enter the inlet steam or exit condensate flow rate for the vacuum steam jet. 
5.If air, nitrogen, or a different gas is allowed into the vacuum system for vacuum level control then this gas stream should be specified in the 'Gas Sweep' section. 

 

 

Technical Considerations

Some control devices and process equipment operate in such a way that contaminants are removed from the exit vent stream by way of a phase change from the vapor to the liquid.  In most cases the liquid consists of only a single phase, although two separate liquid phases can be encountered. 

 

When a control device (such as a condenser, direct contact condenser, liquid ring vacuum pump) operates from to a phase change then Emission Master applies the following criteria and equilibrium relationships to calculate a solution. 

 

1.The material balance around the equipment item is assumed to remain constant during the operation.  The process inlet streams (liquid and vapor) are equal to the exit streams (liquid and vapor). 

 

2.Single stage equilibrium concentration between the exit liquid and vapor streams are maintained for all condensable components.  For example, if the process stream contains methanol, water, and ethanol then the partial pressures in the exit vapor stream are calculated to be equal to the vapor pressures in the exit liquid stream for each component.  If the exit liquid stream contains two liquid phases then these two liquid streams are calculated to be in equilibrium as well. 

 

3.Noncondensable components such as air, nitrogen, or other gases in the system are considered not to be soluble in the exit liquid stream and are calculated to exit the equipment through the exit vapor stream. 

 

4.Emission Master enables the user to specify activity coefficients for each compound in the system for each liquid phase.  This flexibility enables the program to determine presence of two phases in the exit liquid.