Oh, I, I got work to do
I got work baby
I got a job baby
I got work to do
Said I got work to do

("Work To Do," by the Isley Brothers,1972)

COMPRESSOR JOBS FOR A GAS
ARE MOSTLY THE SAME JOBS PUMPS PERFORM FOR A LIQUID

Compressors move gases like Pumps move liquids. The Process Industry Plant Owner invests in Pumps to service liquids and Compressors to service gases and vapors.

Most industrial gases are colorless. Compressed gases that blanket a tank or flow to where they are needed more than likely cannot be visually observed like a container full of liquid or a liquid flowing into that container could be observed.

Because gases in the action of working cannot be easily observed, learning about the jobs Compressors and compressed gases perform is best understood by studying simple flow diagrams.

Compressor jobs include:

"Boosting" the PV Pressure of a Gas Process Stream

No gas will ever flow into an area of higher PV Pressure because … as all PTOA Readers and Students know … fluids will only flow from an area of high PV Pressure to an area of lower PV Pressure.

"Boosting" the PV Pressure of a gas or vapor enables that gas or vapor to flow into a process line or vessel that has somewhat lower PV Pressure.

Notice the arrowheads on the process lines in the nearby schematic. The arrowheads that point OUT of the process reveal the products that the process makes.

The product line labelled  "to hydrogen-consuming unit" reveals that this industrial process makes more hydrogen than it needs. Ergo, hydrogen product is exported as a feedstock to another process in the integrated industrial complex

The "low PV Pressure" side of this process begins at the vessel labelled "Product Separator." The hydrogen product leaves the top of the separator. More than likely the PV Pressure of the hydrogen product will need to be "boosted" so that it can flow to the place where it is needed.  Although it is not shown, a Hydrogen Booster Compressor will be needed to transport the hydrogen product to users.

The above paragraph segues nicely with the next Compressor job:

 

Compressors Infuse Gas Process Streams With the Energy Needed to Move from Point A  to Point B

Not all gases need to have their PV Pressure "boosted."  Some gases just need to flow from the Compressor Discharge to where the gas is needed. These gases flow out of the Compressor's Discharge Valve and into a Gas Header so that the gas can be distributed where it is needed in the industrial complex. Otherwise stated, some gases just need to be transported from the Compressor to the industrial users that need the gas.

An example of distributing a gas where it is needed would be the gas that flows through a Gas Header and into a tank to supply the "Gas Blanket" which fills the vapor space above a volatile liquid.  PTOA Segment #162 described how a "Gas Blanket" of natural gas or much more expensive inert nitrogen keeps air from contaminating the liquid contents of the tank.

As the liquid contents of the tank are drawn off, more blanket gas will enter the tank to fill up the vapor space. When the tank is filled with more liquid, some of the "Gas Blanket" is displaced by the liquid and vented to atmosphere.

To recap, the gas that is supplied as a "Gas Blanket" via the Gas Blanket Header originates from the discharge of whichever Compressor is supplying "Once-Through Gas" to the Gas Blanket Header.

"Once-Through Gas" means that the gas which flows through the Header is not  recycled but rather used once and then vented to the atmosphere.

The trusty Instrument Air Compressor (which might well be a Liquid Ring Pump/Compressor as described in PTOA Segment #215) will be situated in the Utility Area of a Process Plant.

The Instrument Air Compressor provides "once-through Instrument Air" to the Instrument Air Header. The Instrument Air Header distributes dry, filtered air to the pneumatic instruments used in automatic control loops …  specifically, the Final Control Valve.

PTOA Readers and Students who do not remember how an instrument control loop works can review PTOA Segment #16 which described how a PV Temperature Control Loop works. Instrument Air supplied by the Instrument Air Header makes it possible for the Final Control Valve to work.

The Final Control Valve in the nearby schematic is labelled TV10045, which is a Temperature Control Valve on control loop #10045.

 

Recycling a Gas with a "Recycle Compressor"

Any gas that is not air or natural gas is typically too expensive to be "Once-Through Gas."  For example, "once-through nitrogen" is extremely expensive but sometimes required for safety.

Wherever possible, a Recycle Gas Compressor will be included in an industrial process which makes it possible to use a circulating gas over and over again.

Most of the gas supplied to the Recycle Gas Compressor's Suction Line and Suction Valve has been through the process multiple times.

While the gas flows through the Recycle Gas Loop:

• Some of the gas will be consumed in the process.
• A small stream of gas will be intentionally vented, possibly referred to as "Purge Gas."
• Some of the gas will leak through fittings that should be tightened or gaskets that need to be replaced.

 

Because of these gas losses, fresh Make Up Gas will be added to the Recycle Gas Loop.

The job of a a Recycle Gas Compressor (RGC) is better understood by studying the nearby Simple Reactor Process Flow Schematic.

Brilliant PTOA and Readers …meaning those who have been reading the PTOA Segments in the intended, sequential order … finished the PTOA PV Temperature Focus Study Area a while back and thus will have no problem decoding the ISA symbols that represent the varied process operations shown in the nearby Simple Reactor Process Flow Schematic,

The nearby Simple Reactor Process Flow Schematic includes a Recycle Gas Compressor (labelled RGC). Find the RGC in the bottom right quarter of the schematic.

PTOA Readers and Students can use what they just learned in PTOA Segment #216 to conclude that the Recycle Gas Compressor is a Dynamic Compressor:

The big hint is the RGC's ISA symbol which is cone-shaped with a decreasing radius from the taller vertical line at Compressor Suction to the shorter vertical line a the Compressor Discharge. Remember! The vertical line represents the volume of the gas. The volume of the gas becomes increasingly smaller as the PV Pressure of the gas becomes greater while the gas flows from Compressor Suction to Compressor Discharge.

The discharged gas (Hydrogen) flows into Line 1 and eventually is mixed with a (liquid) Feed stream (Hydrocarbon). The combined liquid and gas stream becomes Line 2.

The combined stream of liquid and gas is pre-heated in a Reactor Feed/Reactor Effluent Shell and Tube Heat Exchanger (labelled F/E exchanger). The pre-heated mixture is labelled Line 3. PTOA Readers and Students were introduced to the form and function of Shell and Tube Heat Exchangers in PTOA Segments #30 through PTOA Segment #36.

The pre-heated mixed liquid and gas Line 3 is heated to at target PV Temperature in a Fired Heater. PTOA Readers and Students were introduced to the form and Function of Fired Heaters in PTOA Segment #22 and learned how to operate Fired Heaters in PTOA Segments #68 through PTOA Segment #73.

The significantly hotter effluent from the Fired Heater becomes Line 4, the Reactor Feed Stream. A more detailed Process Flow Diagram would be needed to ascertain if the contents of Line 4 was totally in the  gas phase or if the composition of this process line is still part liquid and part gas.

PTOA Readers and Students were introduced to Reactors in  PTOA Segments #27 and #28. Chemical reactions... like the "Endothermic and "Exothermic" reactions featured in PTOA Segment #37 … take place in the Reactor so the effluent that flows out of the bottom of the Reactor as Line 5 is chemically different than the components of the Reactor Feed.

Up through entering the Reactor, the PV Temperature and the PV Pressure of the hydrogen and hydrocarbon stream have both been increased.

As the Reactor Effluent flows into the tube side of the F/E Exchanger the process stream has entered "the cool-down, de-pressuring side" of the process.

The Reactor Effluent in Line 5 flows into the tube side of the F/E Exchanger and flows out of the exchanger as much cooler Line 6. The mixed gas and liquid in Line 6 flows into the Hot Separator.

The job of the Hot Separator is to separate the desired liquid product (drawn off as "Product to Extraction") from the hydrogen gas that will be recycled.

The desired PV Pressure of the Hot Separator will be controlled by automatic instrumentation which is not shown in the simple schematic.  The PV Pressure of the Hot Separator is considerably less than PV Pressure of the Reactor and the mixed stream labelled Line 6.

The abrupt decrease in the target PV Pressure of the Hot Separator causes the hydrogen and lightest hydrocarbons to "flash off". That means the hydrogen and lightest hydrocarbons suddenly depart from the liquid components of the process stream that entered the Hot Separator as labelled Line 6.

Why do the hydrogen and light hydrocarbons "flash off?"  Hydrogen and the hydrocarbons with the lowest individual vapor pressures will "flash off" from the heavier hydrocarbons as was explained in PTOA Segment #162. The hydrogen and light hydrocarbons are just so happy to suddenly be under much less PV Pressure that they cannot help but disengage from the heavier hydrocarbons  and enter the vapor phase at the PV Temperature of the Hot Separator.

The "flashed off gas" exits the top of the Hot Separator and is labelled Line 7. The desired product … "Product to Fractionator"... flows from the bottom of the Hot Separator to an unseen Fractionator for polishing.

The "flashed off" hydrogen gas and vaporized light ends must be cooled so that any water and entrained hydrocarbon light ends can be condensed and removed before the recycled hydrogen flows into the Recycle Gas Compressor's Suction Line.(Line 10).

Any gas that is going to be compressed by any type of Compressor must be dry  (no water) and liquid free (no other kinds of liquid). Compressors cannot compress liquids, only gases.

For this reason, the "flashed off" Hot Separator Gas labelled Line 7 is cooled via heat exchange in Cooler 1. The effluent from Cooler 1 (labelled Line 8) is further cooled to a target PV Temperature in an Air-Cooled Fin Fan Heat Exchanger (labelled Cooler 2). PTOA Readers and Students already learned in PTOA Segment #38 how Fin Fan Heat Exchangers condense liquids like entrained hydrocarbons and water.

The much cooler effluent that exits Cooler 2 is a mixture of hydrogen gas, water in the liquid phase, and light hydrocarbons in the liquid phase. This much cooler effluent that exits is labelled Line 9 and flows into the the Cold Separator.

The job of the Cold Separator is to separate liquid water and liquid light hydrocarbons from the hydrogen gas which will be recycled.

The PV Pressure of the Cold Separator will be controlled by instrumentation that is not shown. The target PV Pressure of the Cold Separator is the target Suction PV Pressure of the Recycle Gas Compressor.

The condensed light hydrocarbons are drawn off the bottom of the Cold Separator and added to the desired product that is flowing to a fractionator. The water that was entrained in the gas stream is drawn off the bottom of the Cold Separator and sent to oily sewer.

The hydrogen gas that flows out of the top of the Cold Separator as Line 10 thence flows into the Compressor Suction.

And Ta-dah! … and this is why you studied the schematic in the first place …

The hydrogen gas product that exits the top of the Cold Separator as Line 10 is dry, hydrocarbon-free, and at the desired  RGC Suction PV Pressure and PV Temperature.

Thus the vast majority of the hydrogen gas is not wasted and exhausted to the atmosphere.

 

ALERT! Did I hear the phrase "Recycle Loop?"

The most brilliant PTOA Readers and Students are alerted by the phrase "Recycle Loop" because they already learned in PTOA Segment #40 the tips and trip-ups about every Recycle Loop, including recycled cooling water and recycled lubrication oil:

• The PV Pressure at Compressor Suction is the lowest PV Pressure in the recycle loop and the highest PV Pressure is at the Compressor Discharge.

• Because the gas is being recycled, "heebie-jeebies" will build up over time.  Ergo, some means of venting or purging a small stream of recycled gas must be provided. A "purge gas line" is shown in the schematic.
• As was stated above, fresh Make Up Gas must be added to any Recycle Gas Compressor Loop.
• PTOA Readers and Students also know that recycled fluids remove heat from a process. This heat will be exchanged with another process stream that needs to be heated up. Otherwise stated, the Recycle Gas Compressor helps to keep the target PV Temperature of the process within desired specifications.

 

Compressed Gas is needed for "Pressuring-Up" an Industrial Process Prior to Start-Up

A source of flowing, compressed gas is used for the Purging Step and  "Pressure-Up" Step of a Process Unit Start-Up.

Although a liquid level will be established in Separators and Fractionators, the main job of "Pressuring-Up" a process … for example the Simple Reactor Process … is achieved with compressed gas that is flowing from either a Compressor or very expensive cylinders of compressed gas.

Think about it!  Imagine all of the components and piping of the Simple Reactor Process  are air-filled after a maintenance interval  (aka "Turn-Around").

Hydrogen gas can be used to first purge out the air and then "Pressure Up" the lines and components to the target Start-Up PV Pressure. Eventually the Fired Heater can be ignited to bring the Reactor up to the PV Temperatures needed to introduce Reactor Feed and commence chemical reactions.

The very first task to perform prior to Pressure-Up is to close all the valves on the Product Lines pointing out of the Simple Reactor Process Diagram. Otherwise it will be impossible to achieve a target PV Pressure!

Start-Up Hydrogen will flow into the Simple Reactor Process via the Make Up Gas line. As stated above, the Start-Up Hydrogen will be provided by an unseen Compressor or by compressed gas cylinders.  .

The Start-Up Hydrogen will flow from the Compressor Discharge through the piping that links the various components shown in the diagram.

"Pressuring-Up" and displacing air takes hours!  In not so rapid succession the Start-Up Hydrogen will flow to the shell side of the F/E Exchanger → the tubes of the Fired Heater → the Reactor → the tube side of the F/E Exchanger→ the Hot Separator → the shell side of the Cooler → the tube side of the Fin Fan Exchanger→ the Cold Separator … which is the Suction PV Pressure for the Recycle Gas Compressor!

 

Compressed Gas Creates the PV Pressures Required For Chemical Reaction Processes to Work 

The target PV Pressure of an industrial process is chosen to preferentially produce the desired product.

The design PV Pressure for Reactors (and Separator Vessels and Fractionating Towers) will be determined during the design phase of the process. The PV Pressure that yields the most of the desired product will be the PV Pressure of the process.

In the Simple Reactor Process Schematic, the significantly high PV Pressure in the Reactor will insure that the chemical reactants are pressed into the catalyst to promote a high yield of desired product.

The desired Start-Up PV Pressure is first established with compressed gas generated by a Compressor or compressed gas cylinders which are located external to the process. After successful Start-Up, the PV Pressure of the process is mostly maintained by the recycled, compressed gas.

 

TAKE HOME MESSAGES: Compressors add PV Pressure to gases like Pumps add the PV Pressure to liquids. PV Pressure is needed so that the gas can be move through pipes or distributed via utility Headers. 

Booster Compressors are used to boost the PV Pressure of a gas so that it can flow into an area of higher PV Pressure.

Utility Compressors are used to add PV Pressure energy into gases that are thence distributed by Headers to where the gas is needed to perform a job like Gas Blanketing or Pneumatic control.

"Gas Blanketing" and supplying a gas for pneumatic instrument activation are examples of "Once-Through Gas" use. "Once-Through Gas" originates from a source Compressor and is distributed through a Header system for the purpose of being used once and afterward vented to the atmosphere.. "Once-Through Gas" is typically generated and supplied by Compressors which are providing a utility gas to the industrial process facility.

Recycle Gas Compressors reuse a gas that is needed in a process system over and over again.  Recycle Gas Compressors will be incorporated into industrial processes which require a constant flow of compressed gas to make the desired process product. A Recycle Gas Loop will require additional investment costs of piping, coolers, and separators. The investment in Recycle Gas Loop hardware is quickly paid back compared to the increased operating cost of providing "Once-Through Gas" when there is a continuous high volume demand for the gas.

All Recycle Gas Systems:

• Have the highest PV Pressure at the Compressor Suction and the lowest PV Pressure at the Compressor Discharge.

• Because the gas is being recycled, "heebie-jeebies" will build up over time.  Ergo, some means of venting or purging a small stream of recycled gas must be provided.
• Fresh Make Up Gas must be added to any Recycle Gas Compressor Loop. Make Up Gas replaces gas losses due to consumption, leakage, and intentional venting.
• Recycle Gas removes heat from process. Heat Exchange is used to control the heat generated by compression of gases and other unit operations ... like Reactors.

 

Compressors provide the gas needed for the purging and  "Pressure-Up" step of Start-Up and the target PV Pressure needed for Reactors to make the most of a desired product.

Before a gas enters the Suction Line and Suction Valve of a any type of Compressor, the gas must be dry (water and hydrocarbons removed).  Liquids DO NOT compress! Liquids entrained in gases will ruin an industrial Compressor.

 

©2021 PTOA Segment 0217

PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT COMPRESSORS

 

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