Day: August 22, 2023

Written by: Jackson Udy, PE

Process Engineer at Process Engineering Associates

October 24, 2020

Due to the current economic climate, many refineries and petrochemical plants are running at reduced capacities. Operating companies are more closely scrutinizing operating expenses in this low margin environment. As a process engineer or operations professional, we have the opportunity to find ways to reduce operating expenses by reducing energy consumption.

One of the areas inside process plants that takes up significant energy is distillation and fractionation. Distillation consumes over 40% of the total energy in the refining and chemical industry(1). In fact, distillation is 6% of the total energy usage in the United States. This is an incredible amount of energy! Thus process engineers should be consistently asking, am I minimizing energy usage in my distillation columns?

Two ways to reduce distillation energy usage are eliminate excess reflux and reduce tower pressure.

Is Your Distillation Column Over-refluxed?

Is your distillation column over-refluxed? To answer this question, determine the specifications required for each stream exiting a column. For example, a depropanizer column in a refinery may require a 95% propane product purity and at least 95% butane concentration in the bottoms stream. Suppose the column is operating at 99% propane purity while meeting bottoms specifications. How much energy is being wasted in this scenario?

The figure below shows the difference in duty for the two cases described above. The 99% purity case consumes 33% more energy than the 95% purity case. For the 10,000 BPD example below, this translates to a savings of 5 MMBTU/HR. If energy in this scenario costs $3/MMBTU, this would come out to $130K per year in energy savings.

Typically operators will target above the required product purity to absorb any swings in tower operation while keeping product streams on spec. Reducing this ‘overshoot’ as much as feasible can result in great energy savings.

Over refluxing towers wastes a significant amount of energy, and sometimes with little or unnecessary improvement in product purity or yields.

Reducing Energy Usage by Decreasing Operating Pressure

Another way to reduce energy usage is by reducing tower pressure. This can be especially effective in the winter months, when cooling water and ambient temperatures are lower, increasing the available process cooling. Reducing tower pressure reduces energy consumption because the relative volatility of hydrocarbons increases at lower temperatures. This makes them easier to separate, i.e. requiring less energy. This is especially true of lighter hydrocarbons (C1-C6).

The Cox Diagram(3) above shows why decreasing column pressure reduces energy consumption. For any given compound, as column pressure decreases, the flash temperature also decreases. The diagram above shows the relationship between flash temperature and vapor pressure. As flash temperature decreases, so does the vapor pressure for each hydrocarbon. But more importantly, the difference in vapor pressure between the compounds increases. For example at 180 deg F, The relative volatility between propane and butane is

Relative Volatility @ 180 F= VP C3/ VP C4

= 400 psig/150 psig

= 2.7

Suppose the overhead pressure in a column is lowered until the overhead temperature reaches 100 deg F.

Relative Volatility @ 100 F= VP C3/ VP C4

= 200 psig/ 50 psig

= 4.0

As shown above, decreasing operating pressure will increase relative volatility. This in turn reduces the required energy to perform a given separation. Let’s return to our depropanizer example. Suppose we drop the operating pressure in this tower from 300 psig to 275 psig. This reduces the required energy in our tower by 7%, saving $25k/year in our example.

One thing to consider when reducing tower pressure is that the reduction in tower pressure will cause an increase in vapor velocity inside the tower. This increases the chance of flooding inside the tower.

Savings energy is not only good for the bottom line, it is also good for the environment, conserving natural resources. If you find this article useful, let me know in the comments below. What ways have you reduced energy usage in distillation columns?

References

1) https://www.emersonautomationexperts.com/2010/industry/downstream-hydrocarbons/reducing_distil/#:~:text=Did%20you%20know%20that%20there,energy%20consumed%20by%20U.S.%20manufacturers.

2) Lieberman, Norman and Elizabeth Lieberman. A Working Guide to Process Equipment. McGraw-Hill Education, 2014.

3) https://petrowiki.org/Vapor_pressure

Recently I saw a safety article that resonated with me. The topic, Trusting Plant Instruments.

Few days go by where myself or a coworker don’t question an instrument reading at the plant I work at. And how could you not? Everyday, process engineers and operations personnel are looking at dozens or hundreds of instrument outputs. And every so often, we come across a reading that doesn’t make sense. But how operation personnel respond to an instrument that “doesn’t make sense” can have serious process safety implications.

I had an experience that made this principle quite clear to me. The process flow diagram below shows the process I was working with.

I was helping with the start up of a new bypass around an exchanger. We were monitoring the feed temperature to the stabilizer tower. If the stabilizer feed temperature got too high, liquid traffic in the stripping section of the tower would be reduced, and stripping efficiency would suffer. Our goal was to keep the feed temperature at the design value of 215 deg F.

As shown above, the unit described above has two feed temperature indications. But one was reading above the design value, and one was below. So then, which one is correct? Both values can not be correct, as there is no heat exchange occurring between the temperature indications.

I looked through the process graphics and trends, self-assured that I would determine which instrument was faulty. But in my search to justify my own assumptions, I realized something unexpected, both instruments were correct.

How can this be? Well dear reader, if you study the process flow diagram above, you will notice a control valve between the two thermocouples. A small portion of the stream flashed as it flowed through the control valve, cooling the stream before it entered the tower. Indeed, both thermocouples were correct!

So why am I telling this story? I believe it teaches a valuable concept. I didn’t understand how both instruments could be correct, so I immediately assumed one reading was false. This is the wrong way to think about instrument readings. When plant personnel ignore instrumentation, it is often because we don’t understand how the reading is possible. We then discard the reading as erroneous to justify our own notions of what is occurring inside our processes. This can lead to serious process safety implications, as the two examples below show.

In the 1960’s, a fire and explosion occurred chemical plant in Tennessee during a unit start-up (1). a thermocouple inside a distillation column was reading 250 degrees when it “should have” read 215 degrees. An instrument technician was sent into the unit to troubleshoot the instrument just before the explosion. A post-incident investigation revealed that the high temperature readings were consistent with increased Nitrobenzene content on that tray, likely due to tray damage. Had plant personnel not ignored this information, the incident may have been avoided.

Before and after of a Tennessee chemical plant fire

Another incident occurred in the 1990’s at a California Oil Refinery (2). A hydrocracker reactor outlet temperature went off-scale high, indicating a run away reaction. The control board operator was hesitant to believe the temperature reading, and did not depressure the reactor to stop the reaction. The ensuing fire and explosion resulted in one fatality and 46 injuries.

Not trusting process instrumentation can lead serious incidents. This is especially true for instruments that indicate an unsafe condition. So what can you do to make sure this doesn’t happen to you?

1. Assume an instrument is working until proven otherwise, especially when the reading could indicate an unsafe process condition. Ask yourself, What are the consequences if this instrument is reading correctly? Then take appropriate action.
2. Use other instruments and samples to confirm suspicious readings.
3. And finally, don’t assume an instrument isn’t working because you don’t understand how the reading could be correct!

Sources

Center for Chemical Process Safety April 2019 Safety Beacon
EPA Chemical Accident Investigation Report Tosco Avon Refinery, Martinez, California