Function of Reflux in Distillation Towers

Function of Reflux in Distillation Towers

3 Function of Reflux in Distillation Towers “Bobby why are you increasing the reflux on the de-prop tower?” “Mornin’ Mr. Lieberman. I’m increasing t...

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Function of Reflux in Distillation Towers

“Bobby why are you increasing the reflux on the de-prop tower?” “Mornin’ Mr. Lieberman. I’m increasing the reflux because there’s too much C4 in my LPG.” “O.K. that’s good Bobby. But how about the reboiler duty. The steam flow to the reboiler is on manual. You’ll have to increase the steam yourself. Remember?” “No Mr. Norm. I only have 2% propane in my butane bottoms product. It’s right on spec. I don’t need more reboiler steam. Check the 7:30 a.m. lab sample result. I’m fine with that.” “Bobby, we’ve discussed this before. You can’t just crank-up the reflux rate without also increasing the heat to the reboiler. Remember? It was just last week on nights, we talked about this.” “Uh no… maybe you can explain that again.” “That’s fine. Let me try to explain it a different way this time. Maybe I should illustrate the point with some examples. I’ll explain it to you just the way I explained it to Jay Holcomb to help him with his crude unit and preflash tower. I used an example of an old still in Peru to help Jay. I’ll tell you Jay’s story and the story of the Peruvian still.” Jay Holcomb had been married twice, had four troublesome grown sons, and was the operations manager of a 30,000 BSD refinery, surrounded by the soy bean fields in southern Illinois. “Norman,” Jay said, “the project you suggested last year has been a disappointment. I diverted the crude flash drum vapors to the new air cooled condenser we installed in the October turnaround (see Fig. 3.1). Diverting the cool 350°F flash drum vapors out of the crude tower flash drum has, as planned, increased the flash zone temperature in the crude unit by 30°F. Also, we’ve unloaded the crude tower trays and have been able to increase the crude rate by 3000 BSD.” “Jay, that all sounds good. What’s the trouble?” “My trouble is that the light naphtha from the new flash drum vapor air cooler has a long tail. It has a 420°F end-point. That heavy tail is coking up my naphtha reformer catalyst. Also, my youngest son is getting divorced again.” “You should have had daughters. They’re a lot less trouble.” “I hope your advice about my high naphtha end-point problem will be more useful. I need to get the ASTM D-86 Distillation end-point below 380°F max.” “Jay, I’ll tell you a story that will illustrate the idea that I have in mind,” I said. Understanding Process Equipment for Operators and Engineers. © 2019 Elsevier Inc. All rights reserved.





Flash drum


Ago Flash zone Steam

Resid FIG. 3.1 Light naphtha has an excessive end-point.

I was visiting Peru a few years ago. On the road to Machu Piccu, a famous tourist destination, my driver suggested we stop for a tour at a distillery that made Pisco, the main ingredient of “Pisco Sour” the national drink of Peru. I had drunk “Pisco Sour” at our hotel in Lima. I would normally choose Whiskey but thought I should try the local spirit. It would be interesting to see the distillery, particularly as this one was very old and in the remote countryside. I was introduced to the lead operator, Juan Ramirez—a middle-aged man who appeared to be of Indian extraction, and very knowledgeable in regard to the distillery, which was indeed very ancient. Hundreds of what appeared to be clay jars or pots were piled in one corner of the site. Huge stacks of firewood in another area. Several old Ford trucks were lined up in front of the plant. And then there was the Still. The Still was a huge copper pot, 8 ft wide and 5 ft high. It was set on a red brick structure above a brick-lined chamber into which firewood was being fed. A long coil of copper tubing condensed the vapors from the Still, which drained the condensed Pisco vapors, into a man-sized vertical copper pot. This second pot was elevated 16 ft above the top of the Still (Fig. 3.2). Pisco is 25%–30% alcohol. Whiskey (100 proof ) is 50% alcohol. At this distillery, on the road to Machu Piccu, they measured the alcohol content of the Pisco with a float.

Chapter 3 • Function of Reflux in Distillation Towers


Condenser Pot

Pisco sour Still

Fire FIG. 3.2 Ancient still. Yield reduction for increased product purity.

If the Pisco was too weak, the line on the float would be above the liquid level in their test glass, indicating too high a water content. Meaning the Pisco density was too high. “So Juan, what do you do to bring the Pisco up to 30% alcohol, if it’s too weak?” I asked. “So maybe, next batch, we don’t cook it so hard. Use less wood. So, juice it cooks slower ˜ or, I put weak Pisco back and colder. Mucho caliente makes Pisco too weak. Or maybe Sen into the Still, heat it up a second time. The second trip through our Still will make Pisco muy strong,” Juan explained. For me this is what Process Engineering is all about. It’s that interface where I meet the plant operator. It’s where we resolve our understanding of a problem using applied technology. Albert Einstein once said that which made America great is not freedom, or equality, or democracy. He said that America is the great land of applied technology.

Vapor Pressure Limitations “Juan,” I asked, “Why do you sometimes cook the juice too hard if it makes weak Pisco? If it vaporizes too much water with the alcohol?” “Because Sen˜or Norm, we make mucho dinero, by making more Pisco product.” This is the original problem of chemical and process engineering—the balance between product recovery and product purity. To keep his product on-spec, at a minimum of 30% alcohol, Juan had to reduce his product recovery by burning less firewood underneath the Still. The problem is that alcohol has a large vapor pressure and water has a smaller vapor pressure. When the Still is fired harder, the vapor pressure of the water increases faster



than the increase in vaporization rate of the alcohol from the fermented juice. So while the amount of juice vaporized increases, the amount of water vaporized increases faster, because the vapor pressure of the water is increasing at the hotter temperature in the Still. Reducing the Still temperature reduces water vaporization, but also reduces the Pisco production. But, that’s the price paid by Juan for making the Pisco stronger to meet his alcohol spec. It’s a compromise between product yield and product purity. I explained the concept of vapor pressure to Juan Ramirez, but perhaps I didn’t offer a good explanation, as Juan not only didn’t really grasp the concept, but began to show disinterest, and even possible signs of hostility! ˜ or Juan, by using process engineering So I said, “Maybe we can fix the problem Sen technology. Shall I explain?” And Juan said, “No!” So I explained anyway.

Two-Stage Distillation Process “Sen˜or Juan, we are going to create a two-stage distillation process using a partial condenser that will run continuously, process the Pisco all the time on-spec, and cost almost nothing to install. We can do it today.” When I said that we could do it today and that it would cost nothing, Juan smiled and said, “Bueno! Si! Si!” • •

Step One—I’ll cut the long coil of copper tubing in half. Step Two—The first half, I’ll use to connect the Still to the vertical pot (Fig. 3.3)


Second stage condenser


Clay pot

Flux Loop seal


Pisco sour

Fire FIG. 3.3 Two-stage distillation apparatus increases purity without reduction in product yield.

Chapter 3 • Function of Reflux in Distillation Towers

• •


Step Three—The second half I’ll leave connected to the top of the vertical pot and have it drain, by gravity, into one of those clay pots. Step Four—The liquid, which condenses (called “FLUX”), that accumulates in the vertical pot, I’ll use a rubber hose with a 2 ft long loop, to continuously drain back into the Still (see Fig. 3.3).

“Sen˜or Juan, the liquid that drains back into the Still will be re-vaporized in the Still, and is called flux or “Reflux.” The purpose of the loop is to keep vapors from the still from backing-up into the clay pot’s liquid drain line, which would stop the liquid draining through the hose. That is, the vapor would push the liquid back into the pot.” ˜ or Norm, suppose the Pisco is too weak. What should I do then?” “But Sen “Juan, just spray some water on the copper tubing between the Still and the vertical pot.” That coil being like a first stage condenser, spraying water on the coil to remove more heat will make more flux, but less product. “You see Juan, I’m just guessing about reducing the size of the existing first stage condenser (Fig. 3.2), when I said to cut the long coil of copper tubing by half. Maybe it should be a little bit longer to generate the right amount of reflux. But using an external source of cooling, the water spray, will allow you to compensate for lack of cooling by the air.” ˜ or Norm, suppose the Pisco becomes to strong! What should I do then?” “But Sen “Juan, just add more firewood to the Still. That will make more Pisco product too, by distilling over some extra water.”

Origin of a Distillation Column The technology of the partial condenser to produce a very strong alcoholic beverage originated in Arabia around AD 1000. The origin of the multistage, trayed distillation tower which we still use today, originated in Scotland in 18th century. It’s called the “Patent Still.” It had a reboiler, condenser, six bubble cap trays, and a feed preheater. I saw one being used to make apple brandy on a farm in the county of Somerset in England just a few years ago. It worked just fine. The only things that have changed in distillation since the 18th century is that we now call “Flux,” “Reflux,” the Still now has a separate reboiler, and we use valve trays, instead of bubble cap trays. My idea for updating the Pisco production technology was not original. I reproduced the small, two-stage, home-made distillation apparatus I had seen in a store front window of a liquor shop in Vilnius (Lithuania), a few years before my visit to Peru.

Removing the Naphtha’s Tail “Jay, we can do the same thing on your unit. Shut off three of the four fans on the air cooler. Then, we’ll take the liquid, that was partially condensed in the air cooler outlet header box, and drain it back into the flash drum. Kind of like reflux. The remaining naphtha vapor, we




Flux Jet Diesel Flash drum

Ago Flash zone Steam

Resid FIG. 3.4 The air cooler is acting as a theoretical separation stage.

can route to the overhead of the crude tower (Fig. 3.4) and comingle it with the crude tower overhead naphtha. The air cooler will be like the first stage condenser in Peru, which is also working like a theoretical, fractionation stage.” “But Norman. I’m going to wind-up with less naphtha from the preflash tower, by using the air cooler as a partial condenser. I don’t know if I like that.” Jay said. “It’s all a balance between product yield, and product purity. You can’t have it both ways Jay. It’s kind of like being married. You have to trade a part of your freedom for companionship.” “Ah yes Mr. Lieberman. I see it now. Wisdom comes with age.” Said Jay, nodding his head in agreement.

Conclusion “You certainly are quite the world traveler, Mr. Norm. Imagine you working all the way down in Peru,” Bobby said, after listening to my story. “Yes, Bobby. Just like Jay’s story, the travelling has its good and not so good points. But the point of this story, is that the REFLUX COMES FROM THE REBOILER. If you increase the tower top reflux flow, to reduce the C4’s in your LPG Bobby, the reflux drum is going to go empty, unless you refill it with the vapors generated from the steam reboiler. The reflux pump is going to lose suction and cavitate, and you’ll tear up the pump seal again due to vibration.”

Chapter 3 • Function of Reflux in Distillation Towers


“I sure am going to remember your story, and add steam to the reboiler next time I add reflux to the depropanizer. But Mr. Lieberman, wouldn’t it be best to get the temperature control loop fixed on the reboiler steam, so it will add the steam automatically? It won’t be that expensive to fix.” “Uh Bobby. We’ll have to discuss that another time. I have to go to an important meeting at 9:30 this morning, I’d better get going.”