Process Engineering: Facts, Fiction and Fables

Introduction

I started work as a process engineer for the American Oil Company in 1965. Now, after 52 years, I’m still a process engineer. Still working in the same way, on the same problems:

Distillation Tray Efficiency

Shell & Tube Heat Exchangers

Thermosyphon Reboilers

Draft in Fired Heaters

Steam Turbine Operation

Vacuum Steam Ejectors

Centrifugal Pump Seals

Surge in Centrifugal Compressors

Reciprocating Compressor Failures

Process Safety

Fluid Flow

Most of what I need to know to do my job, I have still to learn. And I’m running out of time! So, with the help of my little friends in this book, I’ve recorded what I have learned so far. I hope this will help you in solving process problems.

The difficulty of being a process engineer is that our job is to solve problems. Not with people, but with equipment. Within minutes, or hours, or days, the validity of our efforts are apparent. More like plumbing, less like other branches of technology.

Most things I’ve tried as a process engineer haven’t worked. But those that have been successful I remember, and use again. And it’s insights from these successful plant trials and projects that I have shared with you in my book.

One thing’s for certain. The money paid for this book is nonrefundable. But should you have process questions, I’ll try to help.

Norm Lieberman

1-504-887-7714

[email protected]

PART I

CHAPTER ONE

PROCESS OPERATIONS & DESIGN

CARL & CLARE

Hello! I’m Clare! I work for Carl. We troubleshoot refinery process equipment! We’re a team!

Hi! I’m Carl! I know everything, because I’m really, extremely, smart! Clare is my associate!

INCREASING COOLING WATER FLOW THRU AN ELEVATED CONDENSER OR COOLER

Clare! Let’s open the cooling water outlet valve to get more water flow.

No, Carl! The Condenser is 60 feet above grade. The pressure at P1, is under vacuum! Opening that valve will give us less cooling water flow!

NO! Opening a valve will always increase flow!

Sorry, Carl! Opening that valve reduces the pressure at P1, further below the atmospheric pressure. This causes the air to flash-out of the cooling water, which chokes back water flow!

Clare! WRONG! I’m really smart! Anyway, where’s the test to prove you’re right?

OK. I’ll close the valve and you’ll see the temperature at T1 will go down. But don’t close it too much! Otherwise, you will throttle the water flow. Then, T1 will get hotter!

But Clare! How do I know how to adjust that stupid valve?

Carl, dear! Set the valve to hold a backpressure of about 3″ Hg. That’s minus 0.10 atmosphere. At 100 °F, that will stop air evolution from the water, but not throttle the water flow too much!

HOT VAPOR BY-PASS PRESSURE CONTROL

Clare! Close the hot vapor by-pass valve! We need to lower the tower pressure. Do it now!

Sorry, Carl! When I closed the valve the tower pressure went up … not down!

No, Clare! Closing the valve will cool off the reflux drum! The pressure at P2 will drop, and draw down the pressure at P1. Understand?

But Carl! How about the pressure drop across the air cooler? It increases as more flow is forced through it. True, the pressure at P2 will always fall! But the pressure at P1 may go up or down—depending on the air cooler DP!

But, but …? Closing the hot vapor by-pass is supposed to lower the tower pressure, according to my design manual!

But suppose the tubes get full of salts and scale? Then what? Also, Carl, we now have a vacuum in the reflux drum, which can be quite dangerous! Air could be sucked into the drum and an explosive mixture could form! Don’t forget there’s pyrophoric iron sulfide deposits (Fe(HS)2) in the drum! They’ll auto-ignite at ambient temperatures!

STALLING A THERMOSYPHON REBOILER

Clare! Open the steam supply valve! Quick! We need more reboiler heat. The reflux drum is going empty!

Sorry, Boss! That won’t help! The Once-Thru Thermosyphon Reboiler is STALLED OUT!

Clare! More steam flow will have to give us more heat to the reboiler! Open that valve!

Opening the steam valve will not increase steam flow when the reboiler is STALLED-OUT!

STALLED-OUT? What does that mean?

Stalled-out means heat duty is limited by the process flow to the tube-side of the reboiler! The process flow rate to the reboiler is real low now and limiting the steam condensation rate!

How do you know that, Clare? Do you have X-ray vision?

Carl! Look at the reboiler outlet. It’s 450°F! The tower bottoms are only 330°F. Most of the 300°F liquid from tray #1 is leaking past the draw pan, and dumping into the bottoms’ product!

OK, Clare, OK! But still, the steam inlet valve is only 50% open! Won’t opening it 100% help some?

No, Sir! The pressure at P1 on the steam inlet line is 500 PSIG! The same as the steam supply pressure. There is zero DP across the steam supply valve. The valve position, with no DP, is IRRELEVANT!

I guess we should have used a total trap-out chimney tray for tray #1! I remember you suggested that last year, Clare. Perhaps you’d like a transfer to the Process Design Division? They would probably love to have you! I remember that in the old days we had bubble cap trays, which could never leak and cause this loss in thermosyphon circulation, or stalling-out.

OPTIMIZING FRACTIONATOR PRESSURE

Clare! The best way to optimize tower pressure is to target for the lowest pressure!

Why is that, Carl?

Because, Clare, as we learned at university, the lower the pressure, the greater the RELATIVE VOLITILITY between propane and butane!

But Carl! Suppose the lower tower pressure causes entrainment? Then, a lower pressure will reduce tray separation efficiency and make fractionation worse!

Well! What do you suggest? It takes too long to wait for lab sample results.

Carl, I suggest:

At a constant reflux rate, start lowering the fractionator pressure.

Now, watch the delta T (T1 - T2).

That tower pressure, that maximizes delta T, will give the best split between butane and propane. But make the moves slowly!

Clare! You really should take a more positive attitude towards your engineering degree, and show more respect for the principle of relative volatility!

ADJUSTING STEAM TURBINE SPEED TO MINIMIZE STEAM CONSUMPTION

Clare! Always run steam turbines at a constant speed! In the U.S., 3,600 rpm, Europe 3,000 rpm!

I’m sorry, Carl! But I don’t agree!

Exactly what’s your problem? All process plants run their turbines at 5% below their maximum rated speed. That’s best!

Carl! The best way to set the speed of the turbine is to slow the turbine down until the control valve on the discharge of the pump it’s driving is in a mostly wide-open but still controllable position!

All to what purpose?

Well, Carl, for each 3% speed reduction, the steam required to drive the turbine will fall by 10%. Work varies with speed cubed: W ~ (speed)³ … That’s the Affinity Law!

Thanks, Clare! I’ll write new instructions for the operators!

Carl! Actually, you can get rid of the control valve on the discharge of the pump, and run just on governor speed control, to adjust the upstream level or the pump discharge flow and pressure. That will save even more of the motive steam.

STEAM CONDENSATE DRAINAGE FROM REBOILERS BLOWING CONDENSATE SEAL

Clare, listen up! Open that condensate drain valve more. We need more heat to the reboiler!

Sorry, boss! I think opening the condensate drain valve more will reduce reboiler duty! I’m not too sure!

Not sure? Can’t you just follow my instructions for once? It’s getting late!

Carl, here’s the problem! If I open the drain valve too much, we’ll blow the condensate seal. Steam will blow through the reboiler tubes, without condensing. If I open the valve too little, we will suffer from condensate back-up!

I’m confused! Then how do we know whether to open or close that stupid condensate drain valve?

Well, if we close the valve, and T1 goes up, it means we were previously blowing the condensate seal! If we close the valve and T1 goes down, it means we were previously suffering from steam condensate back-up!

OK! Let’s optimize the drain valve position to maximize the reboiler outlet temperature, and go to lunch. I’m hungry! Seems like it’s all a balance between condensate back-up and blowing the condensate seal. Kinda like eating too much or too little.

EFFECT OF REFLUX ON FRACTIONATOR TOP TEMPERATURE

Clare, dear! To lower the tower top temperature, one should always raise the reflux rate. It’s a basic idea of Process Control!

Sorry to disagree again, Carl! It ain’t necessarily true!

Now Clare! Read any distillation textbook. They all agree with me! What’s your problem?

It’s this: If the top tray is at its flood point, raising top reflux must increase reboiler duty, because the reflux comes from the reboiler. This will increase vapor flow to the top tray!

So what? The reflux will cool off that extra vapor! The reflux will knock back the heavier components in the vapor. What’s your problem, Clare?

What you say is true, boss. Up to a point. The INCIPIENT FLOOD POINT! Above that point, extra vapor promotes ENTRAINMENT. The result is droplets of heavy liquid blow up through the trays, increasing the boiling range of the top product and temperature at the top of the tower.

That’s BAD! Because then the tower top temperature will go up, as the reflux rate is increased. Worse, the top reflux rate, and reboiler duty will then increase more, and make the problem even worse!

Yes, Carl! I call this getting caught up in a POSITIVE FEED-BACK LOOP! The reflux needs to be switched to manual and partly closed to break the feed-back loop.

CENTRIFUGAL PUMP HEAD VS. FLOW PERFORMANCE CURVES

Clare! Where did you get that pump curve from? It’s wrong!

From plant data on the new giant vacuum tower residual pump. I plotted observed flow vs. pump discharge pressure!

Well! It’s contrary to the manufacturer’s curve! The head and flow are both going down, at a low flow. Impossible, I’d say!

The pump is never supposed to operate at such a low flow. But, Carl, you purchased this oversized pump yourself!

Oh! Well then, I guess it’s OK to run it below point A, as we have lots of excess head anyway!

No, Carl! It’s not OK. Below point A, the pump vibrates in a most alarming manner!

No need to mention this to Carl … but Norm first saw this in 1991, at the Coastal Refinery in Aruba! Norm still complains to this day that those pump vibrations loosened the fillings in his teeth! Imagine what that did to the pump’s mechanical seal? The pump had a minimum flow spill-back, but when Norm opened it, the pump lost suction and cavitated is a most alarming manner.

CONDENSATE BACK-UP IN CONDENSERS-THE EFFECT OF SUB-COOLING

Clare! Let’s get condenser B cleaned. Look how high its outlet temperature is!

Actually Carl, it’s A that’s really underperforming!

But the outlet flow of B is hotter than A?

Well, that’s true! But only because A is suffering from CONDENSATE BACK-UP and sub-cooling! About 40% of the tubes in A are submerged in liquid, but only 10% of the tubes in B are covered in liquid!

So, Clare, I suppose that now you have X-ray vision too? How can you know that A is suffering