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Thinking Quickly With Distillation Columns

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    When your basic column controls fail, a quick thinking engineer can save a company alot of money.  Weighing the financial options before the incident happens can help you feel secure in you decisions when you make them.  We'll have a look at three scenarios that you may not have thought of, but may encounter.
    Let's start with a basic, optimized column design as described in Table 1:

Table 1:  Column Design Specifications

Feed Temperature (0C) 90.0
Feed Pressure (kPa) 19.0
Top Pressure (kPa) 6.5
Bottom Pressure (kPa) 15.8
Reflux Ratio 12.2
R/Rmin 1.18
Feed Stage 9
Number of Actual Trays 31
Tray Efficiency 78%
Diameter (m) 1.2
Length (m) 22
Flooding 53%
Condenser Heat Duty (MJ/h) -2726
Reboiler Heat Duty (MJ/h) 2679
Acrylic Acid Purity 99.9%

   Figure 1 shows a material balance around the column:

qdist2.gif (5633 bytes)

Scenario One

Situation:   Following a short time during which capacity was increased, there was some difficulty in adjusting the utility flow rates in order to get the column to operate properly.  It seems that the column is currently using more
cooling water and steam than the current feed flowrate should require to obtain the necessary purity according to the column design.  The utilities had to be taken off of automatic control and adjusted manually causing extensive amounts of downgraded material because the purity specification was not being met.

Consequences:   Lost revenue due to off specification material.

Cause:   During the scale up, the utilities are increased to handle the additional heat duty.  When production returned to normal, the steam flow in the reboiler was reduced according to the feed flow, not the original conditions before the scale up (although there should be no difference).  However, the design utility flowrates can seldom accurately predict fouling.  This means that more utilities may be required than specified by the design specifications.  This is the case here.

Solution:   After increasing the steam flow to it's proper set point, refine the operating procedure for capacity changes. The correlation between feed flow and steam may need to be re-examined. Over time, the steam flow may gradually increase to maintain purity.  Mineral buildup in the reboiler (fouling) may need to be accounted for in the correlation.  For example, the feed flow-utility flow correlations may also need to be time dependent to account for the fouling in the reboiler/condenser.  Examining the column's history may be helpful here.  You may be tempted to say, "Why don't we just use the set points that we used before the scale up?".  That will work if you are returning to exactly the same flowrate, but you may be required to change capacity to flowrate that you don't have a reference point for in the immediate past.

Scenario Two

Situation:   The acrylic acid is stored in holding tanks until shipment to customers.  The purity is tested in the lab everyday and continuously online.  Currently, one of the acrylic acid tanks is off specification at 99.5% pure.  The condenser and reboiler utility streams have been behaving strangely by oscillating up and down in the past two days.  Because these oscillation have been small and barely outside of set point parameters, no changes were made.   The concentration detector that measures the online concentration of the acrylic acid has been operating properly.

Consequences:   If shipped as is, this material would have to be downgraded resulting in a serious financial loss.

Cause:   All to often, engineers forget about one simple upset in columns.  Feed surges can cause small amounts of off specification material to slip through the column.  In this case, feed surges have been occurring frequently in the past 48 hours causing the utilities to oscillate up and down trying to compensate.  During compensation, enough off grade material has made it through the system to contaminate the entire tank.

Solution:   Manually increase the utilities such that the required purity is maintained even through the feed surges.  When the feed is stable, the product will be over purified and may be able to increase the overall purity of the tank.  Although more money will be used in utilities, the profits from upgrading the material in the tank will be much greater over a short time span.   Don't forget to find out what is causing the surges and correct that problem too!

Scenario Three

Situation:   One day while browsing the recent column history, you decide to check the current performance against the design performance.   This seems like a good idea since the column has been in operation for 15 years.   It has always been used for the same purpose and it is currently operating at its design feed flowrate.  You are stunned to learn that the current reflux rate of 80 kmol/h is 20% higher than the design reflux rate!  The purity that the column is producing is no more than the intended design purity.  For some reason, the column is literary straining to perform this separation....but why?

Consequences:   The additional reflux has forced the column to handle heat duties that are much higher than they should be.  This is costing the company thousands of dollars each year.

Cause:   Since the column is under a vacuum, only glass covered view ports above the trays are available to look into the column making it difficult to see the bottom of the plates.  After having a contractor perform a grid scan with radioactive isotopes, you confirm your suspicion....the column is experiencing serious flooding.  Although thoughtfully designed with stainless steel trays, nothing can withstand acid forever and the tray openings have been corroded.

Solution:   At the next shutdown, new trays should be installed, along with view ports situated just below one of the trays so that flooding can be checked visibly.  Although you may think that view port placement should be a "no brainer", I've seen them placed where they were nearly useless!  Once the new trays are in place, watch the utility costs drop and line up for a promotion.   But, why wasn't this checked long ago?  Remember, the column was operating "normally" in terms of purity and often times that is all that management really cares about!

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