MAINTENANCE OF DISTILLATION COLUMN IN AN REFINERY

BY: ASMITA MISHRA

Btech, Chemical Engg IGIT SARANG

CONTENTS Introduction Main problems met on Distillation Column Failure modes associated with distillation column Distillation column troubleshooting layout Some Helpful techniques for problem

identification Measures to deal with the problem Conclusion References

INTRODUCTION Distillation is probably the most widely used separation process in

the chemical and allied industries; its applications ranging from the rectification of alcohol, which has been practised since antiquity, to the fractionation of crude oil.

Distillation columns are considered as one of the most critical components in petroleum refineries, gas processing installations and chemical plants. Plant performance depends to a large extent on the ability of these columns to function as intended.

Defective columns may lead to serious consequences to the plant operation and hence the quality of the product. Thus, when a distillation column experiences irregularities, it is urgently necessary to find out exactly what is happening inside the column. Any kind of problem can result in a large financial loss.The earlier the problem is identified and corrected, the lesser will be the loss and the cost to rectify it.

MAIN PROBLEMS MET ON DISTILLATION COLUMNS

1. Liquid entrainment defined as the physical rise of droplets, provoked by

the ascending flow of vapour. The vapour tends to host droplets upward, while the

gravity tends to pull them downward. If the vapour speed is relatively high, then the

entrainment can surpass the gravity and some droplets can be transported from a tray to the tray above.

Also seen as the excessive accumulation of liquid on trays

A high liquid entrainment can lead directly to a column flooding called "flooding jet type

2. Flooding Defined as the operating mode in which the entrainment is such as there is no downward flow or clear reflux. Caused by the accumulation of deposits (dirt) or a blockage on trays Also caused when the feeding flow of reflux towards the column is upper to the flow coming down from the downcomer. leads to a reduction of the efficiency of trays If the downcomer capacity is inadequate, the level of liquid in this one increases gradually until it limits the liquid flow on the tray above, or on its tray. Flooding can be seen as an indication of ineffectiveness due to an insufficient liquid-vapour contact

3. Other encountered problemsSome other problems can be met. These are mainly:• ‘‘Foaming’’: foam formation• Destruction and collapse of trays• ‘‘Weeping’’ or in other words presence of downward droplets in the vapour space between trays

FAILURE MODES ASSOCIATED WITH DISTILLATION COLUMN

Corrosion Design Fault External Event Fire/Explosion Human Error Impact Impurities Maintenance Faults

Overheating Over-

pressurisation Structural Failure Vibration Wrong/Defective

Equipment

DISTILLATION COLUMN TROUBLESHOOTING LAYOUT

SOME HELPFUL TECHNIQUES FOR PROBLEM IDENTIFICATION1. Gamma scanning can identify a number of

common malfunctions in columns.Mechanical • Displaced or damaged trays and packing problems • Corrosion resulting in partial tray damage• Out-of-place liquid or vapour distributors• Level control problems on chimney trays or base

liquid levelRate related • Entrainment (slight, moderate, severe, jet flooding) • Weeping or dumping trays

• Dry or flooded trays due to loading conditions• Unequal liquid levels on trays Process • Foaming on trays or in reboilers, condensers and accumulators• Misdistributions of vapour and liquid in packing• Liquid hold-up due to plugging and fouling• Superheated or sub-cooled feed or reflux

Radiation Detector Suitable

radioactive source

The quantity of gamma radiation absorbed or transmitted by the material placed between the radioactive source and the detector gives an indication of the nature and the real quantity of this material. •When radiation, issued from a radioactive source, pass through a medium containing a tray with an aired liquid, a good part of this radiation is absorbed and the radiation quantity reaching the detector is relatively small.• If a beam of radiation passes through a non aired liquid, the most part of this radiation is absorbed by the medium and the intensity is weak.• When the radiation beam passes through steam, in this case a small quantity of material is present to absorb the radiation. That means high intensities of radiation are transmitted to the detector

To sum up, a scan using gamma radiation of a column can detect and localize regions of liquid and steam within this one. It can make the discrimination between the liquid aeration and can detect the levels(heights) of foam or aerosol in the steam regions.

Distinguishing features of the technology• Capable of non-disruptive and non-invasive investigation.• Runs even in operating conditions• Full length scanning capabilities.• Insulation or lagging need not be removed.• Performs non-contact measurement• Utilises small amount of “sealed radioactive source” with in-built radiological safety features• Applicable to all column sizes• Uses computer-aided data acquisition, retrieval and analysis system enabling fast, accurate and reliable results.

2. CSI Wireless Vibration Transmitter: o Provides early warning of excessive vibration and

impending damage.o Determines root causes and guides action. o identifies premature bearing wear and failure.3. Micro Motion Coriolis Flow Meter: o Directly measures the mass flow of gas and liquid

streams and liquid density—including compressible or supercritical fluids

o allows quick reactions to process changeso improved product-quality control, and high turndown

capabilities

MEASURES TO DEAL WITH THE PROBLEM1. Dry tray pressure drop should exceed 20% of the available

tray spacing to prevent enrichment2. The interfacial surface between the phase must be made

large3. Column should operate in between the optimum reflux ratio

and minimum reflux ratio conditions4. minimize the height of column per theoretical stage5. minimize pressure drop per theoretical stage and6. maximize operational range7. Proper packing should be selected in case of packed

distillation columns8. Proper control of vapour velocity to prevent entrainment

etc.....

CONCLUSION An operating problem is impossible to

fix until the specific cause is found. So technically we the chemical engineers must develop new and advanced techniques to deal with these problems effectively, efficiently and economically in a safer and quicker manner . So lets join hands and put our innovations and ideas to bring a better , brighter and safer tomorrow.

REFERENCES http://www.hse.gov.uk/comah/sragtech/techmeasmate

rial.htm Hills A., “Practical Guidebook for Radioisotope based

Technology in Industry,” Technical Report,IAEA/RCA/8/078, 2001.

M. Sanches, M. Hanaguchi, F. Beckmann and W. Calvo, "Radiological safety in the gamma scan procedures",

the International Nuclear Atlantic Conference., Santos, Brazil, 2007. Coulson and Richardson vol vi Mass Transfer Operations by R.E.Treybal

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