Things That Can Go Wrong in a Methanol Distillation Column

GBH Enterprises, Ltd.

THINGS THAT CAN GO WRONG IN A
METHANOL DISTILLATION COLUMN

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Things That Can Go Wrong in a Methanol Distillation Column

VULCAN Systems Process Catalyst Technology
Overview
1
1
2

Contents
Contents ......................................................................................................... 2
Introduction ..................................................................................................... 3
2.1 Distillation Systems ................................................................................... 4
2.1.1 Two Column System ........................................................................... 4
2.1.2 Three Column Distillation System ....................................................... 4
3 Hardware Problems ........................................................................................ 6
3.1 Vapor Cross Flow Channelling.................................................................. 6
3.2 Low Tray Separation Efficiency................................................................. 7
3.2.1 Tray weeping ...................................................................................... 8
3.2.2 Liquid Entrainment .............................................................................. 8
3.2.3 Effect of Low of Efficiency ................................................................... 8
3.3 Removal of Topping Column Trays ........................................................... 8
3.4 PH Control ................................................................................................ 9
3.4.1 Corrosion ............................................................................................ 9
3.5 Damage to Tanks.................................................................................... 11
3.5.1 Tank Bursting Due to Excessive Internal Pressure ........................... 11
3.5.2 Tank Collapsing Due to Insufficient Internal Pressure....................... 11
3.5.3 Angle Rings ....................................................................................... 11
3.5.4 Rippling of the Floor .......................................................................... 12
3.5.5 Edge Settlement ............................................................................... 12
3.6 Plant Diagnostics ...................................... Error! Bookmark not defined.
3.7 Leaks in Reboilers .................................................................................. 14
3.7.1 Leaks in Steam Reboilers ................................................................. 14
4 Byproduct Problems...................................................................................... 15
4.1 TMA Removal ......................................................................................... 15
4.2 MEK Removal ......................................................................................... 15
4.3 Fusel Oil Disposal ................................................................................... 15
5 Other Issues ................................................................................................. 16
5.1 Permanganate Fading Time.................................................................... 16
5.2 Jet Flooding ............................................................................................ 16
5.2.1 Asia-Pacific Producer Experience ..................................................... 17
5.3 Weeping .................................................................................................. 17
5.4 Foaming .................................................................................................. 18
5.5 Downcomer Flooding .............................................................................. 18
Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up /
Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance
Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment
Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process
Technology - Hydrogen Catalysts / Process Technology - Ammonia Catalyst / Process Technology - Methanol
Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New
Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com


6
7

Troubleshooting ............................................................................................ 20
Conclusions .................................................................................................. 22

Introduction
This paper details some common problems that can occur in a methanol
distillation columns and the associated storage tanks. The problems have been
grouped into the under the following headings,
•
•
•

Hardware problems,
Byproduct problems,
Other miscellaneous issues.

Some typical examples include, but are not limited to,
•
•
•

High byproducts levels due to old synthesis catalyst,
Damage to the column,
Flooding and Weeping.

Plant reliability could be defined by the following graph,

After plant start up, there are a number of problems formally associated with
commissioning, design issues and the operators learning about the plant.
Towards the end of the plants life, the problems are more associated with ageing
hardware, loss of corporate memory, changes in plant personnel and changes in
operating philosophy. It should be noted that many of these problems that have
occurred in the past are starting to re-occur again. This is a function of the above


issues and the reduction in plant personnel due to the effect of market forces on
fixed costs. See reference
Distillation Systems
1.1.1
Two Column System
Licensors offers two different distillation column configurations, the first is the
standard design comprising of two columns, a topping column for light end
removal and a refining column for heavy end removal; such a scheme is shown
below,
1.1.2



Three Column Distillation System
In order to improve the heat usage within distillation, licensors offer a three
column design as shown below,

In this scheme, the overheads from the refining column is used to provide the
reboil duty for the Topping Column ad the Recovery Column. It is typical that the
methanol recovery efficiency of such a system is higher than for a two column
system – usually a three column system has a recovery efficiency of 99% whilst
the two column system has a recovery efficiency of 98.5%.



2

Hardware Problems

2.1
Vapor Cross Flow Channelling
A Caribbean producer operates a 1,500 MTPD methanol plant.
From the start of full load manufacture a number of operational issues were
encountered with the distillation section amongst which were poor product quality
and unstable operation.
An additional phenomenon was a low apparent efficiency on the topping column.
Investigation of the problem including gamma-ray scans. These aided in the
identification of vapor-cross flow channelling. This effect reduces the efficiency
because the vapor and liquid do not maintain good contact. The effect is shown
in the figure below.



The Gamma scans of the topping column compared with a normal scan are
shown below. The operating company A scan (left) shows a regular density
pattern whilst the operating company B scan (right) shows alternating densities
that characterise vapour cross flow channelling.

The solution undertaken to solve this and the additional operational problems
was to re-tray both the topping and refining column. Once completed the
distillation system immediately produced on specification methanol product.
2.2

Low Tray Separation Efficiency

When a distillation design is undertaken by licensors, the size of the column is
stated in number of theoretical stages. It then becomes the job of the contractor
to select the type of tray to be used. This will have an efficiency less than 100%
and so will need more trays than the theoretical requirement.
The tray efficiency is not only a function of the tray but also a function of the
physical properties of the fluids and the hydraulics through the column both liquid
and vapor. Some of these effects are listed below.


2.2.1

Tray weeping

This is where the liquid passes through the vapor holes instead of passing over
the whole plate and down the down-comer. This is usually experienced at low
vapor rates. See section 5.3 for more details.
2.2.2

Liquid Entrainment

This is where the liquid is carried to the tray above by the vapor flow through the
tray. This is usually experienced with high vapor rates.
The two cases mentioned above will affect the efficiency of the tray without
stopping the operation of the unit although extreme cases of liquid entrainment
can cause flooding.
In addition there are additional effects which can reduce the tray efficiency such
as Vapor Cross Flow Channelling (see above).
2.2.3

Effect of Low of Efficiency

The effect of having low efficiency operations will be a reduction in the
separation. This will be exhibited in all the separation but will appear worst at the
more difficult separations. Separation of the methanol from water should still be
achieved well but this will contain more ethanol. The mass balance through the
unit can be adjusted to reduce this but it will result in more methanol in the fusel
oil and methanol/ethanol in the water stream.
2.3

Removal of Topping Column Trays

Another Asia Pacific operator, two distillation columns were constructed on the
site to allow for processing of crude methanol; originally the plants converted
crude methanol to gasoline. During normal operation, there was a loss of control
of the steam being fed to the distillation column re-boilers and this lead to a high
vapor flow up the column. This lead to high velocities through the holes in the
trays which lead to ‘fluidization’ of the trays. The trays were lifted off their
support rings up into the top of the column. This causes a high pressure drop,
which caused the operators to drop the reboiler heat load.



The trays then dropped back down the column since the velocity through the
holes was now too low to keep the trays fluidized. The trays were then deposited
in the bottom of the column.
It should be noted that the product quality was not affected during the period
when there were no trays in the topping column.
2.4

PH Control

Caustic is added to the feed to the topping column to prevent corrosion by
carbonic acid attack. At the aforementioned Asia Pacific operator, the caustic
pumps were installed some distance away from the Topping Column.
There were significant problems with the control of the refining column pressure
due to breakthrough of carbon dioxide into the refining column. The root cause
was found to be the loss of caustic which lead to breakthrough of carbon dioxide
into the refining column and thereby loss of control of the pressure of the refining
column. Another issue is that the time for the caustic to flow from the pump to
the distillation was approximately two hours. This also caused problems since
there was initially no DCS indication that the pump had tripped and the first
indication that the operators had was the loss of control of the refining column
pressure.
2.4.1

Corrosion

One of the by-products of the methanol synthesis process is formic acid. If left
uncontrolled this would have a significant effect on the carbon steel columns and
trays of which most distillation trains are made. The pictures below show some of
the effects which can be experienced by corrosion. The figure below shows
extensive corrosion to the outside shell of the column.



The figure below shows damage to a valve tray including the valves and the weir.

This corrosion is controlled by the addition of caustic. The amount of caustic
required will vary from plant to plant but the objective of the addition is to achieve
a pH between 9 and 11. A typical addition rate would be 1 litre of 2% caustic
solution per tonne of methanol.



2.5

Damage to Tanks

2.5.1

Tank Bursting Due to Excessive Internal Pressure

If the shell of the tank is not sufficiently strong or has insufficient stiffening (by
use of angle or stiffening of rings), then the tank can ‘explode’ due to excessive
pumping in rate.
2.5.2

Tank Collapsing Due to Insufficient Internal Pressure

The converse of the problem detailed above can also happen if the shell is not
strong enough during pumping out of the liquid due to the loss of liquid level.
This can be prevented by the use of angle or stiffening rings or suitable breathing
vents. See reference 2 for more details.
2.5.3

Angle Rings

As mentioned, one European operator noted on their storage tanks, a number of
areas of damage was found including damage around a bent angle or stiffening
ring. These are included in
the design to stiffen the shell
of the tank to prevent damage
during pumping out or in of
product methanol. The figure
to the left illustrates the
damage to the stiffening ring,
and highlights where water
collected.
The water that collects on the
angle ring leads to local
corrosion (particularly if
external lagging is used on
the tank) and this can lead to
thinning of the shell and loss
of strength.



2.5.4

Rippling of the Floor

The floor is normally supported by the foundation and can fail due to,
•
•
•

Cracks developing in floor or cracks in the fillet welds joining the plates
together,
Failure of the welds due to loss of support during the repeated loading and
unloading,
Tearing of the lap welds by gross strain.

The latter two can occur due to erosion of the foundation edges, differential
settling and leaching away of the foundation with time. Rippling occurs when
there is differential settling rates between the base of the tank and periphery.
This is highlighted in the figure below,

2.5.5

Edge Settlement

Another potential issue is the settling of the edge and the stress that this can
induce on the shell itself and corrosion due to water collection at the base of the
tank. This is shown in the figure below:



2.6
Plant Diagnostics
Radio-isotope scans can be applied to methanol distillation columns in order to
determine problems such as,
•
•
•
•
•
•
•

Carry over in separators and columns,
Heat exchanger leaks (location and rate),
Blockages,
Flowrate variations,
Mal-distributions of process fluids,
In adequate mixing/residence times,
Mal-function columns due to,
•
tray flooding,
•
tray damage including missing trays and tray debris,
•
foaming,
•
liquid weeping.

The following figure illustrates the results that can be achieved from a typical
scan of a distillation column,



2.7

Leaks in Reboilers

On most methanol plants, the majority of distillation heat load for both the topping
and refining columns, is supplied from the synthesis gas with a small trim heat
load from LP steam.
2.7.1

Leaks in Steam Reboilers

If there is a steam leak in the topping column reboiler, then the first effect will be
an increased separation efficiency in the topping column since the water helps to
increase the relative volatility of the light ends. This is not normally an issue
since the light ends are typically volatile enough to be removed.



The second issue is that this will increase marginally the heat load in the refining
column due to the increased separation requirement. If there is a leak in the
refining column steam reboiler then the effect will be the same as above.
3

Byproduct Problems

3.1

TMA Removal

TMA or Tri Methyl Amine is a problem in the product methanol since is gives the
methanol a fishy smell – methanol has a limit in all product specification that it
“shall be free from odor”. TMA is formed by the reaction of ammonia produced in
the primary reformer with methanol formed in the loop. It is impossible to remove
in the distillation section and therefore any TMA produced will pass through to
the product.
Normally TMA is not an issue but at start up high levels of TMA are formed by
the conversion of residual nitrates in the catalyst to TMA. Also after a reformer
catalyst change, the ammonia levels will be high since the reforming catalyst is at
its peak catalyst activity and the reformer is normally being operated at the
highest exit temperature.
3.2

MEK Removal

MEK or Methyl Ethyl Ketone has a volatility that is very close to that of methanol
and as such passes through the Topping Column and Refining Column and into
the product methanol. Normally, MEK is not a problem, but when the synthesis
catalyst is at its end of life, the levels of MEK can be high and cause problems in
achieving the required product specification.
3.3

Fusel Oil Disposal

There are a number of methods of disposing of the fusel oil,
•
•

Feed into the saturator circuit,
Burning in the
•
Primary reformer,
•
Package boiler,
•
Feed preheater.


On many plants, the burning of the fuel oil will lead to carbon formation on the
burner tip. This then requires that the burners be removed on line and cleaned
which is a hazardous operation.
4

Other Issues

4.1

Permanganate Fading Time

One of the tests that is performed on refined methanol is the permanganate
fading time test – the aim of this test is to determine whether oxygenates (such
as acetones and ketones) or unsaturated hydrocarbons are present. The test
consists of adding permanganate to a sample of refined methanol and monitoring
a change in the color of the sample. To pass the test, the color of the sample
must be ‘better’ than a set standard.
The first potential issue is if the sample contains low levels of hydrocarbons since
this will affect the fading rate and therefore cause potential failure. The second
problem is that matching the color between the sample and standard does
include a certain amount of subjectivity.
4.2

Jet Flooding

Jet flooding occurs when the vapor rate is high enough to fluidize liquid on the
tray. This causes liquid droplet to impact on the tray above and prevent some of
the liquid from this tray flowing down. This leads to a loss of separation
efficiency with the consequences of increasing ethanol levels in the product
methanol. If this occurs below the feed point/fusel draw off, there consequences
are not too great since the methanol-water separation that occurs at this point is
relatively easy.
The following figure illustrates jet flooding,



4.2.1
Asia-Pacific Producer Experience
Methanol is transferred from one site to another, for processing in the D2
distillation columns. This transfer was through a long pipeline between the two
sites.
The D2 topping column at Site A was found to be suffering from sudden and
unexplained pressure rises which occurred at high rates. During a shut down,
the internals of the Topping Column were inspected, and around the feed point a
3-5 mm thick deposit was found on trays. This was found to iron which had been
corroded in the transfer pipe line between the two sites.
4.3

Weeping

Weeping can be considered to be the opposite of jet flooding in that in this case,
the velocity of the gas passing through the holes in the tray is too low to prevent
back flow through the tray. The following figure illustrates this problem,



The effect of this is to reduce the effective tray efficiency and thereby increase
the contaminants in the product stream (ethanol in the methanol) and the
bottoms stream (methanol in the water).
4.4

Foaming

Foaming occurs when bubbles rise to the surface of the liquid on a tray and do
not coalesce. The simplest solution is to inject a foam inhibitor but care must be
taken that this does not affect the product quality.
The effect of foaming on the column is to reduce separation efficiency thereby
causing contamination of the top and bottom products.
4.5

Downcomer Flooding

Downcomers can become flooded if the flow of liquid through the column is too
high and there is therefore insufficient space for the liquid to flow across the trays
and down the down comer. The following figure illustrates this,



The general effect is that the will be a high pressure drop across the column and
again the separation efficiency will be reduced.



5
Troubleshooting
The following table should be used as a guide to trouble shooting some common problems that occur in methanol
distillation column.
Effect

Solution

Catalyst at end of life.

Change catalyst.
Commission secondary methanol purge.
Taken methanol from below
pasteurization section.
Raise secondary exit temperature.
Commission secondary methanol purge.
Increase water addition.
Check for tray damage via DPI meter.
Increase reboil load
Increase caustic addition rate.

Problem
Acetone in Refining
Column

Secondary condenser exit temperature too low.
Too little water addition.
Poor separation in Topping Column.
Corrosion products in
Refining Column Bottoms
Inert Gases in Refining
Column

Too little caustic addition.

High Methanol in Refining
Column Bottoms

Too little separation.

Poor separation in Topping Column.

Increase topping column reboil duty.
Taken methanol from below
pasteurization section.
Start manual purging at top of refining
column.
Too low a reboil duty.
Increase fusel oil draw.
Increase product draw.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding
Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment
Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology Ammonia Catalyst / Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology
in the Refining & Petrochemical Industries


High Ethanol in Refining
Column Product


Water in Refining Column
Product


C10-C13 in Product

Too little separation

Failed PFT Test

Unsaturated hydrocarbons.

Saturated/Unsaturated Aldehydes

Carbon Dioxide in top of refining column.

Failed Smell Test

High Amines level.

Reduce product take off.
Increase reboil duty.
Increase fusel oil draw.
Change draw position.
Commission sample line above feed
point.
Increase Topping Column reboiler load.
Check Topping column tray performance
by DPI meter.
Increase water addition.
Commission purge from top of topping
column.
Increase Topping Column reboiler load.
Check Topping column tray performance
by DPI meter.
Purge top of refining column.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding
Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment
Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology Ammonia Catalyst / Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology
in the Refining & Petrochemical Industries

6

Conclusions

To conclude, a number of potential issues have been highlighted that can affect
both the hardware (equipment) in a methanol distillation system.


Things That Can Go Wrong in a Methanol Distillation Column

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