things that can go wrong in a methanol distillation column

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THINGS THAT CAN GO WRONG IN A METHANOL DISTILLATION COLUMN

Things That Can Go Wrong in a Methanol Distillation Column

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VULCAN Systems Process Catalyst Technology Things That Can Go Wrong in a Methanol Distillation Column Overview

1 Contents 1 Contents ......................................................................................................... 2 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




6 Troubleshooting ............................................................................................ 20 7 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.

Problem Effect Solution

Acetone in Refining Column

Catalyst at end of life. Secondary condenser exit temperature too low. Too little water addition. Poor separation in Topping Column.

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

Corrosion products in Refining Column Bottoms

Too little caustic addition. Increase caustic addition rate.

Inert Gases in Refining Column

Poor separation in Topping Column. Increase topping column reboil duty. Taken methanol from below pasteurization section. Start manual purging at top of refining column. Check for tray damage via DPI meter.

High Methanol in Refining Column Bottoms

Too little separation. Check for tray damage via DPI meter. Too low a reboil duty. Increase fusel oil draw. Increase product draw.




High Ethanol in Refining Column Product

Too little separation. Reduce product take off. Check for tray damage via DPI meter. Increase reboil duty.

Water in Refining Column Product

Too little separation. Reduce product take off. Check for tray damage via DPI meter. Increase reboil duty.

C10-C13 in Product Too little separation Reduce product take off. Check for tray damage via DPI meter. Increase reboil duty.

Failed PFT Test Unsaturated hydrocarbons. Saturated/Unsaturated Aldehydes Carbon Dioxide in top of refining column.

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.

Failed Smell Test High Amines level. Purge top of refining column.



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

Technology

column distillation

distillation systems

column system

particular purpose

contents contents

tma removal

mek removal

tray weeping