Glycol dehydration is the most common dehydration process used to meet pipeline sales specifications and field requirements (gas lift, fuel, etc.). Triethylene glycol (TEG) is the most common glycol used in these absorption systems. In this Tip of The Month (TOTM), the effect of stripping gas rate on the regenerated lean TEG concentration for several operation conditions will be studied.
Chapter 18 of “Gas Conditioning and Processing” [1] presents the process flow diagram and the basics of glycol units. A key parameter in sizing the TEG dehydration unit is the water dew point temperature of dry gas leaving the contactor tower. Once the dry gas water dew point temperature and contactor pressure are specified, water content charts similar to Figure 1 in reference [2] can be used to estimate the water content of lean sweet dry gas. The required lean TEG concentration is thermodynamically related to the dry gas water content which influences the operating (OPEX) and capital (CAPEX) costs. The lower the dry gas water content required, the higher the lean TEG concentration must be. This parameter sets the lean TEG concentration entering the top of the contactor and the required number of trays (or height of packing) in the contactor tower.
The rich TEG solution is normally regenerated at low pressure and high temperature. Maximum concentrations achievable in an atmospheric regenerator operating at a decomposition temperature of 206°C (404°F) is 98.7 weight percent. The corresponding dry gas water dew point temperature for this lean TEG weight percent and contactor temperature of 38°C (100°F) is -8°C (18°F).
If the lean glycol concentration required at the absorber to meet the dew point specification is higher than the above maximum concentrations, then some method of further increasing the glycol concentration at the regenerator must be incorporated in the unit. Virtually all of these methods involve lowering the partial pressure of water in the glycol solution either by pulling a vacuum on the regenerator or by introducing stripping gas into the regenerator.
A typical stripping gas system is shown in Figure 1 [1]. Any inert gas, or a portion of the gas being dehydrated, or the exhaust from a gas-powered glycol pump (if used), is suitable. The quantity of gas required is small. The stripping gas may be introduced directly into the reboiler or into a packed “stripping column” between the reboiler and surge tank. In theory, adding gas to a packed unit between the reboiler and surge tank is superior and will result in lower stripping gas rates. If introduced directly to the reboiler, it is common to use a distributor pipe along the bottom of the reboiler. Other stripping gas alternatives can be found in reference [1].
Most regenerators will contain more than 1 equilibrium stage, particularly if a stripping column is installed between the reboiler and surge tank. Stripping gas rates seldom exceed 75 std m3/std m3 TEG [10 scf/sgal] unless lean TEG concentrations in excess of 99.99 weight percent are required. If these concentrations are required, an alternate design such as DRIZO® or a mole sieve adsorption system should also be considered [1].
In this Tip of The Month (TOTM) we study the required stripping gas rate as a function of the lean TEG weight percent, reboiler temperature, number of theoretical trays in the stripping section (NS) and number of theoretical trays in the still (regenerator) column (NR). By performing rigorous computer simulation of TEG regeneration, we have prepared charts for quick determination of stripping gas rates needed for facilities type calculations.
Figure 1. Typical TEG regeneration column with stripping gas [1]
Computer Simulation Results:
In order to study the impact of stripping gas rate on the lean TEG weight percent, we simulated the TEG regeneration process diagram shown in Figure 1. To undertake this study, we used ProMax [3] software with its Soave-Redlich-Kwong (SRK) [4] equation of state (EOS). The corresponding process flow diagram for computer simulation is presented in Figure 2.
Figure 2. Process flow diagram showing sample results using ProMax [3]
Figure 2 also shows sample calculation results for a case study. As shown in Figure 2, the rich TEG solution contained 97.5 weight percent TEG entering the still column at 150°C (302°F) and 104 kPaa (15.1 Psia). The reboiler temperature was set at 204°C (400°F) and boil-up ratio of 0.1 (molar bases). Two theoretical trays in the still column (NR = 2) and two theoretical trays (NS = 2) in the gas striping section were specified. The striping gas enters the bottom of the gas stripping section at 150°C (302°F) and 125 kPaa (18.1 Psia). For the stripping gas (methane was used) rate of 5 std m3/h (175.6 scf/hr), the regenerated lean solution contains 99.65 weight percent TEG and the ratio of stripping gas to lean TEG liquid volume rates is 5.76 std m3 of gas/std m3 of lean TEG solution (0.77 scf/sgal). If stripping gas was spurged directly into the reboiler (NS = 0, no gas stripping section), and everything else remaining the same, the regenerated solution contains 99.2 weight percent TEG and the ratio of stripping gas to lean TEG liquid volume rates is 5.73 std m3 of gas/std m3 of lean TEG solution (0.76 scf/sgal). For the above cases, we increased the number of theoretical trays in the still column from 2 to 3 (NR = 3) and the lean TEG concentration remained almost the same. We also varied the concentration of rich TEG solution from 90 to 98 weight percent, but no appreciable change in the lean TEG concentration was observed for the same stripping gas rate.
Using similar set up, several simulations were performed for a range of stripping gas rates, for NR=3, NS=0, 1, and two were performed at reboiler temperatures of 204, 193, and 182°C (400, 380, and 360°F). The results of these simulation runs are presented in Figures 3 to 5. All of these diagrams are replotted in Figure 6.
Fig 3. Effect of lean TEG weight %, reboiler temperature and number of ideal trays in stripping column
Fig 4. Effect of lean TEG weight %, reboiler temperature and number of ideal trays in stripping column
Fig 5. Effect of lean TEG weight %, reboiler temperature and number of ideal trays in stripping column
Conclusions:
In this TOTM, the effect of stripping gas rate on the regenerated lean TEG concentration for several operation conditions was studied. A series of charts for quick determination of the required stripping gas rate to achieve a desired level of lean TEG concentration was prepared and presented in Figures 3 through 6. These charts are based on the rigorous calculations performed by computer simulations and can be used for facilities type calculations for evaluation and trouble shooting of an operating TEG dehydration unit. In addition, the following observations were made:
- The required stripping gas is independent of rich TEG concentration (for 90 to 98 TEG weight percent)
- As the number of theoretical trays in the stripping column (NS) increased from 0 to 2, the required striping gas rate decreased.
- Increasing the number of theoretical trays in the still column (NR) from 2 to 3 has no appreciable effect on the stripping gas requirement.
- Increasing the reboiler temperature from 182 to 204 ˚C (360 to 400 ˚F), decreases the required stripping gas rate.
Fig 6. Effect of lean TEG weight %, reboiler temperature and number of ideal trays in stripping column
To learn more, we suggest attending our G40 (Process/Facility Fundamentals), G4 (Gas Conditioning and Processing), G5 (Gas Conditioning and Processing-Special), and PF81 (CO2 Surface Facilities), PF4 (Oil Production and Processing Facilities) courses.
John M. Campbell Consulting (JMCC) offers consulting expertise on this subject and many others. For more information about the services JMCC provides, visit our website at www.jmcampbellconsulting.com, or email us at consulting@jmcampbell.com.
By: Dr. Mahmood Moshfeghian
References:
- Campbell, J. M., “Gas Conditioning and Processing”, Vol. 2, The Equipment Module, 8th Ed., Second Printing, J. M. Campbell and Company, Norman, Oklahoma, 2002.
- Campbell, J. M., “Gas Conditioning and Processing”, Vol. 1, The Basic Principles, 8th Ed., Second Printing, J. M. Campbell and Company, Norman, Oklahoma, 2002.
- ProMax 3.2, Bryan Research and Engineering, Inc., Bryan, Texas, 2012.
- Soave, G., Chem. Eng. Sci. Vol. 27, No. 6, p. 1197, 1972.
A very interesting article
Some years ago whilst working for another company, we significantly upgraded the performance of two existing TEG units by replacing the pall rings in the gas stripping section with Sulzer structured packing. The modification was relatively cheap, circa $20k per unit, and worked extremely well. As I recall the structured packing was about 2ft deep and we were able to achieve 99.95% by wt TEG at about 2 scf/gal with a reboiler temp of 400F
Marty: Thanks for sharing your experience. Structure packing has become very popular.
Dear Dr. Mahmood Moshfeghian,
– Is that possible if we can use this system to bring down water content of sales gas from 3000 mG/SCM to 50mG/SCM?
– During this treating process, does that HC dewpoint of sales gas decrease?
If I understand this correctly, the gas contains 3000 kg/10^6 SCM which is too much for typical feed gas temperature of about 35 C and 7MPa to a TEG contactor unit. You may reduce the water content by compression and cooling to about 900 kg/10^6 SCM at 35 C and 7 MPa then using TEG dehydration to lower the water content to 50 mG/SCM or 50kg/10^6 SCM which is equivalent to a water dew point of about -11 C at 7MPa.
Here is the response to the second comment:
No, it does not. It only lowers the water dew point.
For achieving lower HC dew point can TEG dehy system work. or we need DPCU (Refrig system)
Ravi Sharma
30/05/2013
TEG dehydration lowers only the water dew point which may be followed by a turbo-expander for lowering for HC dew point control or NGL recovery. If mechanical refrigeration with MEG injection is used both water dew point and HC dew point temperatures are lowered, simultaneously
Dear prof.
the amount of stripping gas does have any effect on pH of TEG?
for example with increasing of stripping gas pH decreases or not?
thanks
kharg island d3
The stripping gas is normally sweet gas (No CO2 or H2S) therefore it does not affect the pH.
It would be very interesting for a follow-up article on how backpressure affects the lean TEG regenration. This is particularly so with the trend to collect offgas to vapour recovery system, whereby the regeneration process is exposed to pressure fluctuations.
Good topic, it will be on my list to do. Thanks.
Dear Mr. Ravi Sharma,
I am wondering if people could use the Dew Point Control Unit (Refrig system)to control the water content also. With very low temperature, most of water in gas condense and separate into aqueous phase. In order to prevent hydrate formation at low temp piping, people inject EGlycol / Me-OH.
On over the word, is there any system operating as same as above or it always separate into two systems: HC Dewpoint Control Unit (Refrigeration system) and Water removal system (Glycol Contactor / Absorber …) ?
By using mechanical refrigeration (with MEG injection to inhibit hydrate formation), the water dew point and hydrocarbon dew point temperatures will be the same assuming there is no liquid carry over.
Dear Dr.,
There was a trace of doubt only. Now you have made it clear. Thanks a lot,
Ravi
sir, i was going through ur article “Quick determination of methanol injection rate for natural gas hydrates…”
i have a query..
does methanol injection qty beyond certain level, for a lean gas system(99.2 mol% methane)with water content of 2 bbl/mmscf have no significant effect on depressing the hydrate condition
The quantity of required methanol injection depends on the amount of water condensed and the degree of hydrate depression (Hydrate formation temperature minus the cold temperature). Any additional methanol injection is wasted.
For more detail, please see Chapter 6, Vol. 1, Gas Conditioning and Processing by John M. Campbell. You may also refer to March, June and July 2006 Tip of the months.
Sir,
In my TEG dehydration unit the stripping gas contains 5 mol% co2.What might be its effect on dehydration & TEG regeneration ? What is the impact of pH on these parameters ?
Dear Dr. Moshfeghian:
This is a good article on the merits and application of stripping gas on TEG. On June 1, 2013, Ting Sze Tiong commented that it would be very interesting for a follow-up article on how back pressure on the TEG reboiler affects the lean TEG regeneration, particularly with the trend to collect offgas to a vapor recovery system, in the case where the regeneration process is subjected to pressure fluctuations.
This is a very important issue in actual TEG unit operations and continues to increase in importance because of environmental pressures imposing more equipment on the tail end of the TEG Stripper overheads tail gas.
You replied that it would be a good topic and you would add it on your “to do” list.
I believe this article is your response to that issue. However it does not address the effect of pressures higher than atmospheric on the stripping gas application on the lean TEG regeneration to achieve 99.9+ purity. Can you please comment on this?
Art:
Thanks for your interest.
Yes, I remember and have kept my promise. The September Tip of the Month (TOTM) is:
“High Pressure Regeneration of TEG with Stripping Gas” which covers the pros of operating reboiler at high pressure. Hopefully it meets your expectation.
Dr. Moshfeghian:
Thank you for your timely response. I certainly look forward to reading your forthcoming article on “High Pressure Regeneration of TEG with Stripping Gas”. This is a much-needed discussion of the topic as it applies to our industry’s reply to today’s environmental concerns. Keep up the good work.
Dr. Moshfeghian,
Kindly explain how to measure pH of TEG sample briefly ?
pH meter stuck and no response when we put a TEG sample direct.
I am interested in a rule of thumb that can give a budget cost estimate – capital cost (procurement and installation costs) of a typical TEG Absorber in say $/mmsfd. Assume that it is required to achieve a max concentration 7lb H20/mmscfd of gas and the column is required to operate at 85barg.
This is a response from my colleagues:
A real ballpark estimate for an installed onshore glycol dehy package (sweet gas) in the US would be ~ $12,000/MMSCFD based on a 100 MMSCFD unit, ie. ~ $1.2 MM installed. Smaller units would be somewhat more expensive and larger units less expensive on a $/MMSCFD basis. Below is an approximate correlation.
Installed cost, $MM = $1.2 (Q/100)^0.6
Does the stripping gas in the work include any water (e.g. 4#/MM or 7#/MM)to match the dry gas produced?
Dear,
We are using HC gas as a stripping gas.
I want know can we use N2 as a Stripping Gas in glycol reboiler TEG Dehydration system. If yes any affect will take place or what are the problem we can face.
This reply is from a colleague.
“Sure you can use nitrogen.
The main drawback is cost – nitrogen generation is not cheap. I guess the other issue relates to what they are doing with the still overhead vapor. Using N2 for stripping gas will make it harder to condense BTEX if they need to, but this is true for methane as well. If the still overheads are tied into the flare, might be an issue with the heating value of the flared gas/maintaining a stable flame, depending on how much N2 vs other gas sources going to flare.”
We are facing heavy los of TEG in our regeration system(from still column). Is there anyone who could help us in reducing the TEG loss?
please I want to know the effects of adding pentane to fuel gas for glycol regeneration. thanks
Dear Dr. Moshfeghian,
I am a Chemical Engineering student and Iam trying to simulate the TEG regeneration section in Aspen Plus with a stripping gas.
I am currently unable to find a module in Aspen plus which would allow me to simulate the still column with a provision to pass an inert gas through it.
Could you please provide some tips regarding the same
Hi Dr.
I am a chemical engineering student, my research project is on natural gas dehydration. Please I need your help in a unique area that I could work on. I am currently thinking in line of using the stripping method of TEG regeneration.
Right now it looks like Drupal is the top blogging platform available right now. (from what I’ve read) Is that what you are using on your blog?
What affect does striping gas have on the overhead temperature and the reflux section of the still column ?