Gas Dehydration - Chapter 5 - Part 2

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Gas Dehydration - Chapter 5 - Part 2

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Fundamentals of Oil and Gas Processing Book
Basics of Gas Field Processing Book
Prediction and Inhibition of Gas Hydrates Book
Basics of Corrosion in Oil and Gas Industry Book

https://www.amazon.com/Yasser-Kassem/e/ ... scns_share
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Still Column Diameter Size
Diameter size is based on the required diameter at the base of the still, calculated by vapor and liquid loading conditions at that point. Vapor load consists of the water vapor (steam) and stripping gas flowing up through the still. Liquid load consists of the rich glycol stream and reflux flowing downward through the still column. The diameter required for the still is based on the glycol circulation rate (Figure 5-44).

Still Column Packing
1 to 3 theoretical trays (4 to 12 feet) is sufficient for most TEG stripping still requirements.
304 SS packing is normally used.

Amount of Stripping Gas
The amount of stripping gas required to reconcentrate the glycol to a high purity will range from 2 to 10 ft.3/gal TEG circulated (Figure 5-45).

5.7.15 Filters
Microfiber : Sized to remove 5 micron solids.
Activated Charcoal (Carbon) :
Used to remove chemical impurities. Sized for full flow with 10 gpm streams.
Sized for 10 to 25% side streams on large units.

Image
Fig. 5-42 Still column with wet glycol entering above the ceramic saddle packing.

Image
Fig. 5-43 Still column with wet glycol entering below the stainless steel pall rings.
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Fig. 5-44 Determination of stripping still column diameter.
Image
Fig. 5-45 Amount of stripping gas required to reconcentrate glycol to high purity.


5.8 Calculation Examples for Glycol Dehydration
Example 5-1
30 MMscf/d of a 0.65 sp gr natural gas enters a TEG contactor at 600 psia and 100°F. Outlet water content specification is 7 lb H2O/MMscf and the TEG circulation rate is 3 gal TEG/lb H2O. Estimate the contactor diameter and number of bubble cap trays or height of structured packing required to meet this requirement. Assume z = 0.92.
Gas mol. Wt. = 0.65 X 28.96 = 18.8
Gas density, ρg =P (MW)/ RTZ
= (600) (18.8)/ (10.73) (560) (0.92) = 2.0 lb/ft3
Liquid density “from table 4-4”, ρL = 1.119* (62.4) = 69.9 lb/ft3
From fig. 4-8. Water inlet (Win)= 90 lb/MMscf

Solutions Steps:
1. From Fig. 4-8 (McKetta and Wehe chart) is equivalent to a water content of 7 lb H2O/MMscf @ 600 psia)
2- Estimate required TEG concentration from Fig. 5-10, H2O Dewpoint = 24°F,
@ T = 100°F, lean TEG concentration ≈ 98.8 wt%
3. Estimate number of theoretical stages.
Calculate water removal efficiency
(Win −Wout) / Win = (90 −7) /90 = 0.922
From Fig. 5-37 (N = 1.5) at 3 gallon TEG/lb H2O and 99.0 wt% TEG
(Win – Wout)/Win = 0.885
From Fig. 5-38 (N = 2.0) at 3 gallon TEG/lb H2O and
99.0 wt% TEG
(Win – Wout)/Win = 0.925 use N = 2.0
Using equations 5-6, 5-7, and 5-8.
G = C [ρV (ρL - ρV)]0.5 Eq. 5-6
2 theoretical stages ≅ 8 bubble cap trays @ 24 inch tray Spacing Eq. 5-7
2 theoretical stages ≅10 ft of structured packing Eq. 5-8
“C” from table 5-3

4. Size the contactor Bubble caps, 24 inch tray spacing:
G = 576 [2.0 (69.9 −2.0]0.5 = 6712 lb /ft2.hr
Mass flow, m =

M = 62000 lb/hr
Area of flow (A) = m “mass flow” /G “mass velocity” = 62000 / 6712 = 9.2 ft2
Since, area ft2 = π D2 /4
D2 = 9.2 X 4 / 3.14 = 11.7
D = 3.4 ft.

Alternatively
using equation 5-5 ( drag coefficient CD =0.85 and dm = 150 micron)
d2 = 5040 [(T0ZQg)/P] [(ρg / ρL – ρg)(CD/dm)]0.5 Eq. 5-5
d2 = 5040 [(560 x 0.92 x 30 x 106)/600] [(2 / 67.9)(0.85/150)]0.5
minimum diameter D = 3.5 ft.

5. Size For Structured packing: ( “C” from table 5-3 “ use 1260 as an average”)
G = 1260 [2.0 (69.9 −2.0]0.5 = 14683 lb /(ft2.hr)
Mass flow, m = (30 X106) (0.65 X 28.96) / (379.5) (24) = 62000 lb/hr
Area A = m/G = 62000 / 14683 = 4.22 ft2
Area ft2 = π D2 /4
D2 = 4.22 X 4 / 3.14 = 5.38
D = 2.3 ft

Example 5-2
Determine reboiler duty for conditions in the previous example. Assume the rich TEG temperature entering the regenerator is 300°F and the reboiler temperature is 400°F.
Glycol Reboiler Duty: Basis 1 gal. TEG. ( assume 25% reflux ratio)

Solution:
Using equation 5-12, and 5-13
Qs = m Cp Δt Eq. 5-13
- (use 0.67 as an average for the range of 300-400 0F)
Qs = (9.3 lb/gal.) X (0.67 Btu/lb.°F) X (400°F − 300°F) = 623 Btu /gal.
Using eq. 5-14
Qv = (ΔHvap) (ΔW) Eq. 5-14
Qv = (970 Btu/lb H2O) X (1 lb H2O/ 3 gal. TEG) = 323 Btu / gal.
Using eq. 5-15
Qc = % Reflux Ratio X Qv /100 Eq. 5-15
Qc = (25) (Qv)/100 = 81 Btu /gal.
From eq. 5-12
Total Duty Including 10% Heat Loss:
Qr (total regeneration heat duty) = (623 + 323 + 81) (1.1) = 1130 Btu /gal.
Total Duty Based on 30 MMscfd of Gas:
Q = (1130 Btu/gal) (3 gal./lb) (30 MMscfd/24) ((90-7) lb/MMscf) = 350,000 Btu/hr

Example 5-3
Given: Gas Qg = 98 MMscfd at 0.67 SG saturated with water at 1000 psig and 100 0F
Dehydrate to = 7 lb/MMscf
Use triethylene glycol, No stripping gas is available
98.5% TEG concentration
CD (contactor) = 0.852
Z = 0.865
Determine:
1. Calculate contactor diameter
2. Determine glycol circulation rate.
3. Size the still column

Solution:
1. Calculate contactor diameter
d2 = 5040 [(T0ZQg)/P] [(ρg / ρL – ρg)(CD/dm)]0.5 Eq. 5-5
dm = 125 microns, T = 570 0R
P = 1015 psia, Qg = 98 MMscfd
ρL = 70 lb/ft.3
ρg = (0.67 X 1015)/(560 X 0.865) = 3.79 lb/ft3
d2 = 5040 [(560 X 0.865 X 98)/1015] [(3.79/70-3.79)(0.852/125)]0.5
d = 68.2 in.
Use 72.00 ID contactor (standard off-the-shelf)

2. Determine glycol circulation rate and reboiler duty
Wi = 63 lb/MMscf (from McKetta-Wehe) “saturated water content”
W0 = 7 lb/MMscf (spec)
ΔW = Wi - W0 = 63 - 7 = 56 lb/MMscf
ΔW/Wi = 56/63 = 0.889
Using n = 2 (i.e., 8 actual trays) and glycol purity of 98.5% read from Figure 5-38 the glycol circulation rate of about 2.7 gal TEG/lb H20. Use 3.0 gal/lb for design.
Using equation
L = (ΔW/Wi) Wi Qg/24 5-10
L = (3.0 gal/ Lb) (56 lb/MMscf)(98 MMscf/day)(day/24 hr)(hr/60min)
= 11.4 gpm TEG
= 862 Btu/gal (From table 5-4)
= (862 Btu/gal) (11.4 gal/min) (60 min/hr)
= 590,000 btu/hr
To allow for start-up heat loads, increase heat duty by 10% and then select a standard off-the-shelf fire tube.
Thus, select a 750 MMBtu/hr.

3. Design of still column:
Use 12-foot still column (standard packed arrangement)
dm = 125 micron
T = 300 0F = 760 0R
P = 1 psig = 15.7 psia
Qg = (10 scf/gal) (11 gal/min) (60 min/hr)(24 hr/day)
= 0.16 MMscfd
Z = 1.0
Since ρg = Gas density, lb/ft.3 = 2.7 (SP/TZ) or = ρg= 0.093 ((MW)P)/TZ lb/ft3 (Eq. 1-19)
ρg = 2.7 (0.62 X 15.7)/ (760 X 1.0 )
= 0.035 lb/ft.3
ρL = 62.4 lb/ft.3
CD = 14.2 (given)
d2 = 5040 [(760 X 1 X 0.16)/15.7] [(0.035/62.4-0.035)(14.2/125)]0.5
d = 17.7 in.
Use 18 inch OD x 12 feet long still.


5.9 Glycol Unit Operation
5.9.1 Start up
Prior to the initial start up of a new plant, the vessels and lines should be thoroughly washed out with water to remove debris and corrosion products that accumulated during construction. After the system has been cleaned, start up is accomplished in three phases:
Establish glycol circulation throughout the plant.
Apply heat to the reboiler and bring it up to operating temperature.
Open the wet gas stream to the contactor and begin dehydrating the gas.
In order to circulate glycol throughout the system, it will be necessary to pressurize the vessels in the system. Pressuring can be done with wet gas or dry gas. The contactor pressure should be raised to at least 150 psig (10 bars) and the flash tank pressure should be raised to at least 45 psig (3 bars). When the vessels have been pressured, start up procedure is:
Fill the reboiler and surge tank with fresh glycol solution Also add to the flash tank.
Pressure up the contactor column by very slowly opening the gas inlet valve.
Prime and start glycol pump.
When liquid appears at the bottom of the contactor put the bottom level controller in service so the glycol will flow to the flash tank.
Put the flash tank level controller in service when liquid appears in the bottom, so that liquid will flow to the stripper.
Keep surge tank level half full by adding glycol when needed.
When desired circulation rate is established, light the reboiler or put the heat source in service and slowly bring reboiler temperature up to 250°F (121°C). Leave temperature at 250°F (121°C) until all water has been boiled out of glycol.

5.9.2 Routing Operation
Routing operating checks include the following:
Check levels in each vessel and reset level controller as necessary.
Check the pressure drop across the filter and replace the elements as required.
Check the temperature of the lean glycol out of the glycol exchanger to see that the proper transfer rate is occurring in the exchanger.
Check the flow of glycol to the contactor and of stripping has to the reboiler.
Check the pressure of the flash tank to see if it is at proper level.
If water or air is used to cool the glycol prior to its entry into the contactor, check the glycol temperature in order to ensure that it is about 5°C to 7°C (10°F to 15°F) above the inlet gas temperature. Adjust the flow of air or water through the cooler as required.
Check the water content of the outlet gas to see that it is below the design limit.

5.9.3 Shut Down
This procedure is used to shut down a glycol plant:
Block gas to contactor column,
Shut off heat (leave pump running),
When unit cools to safe temperature, less than 200°F, shut off pump.
Drain glycol, if necessary.
5.10 Glycol Maintenance and Care
5.10.1 Preventive Maintenance
Scheduled preventative maintenance reduces glycol losses and operating problems such as foaming, system plugging, corrosion, pump failures.
It also minimizes system down time and maximizes system operation efficiency.

Five Steps to a Successful Preventive Maintenance Program
Record-Keeping
Accurate records can be used to determine the system efficiency and to pinpoint operating problems. Records of prior and existing conditions including dew points, glycol usage, and repairs help establish the system profile. Once the system profile is defined it becomes easier to identify unusual system characteristics that may indicate potential problems.
Mechanical Maintenance
Daily physical inspections are necessary to insure that the system is running properly. Any trouble encountered should be dealt with immediately, thus preventing the problem from escalating.
Glycol Care
Regular chemical analysis (every one or two months) of the glycol provides detailed information on the internal operation of the unit. Many process-related problems can be diagnosed well in advance of mechanical failure. Chemical problems can be diagnosed and corrective action taken before they become costly and detrimental to unit performance.
Corrosion Control
Corrosion is a frequent problem in glycol dehydration systems. If unchecked the damage can be extensive. All units should have provisions for corrosion control.
Communication
Lines of communication between field and technical assistance or office personnel are critical to the smooth operation of any system. Office personnel (production supervisors, facility engineers, purchasing agents) must be kept informed of daily operations and any problems that may arise.
Field personnel must be made aware of technical information that may improve their operations. Training for field operators allows the operator to better maintain the equipment.

5.10.1.1 Record-Keeping
Records necessary to establish a system profile include:
Design information including vessel specifications, equipment drawings, and P&IDs
Filter element or media replacement—type and frequency
Glycol usage—gallons/month
Chemical additives—type and amount
Gas production and flow rate charts—peak, average, and low periods
Outlet gas dew point/water content (lbs/Mscfd)
Mechanical inspections—type, magnitude, frequency, results

Records necessary to establish a system profile include:
Glycol analysis—format, frequency, recommendations, results
Corrosion coupon results—mills per year (MPY), frequency
Materials and labor relating to system repairs— operating costs
With the aforementioned information, a good system profile can be drawn of a specific system.
Updating these records will show any gradual changes in a unit’s system profile and may alert you to a potential or developing problem.
5.10.1.2 Mechanical Maintenance
The following things should be done so as to keep the unit operating properly and to prevent operational problems:
1. Insure that instruments and controls are in good working condition (thermometers and pressure gauges, etc.). Use a test thermometer on the reconcentrator to insure proper reconcentrator heat.
2. Insure glycol filter elements are changed according to average expected need basis:
Microfiber filters should be changed monthly.
Carbon filters should be changed monthly (small cartridge filters) to every six months (large bulk units). Glycol analysis helps determine the frequency.
An upset or sudden change in the operating conditions may foul the filters faster than the preventive maintenance program anticipates. Make sure filter differential pressure is below 15 psi.
3. Look for glycol leaks on and around the glycol skid. Most leaks can be stopped by tightening a union, valve stem packing, or pump rod packing. After the leak has been repaired, clean the affected area so it is easier to notice new leaks.
4. Check the glycol level, at least twice a day, and add glycol as necessary. Maintain a written report of glycol added. This allows operations to detect excessive losses of glycol and take corrective action faster.
5. Insure unit performance by taking a dew point measurement daily.
6. Clean the glycol strainers monthly to prevent accumulation of trash, which can cause the glycol pump to fail.
7. Check the glycol circulation rate daily. Any time the gas flow rate changes or when a drastic change in gas pressure or temperature is experienced, the glycol flow rate should be recalculated and the pumps set accordingly. On multiple pump installations, switch the pumps weekly, thus insuring pump operation when necessary.
8. On direct fired fire tubes, on weekly basis; sight down the flame direction, to insure it is not touching the fire tube for fire tube blisters or hot spots.
9. Cycle the main burner manually to be sure the fuel gas valve works and the pilot light stays lit. Check the fuel gas scrubber pot for fluid build-up that may hinder burner operation.

5.10.1.3 Glycol Care
Operating and corrosion problems occur when the circulating glycol gets dirty. Some contaminated glycol problems can be noticed easily and corrective action taken.
A small glycol sample should be taken daily from the surge tank or dry glycol suction header to the pump. Check closely for fine black particles settling out of the sample, which may be corrosion by-products and indicate an internal corrosion problem.
Uncontaminated glycol is colored like pure kitchen oil. Glycol gets darker and darker due to hydrocarbon contaminates, and/or corrosion products.
Smell the sample, If the smell is sweet and aromatic (similar to rotten bananas) it may be thermally decomposed. If the sample is viscous and black it is probably contaminated with hydrocarbon or well-treating chemicals. If the hydrocarbon contamination is great enough, the sample will separate into two liquids or interphase. Every one or two months send a sample of both the rich and lean glycol to a laboratory for complete analysis. This type of analysis will provide a detailed description of unit performance and glycol condition.

5.10.1.4 Corrosion Control
Corrosion is a major cause of premature equipment failure. Corrosion can occur over the entire system, inside and out. The two most common areas of severe corrosion are:
Still column reflux coil, and vent/fill connection on the surge tank.
This is due to a high concentration of water vapor in the top of the still and the ready availability of oxygen in the air at the vent/fill cap. Three types of corrosion are usually found in glycol systems either individually or in combination with one another are:
Oxidation
Is the corrosion process involved the presence of oxygen molecules. Some metal loss is incurred and the resultant corrosion product is a scale-like residue called oxide, or rust.
Oxidization is characterized by rough, irregular, shallow pitting of the metal scaled over the rust.
Sour Corrosion
Hydrogen sulfide (H2S) are often found in produced natural gas. Glycols are very reactive with sulfur compounds, such as H2S. The resulting materials tend to polymerize (form larger molecules) that form a “gunk” that is very corrosive.
Corrosion in the presence of acid gases is characterized by deep, jagged pitting.
Sweet Corrosion
Water is found in glycol as vapor, free condensed water, or entrained water in glycol. Carbon dioxide (CO2) when dissolved in water forms carbonic acid. Since most produced natural gases contain some CO2, the presence of carbonic acid in glycol systems is very common. The corrosion resulting from carbonic acid is known as sweet corrosion. Sweet corrosion is characterized by deep, round, smooth pitting. Sometimes the pitting will cover a broad area, disguising the depth of the pit.

5.10.1.4.1 Corrosion Monitoring and Control Programs
Monitoring and control programs should include system monitoring through:
Corrosion coupons
Glycol analysis (pH and iron)
Three steps in combating corrosion in glycol systems are:
Use an effective corrosion inhibitor in both the liquid and vapor phases.
Use corrosion resistant alloys (CRA) in construction.
Keep the unit clean to prevent acid formation due to contamination.
Cathodic protection has been attempted but met with little success.
It is impractical to attempt to eliminate corrosion. The rate of corrosion can be slowed to a point that is almost negligible. The maximum acceptable corrosion rate is 6 mils per year (MPY).

Corrosion inhibitors work in several ways.
The two most applicable to glycol units are:
pH buffers which include:
Alkanolamines
MEA (Monoethanolamine)
TEA
The pH buffers fight corrosion by stabilizing the pH near neutral, thereby reducing corrosive environment. Alkanolamines are regenerable as is the glycol and thus can be retained in the system for lengthy time periods. However, they are thermally degraded at normal reconcentrator operating temperatures and if used frequently may leave harmful residues within the system.

Plating Inhibitors
Tallow diamine, unlike the inorganic amines, is an organic amine. It is grouped with the plating inhibitors even though it does not actually plate out on the vessel walls. It flashes out of the glycol at high temperatures. As it vaporizes it contacts the vapor spaces of the reconcentrator and forms a tenacious film over the exposed metal. These inhibitors are strictly liquid phase protection. They will plate out on the vessel walls forming a protective barrier between a corrosive environment and the metal. This film will eventually wear away and must be replenished occasionally to continue protection.
Since the plating inhibitors are all alkalies, some degree of pH buffering will be effected.
The pH buffering will not be as great as through the use of amines.
True plating inhibitors include:
Borax
NaCap (Sodium mercaptobenzothiazole)
Dipotassium phosphate

5.10.1.5 Communication
Communication is the easiest portion of an effective maintenance program and yet it is the most overlooked. Communication can be between management and labor, engineer and foreman, operators on opposite shifts, and office and field personnel. Lack of communication is the single most contributing factor to glycol system failure. Failure to communicate can cause confusion and evolve into major problems.
5.11 Glycol Operation Considerations
Operating and corrosion problems usually occur when the circulating glycol gets dirty.
To achieve a long, trouble-free life from the glycol, it is necessary to recognize these problems and know how to prevent them.
Some of the major areas are:
Oxidation
Thermal decomposition
pH control
Salt contamination
Hydrocarbons
Sludge
Foaming

5.11.1 Oxidation
Sometimes glycol will oxidize in the presence of oxygen and form corrosive acids. Oxygen enters the system with the incoming gas through:
Unblanketed storage tanks and sumps
Pump packing glands
To prevent oxidation:
Bulk storage tanks should be gas blanketed.
Use oxidation inhibitors.
Normally, a 50/50 blend of MEA and 33.5% hydrazine is inserted into the glycol between the absorber and the reconcentrator. A metering pump should preferably be used to give continuous, uniform injection.

5.11.2 Thermal Decomposition
Excessive heat, a result of the following conditions, will decompose glycol and form corrosive products:
High reconcentrator temperature above the glycol decomposition level
High heat-flux rate, sometimes used by a design engineer to keep the heater cost low
Localized overheating, caused by deposits of salts or tarry products on the reconcentrator fire tubes or by poor flame direction on the fire tubes
5.11.3 pH Control
pH is a measure of the acidity or alkalinity of a fluid, based on a scale of 0 to 14. pH values from 0–7 indicate the fluid is acidic. pH values from 7–14 indicate the fluid is alkaline.
To obtain a true reading, glycol samples should be diluted 50-50 with distilled water before pH tests are run. The pH meter should be calibrated occasionally to keep it accurate.
New glycol has a neutral pH of approximately 7. With usage the pH decreases and the glycol becomes acidic and corrosive unless pH neutralizers or buffers are used. Equipment corrosion rate increases rapidly with a decrease in the glycol pH. Acid created by glycol oxidization, thermal
decomposition products, or acid gases picked up from the gas stream are the most troublesome of corrosive contaminants. In addition, a low pH accelerates the decomposition of glycol.
Ideally, the glycol pH should be held at a level between 7.0 and 7.5. A value above 8.5 tends to make glycol foam and emulsify. A value below 6.0 corresponds to system contamination, corrosion, and/or oxidation.
Borax, ethanolamines (usually triethanolamine) or other alkaline neutralizers are used to control the pH. These neutralizers should be added slowly and continuously for the best results.
An overdose will usually precipitate a suspension of black sludge in the glycol. The sludge could settle and plug the glycol flow in any part of the circulating system.
Frequent filter element changes should be made while pH neutralizers are added.
The amount of neutralizer to be added and the frequency will vary from location to location. Normally, 1/4 lb of triethanolamine (TEA) per 100 gallons of glycol is sufficient to raise the pH level to a safe range.
When the glycol pH is extremely low, the required amount of neutralizer can be determined by titration. For best results, the lean rather than the rich glycol should be treated. It takes time for the neutralizer to mix thoroughly with all the glycol in the system. Several days are required before the pH is raised to a safe level. Each time that neutralizer is added, the pH of the glycol should be measured several times.

5.11.4 Salt Contamination and Deposits
In areas where large quantities of brine are produced, some salt contamination will occur. Salt deposits accelerate equipment corrosion. It also reduces heat transfer in the fire tubes. It alters specific gravity readings when a hydrometer is used to measure glycol water concentration. It cannot be removed with normal regeneration. A scrubber installed upstream of the glycol plant should be used to prevent salt carry-over from produced free water. The removal of salt from the glycol solution is then necessary.

5.11.5 Hydrocarbons
TEG will typically absorb about 1 scf of sweet natural gas per gallon of glycol at 1000 psia and 100°F. Solubilities will be considerably higher if the gas contains significant amounts of CO2 and H2S. Heavier paraffin hydrocarbons are essentially insoluble in TEG. Aromatic hydrocarbons, however, are very soluble in TEG, and significant amounts of aromatic hydrocarbons may be absorbed in the TEG at contactor conditions. This may present an environmental or safety hazard when they are discharged from the top of the regenerator.
Vapor-liquid equilibrium constants (K-values) for benzene, toluene, ethylbenzene, and o-xylene in TEG solutions, indicates that at typical contactor conditions approximately 10-30% of the aromatics in the gas stream may be absorbed in the TEG solution.
Aromatic absorption increases with increasing pressure and decreasing temperature. Aromatic absorption is directly related to TEG circulation rate. Higher circulation rates result in increased absorption. Aromatic absorption is essentially independent of the number of contacts in the absorber so one method of minimizing aromatic absorption is to use taller contactors and minimize TEG circulation rates.
Most of the aromatic components will be stripped from the TEG solution in the regenerator.
Flash tank sizing should be sufficient to degas the glycol solution and skim entrained liquid hydrocarbons, if necessary. A minimum retention time of 3-5 minutes is required for degassing. If liquid hydrocarbons are to be removed as well, retention times of 20-30 minutes may be required for adequate separation. Flash tank pressures are typically less than 75 psia.
Regenerator sizing requires establishing the reboiler duty and, when high TEG concentrations are required, providing sufficient stripping gas.
Liquid hydrocarbons, a result of carry-over with the incoming gas or condensation in the contactor, affects glycol by foaming, degradation, and losses. It must be removed with glycol/gas/condensate separator, or hydrocarbon liquid skimmer, or activated carbon beds

5.11.6 Sludge
Solid particles and tarry hydrocarbons (sludge) are suspended in the circulating glycol, and with time will settle out (It looks like asphalt or paraffin sludge). This results in the formation of black, sticky, abrasive gum that can cause trouble in pumps, valves and other equipment, usually when the glycol pH is low.

5.11.7 Foaming and defoamers
Excessive turbulence and high liquid-to-vapor contacting velocities usually cause the glycol to foam (this condition can be caused by mechanical or chemical problems).
The best way to prevent foaming is proper care of the glycol, such as:
Effective gas cleaning ahead of the glycol system
Good filtration of the circulating solution

Defoamers serve only as a temporary control until the conditions generating foam can be identified and removed. Success depends on when and how it is added. Most are inactivated within a few hours under high temperature and pressure, and thus their effectiveness is dissipated by the heat of the glycol solution. Thus, defoamers should be added continuously, a drop at a time, for best results.
The chemical feed pumps should meter the defoamer accurately, improve dispersion into the glycol, and may be activated automatically by differential pressure across the contactor.

5.12 Analysis and Control of Glycol
Analysis of glycol is essential to good plant operation. It helps pinpoint high glycol losses, foaming, corrosion, and other operating problems. Analyses enable operations personnel to evaluate plant performance and make operating changes to obtain maximum drying efficiency.

5.12.1 Visual Inspection
A glycol sample should first be visually inspected to identity some of the contaminants.
A finely divided black precipitate may indicate the presence of iron corrosion products.
A black, viscous solution may contain heavy, tarry hydrocarbons.
The characteristic odor of decomposed glycol (a sweet aromatic odor) usually indicates thermal degradation. A two-phase liquid sample usually indicates the glycol is heavily contaminated with hydrocarbons. The visual conclusion should next be supported by a chemical analysis.

5.12.2 Chemical Analysis
A complete glycol analysis of lean and rich samples, when properly interpreted, can provide a detailed picture of the workings of the dehydration unit and its process.
Glycol analysis should include tests to determine the following (table 5-5):
Test Lean Glycol Rich Glycol Allowable Range Ideal
pH (50/50) 6 to 8 7 to 7.5
Hydrocarbon
(% wt.) 0.1%
Water content
(% wt.) 2% lean - 6% rich
TSS (% wt.) 0.01%
Residue (% wt.) 4% 2%
Chlorides (mg/l) 1500 1000
Iron (mg/l) 50 35
Foam character:
Height (ml) 20 to 30 ml
Stability (sec) 15 to 5 sec
Specific gravity 1.118 to 1.126
Image
Image
Table 5-5.Glycol analysis.

5.12.3 Chemical Analysis Interpretation
5.12.3.1 pH
A pH below 6 generally corresponds with system contamination, corrosion, and/or oxidation.
Below 5.5 autoxidation occurs. Where Chemical decomposition of the glycol occurs within itself, and glycol has the tendency to continue to drop without outside influences.
The causes of low pH are:
Acid gases in the gas stream
Organic acids due to oxidation or thermal degradation
Excessive chlorides (salt) in the glycol
Well-treating chemicals entrained in the gas stream
Thermal decomposition of entrained liquid hydrocarbons in the gas stream and glycol
Oxidation of the glycol due to improper storage
While the causes of high pH are:
Contamination from well-treating chemicals entrained in the gas stream
Overdose of neutralizer added to a system for low pH
Foaming tendencies can result from high pH, due to stabilized glycol-hydrocarbon emulsions.
Sludge and residue build-up can result from both high and low pH.
Glycol pH should be checked periodically and kept on the alkaline side by neutralizing the acidic compounds with alkaline chemicals, such as monoethanolamine (MEA). A pH of about 7.3 is considered a safe level. Raising the pH above 8 to 8.5 is not desirable because of the tendency for an alkaline glycol solution to foam and emulsify more easily.

5.12.3.2 Sludge
Sludge may become abrasive and cause premature pump and valve failure. It may deposit in trays and downcomers, still column packing, and heat exchangers, which cause system plugging.

5.12.3.3 Hydrocarbons content
Enter the glycol stream as a result of inlet separator carry-over or as condensation due to temperature variations. Compressor lube oils and other extraneous organic chemicals such as pipeline corrosion inhibitors, are often stripped out of natural gas as it passes through the contact tower. Oils and organic residues can cause glycol/water emulsions and suspensions, which contribute to foaming, which will results in excessive high glycol carryover from the contactor, and the contaminants may cause plugging in the contactor, still column, and heat exchangers
The light hydrocarbons are usually separated from the glycol stream with an adequately sized glycol/hydrocarbon separator, while heavy hydrocarbons that are referred to as soluble hydrocarbons because they bond with the glycol are usually filtered out with activated carbon.
Light end hydrocarbons (insoluble) are allowable up to 1% by volume.
Soluble hydrocarbons are only acceptable to 0.1 % by weight, since they are primarily responsible for foaming, sludge and residue build up, low pH, loss of hygroscopicity, and glycol decomposition.
Hydrocarbons are often get into glycol in;
Condensation which is caused when glycol enters the contactor colder than the incoming gas. This problem can be eliminated by maintaining a temperature for the entering lean glycol of 10-15°F (5-7°C) warmer than the incoming gas.
Carryover of hydrocarbon contaminants from the inlet separator or gas

5.12.3.4 Water Content
Water content is defined as the quantity of water in the glycol. The difference between the lean sample and rich sample measures the degree of loading in the contactor. It indicates regeneration efficiency. Glycol purity should be at least 98% in the lean stream and at least 94% in the rich. These concentrations will produce the desired dew points in systems that are operating properly. For lower dew points the glycol purity must be increased (or water content decreased). High water content of the lean sample generally indicates low reconcentrator heat or one of the following reasons:
Excessive glycol circulation
Undersized equipment
Carry-over from the separator
Vapor communication from reconcentrator to surge
A leak in the glycol/glycol heat exchanger
Over-refluxing in the still column
Hot inlet gas temperature
High water content in the rich sample usually indicates a low glycol circulation rate or:
Carry-over from the separator
Poor reconcentration
Heat exchanger communication
Undersized equipment
Hot inlet gas temperature
Check values for hydrocarbon, chlorides, iron, and foaming to help pinpoint the problem.

5.12.3.5 Suspended Solids
Considered to be those solids and tarry hydrocarbons that remain suspended within the glycol solution down to 0.45 micron in size. They are result of poor inlet separation, corrosion, and thermal degradation of the glycol. Values greater than 0.01% by weight indicate poor sock/microfiber filtration. Most filters are sized to remove particles to a size of 5 microns.
Particles larger than this in excessive amounts may serve to stabilize foaming tendencies in glycol. When the glycol is allowed to maintain a large concentration of suspended solids, a silty residue is likely to form along vessel walls causing plugging of the contractor trays, heat exchangers, still column, and reconcentrator. Suspended solids are likely (common with low glycol pH).
Problems resulting from a high solids content include:
Increased pump wear from abrasion
Accelerated corrosion and erosion
Increased fouling of fire tube .
Increased glycol loss due to foaming
Increased plugging problems.

5.12.3.6 Residue
The value for residue is a function of system contamination. The glycol sample is distilled, removing all light end hydrocarbons, water, and glycol. Residue represents the remaining contamination, which is comprised of total solids (suspended and residual), salt, and heavy hydrocarbons.
Value for residue is best kept below 2% by weight, however some systems may operate reasonably well at values from 2% to 4%. Units with Glycol containing greater than 4% are prime candidates for failure and should be cleaned immediately.

5.12.3.7 Chlorides
Chloride values indicate the quantity of inorganic chlorides (salts) found in the glycol sample.
As the concentration of chlorides (as NaCI or CaCI) in glycol increases, its solubility decreases.
When heat is added to the glycol solution, the salt begins to form crystals which:
Fall out of the glycol solution
Accumulate on the heat source and can lead to premature heat tube failure
May be swept by the glycol into other areas of the system
Potential problems with excessive chlorides include system plugging, low pH, glycol pump damage, foaming, and loss of hygroscopicity due to rapid glycol decomposition.
Removal of chlorides in high concentrations requires vacuum distillation of the glycol.
Concentrations greater than 1000 ppm will stabilize foaming tendencies, may lead to excessive glycol loss, and may affect glycol pH.
Precipitation of salts from the glycol will begin at approximately 1200 to 1500 ppm, however, the crystals formed are extremely small and rarely troublesome. At concentrations above 2200 ppm, precipitation occurs readily and system failure is a possibility.
Filtration removes large salt crystals, but most of the damage associated with salt will have already occurred prior to the development of crystals large enough to filter.

5.12.3.8 Iron
Iron found in glycol samples can indicate possible corrosion, and/or produced water carry-over.
Iron in excess of 50 ppm generally indicates corrosion. Whether it be in the glycol unit, upstream in the production equipment or downhole in the well string is difficult to determine.
Comparing values for iron content in several points downstream the glycol unit helps to establish the location of suspected corrosion. Corrosion by-products will consist of soluble iron and fine, gritty particulate in systems where oxygen is available. In systems where no oxygen is present, corrosion by-products will include sulfides in addition to the iron.

5.12.3.9 Foaming
More glycol is lost through foaming than any other cause. Foaming is not easily detected without chemical analysis; gradual low-volume glycol loss often goes overlooked. It is usually a result of contamination. Primary contaminants that cause foaming are hydrocarbons (from separator carryover), suspended solids, chlorides, compressor lube oil, well treating chemicals, and iron.
Water content affect foaming tendencies by inducing emulsification of contaminants, particularly hydrocarbons. Carbon filtration is the most effective means of controlling foam. Silicone emulsion–type foam inhibitor is used, but they treat the symptom, not the cause and thus are temporary solutions. Addressing the source of the contamination causing the foam is the only long-term solution.
The foam test consists of bubbling dry air at a rate of 6 liters/min through a graduated cylinder container of 200 mm of the glycol sample until the foam stabilizes at its maximum height.
Volume for both the liquid and the foam is reported as a single value. The original 200 ml is then subtracted. The remaining value is recorded as height and represents the ease at which the solution will foam. Once the maximum foam height is recorded, the dry air is removed from the sample and the time it takes for the foam to break from its maximum volume to a clear surface on the glycol sample is recorded in seconds. This time represents the tendency of the foam and is known as stability.
There are no concrete values given for acceptable foam height and stability. Foam with very low height and moderate stability will result in little glycol loss as will a foam with moderate height and very low stability. Thus, the acceptable range for foam test results are:
Height/ml: 20 to 30 ml
Stability/sec: 15 to 5 sec
For example, a sample with a height of 25 ml and a stability of 10 sec is acceptable, while a sample with 30 ml height and 15 sec stability would have a high foaming tendency and could result in glycol losses.

5.12.3.10 Glycol Weight Percentage
This refers to the amount of glycol in the glycol solution. Lean glycol should contain about 98.5 to 99.9% a glycol. Rich glycol content varies from about 93-96% glycol.
5.12.3.11 Specific Gravity
Specific gravity is used to determine the purity of glycol. A specific gravity of 1.126 to 1.128 at 60 0F indicates a 99% TEG (technical grade).
A specific gravity of 1.124 to 1.126 indicates 97% (industrial grade).
With glycol extracted from an operating dehydration unit, the lean sample should have a specific gravity of 1.1189 to 1.121.
This variance allows for acceptable amounts of system contamination.
Low specific gravity would indicate one or more of the following:
TEG containing excessive amounts of EG and/or DEG (poor quality replacement glycol)
Excessive water in sample
Excessive hydrocarbons in sample
A high specific gravity indicates one or more of the following:
The system is contaminated with excessive amounts of solids or any additives with a greater density than glycol
Thermal degradation of the glycol
Oxidation or chemical degradation of the glycol

5.12.3.12 Glycol Composition
The composition of glycol indicates its quality.
Values are given to the component glycols (EG, DEG, TEG, TTEG) contained within the glycol sample solution. Industrial grade (97%) TEG or better is required for best glycol system results.
In addition to 97% TEG, the glycol solution may contain, in various concentrations, up to 1% EG and 3% DEG, but not to exceed a combined total of 3%.
Glycol degradation will often be reflected by changes in the glycol composition and reduction in pH. Thermal degradation is most common and is characterized by excessive values of EG, DEG, and occasionally the presence of TTEG. The thermal degradation is characterized by:
The glycol pH will be low.
The glycol sample will be dark and have an aromatic smell (ripe bananas).
Chemical degradation is brought about by oxidation and acidic contaminants and is characterized by:
Excessive values for EG and DEG but no TTEG will be present
Low pH
Glycol may not appear to be too dirty
Autoxidation is a form of continuing chemical degradation.
5.13 Troubleshooting
5.13.1 General Considerations
The most obvious indication of a unit malfunction is high water content (dew point) of the outlet stream. High water content is brought about by:
Insufficient glycol circulation
Insufficient reconcentration of the glycol
These problems can be caused by a variety of contributing factors such as mechanical causes or existing operating conditions for which the equipment was not designed

5.13.2 Main approach to troubleshooting:
Timeframe
Determine the approximate date/time at which the problem became apparent.
List Changes
Inventory any changes (things that happened differently than usual). Look for what is different.
Production changes
Operational changes
Maintenance and Repairs
Weather
Investigate by process of elimination reduce the list of changes to determine the factor or factors that manifest the problem.

5.13.3 High Dew Points
5.13.3.1 Insufficient Glycol Circulation
If there is insufficient glycol circulation, check heat exchangers and glycol piping for restrictions or plugging.
On an electric driven piston pump:
Check flow rate indicator (if present) to insure proper glycol circulation. If flow rate indicator is not present, verify circulation rate by closing the glycol discharge valve from the contactor and timing the fill rate in the gauge column.
Check high-pressure dry-glycol bypass valve. Close if necessary.
Check pump prime by shutting pump down, closing the discharge valve, opening the bypass valve and restarting the pump. Allow to run briefly under no load through the bypass line to remove any trapped gas in the pump.
On glycol-gas powered pumps:
Close dry discharge valve. If pump continues to run, open dry discharge bleed valve and allow running a few strokes. Once all gas is purged from put, close the bleed valve. If pump continues to run, discontinue use and send in for repair. If pump will not prime, but continues to run gas through the dry discharge bleed valve then:
Check pump suction strainer for plugging.
Check glycol level in surge tank.

5.13.3.2 Insufficient Reconcentration
Verify reconcentration temperature with test thermometer (3500 to 400 0F). Raise temperature if necessary.
Check glycol-to-glycol heat exchanger for leakage of wet glycol into the dry glycol stream.
Check stripping gas if applicable. Be sure stripping gas is in service at the proper rate.
Check for communication between the reconcentration vapor space and the surge tank vapor space.

5.13.3.3 Operating Conditions Different from Design
Check operation of upstream separators and scrubbers. Be sure not to overload system.
Increase absorber pressure. This may require installation of a back pressure valve.
Reduce gas temperature, if possible.
Increase circulation rate, if possible.
Increase reconcentrator temperature, if possible.

5.13.3.4 Low Flow Rate
Blank off a portion of the bubble caps, if possible.
Lower system pressure.
Add additional cooling to dry glycol and increase circulation rate.
Change out absorber to a small unit designed for a lower flow rate.

5.13.3.5 Absorber Tray Damage
Open inspection ports and/or manway and verify tray integrity. Repair or replace as necessary.

5.13.3.6 Breakdown or Contamination of Glycol
Have lean and rich glycol sample analyzed.
Note evidence of severe contamination, thermal or chemical decomposition. Clean system and/or recharge with fresh glycol as necessary.

5.13.3 Glycol Loss from the Contactor
5.13.4.1 Foaming
Major cause of foaming is contamination.
Remove source of contamination. Clean contactor if necessary, clean system if necessary, replace glycol if necessary.
Increase filter capacity and/or add carbon filtration.
Add antifoam compound (silicon emulsion type).
Adjust high pH to prevent emulsification (use acetic acid).

5.13.4.2 Plugged or Dirty Trays
Manually enter tower and clean.
Open inspection ports and clean with water jet or by hand.
Chemically clean.

5.13.4.3 Excessive Velocity
Decrease gas rate.
Increase absorber pressure.

5.13.4.4 Interrupted Liquid Seal on the Trays (Gas Surge)
If the contactor has a bypass valve, isolate the tower by opening the bypass valve and closing the gas inlet valve. Allow the glycol pump to run 5 minutes then while the glycol is circulating open the gas inlet valve and slowly close the gas bypass valve.
If contactor does not have a bypass valve, stop or greatly reduce the gas flow through the tower (shut wells, flare gas, alternate system, etc.). Allow the glycol to circulate 5 minutes then slowly turn the gas back through the tower.
If unable to stop or reduce the gas flow, increase the glycol circulation rate to the maximum possible for 2–5 minutes (flood trays in attempt to reestablish seal using liquid head pressure).

5.13.4.5 Cold Glycol (Cold Gas)
Increase gas temperature by increasing temperature of flowline heater or add flowline heater, if necessary.

5.13.4.6 Leaks
Perform a pressure test for external gas glycol heat exchanger to check or detect glycol leakage into dry gas stream.
Check drain header (if applicable) at all gauge columns, external float cages (LSLL, etc.).

5.13.4.7 Accumulation in Integral Scrubber
Check for communication between chimney tray and scrubber section.
Check bottom tray leakage.
Check glycol level control and dump valve operation (electric powered glycol pumps units).

5.13.5 Glycol Loss from the Reconcentrator
5.13.5.1 Leaks
Be sure all drain valves are closed.
Be sure gauge column seals are good.
Check heat tube integrity (glycol loss into fire tube or waste heat tube will produce heavy smoke from stack).
Check reconcentrator shell integrity (note glycol leakage from insulation, etc.)
Heat source flange leak (poor gasketing).

5.13.5.2 Bad Glycol Relief Valve
Check glycol relief valve, replace if necessary.

5.13.5.3 Exiting the Still Column
For plugged or fouled still column packing, clean or replace still column packing.
For saturated glycol (droplets blowing out still):
Check reconcentrator heat source. Insure heat is between 3500 and 400 0F.
Check for free liquid or misting liquid carryover into contactor tower.
Repair or replace separator control, if necessary.
Reduce slugging if possible. Add scrubber, if necessary.
Reduce glycol flow through the reflux condenser.

5.13.5.4 Vaporization
Check reconcentrator temperature (below 404 0F).
Check reflux temperature. Increase the glycol flow through the reflux condenser to lower the reflux temperature.
Check stripping gas flow rate.
Check for plugged or fouled glycol outlet from reconcentrator (downcomer or heat exchangers).

5.3.16 Glycol Loss From Glycol Hydrocarbon Separator
5.13.6.1 Improper Control Operation
Repair or replace level control.
Clean, repair, or replace dump valve.
Check gas velocity in separator and mist extractor.

5.13.6.2 Leaks
Check drain valve. Tighten, repair, or replace.
Check gauge columns, external float cages, and level control adapters.
Add antifoam compound to prevent loss through gas outlet.

5.13.6.3 Accumulation in Oil Bucket (Bucket & Weir)
Open vessel and clean glycol passage under oil bucket (horizontal vessels).
Adjust the weir.

5.13.7 Glycol Loss—Miscellaneous
5.13.7.1 Leaks
Check all flanges, unions and associated piping.
Check electric pump rod packing.
Check all drain valves (filter, heat exchanger, etc.).
Check pump bleed valves (and electric pump bypass).
Check external gas-glycol heat exchanger.
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Re: Gas Dehydration - Chapter 5 - Part 2

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