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Chapter 5 113
Gas Dehydration 113
5.1. Gas Dehydration by Absorption 114
5.1.1 Absorption and Stripping 114
5.1.2 Raoult and Dalton’s Laws 114
5.1.3 Glycol-Water Equilibrium 115
5.2 Glycol Dehydration 115
5.2.1 Principles of Operation 115
5.3 Effect of Operating Variables 123
5.3.1 Glycol Selection 123
5.3.2 Inlet Gas Temperature 124
5.3.3 Lean Glycol Temperature 124
5.3.4 Glycol Reconcentrator Temperature 124
5.3.5 Temperature at Top of Still Column 125
5.3.6 Contactor Pressure 125
5.3.7 Reconcentrator Pressure 125
5.3.8 Glycol Concentration 126
5.3.9 Glycol Circulation Rate 128
5.3.10 Number of Absorber Trays 129
5.4 Enhanced Glycol Concentration Processes 132
5.4.2 DRIZO® (wt.-2) Process 133
5.4.3 Coldfinger® Process 135
5.5 Other Considerations of Glycol Dehydration 136
5.5.1 Systems Utilizing Glycol-Gas Powered Pumps 138
5.5.2 Systems Utilizing Electric Driven Pumps 139
5.6 Glycol Gas Contactors 140
5.6.1 Trays and Packing 140
5.7 Glycol Dehydration System Sizing 146
5.7.1 Contactor Diameter 146
5.7.2 Number of Trays and Tray Spacing 148
5.7.3 Downcomers 148
5.7.4 Glycol Circulation Rate 149
5.7.5 Lean Glycol Concentration 149
5.7.6 Glycol-Glycol Preheater 149
5.7.7 Glycol-Gas Cooler 149
5.7.8 Glycol-Glycol Heat Exchanger 149
5.7.9 Gas-Glycol-Condensate Separator 150
5.7.10 Reconcentrator 150
5.7.11 Heat Duty 150
5.7.12 Fire Tube Sizing 151
5.7.13 Reflux Condenser 154
5.7.14 Stripping Still Column 154
5.7.15 Filters 155
5.8 Calculation Examples for Glycol Dehydration 158
Example 5-1 158
Example 5-2 159
Example 5-3 159
5.9 Glycol Unit Operation 161
5.9.1 Start up 161
5.9.2 Routing Operation 161
5.9.3 Shut Down 161
5.10 Glycol Maintenance and Care 162
5.10.1 Preventive Maintenance 162
5.11 Glycol Operation Considerations 165
5.11.1 Oxidation 165
5.11.2 Thermal Decomposition 165
5.11.3 pH Control 166
5.11.4 Salt Contamination and Deposits 166
5.11.5 Hydrocarbons 166
5.11.6 Sludge 167
5.11.7 Foaming and defoamers 167
5.12 Analysis and Control of Glycol 167
5.12.1 Visual Inspection 167
5.12.2 Chemical Analysis 167
5.12.3 Chemical Analysis Interpretation 168
5.13 Troubleshooting 5.13.1 General Considerations 172
5.13.2 Main approach to troubleshooting: 172
5.13.3 High Dew Points 172
5.13.3 Glycol Loss from the Contactor 173
5.13.5 Glycol Loss from the Reconcentrator 174
5.3.16 Glycol Loss From Glycol Hydrocarbon Separator 175
5.13.7 Glycol Loss—Miscellaneous 175
5.14 Glycol System Cleaning 175
5.15 Eliminating Operating Problems 176
5.15.1 Inlet Scrubber/Microfiber Filter Separator 176
5.15.2 Absorber 177
5.15.3 Glycol-Gas Heat Exchanger 178
5.15.4 Lean Glycol Storage Tank or Accumulator 178
5.15.5 Stripper or Still Column 179
5.15.6 Reboiler 180
5.15.7 Stripping Gas 181
5.15.8 Glycol Circulating Pump 182
5.15.9 Flash Tank or Glycol-Gas Separator 183
5.15.10 Gas Blanket 183
5.15.11 Reclaimer 184
5.16 Improving Glycol Filtration 184
5.17 Overview of Adsorption Processes 185
5.18 Properties of Industrial Adsorbents for Dehydration 187
5.19 Solid Bed Adsorption Process 188
5.20 Principles of Operation 188
5.20.1 Introduction 188
5.20.2 Drying Cycle 189
5.20.4 Regeneration Cycle 189
5.21 Adsorption System Performance 190
5.22 Effect of Process Variables 191
5.22.1 Quality of Inlet Gas 191
5.22.2 Temperature 191
5.22.3 Pressure 192
5.22.4 Cycle Time 192
5.22.5 Gas Velocities 192
5.22.6 Source of Regeneration Gas 192
5.22.7 Direction of Gas Flow 193
5.22.8 Desiccant Selection 194
5.22.9 Effect of Regeneration Gas on Outlet Gas Quality 195
5.22.10 Pressure Drop Considerations 196
5.22.11 Equipment 197
5.23 Desiccant Performance 200
5.24 Design 202
5.24.1 Pressure Drop & Minimum Diameter 202
5.24.2 Mass desiccant Required & Bed Length 203
5.24.3 Regeneration Calculations 205
5.24.4 Design Example 208
5.25 Nonregenerable Dehydrator 211
5.25.1 Calcium Chloride Dehydrator Unit 211
5.25.2 Principles of Operation 211
5.26 Dehydration by Refrigeration 213
5.27 Dehydration by Membrane Permeation 214
5.28 Other Processes 214
5.29 Comparison of Dehydration Processes 215
After the liquid (free) water has been removed from the gas stream by separation, 25 to 120 lbs of water per MMscf of gas will remain, depending on the temperature and pressure of the gas.
The warmer the inlet gas and the lower the pressure, the more water vapor the gas stream will contain.
Dehydration is the process used to remove water from natural gas and natural gas liquids (NGLs), and is required to:
1- prevent formation of hydrates and condensation of free water in processing and transportation facilities,
2- meet a water content specification, and
3- prevent corrosion
Techniques for dehydrating natural gas, associated gas condensate and NGLs include:
1- Absorption using liquid desiccants,
2- Adsorption using solid desiccants,
3- Dehydration with CaCl2,
4- Dehydration by refrigeration,
5- Dehydration by membrane permeation,
6- Liquid dehydration by gas stripping, and
7- Liquid dehydration by distillation
Fig.5-1. Dehydration of natural gas.
5.1. Gas Dehydration by Absorption
5.1.1 Absorption and Stripping
In the absorption process, a hygroscopic liquid is used to contact wet gas and remove the water vapor. Through absorption, the water in a gas stream is dissolved in a relatively pure liquid solvent stream. Normally, between 20 to 115 lbs of water per MMscf of gas must be removed before the required dew point of the gas is met. The reverse process, in which the water in the solvent is transferred into the gas phase, is known as stripping. The terms regeneration, re-concentration, and reclaiming are also used to describe stripping (or purification) because the solvent is recovered for reuse in the absorption step. Absorption and stripping are frequently used in gas processing, gas sweetening, and glycol dehydration.
5.1.2 Raoult and Dalton’s Laws
Absorption can be qualitatively modeled by using Raoult’s and Dalton’s laws.
For a vapor liquid equilibrium system:
Raoult’s Law state that the partial pressure of a component in a vapor phase that is in equilibrium with a liquid is directly proportional to the mole fraction of the component in the liquid phase. Or, the partial pressure of a component in a vapor phase that is in equilibrium with a liquid equal the vapor pressure of its pure component multiply by mole fraction of the component in liquid phase.
Dalton’s Law states that the partial vapor pressure of a component is equal to the total pressure multiplied by its mole fraction in the gas mixture.
Raoult’s Law expressed in equation form is:
pi = PiXi Eq. 5-1
Dalton’s Law expressed in equation form is:
pi = PYi Eq. 5-2
pi = Partial vapor pressure of component i
Pi = Vapor pressure of pure component i
Xi = Mole fraction of component i in the liquid
P = Total pressure of the gas mixture
Yi = Mole fraction of component i in the vapor
Combining these laws we have:
PYi = piXi
pi/P = Yi/Xi Eq. 5-3
Since the pure-component vapor pressure and the total pressure are not affected by composition.
Equation (5-3) is significant. It states that the ratio of the vapor mole fraction to the liquid mole fraction for any component is independent of the concentrations of that component and the other components present. The ratio Yi /Xi is commonly known as the K-value.
Since the pure component vapor pressure increases with temperature, the K-value increases with increasing temperature and decreases with increasing pressure.
In physical terms this means:
• Transfer from the gas phase to the liquid phase (absorption) is more favorable at lower temperature and high pressures.
• Transfer to the gas phase (stripping) is more favorable at higher temperatures and lower pressures.
5.1.3 Glycol-Water Equilibrium
Absorption processes are dynamic and continuous.
Gas flow cannot be stopped to let the vapor and liquid reach equilibrium. Thus, the system must be designed to approach equilibrium as closely as possible while flow continues. This is accomplished by using a trayed or packed contactor in which the gas and liquid are in counter current flow. The closer to 100% equilibrium that a tray or packed section approaches, the higher the tray or packing efficiency.
For example, A common tray efficiency is 25%, meaning that 25% of the water molecules that would have been transferred under equilibrium conditions were actually transferred.
Wet gas enters the bottom of the column and contacts the rich glycol (high water content) just before the glycol leaves the column. The gas encounters leaner glycol as it works its way up the column, contacting the leanest glycol (lowest water content) just before it leaves the column.
The equilibrium based on Dalton’s and Raoult’s Laws can be rearranged as follows:
Yi = Xi (Pi / P) Eq. 5-4
Since Pi/P is constant for constant temperature, the concentration of the water in the gas must be directly proportional to the concentration in the liquid. However, the liquid concentration is constantly changing as water is absorbed. The counter current flow in the contactor makes it possible for the gas to transfer a significant amount of water to the glycol and still approach equilibrium with the leanest glycol concentration.
5.2 Glycol Dehydration
5.2.1 Principles of Operation
The most common liquid used in absorption type dehydration units is triethylene glycol (TEG)
Liquid desiccant dehydration equipment is simple to operate and maintain. It can easily be automated for unattended operation; for example, glycol dehydration at a remote production well. Liquid desiccants can be used for sour gases, but additional precautions in the design are needed due to the solubility of the acid gases in the desiccant solution. At very high acid gas content and relatively higher pressures the glycols can also be “soluble” in the gas.
Glycols are typically used for applications where dew point depressions of the order of 60° to 120°F are required.
Diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TTEG) are used as liquid desiccants, but TEG is the most common for natural gas dehydration. The schematics in Figures 5-2 and 5-3 show the flow through a typical glycol dehydration system. The glycol dehydration process can be discussed in two parts gas system (Figure 5-2) & glycol system (Figure 5-3)
1. Gas System (figure 5-2)
Wet gas enters the unit through the inlet gas scrubber/microfiber filter separator, usually vertical, to remove liquid and solid impurities. After passing through the microfiber filter separator, the gas enters the glycol gas contactor near the bottom of the vessel.
Even if the dehydrator is near a production separator. The inlet gas scrubber upstream the contactor (or filter) will prevent accidental dumping of large quantities of water (fresh or salty), hydrocarbons, treating chemicals or corrosion inhibitors into the glycol contactor. Even small quantities of these materials can result in excessive glycol losses due to foaming, reduced efficiency, and increased maintenance. Integral separators at the bottom of the contactor are common. Figure 5-4.
Fig. 5-2. Gas system
The inside of the contactor contains either packing or several trays with weirs that maintain a specific level of glycol so that the gas must bubble through the glycol as the gas flows up.Different types of trays will be presented.
As the wet gas passes upward through each succeeding tray, it gives up the water vapor to the glycol and becomes progressively drier. Before leaving the contactor the gas passes through a mist extractor to remove glycol that may be trying to leave the gas.
Dry gas exits the contactor at the top and passes through an external glycol gas heat exchanger where it cools the incoming dry glycol to increase its absorption capacity (Figure 5-2).
Some installations incorporate a glycol knockout drum (centrifugal separator) which recovers any glycol that has escaped with the gas through the mist extractor.
The dry gas then leaves the dehydrator unit.
Fig. 5-4. Integral separators at the bottom of the contactor
2- Glycol System (figure 5-3)
The glycol pump pumps up dry concentrated glycol to contactor, and then passes through the glycol gas heat exchanger before entering the contractor tower.
The glycol gas heat exchanger cools the glycol to near the temperature of the gas before the glycol enters the contactor.
It is important that the glycol be near the gas temperature to prevent gas from exceeding equilibrium temperature, and to prevent foaming. Dry glycol from the glycol gas heat exchanger enters the contactor tower and flows across the top tray.
This is the first contact between the glycol and gas. Glycol flows downward through downcomers in the tower, absorbing more water as it passes across each tray.
The downcomer seals the glycol passage into the tray below, thus preventing gas from short-circuiting past the bubble caps.
As the glycol flows downward through each succeeding tray, it becomes wetter with the water it has absorbed from the gas and collects in the bottom of the contactor saturated with water.
As the gas moves upward through each succeeding tray, it becomes drier.
The wet glycol that has accumulated in the bottom of the contactor passes through a strainer (filter), which removes abrasive particles, before flowing through the power side of the glycol pump (energy exchange pumps), where it furnishes the power to pump the dry glycol into the contactor. Power comes from the increased head caused by the absorbed water contained in the rich glycol.
From the glycol gas contactor the cool wet glycol passes through a coil (reflux condenser) in the top of the reboiler still column. The coil cools the vapors leaving the still column and condenses the glycol vapors to liquid.
The glycol liquid droplets gravitate back down the still column to the re-concentrator. The water remains as a vapor and continues on out the top of the still column. The cooling coil is commonly called the reflux condenser.
The slightly warmed wet glycol leaving the reflux condenser passes through the glycol-glycol preheater. The hot dry glycol from the glycol reconcentrator heats the wet glycol further, and in turn further cools the dry glycol before it goes to the glycol pumps.
After leaving the glycol-glycol preheater, the heated wet glycol is sent to a low-pressure gas-glycol-condensate separator, where most of the entrained gas and liquid hydrocarbons that were picked up by the glycol on its path through the contactor are removed.
The heat provided by the glycol-glycol preheater helps in the separation of hydrocarbons from the wet glycol. The hydrocarbon condensate is separated from the glycol by a three-phase gas-glycol-condensate separator (Fig. 5-5), or a vertical three-phase separator as in fig.5-3.
Fig. 5-5. Gas-glycol-condensate separator
After the gas and condensate has been separated in the gas-glycol-condensate separator, the wet glycol passes through a microfiber filter (Fig. 5-6). These filters are used to remove solids, tarry hydrocarbons, or other impurities.
Fig. 5-6. Microfiber filter.
From the microfiber filter the wet glycol enters a charcoal or carbon filter. Activated carbon granules in this filter absorb liquid-entrained hydrocarbons, well-treating chemicals, compressor oils, and other impurities that may cause foaming.
From the charcoal filter, the wet glycol flows through the dry glycol to the wet glycol heat exchanger. This heat exchanger preheats the wet glycol as much as possible before entering the glycol reconcentrator, thus reducing the heat duty of the glycol reconcentrator.
From the glycol/glycol heat exchanger, the wet glycol enters the still column which sits vertically atop the glycol reconcentrator (Fig. 5-7). The inside of the still column is packed with either ceramic saddles or stainless steel pall rings, which are used to add surface area and distribute heat to the incoming glycol.
Fig. 5-7. Still column
The incoming wet glycol spreads out uniformly and drips down through the packed section.
The vapors traveling upward from the glycol reconcentrator heats the packing. As the glycol travels down through the heated packing, water begins to be driven off as steam. Units utilizing efficient heat exchangers may remove as much as 75 to 80% of the water contained in the glycol in the still column before the glycol reaches the reconcentrator.
As water vapor travels up through the still column and exits from the top, it carries with it trapped glycol vapor. To prevent the loss of glycol vapor, the still column utilizes a “reflux condenser” located on the top of the packed still column.
Glycol vapors escaping the still column with the steam are attracted to the film of condensed liquid (primarily water) covering the coil surface area where they too are condensed. The liquid droplets gravitate back down the still column into the reconcentrator for further treating, thus preventing excessive glycol loss due to vaporization.
On some units, the glycol enters the still column below the packed section of the column.
Vaporization takes place in the reconcentrator. The reflux condenser operates the same in both types of still columns. Condensed liquid from the reflux condenser drops back into the packed section providing a liquid film over the upper portion of the packing. Glycol vapors escaping with steam from the reconcentrator must pass through the packed section.
The watery film covering the packing recaptures the glycol vapor, condensing it into droplets, which wash back into the reconcentrator. Thus, more glycol vapor can be recovered in this configuration than in the previously described still column.
Since vaporization occurs primarily in the reconcentrator, the operating temperature is lower in this type of still column. This translates into:
- Greater reflux condensation
- Requires larger heat duty
From the packed still column, the wet glycol drops downward into the reconcentrator. The glycol is heated to a temperature at which most of the remaining water and some of the glycol are vaporized. A heat source heats the glycol to between 350 0Fand 400 0F, where:
- It removes the remaining water.
- It is below the decomposition point of TEG.
The temperature of the glycol in the reconcentrator is critical and must be controlled at this point.
Sources of heat include:
- Direct fired (natural draft/forced draft)
- Waste heat (exhaust gases from compressors or generators)
- Electric heaters
The heated vapor (both glycol and water) rises upward through the still column.
As the mixture passes the cool reflux condenser coils, the glycol vapors condense and drop back down. The water vapor leaves the top of the still column as steam.
Some of the steam will condense, so a downspout is provided to drain the water off.
A weir maintains a level of glycol over the heat source, which:
- Prevents over heating of the tubes
- Prevents premature tube failure
As the glycol is purified, it spills over the weir into a separate compartment. From the reconcentrator, the dry (lean) glycol flows to the accumulator surge tank, where the glycol pump raises it to contactor pressure to start another cycle.
Figure 5.8 shows a typical, simplified flow sheet for a glycol absorption unit. The wet gas passes through an inlet scrubber to remove solids and free liquids, and then enters the bottom of the glycol contactor. Gas flows upward in the contactor, while lean glycol solution (glycol with little or no water) flows down over the trays. Rich glycol absorbs water and leaves at the bottom of the column while dry gas exits at the top. The rich glycol flows through a heat exchanger at the top of the still where it is heated and provides the coolant for the still condenser. Then the warm solution goes to a flash tank, where dissolved gas is removed. The rich glycol from the flash tank is further heated by heat exchange with the still bottoms, and then becomes the feed to the still. The still produces water at the top and a lean glycol at the bottom, which goes to a surge tank before being returned to the contactor.
Fig. 5-8. Schematic of typical glycol dehydrator unit.
Fig. 5-9. Glycol dehydration system
5.3 Effect of Operating Variables
Several operating and design variables have an important effect on the successful operation of a glycol dehydration system.
5.3.1 Glycol Selection
Glycols are the most commonly used liquid desiccants in the absorption process because they are:
1. Highly hygroscopic (readily absorb and retain water)
2. Stable to heat and chemical decomposition at the temperature and pressures necessary in the process Low vapor pressures, which minimize equilibrium loss of the glycol in the residual natural gas stream and in the regeneration systebm
3. Easily regenerated (water removed) for reuse
4. Noncorrosive and nonfoaming at normal conditions; impurities in the gas stream can change this, but even then inhibitors can help to minimize these problems
5. Readily available at moderate cost
Hygroscopicity of glycols is affected by the concentration (glycol-to-water ratio), that is, increasing as the concentration increases.
Dew point depression obtainable in a gas stream increases as the glycol concentration increases.
Ethylene Glycol (EG)
Ethylene Glycol tends to have high vapor losses to gas when used in a contactor. It is used as a hydrate inhibitor where it can be recovered from the gas by separation at temperatures <50 0F.
Diethylene Glycol (DEG)
DEG (Diethylene) is somewhat cheaper to buy and sometimes is used for this reason. But, by the time it is handled and added to the units there is no real saving. Compared to TEG, DEG has a larger carry-over loss, offers less dewpoint depression and regeneration to high concentrations is more difficult. For these reasons, it is difficult to justify a DEG unit, although a few units are built each year. Diethylene Glycol reconcentrates at temperatures between 3150 and 325 0F, which yields purity of 97.0%. It degrades at 3280F. It cannot achieve the concentration required for most applications.
Triethylene Glycol (TEG)
Triethylene Glycol is most commonly used in glycol dehydration. It reconcentrates at temperatures between 3500 and 400 0F, which yields purity of 98.8%.
It degrades at 404 0F. It tends to experience high vapor losses to gas at temperatures in excess of 1200F. With stripping gas, dew point depressions up to 1500F are possible.
TEG (Triethylene) is preferred for use in dehydration units because:
• It is more easily regenerated due to its high boiling point and other physical properties.
• It has lower vaporization losses than other glycols
• It has lower capital and operating costs than other glycol systems
Tetraethylene Glycol (TTEG)
TTEG (Tetraethylen) is more viscous and more expensive than the other processes. It reconcentrates at temperatures between 4000 and 4300F. It experiences lower vapor losses to gas at high gas contactor temperatures. It degrades at 4600F. The Only real advantage is its lower vapor pressure which reduces absorber carry-over loss. It may be used in those relatively rare cases where glycol dehydration will be employed on a gas whose temperature exceeds about 50°C [122°F].
Table 5-1. Physical properties of glycols and methanol.
5.3.2 Inlet Gas Temperature
At constant pressure, the water content of the inlet gas increases as the temperature increases.
For example, at 1000 psia and 800F, gas holds 34 lb of water/MMscf. While, at 1200F, gas holds 104 lb of water/MMscf.
If the gas is saturated at the higher temperature, the glycol will have to remove about three times as much water to meet the specifications.
Temperatures above 1150F result in high glycol losses, thus requires tetraethylene glycol.
Temperature should not fall below the hydrate formation temperature range (650 to 700F) and always-above 500F.
Temperatures below 500F cause problems due to an increase in glycol viscosity.
Temperatures below 600 to 700F can cause a stable emulsion with liquid hydrocarbons in the gas and cause foaming in the contactor.
An increase in gas temperature increases the gas volume, which in turn increases the diameter of the glycol contactor.
5.3.3 Lean Glycol Temperature
Dry glycol temperatures entering the top tray of the contactor (approach temperature) should be held low (100 to 150F) above the inlet gas temperature.
Equilibrium conditions between the glycol and the water vapor in the gas is affected by temperature. Glycol entering the top tray of the contactor may raise the temperature of the gas surrounding it and prevent the gas giving up its remaining water vapor.
Inlet glycol temperatures greater than 150F above the gas temperature results in high glycol losses to the gas. Drastic temperature differential also has a tendency to emulsify the glycol with any contaminants subsequent glycol loss.
5.3.4 Glycol Reconcentrator Temperature
Reconcentrator temperature controls the concentration of the water in the glycol.
With a constant pressure, the glycol concentration increases with higher reconcentrator temperature. Reconcentrator temperature should be limited to between 3500 and 4000F.
Minimizes degradation of TEG which begins to degrade at 4040F. At a 400°F (204°C), the typical maximum regeneration temperature, TEG yields a lean-glycol concentration of 98.6 wt% at sea level. Higher purity requires reduction of the partial pressure of water in the reboiler vapor space. The most common way to achieve this pressure reduction is to use a stripping gas, or vacuum distillation, which yields lean glycol concentrations of 99.95 wt% and 99.98 wt%, respectively (Detailed process description will be followed in “Enhanced Glycol Concentration Processes” section).
5.3.5 Temperature at Top of Still Column
A high temperature in the top of the still column can increase glycol losses due to excessive vaporization. A reboiler temperature in the range of 3500 to 4000F insures adequate heat transfer to the ceramic packing in the still column.
The still column operates best (allows the steam to escape) when the vapor outlet temperature is between 2150 and 2250F.
When the temperature reaches 2500F and above, glycol vaporization losses increase.
Still top temperature can be lowered by increasing the amount of glycol flowing through the reflux condenser coil. If the temperature in the top of the still column drops too low, (below 2200F) too much water can be condensed and washed back into the reconcentrator, which increases the reconcentrator heat duty. Too much cool glycol circulation in the reflux condenser coil can lower the still top temperature below 2200F, which can cause the excess water to condense. Thus, most reflux condenser coils have a bypass valve, which allows manual or automatic control of the stripping still temperature.
5.3.6 Contactor Pressure
At a constant temperature, the water content of the inlet gas decreases with an increase in pressure. The lower the pressure, the larger the contactor diameter required.
Good dehydration can be achieved at any pressure below 3000 psig as long as the pressure is constant. Optimum dehydration pressure is often in the range of 550 to 1200 psig.
Sizing calculations should always be based on minimum expected operating gas pressure.
Rapid pressure changes translate into rapid velocity changes in the contactor which:
- Breaks the liquid seals between the downcomers and the trays
- Allows the gas to channel up through both the downcomer and bubble caps
- Allows the glycol to be swept out with the gas
5.3.7 Reconcentrator Pressure
Reducing the pressure in the reconcentrator at a constant temperature results in higher glycol purity. Most reconcentrators operate between 0.25 to 0.75 psig of pressure.
On standard atmospheric reconcentrators, pressures in excess of 1 psig results in:
- Glycol loss from the still column
- Reduction of lean glycol concentration
- Reduction in dehydration efficiency
Pressures more than 1 psig are usually associated with excess water in the glycol and create a vapor velocity exiting the still great enough to sweep glycol out.
Fouled still column packing often contributes to high reconcentrator pressure.
Still column should be adequately vented and packing replaced periodically to prevent backpressure on the reconcentrator.
Pressures below atmospheric will increase the lean glycol concentration because the boiling temperature of the rich glycol/water mixture decreases.(will be discussed later)
Reconcentrators are rarely operated in a vacuum due to the added complexity and the fact that air leaks will result in glycol degradation.
If lean glycol concentrations in the range of 99.5% are required consider:
Operating the reconcentrator at a pressure 500 mm Hg absolute (10 psia), or Using stripping gas Fig. 5-9 can be used to estimate the effect of operating in a vacuum on lean glycol concentration.
Fig. 5-9. Glycol purity versus reconcentrator temperature at different levels of vacuum.
5.3.8 Glycol Concentration
The water content of the dehydrated gas depends primarily on the lean glycol concentration.
The higher the concentration of lean glycol entering the contactor, the greater the dew point depression for a given circulation rate and number of trays. Increasing the glycol concentration above a 99% purity can lead to dramatic results on the outlet dew point (Fig. 5-10).
For example, with a 100 0F inlet gas temperature (110 0F top tray temperature), an outlet dew point of
10 0F can be obtained with 99.0% TEG,
-30 0F can be obtained with 99.8% TEG
-40 0F can be obtained with 99.9% TEG
Higher concentrations of TEG can be obtained by:
- Increasing the glycol reconcentration temperature
- Injecting stripping gas into the reconcentrator
- Reducing the operating pressure of the reconcentrator
Fig.5-10. Equilibrium water dew points with various concentrations of TEG.
Reconcentration temperatures for TEG normally run between 380 0F and 400 0F, which results in glycol purities of 98% to 99%. Figures 5-11 and 5-12 illustrate the effect of stripping gas.
If gas is injected directly into the reconcentrator (via a sparger tube), the concentration of TEG increases significantly from 99.1% to near 99.6% as the gas rate is increased from 0 to 4 SCF/gal.
When the Stahl method is used (counter current gas stripping after the reconcentrator – will be illustrated here later), concentrations as high as 99.95% TEG can be attained at a 400 0F reconcentrator temperature.
Fig. 5-11. Effect of stripping gas on TEG concentration.
Fig. 5.12. Effect of stripping gas on the concentration using Stahl column
5.3.9 Glycol Circulation Rate
When the number of absorber trays and lean glycol concentration are held constant, the dew point depression of a saturated gas is increased as the glycol circulation rate is increased.
The more lean glycol that comes into contact with the gas, the more water vapor is absorbed out of the gas. Whereas the glycol concentration mainly affects the dew point of the dry gas, the glycol rate controls the total amount of water that can be removed. The normal operating level in a standard dehydrator is 3 gallons of glycol per pound of water removed (Range 2-7).
Figure 5-13 shows that a greater dew point depression is a factor of both glycol concentration and glycol circulation rate.
Fig. 5-13. Calculated dew point depression versus circulation rate (1 equilibrium tray (4 actual trays)).
Excessive circulation rates:
- Overload the reconcentrator and/or decrease the reconcentrator temperature
- Prevent good glycol regeneration, resulting in decrease lean glycol concentration.
- Prevent an adequate glycol gas contact in the absorber; this may results in a decrease of water removed by glycol from the gas.
- Increase pump maintenance problems
- Increase glycol loses
- Only if the reconcentrator temperature remains constant, an increase in circulation rate will lower the dew point of the gas.
5.3.10 Number of Absorber Trays
When the glycol circulation rate and the lean glycol concentration are held constant, the dew point depression of a saturated gas is increased as the number of trays is increased.
Actual trays do not reach equilibrium, and their approach to it is expressed as a fraction of a theoretical tray. A tray efficiency of 25% is commonly used for design.
Four actual trays with efficiencies of 25% would accomplish the job of one theoretical tray.
The number of actual trays in a design ranges from 4 to 12.
An approximation of the actual number of valve trays per foot of packing can be obtained from
Fig. 5-14 Trays of packing required for glycol dehydrator.
For high performance units, the specification of more than 4 trays in a new design can achieve fuel savings (for the same dew point depression) due to:
- Lower circulate rate
- Lower reconcentration temperature
- Lower stripping gas rate
Figure 5-15 shows that specifying a few additional trays in the contactor is much more effective than increasing the glycol circulation rate. The additional investment for a taller contactor is often justified by fuel savings.
Evaluation of a TEG system involves first establishing the minimum TEG concentration required to meet the outlet gas water dewpoint specification. Fig. 5-10 shows the water dewpoint of a natural gas stream in equilibrium with a TEG solution at various temperatures and TEG concentrations. Fig. 5-10 can be used to estimate the required TEG concentration for a particular application or the theoretical dewpoint depression for a given TEG concentration and contactor temperature. Actual outlet dewpoints depend on the TEG circulation rate and number of equilibrium stages, but typical approaches to equilibrium are 10-20°F. Equilibrium dewpoints are relatively insensitive to pressure and Fig. 5-10 may be used up to 1500 psia with little error.
Fig. 5-15 Effect of number of absorber trays on dew point depression.
When dehydrating to very low dewpoints, such as those required upstream of a refrigeration process, the TEG concentration must be sufficient to dry the gas to the hydrate dewpoint.
Once the lean TEG concentration has been established, the TEG circulation rate and number of trays (height of packing) must be determined. Most economical designs employ circulation rates of about 2-5 gal. TEG/lb H2O absorbed. The relationship between circulation rate and number of equilibrium stages (N) employs the absorption calculation techniques set out in Engineering Data Book “GPSA 2004” Chapter 19. Calculation results for TEG systems are presented in figures 5-36 through 5-41 where they show the fraction of water removed versus TEG rate with respect to different glycol purities. (N=1 theoretical trays, 4 actual trays)
The graphs in these figures apply only if the feed gas is water saturated. They are based on feed-gas and therefore contactor temperatures of 80°F, but are essentially independent of temperature.
Although the K values in the absorption factors (i.e., L/VK) do increase with temperature, the required TEG rates also increase, and this tends to compensate for the increasing K values and keep the absorption factors fairly constant.
Conversion from equilibrium stages to actual trays can be made assuming an overall tray efficiency of 25-30%. For random and structured packing, Height of Packing Equivalent to a Theoretical Plate (HETP) varies with TEG circulation rate, gas rate, gas density, and packing characteristics but a value of about 60 inches is usually adequate for planning purposes.
When the gas density exceeds about 6 lb/ft3 (generally at very high pressures), the above conversions may not provide sufficient packing height and number of trays. Also when the contactor temperature is less than about 60°F, the increased viscosity of the TEG can reduce mass transfer efficiency, and temperatures in this range should be avoided.
Typical tray spacing in TEG contactors is 24 inches. Therefore, the total height of the contactor column will be based on the number of trays or packing required plus an additional 6-10 ft to allow space for vapor disengagement above the top tray, inlet gas distribution below the bottom tray, and rich glycol surge volume at the bottom of the column. Bubble cap trays have historically been used in glycol contactors due to the low liquid rates versus gas flow, but structured packing is widely used. Generally, structured packing allows a significantly smaller contactor diameter and a slightly smaller contactor height. Calculation examples will be followed.
5.4 Enhanced Glycol Concentration Processes
The enhanced glycol concentration processes include stripping gas, vacuum, Drizo, and cold finger.
5.4.1 Stripping Gas and Vacuum
Normal dehydration systems with TEG glycol purity of 98.5% are capable of achieving dew point depressions up to 70 0F.
If very pure glycol (up to 99.9% TEG) is required and cannot be achieved by the standard regeneration system, stripping gas may be used. A small amount of dry natural gas, normally taken from the fuel stream, is injected into the reconcentrator. Since hot gas has an affinity for water, the stripping gas is bubbled through the hot glycol, which strips the remaining water from the glycol.
This gas can be put directly into the reconcentrator or it can be added to the storage tank where it can percolate through the packed column between the two vessels (Stahl column) fig 5-17.
The Stahl column also serves as a weir where the dry glycol spills downward by gravity over packing while the gas goes upward, removing even more water.
This method prevents air from coming into contact with the dry glycol in the storage tank, thus preventing oxidization of the glycol.
Oxygen entry into the glycol system will:
- Decompose the glycol to some extent
- Cause corrosion within the system.
Stripping gas can:
- Reduce the temperature at which the reconcentrator must operate
- Reduce the glycol circulation rate necessary to dehydrate the gas adequately
Stripping gas may be used to obtain higher dew point depressions.
Vacuum-operated glycol units can achieve glycol purities of up to 99.9% but are rarely used because of:
- High operating costs
- Problems associated with achieving the necessary vacuums
Fig. 5-9 shows the glycol concentrations that can be obtained with various reboiler temperatures.
Fig. 5-11. Shows the effect of stripping gas on TEG concentration
Fig. 5.12. Shows the effect of stripping gas on the concentration using Stahl column
Fig. 5-16. Simplified Process Flow Diagrams of Enhanced TEG Regeneration Systems.
Fig. 5-17. Two different overhead vapor configurations for Stahl column; (a) Vapour flows directly into the bottom of the regeneration column; and (b) Vapor flows into reboiler of the regeneration column.
5.4.2 DRIZO® (wt.-2) Process
DRIZO®, achieves glycol enrichment by means of its own internally generated stripping medium, a mixture of paraffinic and aromatic (BTEX) hydrocarbons of a C5+ boiling range, which are absorbed by the glycol. The heavy hydrocarbons and water are condensed from the regenerator overhead while the non-condensables discharge to atmosphere essentially free of BTEX. The condensed hydrocarbons (with BTEX) are separated from the water, are vaporized and superheated, and flow to the lean-glycol stripping column to serve as the stripping medium, (The mixture of hydrocarbons form an azeotrope with water in the reconcentrator, thus lowering the effective boiling point of the mixture.), which results in TEG purities higher than 99.99 weight percent. As the liquid hydrocarbons build up, they are drawn off as an NGL product. Various options to further enhance the lean TEG purity are available, such as drying the hydrocarbon liquid solvent with solid desiccant, which achieves TEG purities as high as 99.999 weight percent. It is claimed that this high TEG purity level permits water dewpoint depressions in the range of 250°F.
- May be favored over stripping gas.
- Existing units can be retrofitted to increase the dehydration capacity.
- Must be evaluated on a case-by-case basis since Drizo (wt.-2) is a Dow-patented process and a license fee is required.
As shown in Figure 5-18, the Drizo process is the same as a conventional TEG dehydration system until the wet glycol flows into the reconcentrator.
Wet glycol is reconcentrated to 98.5% by conventional distillation.
The semi-lean glycol is then counter currently contacted with hydrocarbon solvent (iso-octane) vapors at 400 0F.
The hydrocarbon and water are taken overhead, condensed, and then phase separated.
The water is discarded, and the solvent is recycled into the system.
Drizo system is Competitive with applications that utilize a conventional TEG unit with stripping gas and it is most competitive in the range of -40 0F to -80 0F.
Fig. 5-18 Dow Drizo (wt.-2) gas dehydration process.
Fig. 5-19. Simplified Process Flow Diagrams of Enhanced TEG Regeneration Systems.
5.4.3 Coldfinger® Process
Based on the water TEG, vapor liquid equilibrium diagrams which shows that for any TEG liquid concentration the vapor concentration is richer in water.
Incorporates a closed vessel one-half filled with vapor and liquid at equilibrium with a condenser tube bundle in the vapor space (Figure 5-20).
Fig. 5-20 Cold finger condenser.
The condenser causes water condensation, which is removed from the vessel to a trough placed under the condenser tube bundle. As the condensate is removed:
- System’s equilibrium is upset
- Liquid phase releases more water to the vapor in order to reestablish equilibrium Consequently, the liquid phase has a lower water content than it did originally.
With a limited residence time, the water in the liquid phase is exhausted, and the residual liquid (lean TEG) approaches better than 99.7% by weight TEG concentration. In the most common applications, dilute glycol (rich TEG) from the glycol contactor is used as the coolant in the Coldfinger® tube bundle. The Coldfinger® process does not use stripping gas.
Numerous variations based on this principle exist. One design is shown in Figure 5-21.
Contact between gas and glycol is the same as in a conventional TEG system.
Wet glycol leaves the contractor and flows to the condenser-tube bundle of the cold finger, where it acts as a coolant, and it is used as a coolant in the glycol still before the hydrocarbon liquid phase, hydrocarbon vapor phase, and glycol/water phase are separated in a three-phase separator.
The glycol/water phase is mixed with the cold finger condensate, and is heated by the cold finger liquid product before it is fed to the still. The hot semi-lean glycol (which is near its boiling point) from the still bottoms is fed to the cold finger. The liquid product is cooled, pumped, cooled again, and fed to the contactor.
The main benefit of this system is that it is more fuel-efficient then the conventional TEG system.
However, it is more complex and not as well-proven as the conventional system.
5.5 Other Considerations of Glycol Dehydration
Under conventional dehydration conditions, 40 to 60% of methanol in the feed gas to a glycol dehydrator will be absorbed by the TEG. This will add additional heat duty on the reboiler and additional vapor load on the regenerator. High methanol injection rates and slug carryover can cause flooding.
Glycol losses can be defined as mechanical carryover from the contactor (normally 0.10 gallon/MMscf for standard mist eliminator) plus vaporization from the contactor and regenerator and spillage. Glycol losses, exclusive of spillage, range from 0.05 gallon/MMscf for high pressure low-temperature gases to as much as 0.30 gal/MMscf for low pressure, high temperature gases. Excessive losses usually result from foaming in the absorber and/or regenerator. Anti-foam agents are sometimes used.
TEG vaporization losses at the contactor are minimal unless the gas temperature exceeds about 120°F. These losses are more significant at lower pressures. Tetraethylene glycol (TTEG) has been used in some cases to minimize losses in high temperature, low pressure systems. Vaporization losses at the regenerator typically result from excessive stripping gas rates and/or inadequate reflux.
Glycol losses in CO2 dehydration systems can be significantly higher than in natural gas systems particularly at pressures above about 900 psia. This is due to the solubility of TEG in dense phase CO2. Glycerol is much less soluble and has been used successfully as a desiccant in some CO2 dehydration systems.
Glycol becomes corrosive with prolonged exposure to oxygen. A dry gas blanket on the glycol surge tank will help eliminate oxygen absorption. Special precautions should be taken if oxygen is in the gas to be dehydrated. Thermal decomposition of TEG can become a problem if TEG is heated to temperatures above 400°F.
A low pH accelerates decomposition of glycols. Bases such as triethanolamine, borax, or sodium mercaptobenzothiazole may be added to maintain pH, but they should be added sparingly.
Table 5-2. Glycol regeneration process.
Fig. 5-21. Cold finger condenser process.
5.5.1 Systems Utilizing Glycol-Gas Powered Pumps
Cool wet glycol leaves the bottom of the contactor, passes through a strainer, and powers the pump. The wet glycol takes a pressure drop through the pump, then passes through the reflux condenser coil in the reconcentrator still column.
Fig. 5-22 System utilizing glycol-gas powered pumps.
5.5.2 Systems Utilizing Electric Driven Pumps
Cool wet glycol leaves the bottom of the contactor, passes through a choke and level control operated motor valve, where a pressure drop occurs.
The glycol then passes through the still column reflux coils. From the reflux coil it flows through the first dry glycol to wet glycol heat exchanger, then into the gas/glycol/condensate separator, where insoluble hydrocarbons are removed.
From the separator, the glycol passes through the filter to remove tarry hydrocarbons, then through the second dry glycol to wet glycol heat exchanger and into the reconcentrator still column.
In the top of the reboiler still column, cool wet glycol flows through the reflux condenser coils, preventing glycol from leaving as a vapor. The wet glycol enters the column below the coils and spills downward through the packing and into the reconcentrator.
Heat is circulated through the tube to boil the water from the glycol. A weir holds the level of glycol above the heating tubes. Regenerated glycol flows over the weir and leaves through the outlet in the bottom.
Fig. 5-23 Systems utilizing electric driven pumps.
5.6 Glycol Gas Contactors
There are two basic types of glycol gas contactor:
1- Trayed towers
2- Packed towers
Some contactors have an “internal scrubber” which occupies approximately the lower one-third of the vessel (Figure 5-4). They are usually installed on units where the inlet gas flow rate is less than 50
MMscfd. “Chimney” is included on the scrubber/contactor combination:
Consists of a large stack that covers the top of the inlet scrubber.
Allows the gas to pass upward from the scrubber section to the absorber section. Prevents glycol from being lost out of the scrubber section.
Some contact towers have an internal three phase separator:
Distinguishable in that the lower section has two sets of level controls and two liquid dump valves, this design is not recommended as it is difficult to operate and maintain.
A separate two-phase microfiber filter separator located immediately upstream of the contactor is the most efficient configuration
The contactor is usually a tray column containing 4 to 12 trays on which up flowing gas bubbles through down flowing glycol, The number of trays in the contactor will affect the amount of moisture removed from the gas by the glycol; more trays mean more moisture removal.
In smaller capacity units, that is, contactors having a diameter of 18 inches or less, random packing may be used instead of trays. The packing is metal, plastic, or ceramic structures that are designed to furnish a large surface area for the glycol solution to spread out and make better contact with the gas. Random packing is poured into the contactor onto a support gad.
Four feet packing is usually standard and sufficient for dew point depressions up to 55°F to 65°F (13°C to 18°C). If higher dew point depressions are required, additional packing may be required.
Packed columns utilize the same process as tray columns, that is, glycol flows down over the packing and gas flows up through the packing contacting the glycol. Packed columns are less expensive; however, the glycol tends to channel easier and have poorer flow distribution. Therefore, special attention must be given to the design of a glycol distribution header above the packing so that gas/glycol contacting will be continuous throughout the packing and the glycol will not channel.
Contactors being designed today may contain structured packing. Structured packing is a group of corrugated metal sheets welded into a specific pattern and placed in the contactor on edge. Glycol coats these sheets and the gas flows between them. This type of packing is much more efficient than bubble caps or random packing. Structured packing is used in columns from six inches (15 centimeters) in diameter up to ten feet (three meters).
5.6.1 Trays and Packing
The more stages, the more complete the absorption, but the taller and more costly the tower.
For most trays, liquid flows across an “active area” of the tray and then into a “down-comer” to the next tray below, etc. Inlet and/or outlet weirs control the liquid distribution across the tray. Vapor flows up the stabilizer tower and passes through the tray active area, bubbling up through (and thus contacting) the liquid flowing across the tray. The vapor distribution is controlled by:
• Perforations in the tray deck (sieve trays),
• Bubble caps (bubble cap trays), or
• Valves (valve trays).
Sieve trays are the least expensive tray option. In sieve trays, vapor flowing up through the tower contacts the liquid by passing through small perforations in the tray floor (Figure 5-24). Sieve trays rely on vapor velocity to exclude liquid from falling through the perforations in the tray floor. If the vapor velocity is much lower than design, liquid will begin to flow through the perforations rather than into the downcomer.
This condition is known as weeping. Where weeping is severe, the equilibrium efficiency will be very low. For this reason, sieve trays have a very small turndown ratio.
Fig. 5-24. Vapor flow through a sieve tray.
Valve trays are essentially modified sieve trays. Like sieve trays, holes are punched in the tray floor. However, these holes are much larger than those in sieve trays. Each of these holes is fitted with a device called a “valve.” Vapor flowing up through the tower contacts the liquid by passing through valves in the tray floor (Figure 5-25). Valves can be fixed or moving. Fixed valves are permanently open and operate as deflector plates for the vapor coming up through the tray floor. For moving valves, vapor passing through the tray floor lifts the valves and contacts the liquid. Moving valves come in a variety of designs, depending on the manufacturer and the application. At low vapor rates, valves will close, helping to keep liquid from falling through the holes in the deck.
At sufficiently low vapor rates, a valve tray will begin to weep. That is, some liquid will leak through the valves rather than flowing to the tray down-comers. At very low vapor rates, it is possible that all the liquid will fall through the valves and no liquid will reach the down-comers. This severe weeping is known as “dumping.” At this point, the efficiency of the tray is nearly zero.
Fig. 5-25. Vapor flow through valve tray
Bubble Cap Trays
In bubble cap trays, vapor flowing up through the tower contacts the liquid by passing through bubble caps (Figure 5-26).
Each bubble cap assembly consists of a riser and a cap. The vapor rising through the tower passes up through the riser in the tray floor and then is turned downward to bubble into the liquid surrounding the cap. Because of their design, bubble cap trays cannot weep. However, bubble cap trays are also more expensive and have a lower vapor capacity/higher pressure drop than valve trays or sieve trays.
Figure 5-26. Vapor flow through bubble cap tray
Bubble Cap Trays (Figures 5-26 through 5-29) Most commonly used design
Better than conventional packing (Figures 5-30 and 5-31)
Fig. 5-27. Bubble cap components.
Fig. 5-28. Bubble cap tray.
Fig. 5-29 Bubble cap tray inside the contactor tower.
Bubble Cap Trays vs. Valve Trays
At low vapor rates, valve trays will weep. Bubble cap trays cannot weep (unless they are damaged). For this reason, it is generally assumed that bubble cap trays have nearly an infinite turndown ratio. This is true in absorption processes (e.g., glycol dehydration), in which it is more important to contact the vapor with liquid than the liquid with vapor. However, this is not true of distillation processes (e.g., stabilization), in which it is more important to contact the liquid with the vapor.
As vapor rates decrease, the tray activity also decreases. There eventually comes a point at which some of the active devices (valves or bubble caps) become inactive. Liquid passing these inactive devices gets very little contact with vapor. At this point, it is possible that liquid may flow across the entire active area without ever contacting a significant amount of vapor. This will result in very low efficiencies for a distillation process. Nothing can be done with a bubble cap tray to compensate for this.
However, a valve tray can be designed with heavy valves and light valves. At high vapor rates, all the valves will be open. As the vapor rate decreases, the valves will begin to close. With light and heavy valves on the tray, the heavy valves will close first, and some or all of the light valves will remain open. If the light valves are properly distributed over the active area, even though the tray activity is diminished at low vapor rates, what activity remains will be distributed across the tray. All liquid flowing across the tray will contact some vapor, and mass transfer will continue. Of course, even with weighted valves, if the vapor rate is reduced enough, the tray will weep and eventually become inoperable.
However, with a properly designed valve tray this point may be reached after the loss in efficiency of a comparable bubble cap tray. So, in distillation applications, valve trays can have a greater vapor turndown ratio than bubble cap trays.
Packing typically comes in two types: random and structured. Liquid distribution in a packed bed is a function of the internal vapor/liquid traffic, the type of packing employed, and the quality of the liquid distributors mounted above the packed bed. Packing material can be plastic, metal, or ceramic. Packing efficiencies can be expressed as height equivalent to a theoretical plate (HETP).
A bed of random packing typically consists of a bed support (typically a gas injection support plate) upon which pieces of packing material are randomly arranged (they are usually poured or dumped onto this support plate). Bed limiters, or hold-downs, are sometimes set above random beds to prevent the pieces of packing from migrating or entraining upward. Random packing comes in a variety of shapes and sizes. For a given shape (design) of packing, small sizes have higher efficiencies and lower capacities than large sizes.
Figure 5-30 shows a variety of random packing designs. An early design is known as a Rasching ring. Rasching rings are short sections of tubing and are low-capacity, low-efficiency, high-pressure drop devices. Today’s industry standard is the slotted metal (Pall) ring. A packed bed made of 1-inch slotted metal rings will have a higher mass transfer efficiency and a higher capacity than will a bed of 1-inch Rasching rings. The HETP for a 2-inch slotted metal ring in a stabilizer is about 36 inches. This is slightly more than a typical tray design, which would require 34 inches (1.4 trays × 24-inch tray spacing) for one theoretical plate or stage.
A bed of structured packing consists of a bed support upon which elements of structured packing are placed. Beds of structured packing typically have lower pressure drops than beds of random packing of comparable mass transfer efficiency. Structured packing elements are composed of grids (metal or plastic) or woven (metal or plastic) or of thin vertical crimped sheets (metal, plastic, or ceramic) stacked parallel to each other. Figure 5-31 shows examples of the vertical crimped sheet style of structured packing. The grid types of structured packing have very high capacities and very low efficiencies, and are typically used for heat transfer or for vapor scrubbing. The wire mesh and the crimped sheet types of structured packing typically have lower capacities and higher efficiencies than the grid type.
Trays or Packing ?
There is no umbrella answer. The choice is dictated by project scope (new tower or retrofit), current economics, operating pressures, anticipated operating flexibility, and physical properties.
For distillation services, as in hydrocarbon stabilization, tray design is well understood, and many engineers are more comfortable with trays than with packing. In the past, bubble cap trays were the standard. However, they are not commonly used in this service anymore. Sieve trays are inexpensive but offer a very narrow operating range when compared with valve trays. Although valve trays offer wider operating range than sieve trays, they have moving parts and so may require more maintenance. High capacity/high efficiency trays can be more expensive than standard valve trays. However, high capacity/high efficiency trays require smaller diameter stabilization towers, so they can offer significant savings in the overall cost of the distillation tower. Random packing has traditionally been used in small diameter (<20 inches) towers. This is because it is easier and less expensive to pack these small diameter towers. However, random packed beds are prone to channeling and have poor turndown characteristics when compared with trays. For these reasons, trays were preferred for tower diameters greater than 20 inches.
For stripping service, as in a glycol or amine contactor, bubble cap trays are the most common. In recent years, there has been a growing movement toward crimped sheet structured packing. Improved vapor and liquid distributor design in conjunction with structured packing can lead to smaller-diameter and shorter stripping towers than can be obtained with trays.
Figure 5-30. Various types of random packing.
Figure 5-31. Structured packing can offer better mass transfer than trays.
5.7 Glycol Dehydration System Sizing
Glycol gas contactor diameter
Number of absorber trays (which establishes the tower’s overall height), tray spacing, and downcomers sizing
Glycol circulation rate
Lean glycol concentration
Reconcentrator heat duty
The number of absorber trays, glycol circulation rate, and lean glycol concentration are all inter-related.
5.7.1 Contactor Diameter
The minimum diameter for trayed towers and conventional packing can be determined from the following equation:
d2 = 5040 [(T0ZQg)/P] [(ρg / ρL – ρg)(CD/dm)]0.5 Eq. 5-5
d = Contactor inside diameter, inches
dm = Drop size, microns =120 to 150 micron range
To = Contactor operating temperature, 0R
Qg = Design gas flow rate, MMscfd
P = Contactor operating pressure, psia
CD = Drag coefficient
ρg = Gas density, lb/ft.3 = 2.7 (SP/TZ) or = ρg= 0.093 ((MW)P)/TZ lb/ft3 (Eq. 1-19)
ρL = Glycol density, lb/ft.3 = 70 lb/ft3
Z = Compressibility factor
S = Gas specific gravity (air = 1)
Structured packing can handle higher gas flow rates for the same diameter contactor.
Contactor diameter is set by the gas velocity. Sizing is calculated using recommended values for K-factors and C-factors are shown in table. 5-3, and equations 5-6
Table. 5-3. Recommended sizing parameters for TEG contactors.
G = C [ρV (ρL - ρV)]0.5 Eq. 5-6
G = mass velocity, lb /(ft2 • hr)
C = constant from table 5-3,
ρV = Vapor density, lb/ft3
ρL = Liquid density, lb/ft3
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
Note: Structured packing vendors frequently quote an Fs value for sizing glycol contactors, where Fs is defined in Eq 5-9.
Fs = v (ρv)0.5 Eq. 5-9
v = vapor velocity, ft/sec
ρv= density, lb/ft3
Values of Fs = 2.5 to 3.0 will generally provide a good estimate of contactor diameter for structured packing.
Figures 5-32 through 5-35 are correlations prepared by vessel manufacturers that allow graphical solutions of glycol gas contactor diameters.
Fig. 5-32 Determination of contactor diameter—Sivills.
Fig. 5-33 Determination of contactor diameter—Smith Industries.
Fig. 5-34 Determination of contactor diameter—NATCO.
Fig. 5-35 Determination of contactor diameter—BS&B.
5.7.2 Number of Trays and Tray Spacing
6 to 8 trays are used to meet normal dew-point depressions.
12 trays are typically required for high dew-point depressions.
Spacing ranges from 20 to 30 inches
24 inches is preferred, while 30-inch spacing is recommended if foaming is anticipated
Sized for a maximum velocity of 0.25 ft./sec.
5.7.4 Glycol Circulation Rate
For a given dew-point depression, the circulation rate is dependent upon:
Lean glycol concentration
Number of trays
When the lean glycol concentration and number of trays are held constant, the required glycol circulation rate can be determined from the following equation:
L = (ΔW/Wi) Wi Qg/24 Eq. 5-10
L = Glycol circulation rates, gal/hr
ΔW/ Wi = Circulation ratio, gal TEG/lb H2O (see Figures 5-36, through 5-41)
Wi = Water content of inlet gas, lb H2O/MMscf
W0 = Desired outlet water content, lbH2O/MMscf
ΔW = Wg / W0
Qg = Gas flow rate, MMscfd
5.7.5 Lean Glycol Concentration
Equilibrium water dew points for various concentrations of TEG are shown in Figure 5-10.
Glycol purity (lean glycol concentration) is a function of the temperature of the reconcentrator (Fig. 5-9).
Glycol purity can be increased by:
a- Adding stripping gas
b- Reducing the pressure in the reconcentrator
c- Reducing the glycol circulation rate
5.7.6 Glycol-Glycol Preheater
Cool wet glycol from the contactor enters the preheater (heat exchanger) at 100 0F and the warm glycol leaves at 1750 to 200 0F en route to the gas/glycol/condensate separator.
Hot dry glycol from the glycol/glycol heat exchanger enters the preheater at 250 0F and the warm dry glycol leaves at 150 0F to the glycol pumps en route to the contactor.
Temperature limitations to the glycol pump:
Glycol powered pumps (Kimray) limited to 200 0F.
Electric plunger pumps limited to 250 0F.
Overall heat transfer coefficient (U = 10 to 12)
5.7.7 Glycol-Gas Cooler
TEG to gas contactor is limited to 10 0F to 15 0F above the inlet gas temperature. If hotter, some TEG will vaporize with gas. If colder, gas condensation of the hydrocarbons may cause foam and glycol loss.
Overall heat transfer coefficient (U = 45).
5.7.8 Glycol-Glycol Heat Exchanger
Hot dry glycol from the reconcentrator enters the heat exchanger at 390 0F and leaves at 250 0F en route to the glycol/glycol preheater. Warm wet glycol from the charcoal filter enters the heat exchanger at 200 0F and the hot wet glycol leaves at 350 0F en route to the still column.
5.7.9 Gas-Glycol-Condensate Separator
Separator should be sized using procedures for sizing gas-liquid separation.
Liquid retention times between 20 and 30 minutes are recommended, depending on API gravity of the condensate. Operating pressure of 35 to 50 psig is recommended.
The reconcentrator should be designed to operate 350 0 to 400 0F with TEG, and 305 0F with DEG. Design temperature should be sufficiently below the decomposition point so that hot spots on the fire tube and poor mixing in the reconcentrator will not cause decomposition of the glycol. With everything else operating normally, the reconcentrator temperature is raised to lower the water content of the treated gas, and vice versa. Specific reconcentrator operating temperature is determined by trial and error.
Temperatures up to 400 0F are common
400 0F yields 99.5% TEG purity
375 0F yields 98.3% TEG purity
5.7.11 Heat Duty
Estimated from the following equations
qt = LQL Eq. 5-11
qt = Total heat duty on reconcentrator, Btu/hr
L = Glycol circulation rate, gal/hr
QL = Reconcentrator heat load, Btu/gal TEG (Table 5-4)
Heat duty estimated from Equation (5-11 above) is normally increased by 10 to 25% to account for start-up, fouling, and increased circulation.
Table. 5-4. Reconcentrator Heat Load
Alternatively, Heat duty can be determined as follows:
Total heat duty =
Sensible Heat, Qs + water vaporization heat duty, QV+ Condenser Duty, QC, Eq. 5-12
Qs = m Cp Δt Eq. 5-13
Qs = Sensible Heat, Btu / gal
m = density lb/gal (table 5-1)
CP = Heat capacity, (Btu/lb.°F)
CP (95.1% TEG) = 0.56 at 110 0F (from physical property of TEG), 0.63 Btu/hr 0F at 200 0F, and 0.70 at 300 0F.- (use 0.67 as an average for the range of 200-300 0F).
Δt = Temperature difference (0F)
Vaporization of Absorbed H2O:
Qv = (ΔHvap) (ΔW) Eq. 5-14
Qv = vaporization of water heat duty, Btu/gal.
ΔHvap = latent heat of vaporization, Btu/lb
ΔW =change of water content
Condenser Duty @ 25% Reflux Ratio:
Qc = % Reflux Ratio X Qv /100 Eq. 5-15
5.7.12 Fire Tube Sizing
The actual surface area of the firetube required for direct-fired heaters can be calculated from the following equation:
A = qt / 6000 Eq. 5-16
A = Total firetube surface area, ft.2
qt = Total heat duty on reconcentrator, Btu/hr
By determining the diameter and overall length of the U-tube fire tube, one can estimate the overall size of the reconcentrator. A heat flux of 6000 to 8000 Btu/hr-ft.2 is often used, but the 6000 value is suggested to ensure against glycol decomposition.
Fig. 5-36. Water Removal vs. TEG Circulation Rate at Various TEG Concentrations (N = 1.0)
Fig. 5-37. Water Removal vs. TEG Circulation Rate at Various TEG Concentrations (N = 1.5)
Fig. 5-38 . Water Removal vs. TEG Circulation Rate at Various TEG Concentrations (N = 2.0)
Fig. 5-39 . Water Removal vs. TEG Circulation Rate at Various TEG Concentrations (N = 2.5)
Fig. 5-40. Water Removal vs. TEG Circulation Rate at Various TEG Concentrations (N = 3.0)
Fig. 5-41. Water Removal vs. TEG Circulation Rate at Various TEG Concentrations (N = 4.0)
5.7.13 Reflux Condenser
Wet glycol inlet from the gas contactor enters at 115 0F and leaves at 125 0F.
It controls TEG losses. Reflux rate should be 50% of the water removal rate. The condensing coil provides lowest TEG loss and most economical reconcentrator operation.
5.7.14 Stripping Still Column
Temperature is critical to the operation of the still column. Heat is provided by the reconcentration. Reconcentrator temperatures in the range of 350 0F to 400 0F insures adequate heat transfer to the ceramic packing in the still column.
Still columns whose wet glycol inlet enters above the packed section (Figure 5-42):
Operate best with a vapor outlet temperature between 2250 and 250 0F
Purpose of glycol falling over ceramic packing is the efficient use of available heat
Backpressure should be kept to a minimum (1 psig is maximum)
Still columns whose wet glycol inlet enters below the packed section (Figure 5-43):
Allow pall ring type packing to be solely involved in the reflux process
Operate best with a vapor outlet temperature between 185 0F and 195 0F
This temperature allows a greater volume of condensation by the reflux coil while still permitting the majority of the steam to escape