Glycol Dehydrators

Glycol Dehydrators

The still column vent on glycol dehydrators can be a notable source of organic emissions at some sites; however, overall, it is more significant as a potential source of odoriferous emissions and toxic organic air pollutants.

The specific control technologies that may be used to reduce or eliminate emissions from the still column vent include,

• Optimization of key process variables.
• Disposal of the vented vapours by thermal oxidation.
• Recovery of the vapours using a condenser unit.
• Specification of enhanced, low-emission glycol dehydration technology (i.e., the DRIZO process).
• Specification of an alternate, more environmentally friendly dehydration technology (i.e., membrane dehydration process).

Process optimization offers the most immediate and cost-effective means of reducing the amount of emissions. Disposal of the vapours by thermal oxidation is a simple, effective means of controlling the emissions at a moderate cost. However, it has the disadvantage of creating combustion emissions. Recovery of the vapours using a condenser is an interesting solution that may actually have a positive economic benefit to it. Use of the DRIZO process or possibly membrane dehydration technology may be worth considering for use at new facilities.

An additional advantage of the vapour condenser and DRIZO processes is that they may allow dehydration of some sour gas streams.

Source Characterization

Glycol dehydration is the most common process for removing water vapour from natural gas. It is frequently used in the field to prevent hydrates in gas gathering systems, and at gas processing plants to satisfy sales specifications.

Process Description

Glycol dehydration is a continuous liquid desiccant process in which water or water vapour is removed from hydrocarbon streams by selective absorption and the glycol is regenerated or reconcentrated by thermal desorption. The most common application of this process is the dehydration of natural gas streams; however, variations of the process are also used to dehydrate natural gas condensates at gas processing plants. The discussions that follow are directed towards natural gas applications. Nonetheless, the basic glycol regeneration process is the same regardless of the application so many of the available emission reduction methods and technologies will apply to both types of applications.

The use of triethylene glycol (TEG) is standard for dehydration of natural gas.

The absorption occurs in a trayed vessel called the contactor or absorber column. The lean dry glycol liquor enters the top of the column and the wet gas enters the bottom of the column. As the lean glycol flows down through the trays, it contacts the upflow of the wet gas. The lean glycol absorbs the water from the wet gas and exits through the bottom of the column as rich glycol. The gas exits the top of the column as a dry product with an acceptable residual moisture content range of 0.03 to 0.06g/m3 (2 to 4lb/mmscf) of gas.

A low pressure flash separator is sometimes installed between the contactor and the regenerator to release any solution gas that may be entrained in the rich (wet) glycol, especially if a glycol energy exchange pump is used in the system. This type of pump uses pressure energy in the rich glycol to pump dry glycol into the contactor. The gas separated in the flash separator may be used to supplement the fuel and stripping gas required for the reboiler. Any excess vent gas is discharged through a back pressure valve to atmosphere.

From the flash separator, the rich glycol is passed through a heat exchange coil in the surge tank to preheat the rich glycol. The rich glycol then flows to a packed stripping still attached to the top of a reboiler. In the still column, the wet glycol flows down to the reboiler while contacting hot gases (mostly water vapour and glycol) rising up from the reboiler. The mixing of these two streams helps to further preheat the wet glycol and to condense and recover any glycol vapours before the gases are vented from the top of the still.

In the reboiler the glycol is heated to approximately 175o to 205oC to remove enough water vapour to reconcentrate it to 99.5 percent or more. Sometimes a small amount of natural gas is injected into the bottom of the reboiler to strip water vapour from the glycol. The water vapour rises through the stripping still and the lean glycol flows to the surge tank where it is cooled down by preheating the rich glycol from the flash separator.

Finally, the glycol is pumped back to the top of the absorber column to repeat this circuit. The required circulation rate is determined by the actual purity of the glycol at the inlet to the contactor, the number of trays or packing height in the contactor, and the desired dew point depression (API, 1990). Typical values for plant applications are 17 to 50 L TEG/kg H2O removed, and for field applications are 20 to 35 L TEG/kg H2O.

Atmospheric Emissions

The type and amount of emissions from the still column vent depends on the actual process design, operating conditions, and the composition and flow rate of the raw natural gas. For a typical unit equipped with a flash separator, the composition of the vented vapours may be expected to vary (on a relative mass basis) as follows (Schievelbein,1992): 50 to 90 percent water, 5 to 50 percent condensable hydrocarbons, and small amounts of non-condensable hydrocarbons. The concentration of non-condensable gas increases dramatically if a flash separator is not used. Toluene is usually the most concentrated condensable hydrocarbon component, but rarely does it exceed 10 percent by weight. Dehydrators in field applications might be vaporizing 0.08 to 0.8 m3/d (0.5 to 5 bbls/d) of hydrocarbons.

The primary causes of hydrocarbon emissions are secondary absorption/desorption by the TEG, entrainment of some gas from the contactor in the rich TEG, and use of stripping gas in the reboiler. However, secondary absorption/desorption by the glycol is perhaps the most important single cause.

The key secondary compounds that may be removed by the TEG are aromatic hydrocarbons (e.g., benzene, toluene, ethylbenzene, and xylenes [BTEX]), which are notable toxic substances. TEG also has a strong affinity for reduced sulphur species (e.g., hydrogen sulphide, mercaptans and carbon disulphide), which are also toxic and highly malodorous. However, for the purposes of this document, attention is focused on the BTEX and other hydrocarbon emissions.

Through the concentrating effect of the absorption process, the presence of even small amounts of BTEX in the natural gas can result in very high concentrations in the vented effluent. The molar concentration of BTEX at the outlet of the vent can easily be several hundred times higher than in the raw gas.

The amount of BTEX emitted depends largely on its concentration in the raw natural gas and on the glycol circulation rate. BTEX does not occur in all raw natural gas streams, but where it does occur, its concentration is usually less than one percent by volume (Thompson et al., 1992). The glycol circulation rate is determined by the gas flow rate and the amount of water to be removed. At 6900 kPa, glycol can absorb up to 7.5 standard litres of gas for every litre of glycol circulated (Sams, 1992). A gas-driven glycol pump adds additional gas during its pumping operation. Approximately 0.5 actual litres of gas (i.e., at process conditions in the contactor) is required to pump one litre of glycol. Stripping gas flows are normally 7.5 standard litres per litre of glycol.

Since the boiling points of BTEX range from 80oC to 140oC, very little of this material is released in the flash tank (Thompson et al., 1992). Most of the BTEX is separated from the glycol in the still. Although many of the lighter hydrocarbons (e.g., entrained gas from the contactor) may be removed from the glycol in the flash tank, some remain in the glycol and are separated in the still.

The amount of BTEX emitted by a glycol dehydrator in a given application may be estimated using several different commercially available process simulation packages which have been specifically tuned for this type of problem (e.g., HYSIMTM by Hyprotech, PRO/IITM by Simulation Sciences, and PROSIMTM by Bryan Research & Engineering, Inc.). However, a somewhat less accurate, although much less costly, package named GRI-DEHY has been developed for this same purpose and is available from Gas Research Institute (Thompson et al., 1992). If the composition and flow rate of the inlet gas and the design and operating conditions of the dehydrator are known, GRI-DEHI is reportedly able to calculate benzene emissions to within ±20 percent of actual values, and toluene, ethylbenzene, and xylenes emissions to within ± 20 to 50 percent.

A variety of test methods are also available for establishing aromatic emissions from glycol dehydrators (Schievelbein, 1992; Grizzle, 1992; Rueter and Evans, 1992). The simplest and most cost effective approach of reasonable accuracy is perhaps the rich/lean glycol mass balance method. This method consists of analyzing samples of the rich and lean glycol and then performing a mass balance based on the glycol circulation rate. The results are estimated to be low by as much as 20 percent for BTEX and as much as 40 percent for VOC values (Schievelbein, 1992). The most accurate methods are total capture condensation and partial stack condensation/flow measurement; however, these methods are also much more costly to perform. The limited data available (Grizzle, 1992) indicate that direct sampling and analysis of the vent stream may provide good VOC values but poor BTEX results.

Emission Reduction Methods and Technologies

The amount of emissions and the concentration of toxic and odoriferous compounds in the vapours are major factors in choosing the best option for reducing venting emissions from the still column on glycol dehydrators. Maintenance needs in field applications can also be important, particularly for remote locations.

Some simple process modifications and optimization work may be done to help minimize the amount of emissions and lower the cost of any add-on options. If resulting total hydrocarbon emissions are low, then the simplest and most cost effective solution may be to dispose of the emissions by thermal oxidation. This may be a minimum requirement if malodour problems arise or if there is a risk to human health due to toxic emissions. However, the use of condensers to recover the condensable hydrocarbons may offer a very attractive payback in some cases. Moreover, the non-condensable hydrocarbons can be used as fuel to improve the energy efficiency of the dehydration process.

The DRIZO process may be considered for new applications or may possibly be retrofit to existing units. It has the benefits of reduced emissions and increased water removal.

Process Optimization

There are several process options that may be implemented and process variables that may be optimized to help minimize the amount of venting emissions from the still column. These are itemized below and generally pertain to directly or indirectly minimizing the glycol circulation rate and the heating load on the reboiler. For remote field dehydrators, it may be worthwhile to install a SCADA system to allow central monitoring of some of the key variables.

• Flash Tank: If appreciable quantities of natural gas are absorbed or entrained in the glycol, a two-phase flash separator should be installed downstream of the glycol-glycol heat exchanger and glycol pump (especially if a glycol energy exchange pump is used). The separated gas can be used as fuel for the reboiler or as stripping gas, or sent to flare for disposal.

If significant amounts of hydrocarbon liquids are encountered at this point, the flash tank should be designed for three-phase separation. Otherwise, the hydrocarbon liquids could cause problems in the reboiler (i.e., gradual coke accumulation on the fire tube [Pearce and Sivalls, 1984]) and lead to reduced boiler efficiency and increased combustion emissions.

• Glycol Circulation Rate: The amount of moisture control provided by a glycol dehydrator is, for the most part, determined by the glycol circulation rate. However, since this rate also determines the amount of venting emissions, it is important that the circulation rate not be set any higher than is needed.

Notwithstanding this, usual practice is to set the circulation rate for peak flows plus a comfortable safety factor (e.g., 10 percent [Pearce, 1982]). Thereafter, adjustments to the circulation rate with changes in throughput are made infrequently, if at all. Consequently, there is considerable potential for optimization of the glycol circulation rate. This will not only reduce venting emissions, but will remove unnecessary load from the reboiler and thereby lower combustion emissions and conserve fuel.

One option is to conduct periodic performance tests (e.g., sample the rich and lean glycol) on each unit and manually adjust the glycol circulation rate. Sometimes it may even be necessary to replace the existing pump with a smaller pump (i.e., it is not uncommon for the glycol pump and other components of the dehydrator to be oversized because of production declines or low gas demand). According to Ballard (1986), the lean glycol weight percent should be about 98 to 99.5+. The rich glycol will vary from about 93 to 97 percent. If the spread between the lean and the rich glycol content is too narrow (about 0.5 to 1.5 percent), it usually means the glycol circulation rate is too high and should be decreased. If the spread is too wide (over 4 to 5 percent) it usually means the glycol circulation rate is too low and should be increased.

Another option is to implement a continuous feedback control loop using a dependable moisture analyzer. The analyzer is installed on the sales gas line and the output signal is used to regulate a variable speed glycol recirculation pump to maintain the moisture setpoint within specified limits. These limits may be established based on various factors directly related to weather conditions and contract requirements.

Lastly, when glycol dehydrators are bought, consider using more trays, usually about 10, to lower the required glycol circulation rate (Ballard, 1986). The required circulation rate rapidly decreases as the number of trays are increased from 6 trays up to 10 trays.

• Inlet Gas Temperature and Pressure: The amount of emissions that may occur for a well posed dehydration application involving properly trained operators is ultimately determined by the amount of water in the inlet gas. This in turn is determined by the inlet temperature and pressure of the gas. The water content of the gas will decrease through condensation as temperature is lowered and pressure is increased. So, also, will the concentration of higher boiling point aromatics and their corresponding emissions. Accordingly, the operating temperature should be minimized and the operating pressure maximized to the fullest extent possible. However, in most cases temperature will be the easiest to adjust and will likely offer the greatest gains.

Sometimes if the pressures are relatively low, it may be feasible to install an inlet air cooler when the inlet gas temperature is too high.

It should be noted, however, that TEG becomes sufficiently viscous to impair efficient contact at temperatures below about 15oC (API, 1990).

• Lean Glycol Temperature: It is important that the glycol entering the contactor be cooled to 5o to 15o C above the temperature of the gas stream to prevent hydrocarbon condensation in the absorber and subsequent foaming (Ballard, 1986; API, 1990). At higher temperatures, more water vapour will remain in the gas stream resulting in the need for higher glycol circulation rates and increased loading of the reboiler. Higher temperatures will also contribute to higher glycol losses through vaporization into the sales gas stream.

• Stripping Gas: Stripping gas may be used to obtain higher glycol purities to help lower glycol circulation rates. This will tend to lower BTEX emissions but increase methane emissions. The benefits of using stripping gas diminish rapidly beyond rates of about 15 to 20 L/L of TEG solution circulated (Hernandez-Valencia, 1992).

• Reboiler Temperature: The operating temperature of the reboiler should be as high as possible without exceeding 205oC to ensure maximum reconcentration of the TEG and thereby suppress the necessary glycol circulation rate. Higher temperatures will lead to excessive glycol losses into the still column vent gas and possibly thermal decomposition of the glycol.

On a standard unit (i.e. one with a gas fired reboiler) this temperature is thermostatically controlled and fully automatic. Nonetheless, the reboiler temperature should be occasionally verified with a test thermometer to make sure true readings are being recorded (Ballard, 1986). The reboiler operates best when it is able to achieve a uniform temperature. If the temperature fluctuates excessively when operating below the design capacity, the fuel gas pressure should be reduced. Conversely, if the reboiler temperature cannot be raised as desired, it may be necessary to increase the fuel gas pressure up to about 200kPa and readjust the dampers on the air intake.