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Comment PDF Separation Processes

Operating Strategies for Gas Dehydration Units

By Krishnan Madan Mohan and Suman Pachal, WorleyParsons Engineering Oman |

The presence of certain contaminants can wreak havoc on gas dehydration systems and other similar separation processes, but there are design measures that can help to minimize the issues caused by these substances

The presence of the aromatic compounds benzene, toluene, ethylbenzene and p-xylene (BTEX) and acid gases, such as hydrogen sulfide (H 2 S) and carbon dioxide (CO 2), in the wet gas of tri-ethylene glycol (TEG) gas-dehydration units (Figure 1) can result in numerous operating problems. These problems can be minimized by optimizing certain operating parameters and paying careful attention to details during the equipment design stage. This article presents several ways to minimize operating and maintenance problems in TEG gas-dehydration processes.

FIGURE 1. This schematic shows the layout of a typical gas-dehydration unit

FIGURE 1. This schematic shows the layout of a typical gas-dehydration unit

In TEG-based natural-gas dehydration units, most operating and maintenance problems usually occur when the circulating glycol becomes contaminated. The contaminated glycol has a tendency to cause foaming and fouling. Foaming can increase glycol loss and reduce plant capacity. Foaming can also result in poor mass transfer between the gas and the glycol solution and can affect treated-gas quality. Furthermore, contaminated glycol will aggravate fouling in heat exchangers. Fouling in the exchanger for the lean and rich glycol streams will result in poor heat transfer, which in turn will increase reboiler duty and affect the purity and quality of lean glycol, and even potentially cause pump failure. For longterm trouble-free operation of glycol units, it is important to eliminate or minimize the occurrence of foaming and fouling.

Process basics

Figure 1 depicts a typical layout of a TEG-based dehydration unit. After the removal of oil and some condensate from the wet gas stream, it is necessary to remove most of the associated water. The free water associated with the extracted natural gas is removed by simple separation methods at production stations or near the wellhead. The equilibrium water vapor that exists in the natural gas is removed by a gas dehydration process. The glycol is used as a dehydrating agent, since it has high chemical affinity toward water. Typically, a TEG unit follows these conventions.

Wet gas from the wet-gas separator is sent to the bottom of the contactor in the glycol dehydration unit. Lean and water-free glycol is fed to the top of the contactor, where it countercurrently contacts the wet gas stream flowing from the bottom to the top of the contactor. The lean glycol removes water from the natural gas by physical absorption and will flow to the bottom of the contactor. Upon exiting the contactor, the glycol stream is referred to as “rich glycol.”The dehydrated gas leaves from the top of the contactor through the exchanger and is routed to the hydrocarbon-gas dewpointing unit.

The rich glycol from the bottom of the contactor is routed to the TEG regeneration unit for initial heating in the glycol reflux-condenser tube bundle and passes to the glycol flash vessel, where hydrocarbon vapors will be flashed off and liquid hydrocarbons (HC) will be skimmed from the glycol. This step is necessary because the contactor is operated at high pressure, and the pressure must be reduced for adequate separation in the regeneration process. Due to the composition of the rich glycol, a vapor phase with high hydrocarbon content is formed when the pressure is lowered. The gas-free rich glycol is filtered through particulate and activated-carbon filters to remove contaminants.

The rich glycol is then routed to the glycol still column (also known as the glycol regenerator) through the lean/rich glycol exchanger, where rich glycol is heated. The rich glycol flows downward through the glycol-regenerator stripping column for water removal.

The lean glycol from the glycol regenerator is transferred to the glycol contactor by the glycol booster and circulation pumps through the cooler and particle filter. An air cooler is deployed to cool down the lean glycol before it enters into the glycol contactor for the effective gas-dehydration process.

 

Solubility of BTEX and acid gas

The amount of BTEX absorbed in the contactor is a function of several parameters, including the solubility of BTEX in the glycol used, the BTEX concentration in the feed gas, the absorption pressure and temperature and the glycol circulation rate (see Figures 2 and 3). Note that operating pressure does not have a strong effect on aromatic absorption. Commonly used glycols for dehydration applications are TEG, ethylene glycol (EG) or diethylene glycol (DEG). While TEG is the most common glycol used in gas dehydration applications, it also absorbs significantly more hydrocarbons than EG or DEG.

dehydration

FIGURE 2. The solubility of contaminant species in glycol should be considered when specifying the glycol circulation rate for a system

 

dehydration

FIGURE 3. The contactor temperature is another parameter that affects BTEX concentration in the glycol

Glycol has a tendency to absorb acid gases. Acid-gas solubility is a major concern when TEG is used as an absorbent in gas dehydration plants. The solubility of acid gas in TEG is favored at low temperatures, high pressures, higher TEG concentration, more TEG solution circulation rate and higher partial pressure of acid gas in the feed gas (see Figures 4 and 5). If acid gas is present in significant quantities in the wet gas, then it will increase the saturation water content of the natural gas. The presence of acid gases in the TEG solution lowers its pH and enhances corrosion in the glycol circuit.

In addition, another major concern is dealing with the emission of BTEX and H 2 S from the still regenerator. In most countries, these components are considered hazardous air pollutants, and emissions of these components are strictly regulated.

 

FIGURE 4. The concentration of acid gas in the glycol negatively impacts the effectiveness of the process to remove water from the gas

FIGURE 4. The concentration of acid gas in the glycol negatively impacts the effectiveness of the process to remove water from the gas

FIGURE 5. As with BTEX compounds, the contactor temperature impacts the concentration of acid gases present in the glycol

FIGURE 5. As with BTEX compounds, the contactor temperature impacts the concentration of acid gases present in the glycol

 

 

Operating and maintenance issues

TEG units are typically capable of operating with few issues. However, there are some problem areas that can occur when the circulating glycol solution gets dirty. To ensure trouble-free operation, it is necessary to recognize these problems and know how to prevent them. Some of the major problems are as follows:

  • Foaming
  • Corrosion and fouling
  • Glycol losses
  • Thermal degradation of glycol
  • Salt contamination
  • Sludge formation
  • Oxidation
  • Low pH

Although there are several additional concerns, this article limits the discussion to the mitigation of foaming, corrosion and fouling, low pH problems and the effects of aromatics and acid gases.

 

Foaming

One of the most serious and common problems encountered in gas dehydration units is foaming. The root cause of foaming is often difficult to identify. However, if the circulating glycol solution is not continually cleaned by filtration, then it can cause foaming. Some of the major factors that promote foaming are entrained hydrocarbon liquids, dissolved aromatics, H 2 S in the glycol, salt contamination, field corrosion inhibitors, excessive turbulence and high vapor-to-liquid contacting velocities.

 

Corrosion and fouling

Corrosion is another major issue in glycol dehydration units. A pure glycol solution is non-corrosive to carbon steel. However, the presence of impurities in the glycol solution causes corrosion to occur. The impurities may come from oxidation or thermal decomposition of the glycol, or they may enter into the solution from the gas stream, which is subjected to upstream purification and processing. Glycol can react with sulfur compounds present in the feedgas stream. The resulting materials tend to polymerize during the regeneration process, and form a thick, messy substance that is very corrosive. This substance also inhibits effective heat transfer in the reboilers.

The corrosion rate depends upon various factors, including temperature and velocity. If the glycol solution is not properly cleaned, it may result in glycol oxidation or thermal decomposition in the reboiler due to higher temperature, which will lead to corrosion.

Fouling can result in leakage and poor heat transfer in plate-and-frame heat exchangers, which will increase heat flux in the reboiler system.

 

Low pH

The most troublesome corrosive contaminants, including products of glycol oxidation or thermal decomposition, as well as acid gases absorbed from the gas stream, can also lead to conditions of low pH. The ideal glycol pH should be in the range of 7 to 7.5. Practically, it may not be feasible to maintain this range for a long period, but it is both recommended and possible to maintain system pH above 6 continuously. If glycol circulation rates result in more acid-gas absorption, low operating temperatures may occur, which may in turn lead to accelerated corrosion and glycol decomposition.

 

Recommendations

There are some suitable design and operating strategies to minimize the operating and maintenance problems that result from the presence of contaminating species in the glycol. The main design and operating recommendations are detailed in the following paragraphs.

Optimize glycol circulation.The glycol circulation rate and the absorption of BTEX and acid gas in the circulating glycol are directly proportional, as seen in Figure 2. Hence, reducing the glycol circulation rate is the most effective way of decreasing the absorption of BTEX and acid gas. While reducing the circulation rate will increase the number of theoretical stages (mass transfer stage) in the contactor column’s design to achieve the desired outlet water content specifications, it does help to reduce issues like foaming, fouling and high glycol losses. A lower circulation rate can also decrease BTEX emissions and reduce the reboiler duty. However, it should be ensured that when adjusting the glycol circulation rate, the system can still meet the minimum wetting rate and achieve adequate liquid distribution in the contactor, which is essential for effective mass transfer between the gas and glycol to facilitate the required water removal.

Avoid hydrocarbon carryover and condensation. The glycol is chemically reactive and needs to be protected against contamination. Major operating problems can arise due to inadequately designed glycol contactor-inlet scrubbers or separators. An inlet-gas scrubber or separator can be provided upstream of the contactor to avoid liquid hydrocarbon carryover to the contactor. The integral scrubber, as part of the contactor, should not be used as the primary separator. The primary separator must be be properly sized with suitable internals to remove liquid hydrocarbons, free water, solids and other chemical agents. Even small quantities of contaminated materials can result in excessive glycol losses due to foaming, reduced efficiency and additional maintenance problems. Heavy hydrocarbons in the glycol can cause coking on the reboiler surface, creating hot spots on the firebox and plugging in the regeneration system. Heavy hydrocarbon presence can also increase reboiler heat load due to elevated boiling points, and result in glycol losses.

Condensation in the contactor can be prevented by maintaining the inlet glycol temperature 3 to 6°C above the feedgas inlet temperature. If not maintained, condensation of the hydrocarbon might occur, which can cause foaming and increase glycol losses.

Flash vessel considerations. The glycol flash vessel is used to remove light hydrocarbons, acid gases and small amounts of aromatics by rapid reduction in pressure (flashing). Degassing in the flash vessel before the rich glycol enters the lean/rich exchanger helps to prevent foaming and fouling in the exchanger and reboiler. If hydrocarbons such as BTEX are present along with CO 2 and H 2 S in the rich glycol, then a preheating step is more efficient for the degassing process. The recommended preheating temperature is about 70 to 75°C, and the recommended operating pressure of the flash vessel is around 3 to 5 bara.

Activated-carbon filter. A properly designed activated-carbon filter can effectively remove most foaming- and fouling-promoting compounds in the glycol. The filter should be installed downstream of the particle filters in the rich glycol line. Carbon filters are usually sized for glycol loading of 2.5 to 5.0 m 3 /h per m 2 of filter cross-sectional area. If the rich glycol contains dissolved components, such as BTEX, H 2 S, CO 2 or heavy hydrocarbons, it is suggested to install two full-flow activated-carbon filters in parallel with no bypass line. If the filters are not designed for full flowrates, then foaming and corrosion impurities in the rich glycol will enter into the lean/rich exchanger, regenerator system and circulation pumps. The impurities will cause exchanger fouling, leading to poor heat transfer and poor regenerator performance, which affects overall glycol purity. If the lean/rich exchanger is of the plate-and-frame type, then it will cause frequent maintenance and mechanical damage. This will also result in failure of the glycol circulation pump in the long term.

Canister- or cartridge-type synthetic-carbon filters are generally preferred when compared to loose charcoal beds (installed as fill into a vessel) because they are easier to maintain and avoid unnecessary exposure of workers to BTEX components. Contaminated spent charcoal is difficult to dispose of unless it is contained in a canister.

A synthetic-type carbon filter derived from petroleum products can be used effectively for aromatic and acid-gas removal. The main advantages of these types of filters are high surface area and high adsorption capacity compared with activated carbon derived from wood-based charcoal, coconut shell or bituminous coal.

The replacement of a synthetic-type carbon filter cannot be determined using the conventional approach of filter differential-pressure measurement, because synthetic-based carbon does not create a pressure differential after filter exhaust. The filter replacement interval should be determined based on visual examination (color comparison between inflow and outflow) or laboratory analysis of glycol samples, which can indicate high hydrocarbon content.

The ideal solution for the operational problems described in this article involves good mechanical design of the inlet separator, effective preheating of the rich glycol before entering the glycol exchanger and proper selection of a synthetic carbon filter with two filters in parallel and full-flowrate capacity with no bypass line. This will effectively eliminate most foaming and corrosion problems by removing the hydrocarbons and other troublesome impurities from the glycol, which will result in minimized operating and maintenance problems. Reducing the glycol circulation rate is the most effective way of decreasing the absorption of BTEX and acid gases in circulating glycol. ■

Edited by Mary Page Bailey

 

References

1. Campbell, J.M., “Gas Conditioning and Process — Vol. 2: The Equipment Modules,” 8th Ed., 2004.

2. Gas Processors Suppliers Association (GPSA) Data Book, 13th Ed., 2012.

3. Stewart, M. and Arnold, K., “Gas Dehydration Field Manual,” 1st Ed., Gulf Professional Publishing, August 2011.

4. Kohl, A.L. and Nielsen, R.B., “Gas Purification,” 5th Ed., Elsevier Science, August 1997.

 

Author

Krishnan madanmohna photoKrishnan Madan Mohan is a senior process engineer at Worley-Parsons Engineering Oman (P.O. Box 795 Al-Khuwair, Muscat, Oman; Telephone: 968-24473394; Email: k.mohan@worleyparsons.com). He has 15 years of professional experience in the oil-and-gas industry, including activities related to feasibility studies, concept selection, basis of design and front-end engineering design (FEED) and detail design. His experience includes debottlenecking, upgrades, design modifications for glycol-based gas dehydration units and sour-water stripper units, flare and relief studies, including detailed blowdown studies, equipment sizing and thermal design for heat exchangers and air coolers. He holds a B.Tech. (chemical) degree from the National Institute of Technology (NIT), Tiruchirappalli, India.

Suman PachalSuman Pachal is a process engineer with WorleyParsons Engineering Oman (Same address as above; Email: suman.pachal@worleyparsons.com). He has more than 10 years of design consultancy experience in the oil-and-gas, petroleum-refining and petrochemicals sectors. Before joining WorleyParsons, he worked as a process engineer for Technip India in New Delhi. His areas of expertise include line hydraulics, equipment sizing, relief valve sizing and vent and blowdown calculations. He holds a B.Tech. (chemical) degree from the University of Calcutta, Kolkata, India.

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