THE ROLES OF ACTIVATED CARBON IN GAS CONDITIONING
by
MARTA J. BOURKE
AND
ANTHONY F. MAZZONI
CALGON CARBON CORPORATIONPITTSBURGH, PA 15230
For presentation at the Gas Conditioning Conference
March 6-8, 1989
INTRODUCTION
Activated carbon has been successfully used in numerous gas processing applications for many years. Carbon's highly porous nature and large surface area make it an ideal adsorbent for removing trace contaminants from liquid and gaseous streams. For some inorganic contaminants encountered in gas processing operations, removal can be enhanced by impregnation of the carbon to promote both physical adsorption and chemisorption.
This paper will review uses for activated carbon in gas conditioning in the following areas:
Purification of recirculating amines and glycols
Desulfurization of natural gas
Mercury removal from natural gas
Equipment and catalyst protection.
Calgon Carbon has extensive experience in these applications. Specific examples will be reviewed to provide basic guidelines and understanding of the proper utilization, selection, and design of activated carbon and related systems. Operating parameters, performance results, and benefits for a number of actual cases will be discussed.
ACTIVATED CARBON AND CARBON SELECTION
Activated carbon can be manufactured from any carbonaceous material -coal, wood, peat, coconut shell, etc. Granular activated carbon is generally produced by grinding the raw material, adding a suitable binder for hardness, recompacting, and crushing to the correct mesh size. The carbon-based
material is converted to activated carbon by thermal decomposition of the organic materials present in a controlled non-oxygen atmosphere. The resultant product has an incredibly large surface area per unit volume and a network of submicroscopic pores where adsorption occurs. One pound of carbon provides a surface area equivalent to 125 acres.
Because activated carbon can be made from various raw materials, differences will exist in the finished product. Domestically, most granular carbons are manufactured from various grades of coal. Some grades of coal need a chemical pretreatment prior to thermal activation to develop proper pore structure. Table I shows some of the properties of activated carbon and how these vary with the raw material. These differences in carbon properties make some products better suited than others for specific applications.
Iodine Number is the most common standard for indicating total surface area available for adsorption. It is defined as the milligrams of iodine adsorbed by one gram of carbon and it approximates the internal surface area (micro pores) in square meters per gram. Table I shows how Iodine Number can vary for activated carbons produced from different materials.
Densities of the carbon can vary drastically; fewer pounds of a carbon with a low density will fill the same volume as a higher density product. This is significant because the contaminant removal capacity of this volume of low density carbon is severely reduced. The concept of volume activity then becomes important in selecting carbons. A simple calculation for determining the volume activity of carbon is to multiply the bulk density by the Iodine Number. Thus, two containers having the same volume of carbon will have different total surface areas if the densities are different.
Abrasion resistance refers to a carbon's ability to withstand degradation during handling and is expressed in terms of an Abrasion Number; the higher the Number, the more resistant a carbon is to abrasion. This characteristic is especially important for high pressure applications, where carbon fines in the effluent can cause process problems and where carbon is on line for extended periods.
Ash content of activated carbon can be significant as some carbons have a high total ash content which contribute nothing for adsorption. These inorganic materials can leach from the carbon and cause operational problems such as foaming.
PURIFICATION OF AMINES AND GLYCOLS
In the process of absorbing acid gas constituents, amine streams becomes contaminated with thermal and chemical degradation products, organic acids and/or iron sulfides. Contamination of the amine stream can cause foaming and carryover in the absorber or stripper, high steam and circulation rates, increased system corrosion and heat stable salt formation. A properly designed activated carbon system can reduce, and in many cases eliminate, the need for anti foam agents, reduce amine make-up rates, reduce total system energy consumption, reduce corrosion, and improve scrubbing efficiencies and product quality.
Historically, many carbon systems have been under-designed and/or operated incorrectly. The carbon systems had insufficient contact time, and in many cases, the wrong carbon was used and the carbon was rarely changed. A properly designed carbon system should treat a 10-20% slipstream of the amine solution, have a minimum of 15 minutes empty bed contact time, and a superficial velocity of 2-4 gpm/ft2. When the amine solution changes color and clarity, it's usually time to change the carbon, since this is usually indicative of the carbon being exhausted.
Figure 1 shows an effective carbon system. Besides having a properly designed and sized adsorber, the system also includes mechanical filtration both upstream and downstream of the carbon adsorber. The upstream filter removes suspended solids to ensure the carbon is used to adsorb contaminants, and the downstream filter will remove carbon fines which could cause foaming. A feature of this system is the carbon handling equipment which allows quick and easy change out of the carbon without exposing personnel to spent carbon.
The carbon recommended is a hard, small mesh, steam activated carbon, having a broad range of pore diameters. This prevents short bed life, excessive fines, foaming, and the ability to remove a wide range of organic contaminants. The carbon should also be free of phosphorus, which could leach from the carbon bed in the form of phosphates. These phosphates could cause additional foaming in the amine stream.
Cost savings which can be realized with a properly designed carbon system were presented in an article titled "Gas Treaters Need Clean Amines" in the December 1987 issue of Hydrocarbon Processing. The article by R. L. Bright and D. A. Leister of Calgon Carbon Corporation, illustrated actual cost savings of a refinery in the western United States. The carbon system was installed to reduce a severe foaming problem in the amine contactor. The amine solution was black in appearance and operating costs were very high. Table II shows the operating cost elements before and after the activated carbon system was put into service. Because the carbon treated amine did not carry over from foaming, the amine make-up and anti foam consumption was reduced. The refinery was able to increase the amine strength and reduce circulation which resulted in steam savings. Finally, the filter cartridge usage was reduced because the activated carbon removed organic acids from the amine stream resulting in lower tendency to create iron sulfide. All of these reductions resulted in a first year savings of $147,800 (Table III). Second year savings increased to $205,800 as the refiner was able to further reduce anti foam addition, filter cartridge replacement and amine make-up.
As a side benefit, the product gas quality improved to where the refiner was able to reduce H2S concentration from 10-40 ppm to at or near non-detectable levels. Heat stable salts were reduced to a low level of 5 ppm. Although activated carbon does not adsorb heat stable salts, it will remove the precursors to their formation.
A similar application for activated carbon is the removal of dissolved impurities from glycol streams which could cause foaming. The reduced foaming tendency of the glycol provides better contact with the gee which improves drying efficiency. In this application, the activated carbon system can be installed on the rich side. Due to the higher viscosity of the glycol, the maximum superficial velocity recommended is 2gpm/ft2.
DESULFURIZATION
Natural gas feed stocks are used for the production of many chemicals including ammonia, methanol and hydrogen. A feed stock virtually free of sulfur is required to protect the catalysts from deactivation. Precious metal catalysts are highly susceptible to sulfur poisoning and subsequent loss of production and shortened life.
Depending on the sulfur concentration, various sulfur control technologies can be applied. When H2S or other low molecular weight sulfur compounds are present at 10 ppm or less (as total sulfur), plants have successfully used granular activated carbon to remove the sulfur compounds. Metal oxide impregnated carbons such as Calgon Carbon Type FCA are specifically designed to chemisorb hydrogen sulfide and mercaptans.
As shown in Figure II, the metal oxides within the carbon pores react with the sulfur compounds to form sulfides and sulfates. The carbon can then be regenerated in situ with hot gee containing traces of oxygen to restore the impregnants to the metal oxide form. If large amounts of undesirable high molecular weight organics are present in the feed gas, a guard bed of unimpregnated carbon is used.
This method of sulfur control is being utilized at more than 60 plants in the United States. At a Louisiana chemical plant, several beds of FCA are being used for desulfurization of the methanol and ammonia plant feed stocks. Table III details the design conditions. The influent H2S concentration in the natural gas varies up to 10 ppm. Each unit has 2 carbon vessels in series operation. When the concentration of H2S in the natural gee leaving the lead vessel reaches 0.22 ppm, the vessel is regenerated with steam and traces of oxygen. The regenerated carbon bed is then put into the polish position. Currently, the time between regenerations is 6 months, but this can vary depending on H2S concentration in the natural gas.
MERCURY REMOVAL
In years past, the presence or absence of mercury received little attention in the oil and gas industry. In the early seventies, natural gee processors began to use cyogenic techniques utilizing aluminum core heat exchangers (cold boxes). After the failure of several cold boxes, metallurgists determined that mercury corrosion was the source of the problem. Initially, it was believed that the mercury was present due to leaking instrumentation; however, further testing revealed mercury was present in the reservoir.
Extremely low levels of naturally occurring elemental mercury in the inlet gas can change to a solid phase or liquid phase under normal plant operating conditions. When the mercury accumulates as a liquid in the heat exchangers, it can amalgamate with the aluminum and destroy the heat exchangers. The exchanger is not only extremely expensive to replace or repair, but a failure is an obvious safety hazard as well. Traditionally, it was believed that only Indonesian and North African gas contained sufficient amounts of mercury to cause equipment problems. However, several Gulf Coast gas and olefin plants have experienced cold box failures which have been traced to mercury corrosion within the past two years. Unacceptable levels of mercury have also been detected in tertiary oil recovery plants in the western United States.
The suspected mechanism of mercury corrosion is multi-step. Being an extremely heavy metal with a very high vapor pressure, mercury tends to accumulate in pipe bends and along the welds of elbows -areas where the aluminum may already be stressed. For liquid metal embrittlement (LME) or intergranular mercury corrosion to occur, the mercury must be present in a liquid state, i.e. above -40°F, and must wet the surface of the aluminum. Aluminum is slightly soluble in mercury and amalgamatea with the mercury. The aluminum then forms an aluminum oxide and frees the mercury to attack the next layer of metal. Mercury is not permanently bound with the aluminum, but actually acts as a catalyst for the corrosion or embrittlement to occur.
Even minute amounts of mercury in the gas can threaten the integrity of the plate-fin aluminum exchangers. Conditions which favor amalgamation are: temperatures in excess of -40°F, an oxide free or stressed aluminum surface, presence of moisture or air at the mercury/aluminum interface, small amounts of mercury in the liquid state.
Activated carbon is effective at physically adsorbing trace components from gas streams due to its high population of micro pores; however, the capacity is low. By impregnating the carbon with a compound which will chemically react with the mercury, capacity can be enhanced. It is imperative that not only must the mercury be removed to non-detectable levels but that it is adsorbed in a form which will not elute from the carbon bed under fluctuating conditions. In order to obtain long bed life and high removal efficiencies, a carbon is required with a pore structure which can accept substantial amounts of suitable impregnant while allowing access of the process gas to the complex pore structure. Type HGR impregnated activated wee specifically designed for this service. Under normal operating conditions, a properly designed HGR bed will remove mercury for many years. HGR carbon has been used successfully since 1975 to protect aluminum exchangers from corrosion due to low level mercury contamination in LNG and, more recently, olefin plants.
A south Texas pipeline company had a failure of their aluminum cold box, which through metallurgical analysis, was traced to mercury corrosion. Calgon Carbon worked with the pipeline company and, through on-site analysis, determined there was over 50ug/Nm3 of mercury in the natural gas. Using the operating conditions of Table V, Calgon Carbon assisted in designing a mercury removal system and determining the best location of the system in the plant. The mercury removal system has been operating since 1984 removing mercury to non-detectable levels.
CATALYST AND EQUIPMENT PROTECTION
Process and plant engineers have found that using activated carbon is an excellent way to protect expensive catalysts and process equipment from other low concentration poisons which can deactivate catalysts or damage equipment. Carbon is highly cost-effective and relatively simple to use. Some of the applications for utilizing carbon as a guard bed include:
• Removal of organic compounds that can foul downstream equipment
• Arsine control in acetylene, ethane-propane, and other hydrocarbon streams
Aromatics removal to prevent "charring" of Claus catalyst
• Compressor oil removal for high purity gases
• Acid gas removal to protect sensitive electrical equipment
• Purification of process air.
Calgon Carbon utilizes impregnated and non-impregnated carbons to control a wide variety of contaminants to very low effluent levels. Since these carbons are extremely versatile and can operate at existing plant conditions, capital investments and operating costs are minimal. All of the multitude of applications cannot be discussed here; however, a few examples are presented.
One equipment protection application involved a tertiary oil project which utilized triethylene glycol (TEG)dehydration prior to the cryogenic separation plant. Plant personnel and the design engineers became concerned that the TEG would plate out in the cryogenic processes and foul equipment. They installed Calgon Carbon's pelletized activated carbon upstream for removal of the TEG. The estimated influent TEG concentration to the carbon beds was 9 ppm with an effluent objective of 0.02 ppb. Data collected from the site indicates high loading of the TEG (15 wt.%) on the activated carbon and excellent removal efficiencies. See Table VI.
With the advent of sophisticated computer and motor control systems, the need has arisen to protect this equipment from acid gas corrosion. The Instrument Society of America has issued guidelines for computer and motor control room air quality. The concentrations listed below are for a G1 (mild) environment:
COMPONENT MAXIMUM CONCENTRATION (ppb)
Hydrogen Sulfide 3Chlorine 1SO2, SO3 10NOX 50HF 500Ozone 2
Failures of sensitive circuitry in computers, laboratory equipment, and other electronic equipment can lead to long downtimes and high maintenance costs.
Calgon Carbon recommends an all-fiberglass system utilizing a specially impregnated carbon specifically developed for acid gas removal. Air contaminated with corrosive gases is filtered at the fan inlet to remove particulate matter. The air is then delivered at a controlled rate and pressure to a deep bed of Calgon Carbon IVP Granular Activated Carbon to remove the corrosive gases (see Figure III). Treated air is then passed through a 30% and a 99% ASHRAE filter to remove micron size particulates. As shown, the carbon system can be placed on the make-up air and/or the recirculating air depending upon the level of severity. Sulfur plants are an obvious source of hydrogen sulfide, SO2, and SO3.
Calgon Carbon's IVP Carbon is impregnated with sodium hydroxide to enhance adsorption of acid gases. Figure IV shows the chemical reactions which occur. Since the gases are chemically bonded to the carbon surface, there is no potential for desorption of the acid gases and the carbon can effectively remove hydrogen sulfide to non-detectable levels even under varying influent conditions.
SUMMARY
Granular activated carbon has been employed in many gas conditioning and related processes. To obtain optimum results, each application should be individually evaluated by a carbon expert to determine the appropriate carbon and design parameters. A properly designed carbon system can reduce downtime, improve gas quality, protect equipment and catalysts, and reduce overall operating costs.
TABLE I
TYPICAL PROPERTIES
OF
ACTIVATED CARBONS
BITUMINOUS SUB-BITUMINOUS LIGNITE
Iodine number 1,000 1,000 600
Bulk density aspacked (#/ft3) 26 25 23
Volume Activity 26,000 25,000 13,800
% Ash 6.7 12.3 20.1
% Phosphorus* <0.05 1-5 <0.05
Abrasion Number 80 75 60
* Component of ash which can cause foaming in amine solutions
TABLE II
OPERATING COST ELEMENTS BEFORE ANDAFTER CARBON SYSTEM INSTALLATION
PRE-CARBON POST CARBON SYSTEM SYSTEM REDUCTION
AMINE CIRCULATION 300 265 35(GPM)
ANTIFOAM CONSUMPTION 4-5 1 -2 3(DRUMS/YEAR)
FILTER CARTRIDGES 30-40 12-15 20(NO. USED/DAY)
AMINE MAKE-UP 1,400 1,000 400(GAL/MONTH)
STEAM USAGE 18,000 16, 000 2,000(LBS./MONTH)
UNIT COST
DEA -$0.40/LB.
STEAM -$4.44/1,000 LBS.
FILTER CARTRIDGES -$7.37
ANTIFOAM -$4,000/DRUM
TABLE III
FIRST YEAR SAVINGS
AMINE MAKE-UP $20,000
STEAM 77,800
FILTER CARTRIDGES 54,000
ANTIFOAM 12,000
CARBON USAGE -16,000
TOTAL $147,800
TABLE IV
H2S REMOVAL FROM NATURALGAS LOUISIANA CHEMICAL PLANT
DESIGN BASIS Unit 1 Unit 2 Unit 3Gas Flow (SCFM) 6900 6800 3200
Gas Temp. (°F) 49 39 45
Gas Pressure (PSIG) 190 190 210
H2S Conc. <---Varies between .1---> and 10 ppm
FCA CARBON SYSTEM
Pounds of FCA/Vessel 8450 8450 65202 Vessels Each Unit
H2S Concentration <---Non-detectable to--->Out of Vessels .22 ppm
Regeneration <---Steam + Trace O2 --->About 6 months
TABLE V
MERCURY REMOVAL FROM NATURAL GASSOUTH TEXAS PIPELINE COMPANY
DESIGN CONDITIONS
30 MMSCFD1100 PSIG110°F*Mercury content over 50 ug/Nm3
HGR CARBON SYSTEM (installed 1984)Carbon Steel adsorber1500 pounds Type HGR*Mercury level after carbon 0.001 ug/Nm3
* Analyses are using a Jerome Model 301 Gold Film Mercury Analyzer
TABLE VI
WEST TEXAS TERTIARY OIL RECOVERY
TEG REMOVAL
CARBON DIOXIDE INJECTION FACILITY
DESIGN CONDITIONS
2 Trains93 MMSCFD, each train370 psia100°F
GAS COMPOSITION
COMPOUND MOLE PERCENT
Carbon Dioxide 77Hydrogen Sulfide 1Methane 12C2+ 10
TEG Contamination 9 ppm
CARBON SYSTEM
8.5' diameter10' bed depth16,000 pounds of carbon per bed
