MERCURY REMOVAL FROM CRACKED GAS AND LIQUID STREAMS
Steve Coleman and Jason Ries, Equistar Chemicals LP Channelview, Texas, USA and Don Campbell and Keith R. Clark, UOP LLC Houston
, Texas, U.S.A.
To maximize profits in today’s business climate, ethylene producers are cracking heavier, more cost-effective “opportunity” feedstocks. Mercury is often present in these liquid feed streams, causing significant quantities to be carried forward into the cracked gas and liquids. Mercury is known to deactivate metal-based hydrogenation catalysts and damage aluminum heat exchangers and cold boxes to the point of catastrophic failure. Removal of this contaminant upstream will improve the safety of unit operation and reduce the potential for personnel exposure during routine maintenance or shut downs. HgSIV? regenerative mercury removal adsorbents are regenerable and remove both mercury and water. Since the sorption sites for mercury removal are separate from and additive to the dehydration sites, mercury removal is accomplished by replacing a portion of the dehydration grade molecular sieve with HgSIV adsorbents. This paper will discuss a case study where mercury is being successfully removed from cracked gas at an ethylene plant.
Mercury, present in nature, is found in trace concentrations in various ethylene feedstocks, in particular natural gas field condensates. Removal of this mercury is desirable to protect metal-based hydrogenation catalysts from deactivation. In addition, aluminum equipment and piping is subject to attack by mercury via a variety of mechanisms including liquid metal embrittlement, amalgamation and amalgam corrosion (Crippen, 1997). Such attacks have resulted in equipment failures in ethylene plants (English, 1989) and similar processing facilities (Wilhelm, 1990; Kinney, 1990). Moreover, the presence of mercury in process equipment being cleared and cleaned during plant shutdowns requires steps to prevent personnel exposure. For these reasons, ethylene plant operators prefer to exclude mercury from their facilities or reduce it to a level that can not be detected with available analytical capabilities. Currently, this means reducing the mercury to less than 10 ng/Nm3, which corresponds to about 10 ppt by volume. UOP has been testing for the presence of mercury for decades, but historically, low level mercury analysis had to be done in a laboratory setting. Initially, this work was done in UOP’s
labs in Des Plaines, Illinois. However, for over a decade, UOP has been measuring mercury in gas streams in the field, both to validate the effectiveness of its HgSIV adsorbent and to determine if mercury is present in new facilities. UOP started with natural gas streams and now has experience working with cracked gas streams from ethylene plants. Over time, UOP has upgraded its techniques and analytical equipment, allowing even greater precision and accuracy in the field. UOP has learned much from practical experience and has integrally incorporated these discoveries into perfecting field mercury analysis. Due to the complex hydrocarbon matrix, this latest venture in cracked gas measurement presented unique challenges.
Obtaining precise and accurate measurements requires the utmost care and a precision sampling system. In adsorbent bed outlets, levels of mercury are extremely low and any introduction of error can dramatically skew the results. Obtaining a representative sample from a process line requires a special sample tap (stainless steel quill), as shown in Figure 1, that has been thoroughly purged. This quill is imperative because pipe wall effects and condensed liquids in sample points will affect the distort the mercury readings for a significant period of time. Figure 1 Sample Point Configuration for Hg Analysis
1/4" SS Swagelok Valve Bored-Through Male Connector 1/4" Tube to 1/2" NPT
Bushing 1/2" x 3/4" NPT
Existing Gate Valve
1/4" O.D. SS Tubing Extended Away from Wall
Field measurements are performed by first concentrating the mercury in the sample stream on a gold surface trap, a technique utilized by many available mercury analyzers. The detector quantifies the amount of mercury..
The best technology to analyze low levels of mercury in hydrocarbons is cold vapor atomic fluorescence spectroscopy (CVAFS), but the CVAFS’s size has historically limited its use to the laboratory. Complementing its long experience measuring mercury in the field, UOP added CVAFS technology to its testing capabilities about four years ago with the implementation of a new analyzer. As with other techniques, the sampling protocol is critical and it has been optimized through practical experience in hydrocarbon gas streams. At the plant site, UOP reliably measures mercury concentrations as low as 10 ng/Nm3. Each sample can be collected and analyzed within an hour, compared to days or weeks with an off-site facility. Before the analytical team departs, the level of mercury in a treated or untreated stream has been quantified. This information is used to determine whether streams need to be treated for mercury or confirm that an existing mercury removal system is functioning as designed.
UOP HgSIV ADSORBENTS
HgSIV adsorbents were created for effective mercury removal in existing molecular sieve adsorption units. The HgSIV products are molecular sieves that contain silver on the outside surface of the molecular sieve pellet or bead. Mercury from the process fluid (either gas or liquid) amalgamates with the silver, yielding a dry, mercury-free stream. Adding a layer of HgSIV 3 pellets to an existing cracked gas dryer results in the simultaneous removal of the design water and mercury load, without requiring an additional downstream vessel. Mercury and water are both regenerated from the HgSIV adsorbents using conventional gas dryer techniques. Physically, HgSIV adsorbents have a similar appearance and integrity to standard molecular sieves and are available in a beaded or pelletized form. These HgSIV adsorbents are loaded into an adsorption vessel in the same manner. There is no need for special care, such as the use of nitrogen blanketing during the installation. For unloading, only the same precautions need to be taken as with conventional molecular sieves. The disposal requirements are also the same as for dehydration grade molecular sieves, provided the material is properly regenerated prior to unloading. Analysis of fresh and used samples of HgSIV adsorbents have shown that they pass the EPA Toxicity Characteristic Leaching Procedure test (TCLP), and are thus classified as non-hazardous waste for disposal purposes.
UOP first installed HgSIV adsorbents in the natural gas market two decades ago. Currently there are over thirty units operating successfully around the globe. Now UOP is applying this expertise to the ethylene market and has four units in operation. HgSIV 3 molecular sieve is successfully used to remove mercury from cracked gas streams, thereby preventing mercury from entering into the system downstream of the cracked gas dryers. Protection of the downstream cold fractionation section, aluminum heat exchangers and hydrogenation catalysts is thereby readily accomplished, without significantly altering the configuration or operating procedure of the dryers.
Mercury removal is accomplished by a “drop-in” substitution of adsorbent products, without affecting the dehydration performance of a cracked gas dryer, because the sorption sites for mercury removal are separate from and additive to the dehydration sites. The loading diagram for a typical cracked gas dryer is shown in Figure 2. This is a common configuration consisting of a compound main adsorbent bed with a segregated guard bed. A moisture probe is located in the plenum space between the main bed and guard section, and the bed is operated until breakthrough is detected by the moisture analyzer. The guard section sees minimal water and is only used in emergencies. Figure 2 Typical Cracked Gas Dryer Configuration
Figure 3 illustrates a loading configuration which incorporates the HgSIV 3 product. The dryer is operated normally, and the mercury effluent levels achieved are 10 ng/m3 or lower. Although HgSIV 3 pellets have the same dehydration performance as 3A-EPG MOLSIVTM adsorbent, it is positioned in the bottom portion of the bed to minimize the hydrothermal aging that occurs during regeneration. The use of a floating-screen in the main bed facilitates the reuse of the mercury adsorbent on subsequent recharges of the 3A-EPG molecular sieve.
Figure 3 Cracked Gas Dryer Configurationwith HgSIV 3 Pellets
CASE STUDY: EQUISTAR CHANNELVIEW, TX
Equistar processes a variety of feedstocks through its olefins plants in Channelview, Texas. Some of these feeds, including gas field condensates from Algeria, contain trace levels of mercury, some as high as 50 ppb. Recognizing the potential hazards mercury can present to aluminum exchangers, Equistar replaced its brazed aluminum core exchangers in 1989 with a mercury tolerant design and has developed procedures for clearing equipment where mercury might be present. This includes extensive monitoring of equipment for mercury during plant turnarounds and outages. In March 2003, Equistar’s Olefins 2 plant was shutdown for a scheduled turnaround. The plant had been on-line since November 1995. The plant was cleared in a typical manner to
remove residual hydrocarbons in preparation for maintenance. During clearing activities, a significant quantity of mercury, both liquid and vapor phase, was found throughout the cold fractionation area of the plant (Figure 4). Figure 4 Typical Olefins Flow Diagram
Ethylene Cracked Gas Dryer Demethanizer Ethylene Fractionator Propylene
Propylene Fractionator Crude C4s Deethanizer
Quench Quench & Compression Cold Fractionation
Depropanizer Crude C5s
The presence of mercury in the cold fractionation area of the plant was expected based on processing feedstocks contaminated with mercury and previous turnaround experiences in 1989 and 1995. The main difference between the 2003 turnaround and historical experience was the location and quantity of mercury found. In previous turnarounds, the majority of mercury had been found in small concentrations upstream of the brazed aluminum core exchangers. During the 2003 turnaround, mercury was found downstream of the core exchangers and throughout the cold fractionation area of the plant. Mercury concentrations were high enough in many areas to require additional remediation efforts and personal protection equipment to limit personnel exposure. The American Conference of Governmental Industrial Hygienists has assigned mercury vapor a threshold limit value (TLV) of 0.025 mg/m3 as a time weighted average for a normal 8-hour workday and a 40-hour workweek The mercury concentrations in the cold fractionation vessels ranged from less than 0.010 mg/m3 to 0.700 mg/m3. Coping with this unexpectedly high level of mercury delayed a number of turnaround activities. Based on the high levels of mercury encountered in the cold fraction section of the ethylene plant and the desire to prevent mercury from contacting aluminum exchangers and deactivating selective hydrogenation catalyst systems, Equistar initiated an investigation of
options for excluding mercury from the cold fractionation area of the plant. Because of the high mercury removal efficiency (>99.9%) and the speed and ease of implementation, Equistar decided to install UOP’s HgSIV molecular sieve in the Olefins 2 plant’s process gas dryers during the March 2003 turnaround. The HgSIV pellets were a “drop in” solution requiring no additional vessels or equipment. A portion of the dehydration grade molecular sieve, which was coincidentally being replaced during the turnaround, was substituted for with HgSIV material. The HgSIV adsorbent was installed in the guard bed and the lower portion of the main bed, as shown in Figure 3. Loading the sieve in the outlet (bottom) portion of the dryer vessel will extend its life by minimizing degradation due to hydrothermal aging. While the conventional 3A desiccant loaded above the HgSIV material will require replacement in 3 – 4 years, the HgSIV adsorbent can be left in place for another 3 – 4 years, or until the next scheduled turnaround. Several months after the dryers containing HgSIV adsorbent were put in service, UOP Field Technical service was on-site to conduct field mercury analyses of the cracked gas dryer inlet and outlet using the CVAFS instrument. Several trips were made to the Channelview facility, once while running Algerian condensate and once while not. The mercury concentration of the inlet to the cracked gas dryers ranged from 1,500 ng/nm3 to 500 ng/nm3, with significant quantities of mercury detected in the system even when mercury-free feed was cracked. Measurable amounts of mercury remain in the plant, an indication that mercury adheres to equipment and is not readily purged from the system. Testing was also conducted on the regeneration gas inlet; Figure 5 illustrates evidence of mercury detected in the regeneration system approaching 100 ng/nm3. This complicates matters by reintroducing mercury into the adsorber vessel on each regeneration cycle. Mercury contained in the regeneration gas accumulates in the ceramic support material and grating, and then is bled slowly back into the dryer effluent. This residual mercury presented analytical measurement challenges which were overcome with modified sampling and analytical techniques.
Figure 5 Bed A Regeneration Inlet
80.00 Mercury Level (ng/nm )
0.00 10/13/03 09:36 10/13/03 10:48 10/13/03 12:00 Time 10/13/03 13:12 10/13/03 14:24 10/13/03 15:36
To illustrate the complexity of the measurements and for the utmost importance of achieving an equilibrium condition with respect to the mercury levels in the process, Figure 6 shows the effluent measurements of one cracked gas dryer bed through two adsorption cycles. Initial mercury readings were twenty times what we typically expect to see on a bed outlet due to sampling point contamination. Once the system equilibrated, the mercury levels were at or below the 10 ng/nm3 detection limit. To facilitate future mercury determinations, a quill-type sample tap (Figure 1) was modified on the common outlet of the cracked gas dryers. Analysis of the cracked gas confirmed mercury levels at or below 10 ng/nm3. In addition, there was no reported moisture breakthrough or elevated pressure drop. The results in Figure 7 verify that the HgSIV 3 adsorbent removes mercury from cracked gas streams down to very low levels, without loss of moisture capacity or pressure drop penalties.
Figure 6 Bed B Outlet
Mercury Level (ng/nm3)
0.00 10/08/03 00:00 10/09/03 00:00 10/10/03 00:00 10/11/03 00:00 10/12/03 00:00 Time 10/13/03 00:00 10/14/03 00:00 10/15/03 00:00 10/16/03 00:00
Figure 7 Mercury Concentration of Cracked Gas Dryer Outlet
Mercury Concentration (ng/nm3)
0 11/6/03 14:24
11/6/03 21:36 Time
In summary, the unit has currently been in service for one year. Equistar confirms that the dryers are performing well. Mercury continues to be removed, and cycle times and pressure drop are tracking with the design projections.
REFERENCES Crippen, K., and S. Chao, “Mercury in Natural Gas and Current Measurement Technology,” 1997 Gas Quality and Energy Measurement Symposium, February 3-5, Orlando, Florida, pp. 1-16. English, Jerome J., “A Mercury Induced Aluminum Alloy Piping Failure in an Ethylene Manufacturing Plant”, Proceedings of the First Ethylene Producers Conference, presented on April 4, 1989. Wilhelm, Dr. S. Mark, “The Effect of Elemental Mercury on Engineering Materials Used in Ammonia Plants”, prepared for presentation at the 1990 Summer National AIChE Meeting in San Diego, CA, presented on August 1, 1990. Kinney, G. T., “Skikda LNG Plant Solving Troubles,” Oil & Gas Journal, September 15, 1975. American Conference of Governmental Industrial Hygienists (ACGIH),“1994-1995 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices,” ,Cincinnati, Ohio.