Canadian gas plant handles NORM in replacing C3 treater’s mol sieve

Of the streams recovered, sulfur compounds tend to accumulate in the propane and butane streams because they have similar boiling points. Hence the products from the depropanizer and debutanizer are fed through molecular-sieve treaters to reduce total sulfur levels to the customer’s specifications.

Jumping Pound has two identical externally insulated propane treaters, each of which contains 10,206 kg (22,500 lb) of 5A RK-29 ¹⁄₁₆-in. molecular sieve. Each treater has a 1.7 m ID and a height of 9.8 m. At any time one treater is online removing sulfur, while the other is in regeneration or standby. Regeneration involves flowing hot sweet fuel gas (methane) through the bed to release the sulfur compounds that have been adsorbed during the treating cycle. Total regeneration time is 12 hr, including 6.5 hr of heating at 320° C. and 3 hr of cooling at 30° C.

In 2006, a business decision to replace the molecular sieve in the propane treaters was based on two justifications:

1. The molecular-sieve capacity had declined to ~30% of its original capacity.
2. Blended sulfur concentration in the propane sales product was estimated to reach 50% of sales specification by late 2007.

With NORM in the propane stream, specifically in the propane treaters, the planned molecular-sieve change out would require additional health and safety and waste-management measures.

NORM

Radiation exposure to individuals can arise from such natural radiation sources as terrestrial radiation from radionuclides found in soils, cosmic radiation from space, naturally occurring radionuclides deposited in the body from foods and man-made sources such as medical diagnostics. This is known as background radiation.¹

A major component of background radiation, NORM consist of long-lived radioactive elements such as uranium and thorium that are naturally present in the earth’s crust. The presence of these materials in gas-producing formations varies geographically. These elements naturally decay to become more stable.

Although the concentration of NORM in most geological formations is low, higher concentrations may result as radioactive materials scattered across a large area are brought into one facility.

Uranium is typically insoluble and remains largely within the underground reservoir. Radium 226 and Radon 222, however, which are decay products of uranium, are slightly soluble and can thus be mobilized within extracted fluids and gasses, allowing these materials to be brought to surface.

The primary source of NORM in gas processing facilities is Radon 222 (or radon gas). It can be brought to surface with natural gas and concentrated in the propane stream through fractionation because radon and propane have similar boiling points. Radon gas has a short half-life of 3.8 days and quickly decays to Lead 210, which has a half-life of 22 years and forms a thin film that builds up on the inner walls of process equipment (Fig. 2).

The bottle contains Lead 210 dust (Fig. 2; photo from Normcan).

In addition to radon gas, another potential source of NORM accumulation in gas plants is Radium 226, found in produced-water streams. Radium within geological reservoirs is soluble and transported to gas plants with produced water. Radium precipitates out of the produced water as a radium sulfate scale due to changes in temperature, flows, and pressure. Radium 226 has a half-life of 1,600 years (Fig. 3).

Health risk from exposure

Human exposure to NORM sources can occur externally and internally. External radiation exposure occurs when people are exposed to gamma radiation from outside the body. Within the oil and gas industry, NORM typically do not give rise to exposures above the annual exposure limits for an external source. Internal radiation exposure occurs when NORM enter the body, which poses a far greater concern than external radiation exposure because radioactive isotopes that enter the body may not be eliminated from the body for several decades and result in a cumulative dose build up.

There are three possible internal exposure pathways:

• Inhalation of NORM-contaminated dust or radon gas.
• Ingestion of NORM-contaminated particulates or liquids.
• Absorption of NORM-contaminated particulates or liquids through open cuts on the skin.

Long-term exposure to NORM can lead to increased risk of cancer, just as similar exposure to asbestos, coal dust, and cigarette smoke can cause lung cancer.²

Guidelines in Alberta

In Canada, working with man-made radioactive sources falls under federal jurisdiction and is regulated by the Canadian Nuclear Safety Commission. NORM are exempt from CNSC legislation except for the import, export, and transportation of the material. The jurisdiction over radiation exposure to NORM is thus the responsibility of each Canadian province and territory. To date, the province of Alberta has not yet implemented NORM-specific legislation or regulations.

In 2000, Health Canada published the Canadian Guidelines for the Management of NORM, prepared by the Federal Provincial Territorial Radiation Protection Committee. These guidelines establish safe practices for management of NORM in Canada to protect workers and the public from situations in which NORM have been concentrated as a result of industrial activities.

The guidelines establish four NORM thresholds; natural background radiation is excluded from the dose limitations.

• Investigation threshold: A site-specific assessment should be carried out where doses exceed an incremental dose of 0.3 millisievert/year.
• NORM management threshold: an assessed incremental dose of greater than 0.3 millisievert/year.
• Dose management threshold: an assessed incremental dose of 1 millisievert/year.
• Radiation protection management threshold: an assessed or measured incremental dose of 5 millisievert/year.

(Sievert is an SI unit measuring the extent of tissue damage resulting from radiation adsorption. In the US, it is measured in units of millirem; 1 mSv = 100 mrem.)

The guidelines also address management of NORM-contaminated waste. The unconditional derived release limit (UDRL) for fixed surface contamination is 1 Bq/sq cm averaged over a 100 sq cm area. NORM-contaminated scales, sludges, and waste above the UDRL of 0.3 Bq/g must be sent to a licensed NORM disposal site.

(A Becquerel, Bq, is an SI unit of radioactivity; 1 Bq = the activity of a quantity of radioactive material in which one nucleus decays/second.)

In Alberta, typical average background radiation is in the range of 60-120 millisievert/hr (60e-6 to 120e-6 millisievert/hr) according to Normcan, Calgary.

NORM survey

In September 2005 and July 2006, a certified radiation safety officer conducted a NORM gamma screening survey at Jumping Pound. The officer used a calibrated Ludlum 3-97 General Purpose Survey Meter that incorporates both an internal 1-in. by 1-in. sodium iodide (Na-I) scintillator and an external 44-9 Geiger-Mueller pancake probe (Fig. 4).

The project employed this Ludlum 3097 Scintillator and an external 44-9 Geiger-Muelier pancake probe (Fig. 4; photo from Normcan).

The survey entailed data logging gamma radiation levels on the exterior of piping, vessels, and equipment. Measurements were taken within 1 to 3 cm from external surfaces and at a distance of 0.5 m. Gamma scintillation surveys provide a rapid and efficient means of screening a facility for the presence of NORM, allowing for an evaluation of the presence of NORM from the exterior of operating equipment without taking the equipment out of service.

The survey revealed that NORM were present within the Deepcut unit, LPG loading areas, propane storage bullets, sour-water slop tank, and inlet slug catcher. The highest readings were found on the downstream propane treater filter.

While the vessels and equipment are in operation and sealed, the exposure levels to incidental workers were determined to be less than the dose-management threshold of 1.0 millisievert/year. There is potential, however, for exposure to higher doses through inhalation and ingestion of NORM when vessels and equipment are open for maintenance.

Job planning

The site’s awareness of NORM affected the job planning, especially in terms of worker safety and waste management. Removal of molecular sieve that is not NORM-contaminated would typically involve workers using supplied-air breathing apparatus (SABA) in addition to regular personal protective equipment (PPE) when blinding the treaters and entering the vessels. The regular PPE would include boots, gloves, hardhat, and safety glasses.

Dust control would not be a major issue to address throughout the removal procedure. Workers around the perimeter of the job site, for example, would not wear additional respiratory protection, and sealed storage bins would not be required to control any dust generated during removal of spent molecular sieve. Finally, spent molecular sieve that is not NORM-contaminated would be sent to a regular industrial landfill.

The presence of NORM in the propane treaters required additional health and safety and waste-management measures. With no plan for dust suppression, NORM-contaminated dust would be deposited around the job site during removal of spent molecular sieve. Moreover, without disposable PPE, NORM-contaminated dust may remain on workers’ coveralls, boots, and gloves, creating a potential for inhalation or ingestion of the dust.

Extensive planning started about 1 year before execution of the project. Due to the collective lack of experience on site with NORM, many of the planning meetings were focused on NORM handling. The planners of the job were unsure how the presence of NORM would affect safe work practices at Jumping Pound. Thus, Normcan, a contractor with experience handling NORM, was brought on site.

Deactivation method

One of the challenges was to select how to deactivate the molecular sieve before unloading. It’s necessary to deactivate the molecular sieve because there may be residual toxic or flammable compounds remaining on it even after a full regeneration cycle. Reasons for this include bed channeling, liquid carryover, or adsorbent agglomeration. Deactivation removes potential hazardous adsorbed materials and renders the molecular sieve incapable of picking up more.

The planning team considered four options for deactivating the molecular sieve, with the help of external contractors that included UOP, Normcan, and Catalyst Services. The four options were N₂ purge, N₂ and CO₂ purge, steam purge, and waterflooding. The table displays the pros and cons of each option; note that N₂ and CO₂ purge has the same pros and cons as N₂ purge.

This approach led to waterflood being preferred because it addressed most of the hazards:

• Waterflood will displace all absorbed material within the cavities of the molecular sieve and within the vessels.
• NORM-contaminated dust can be effectively controlled and minimized by immersing it in water.

Once the method to deactivate the molecular sieve was chosen, several other challenges needed to be addressed.

Personnel protection

One challenge was to minimize personnel exposure to NORM-contaminated materials. Throughout the job, workers were required to wear disposable Tyvek coveralls, disposable gloves, and disposable Tyvek boot covers in addition to their regular protective equipment. Workers who blinded the vessels and entered the treaters to remove spent molecular sieve were required to use SABA.

Inspectors were required to use the high-efficiency particulate arrestor (HEPA) P-100 cartridge half-mask to enter the vessels for their respiratory protection once the atmosphere in the treaters was tested and proven fit for occupancy. All ground workers around the job site were required to don HEPA P-100 cartridge half-masks.

In addition to PPE, good personal hygiene practices were enforced among workers, such as washing their hands before eating and leaving for home.

Flooding rate; exothermic reaction

Another major item discussed during meetings was the water flow rate that would be required to fill the treaters. Much time was spent searching for past practices on waterflooding within the Shell Foothills gas facilities. But none was found because the Foothills facilities have no experience with waterflooding, which led the planning group to consult external contractors.

One contractor’s experience with waterflooding at a different facility led him to recommend filling the treaters with 30.5 cm of water every 15 min. Using this recommendation, the operations engineer had calculated the required flow rate for flooding the treaters at 28 cu m/day.

In conjunction with determining the flooding rate was a concern about the volume of steam that would be generated due to the exothermic reaction created when molecular sieve adsorbs water. Initial calculations were that the existing 25.4-mm line to flare would potentially be insufficient to divert the expected steam to flare. Thus, the job plan included about 61 m of temporary 50.8 mm pipe and 61 m of properly rated hose to be connected to the flare header.

Waste management

The planning group, uncertain if the spent molecular sieve would be NORM-contaminated once deactivated, explored options for disposing of it. After much discussion, however, the group decided Normcan was able to dispose of the spent molecular sieve in either scenario: NORM-contaminated or non-NORM-contaminated.

Normcan was able to provide sealed bins to contain the spent molecular sieve, which would minimize worker exposure to NORM. Three sealed bins, named Vacuum Box, each with a volume of 19 cu m, were to be brought on site.

Filled bins were to be transported off site once the job was completed, and Normcan would complete its lab analysis to determine options for disposal. Potential permanent disposal options for NORM-contaminated waste included salt cavern disposal, abandoned-well disposal, and landfill disposal.

Job execution

The molecular sieve in the propane treaters was replaced between Apr. 23 and Apr. 30, 2007.

Personnel protection

A radiation safety officer from Normcan was present on site throughout the job, from blinding the treaters to transporting the spent molecular sieve off site. A control area was set up around the job site to establish the boundaries of the area within which the officer monitored all personal protection and other equipment for surface contamination. Inside the control area, a small area was set up to collect all disposable PPE and HEPA P-100 cartridges in a bin provided by Normcan.

Blinding was the first opportunity for workers to be exposed to NORM hazards. While blinding, workers wore disposable Tyvek coveralls, disposable gloves, boot covers, and SABA in addition to regular PPE. Once the task was completed, they went into a disposal area, where each worker was measured for NORM contamination with Ludlum 44-9 Geiger-Mueller pancake probe. Once proven clear of contamination, workers were then directed to remove and dispose of all disposable PPE.

Once the treaters were opened to the atmosphere, the manways, mesh screen, molecular sieve, and inner surface of the vessels were tested for NORM readings by the radiation safety officer, who obtained measurements from within 1-3 cm from the surface.

NORM contamination above the UDRL was found on the manways, grating, mesh screen, and interior surface of the treaters that were not in contact with the molecular sieve. The molecular sieve and interior surface of the treaters that came in contact with the molecular sieve, however, had readings below the UDRL limit.

During removal of the spent molecular sieve, workers wore the appropriate PPE and SABA. Once the molecular sieve was removed, inspectors who entered the vessels wore HEPA P-100 filters in addition to the appropriate PPE, as specified by the plan. Once the job was completed, all personnel were measured for surface contamination on their PPE and then disposed of all disposable PPE.

Throughout the job, readings taken on all workers’ regular PPE, disposable PPE, HEPA P-100 filters, and SABA were below the UDRL.

Flooding rate; exothermic reaction

One treater was flooded at a time, starting with V-3020A. Firewater was introduced to V-3020A from the bottom and the vessel filled at a rate of 50 cu m/day. The treater was filled in batch (i.e., it was filled and held, then filled and held).

As the treater was being filled with water, displaced gas flowed to flare via piping connected to the flare header. A temporary level bridle on the treater indicated the water level. When the tower came close to being completely filled, a dedicated operator diverted excess water and gasses to the sealed bins provided by Normcan.

It took 9 hr to fill the treater, which included the holding duration. Once the tower was completely flooded, it was drained. Nitrogen was introduced at the top of the tower to force the water out and into the sealed bin. That process took 1.75 hr.

The second treater, V-3020B, was filled at a slower but continuous rate in 6 hr at 33 cu m/day. This treater was also allowed to overflow to the storage bins, before filling was stopped. The drain process was the same as for treater V-3020A.

During the job execution, two aspects did not proceed as planned:

• Although the operations engineer had calculated a flow rate for flooding, the rate could not be maintained because it was too low to control with a ball valve. The calculated flooding rate based on the contractor’s recommendation may not have been the preferred flow rate for this job.
• Much time and energy was invested in installing temporary piping and hose connection to flare to accommodate the substantial exothermic reaction that had been anticipated. The temporary 50.8 mm piping and hose connection were not used to direct steam generated to flare. Instead, the existing 25.4 mm piping was used to route expelled gasses and steam to flare. After further discussion, it was believed that the waterflood rate was sufficient to dampen the exothermic reaction.

Waste management

Normcan dealt with all solid waste generated throughout the job, placing all used gaskets and mesh screen into the bin, together with disposable Tyvek coveralls, disposable gloves, boot covers, and HEPA P-100 cartridges. Normcan removed the bin for disposal.

Once wastewater was drained from the treaters into the sealed bins, Normcan tested the bin externally with the Ludlum 3-97 scintillator. Absence of readings above the background radiation allowed for the disposal of the water into Jumping Pound’s sour-water disposal well.

Spent molecular sieve was vacuumed from the vessels to the sealed bins, which were transported away by Normcan. Although field readings measured on the spent molecular sieve were below the UDRL, it was suspected that the molecular-sieve pellets might not meet the limit. The radiation safety officer suspected field readings below the UDRL due to the geometric shape of the pellets, absorption of the particles into the molecular sieve itself, and potential shielding of beta particles from the field detector due to other contaminates surrounding the pellets.

Thus, a laboratory analysis verified the contamination levels, confirming that the spent molecular sieve exceeded the UDRL threshold requiring NORM-specific waste-management practices as outlined by Health Canada’s NORM Guidelines. Two samples of spent molecular sieve that were analyzed showed 1.0 Bq/g and 2.1 Bq/g of Lead 210. Since the samples showed readings greater than the unrestricted release limit of 0.3 Bq/g, the spent molecular sieve was not sent to an industrial landfill but to a salt cavern in Unity, Sask.

Lessons

After the molecular-sieve replacement job ended successfully, a review focused on lessons from the job.

• Bringing in an external contractor with experience in dealing with NORM contamination was the right decision. Normcan not only provided expertise in controlling NORM hazards, it also provided excellent education and confidence to workers. Involving Normcan, UOP, and Catalyst Services during planning phase contributed to the success of the job; having Normcan on site before the job execution greatly helped the planning group determine logistics details.

The personal protection equipment used throughout the job worked well in terms of:

• Eliminating workers’ contact with NORM-contaminated dust and equipment.
• Workers feeling comfortable with their tasks.
• Providing confidence to workers to perform the job.
• The deactivation method of waterflooding had proven to minimize personnel exposure to NORM-contaminated dust. The continuous fill method of treater V-3020B proved to be more successful than the fill and hold method of treater V-3020A.

On V-3020A, spent molecular sieve in the upper layers of the bed was easily removed. The pellets, however, were progressively wetter down the vessel. Towards its bottom, the pellets were stuck together and had hardened to a consistency similar to wet clay. This prevented use of a vacuum and led workers to resort to jackhammers. As a result, the job fell behind schedule by a few hours.

On the other hand, the spent molecular sieve in V-3020B was not soggy and thus was easily removed. Because water was immediately drained as soon as the vessel was fully flooded, however, the bed did not have sufficient time to cool.

Its top was too hot for workers to stand on in order to insert the vacuum hose for removal. A Raytek temperature gun measured the top layer of the bed at 95° C. The temperature subsided progressively down the vessel. The high temperature also resulted in a delay of a few hours.

It was hence determined a combination of the two flooding methods would have achieved greater success: continuous fill and a short hold once the vessel was fully flooded before draining the vessel.

• Qualified radiation protection personnel should be used on field test equipment. This equipment only provides information to determine if a laboratory analysis is required to verify if waste meets the UDRL. Lab analysis should be performed to confirm disposal options.

Acknowledgment

Special appreciation is due Cody Cuthill of Normcan, Gerry Nell of UOP, and Mike Hatch of Catalyst Services for supporting the molecular-sieve replacement job and the writing this article. Special thanks and recognition are also due Joanna Williams, Harvey Malino, Greg Hanlon, and Stacey Ferrara for supporting the writing of this article.

References

1. Canadian NORM Working Group of the Federal Provincial Territorial Radiation Protection Committee. Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials (NORM). 1st Ed. Canada: Health Canada, October 2000 (http://www.hc-sc.gc.ca/ewh-semt/alt_formats/hecs-sesc/pdf/pubs/contaminants/norm-mrn/00ehd245.pdf).
2. Cuthill, Cody, NORM Safety Training. 2nd Ed. Canada: Enform Canada, February 2008.

The authors