By – Neil Eckersley and Satyam Mishra
Gas processors require safe, reliable and effective mercury removal process solutions in order to achieve requisite sales gas quality specifications and protect cryogenic equipment. In the last decade this has lead to more plant operators relying on different mercury removal technologies to meet their various contractual obligations. UOP has lead the effort to develop better process solutions with a range of fixed bed adsorbent products to remove mercury from a wide range of process streams.
UOP’s mercury treatment adsorbents can be broadly classified as non-regenerative copper-based Mercury Removal Unit (MRU) GB adsorbents and regenerative molecular sieve based UOP HgSIV™ adsorbents. Each type of product can be used independently of one another or together in combination.
Mercury is a naturally occurring element found in small but measurable concentrations in many oil and gas fields throughout the world. Mercury is most frequently detected in its elemental form and tends to be prevalent in natural gas processing, LNG (Liquefied Natural Gas) and refinery and petrochemical facilities. Due to advances in detection systems, mercury can now be accurately measured down to single digit nanogram levels in the case of gases and to ppbwt levels in the case of liquid hydrocarbons. UOP has developed an in-house analytical expertise in detecting and measuring mercury.
MRU products have long been used to take advantage of the affinity that mercury has with certain surfaces. Modern metallic based products have tended to replace older, activated carbon based technologies that have traditionally been specified into LNG plants. Metallic MRU products have a larger operational envelope when compared to carbon, with the latter only effective in treating dry gas limiting their use to downstream of a dehydration system.
More gas processors require “up-front” mercury removal from the raw gas entering a treatment facility in order to protect all the assets within a plant. This gas presents a challenge to regular MRU adsorbents since it can be water saturated and contain entrained liquid hydrocarbons. In order to reduce capital expenditure, it is important to ensure that MRU reactor volumes are minimized, which is particularly important when positioning an MRU off-shore where spatial constraints are critical. The residence time required by carbon products often leads to a larger than practical MRU footprint. The drive to reduce capital budgets and avoid large volumes of spent material has driven gas processors to examining technologies other than sulfided carbon.
UOP has developed a range of nonregenerable adsorbents to improve on existing MRU technologies, to be used on gaseous and liquid streams commonly found in and around natural gas processing facilities. Gas streams containing thousands of micrograms of mercury per normal cubic metre (NM3) and liquid streams containing thousands of parts per billion by weight (ppbwt) of mercury can be successfully treated using UOP GB adsorbents. Instead of older technology carbon, transition metal oxides and sulphides are utilized. The active component of GB products is copper and it can be supplied in both oxide and sulfide form, depending on the composition of the fluid to be treated. GB products have the ability to remove mercury, in a variety of locations within any gas plant. The products have been developed to be compatible with modern mercury recovery routes. Mercury is removed and recovered from the discharged adsorbent via vacuum distillation and sold into specialty applications for re-use. The remaining active metal is sent for recovery via a smelting process and sold back onto the open market. This process ensures that the MRU product is handled in an environmentally friendly way. In gas phase service, the adsorbents have been developed to treat both wet and dry gases. A commonly suggested flow-scheme location for a UOP GB product is shown in Figure 1.
Mercury not only enters gas plants with raw natural gas but it can also come in through off-shore condensate streams. Since condensates can represent a valuable refinery and petrochemical plant feed-stock, it too must be purified to protect downstream process plant equipment and catalysts.
Once present in a gas processing facility, mercury can be transferred from in-coming natural gas to subsequently produced products for sale. Ethane, propane, butane and LPG as liquids are separated from the gas and separated within the gas plant to sell to downstream domestic and process plant consumers. Petrochemical customers taking gas plant liquids need to be aware that mercury specifications in traded feedstocks are becoming tighter and financial penalties are becoming more common and punitive as awareness grows in terms of the need to better control the quality of process unit hydrocarbons.
GB products have been developed to remove mercury from both gas and liquid streams at various locations within a gas plant. They offer long lasting adsorbent life cycles, resulting in an overall improvement in plant process economics on a $ per Kg of mercury removed basis.
HgSIV adsorbents are unique products developed by UOP for mercury management in natural gas, LNG and ethylene plants and have been used commercially for more than 20 years. They contain silver which forms an amalgam with the mercury. Since mercury is adsorbed on different sites than water, HgSIV adsorbent adsorbs both simultaneously and can be used to replace a portion of the dehydration grade molecular sieve in the dryers. Since HgSIV adsorbents are used in the existing vessels, no additional capital is required and no additional pressure drop encountered. The beds are completely regenerable, transferring the mercury and water to the regeneration gas, shown in Figure 2. Recovery of mercury form the spent regeneration gas can be done with a small MRU.
Use of HgSIV adsorbents enable the flexibility to switch between mercury-laden as well as mercury-free feed stocks. It can easily be used in the dehydration beds of units which were not originally designed for mercury containing feed gas.
Mercury can be removed during different stages of your process depending on several factors, including inlet gas composition and flow rate, inlet mercury levels, site environmental concerns and available capital budget. Please contact UOP and allow us to help you understand the best options for mercury removal in your plant.