Liquids in Gas - Best Practices
GAS PHASE MERCURY REMOVAL UNITS (MRU)
How do Liquids in the Gas affect the performance of mercury adsorbents?
MERSORB® mercury adsorbent has proven its capability for high efficiency mercury removal not only when treating dry gas, but also when treating wet gas or gas near its hydrocarbon & water dew points. However, if the adsorbent particles are coated with a liquid, the resulting increased resistance to mass transfer reduces the efficiency of Hg removal during such episodes of liquid carryover, or under constant misting conditions. This principle applies not just to our mercury adsorbent but to all mercury adsorbents and also to the adsorption efficiency of all adsorbents, in general, including molecular sieves and activated aluminas.
When a Mol. Sieve dehydration unit is immediately upstream of the MRU, then the gas is extremely dry and moisture dew point is not an issue.
TO ILLUSTRATE: When treating a liquid for mercury removal, the fluid contact time required for good mercury removal performance is measured in minutes, whereas, gas phase units require only SECONDS of gas contact time with the adsorbent for high performance mercury removal.
How can we obtain high mercury removal efficiency with a water-saturated feed gas with liquid mist & occasional liquid carry-over from upstream liquid units suchas a TEG unit, an Amine unit or a 3-Phase Separator?
We have worked with clients who preferred to locate the MRU upstream of the mol. sieve unit, and who also chose not to install a Filter-Separator upstream of their MRU. Inevitably, with just a Three-Phase Separator upstream, the MRU is exposed to large-quantities of liquid carry-over & continuous liquid mist. A Filter-Separator alone can be overcome with large amounts of liquid carry-over from a Three Phase Separator.
Once liquids are in the vessel, they will not be removed since the gas is at its dew point (saturated) and cannot take up any more liquids. Therefore, the way to achieve high-efficiency mercury removal with wet feed gas is to Design Reliability into the Process by providing Three Levels of Protection against liquids entering the MRU vessel.
3 Levels of Liquids Protection
• THERMAL: Super-Heat the Feed Gas & Insulate Feed Pipe and MRU Vessel
• SEPARATION-1: Install a vertical Knock-Out Drum with Demister Pad upstream of MRU
• SEPARATION-2: Install a Filter-Coalescer between KO Drum and MRU
How much Superheating, Heat-Tracing & Insulation do you Recommend?
In addition to liquids in the feed gas, liquids may also enter the MRU when the ambient air temperature is below the MRU feed gas temperature, because liquids can condense in the transfer line between the separator and the MRU. Also, during any plant shut down & start-up, the MRU adsorbent bed will be at ambient temperature. When the gas is at, for example, 45 °C and at its water & hydrocarbon dew point, as the gas enters the MRU, the cold adsorbent will chill the gas, and liquids will condense.
If the MRU feed gas is at its dew point and is hotter than the outside ambient temperature, we strongly recommend that clients consider putting some heat into the gas. We recommend enough superheat to get below 90% of the dew point of the feed gas. Often this may only require raising the gas temperature by 2 C° to 5 C°. Some clients have used 20 C° of superheat, but that amount of superheat was used because they chose not to install a Knock-Out Drum and a Filter- Coalescer upstream of the MRU. The 20 C° super-heat worked very well, providing reliable, high- efficiency mercury removal in the MRU, so this is an option for clients who do not want to or cannot install a KO drum and filter-Coalescer upstream of the MRU. In the long run, we believe it is cheaper to install the KO Drum & Filter-Separator with less superheat (only 2-5 C°).
If both a KO Drum & a Filter-Coalescer are installed upstream of the MRU, a separate feed gas super-heater exchanger may not be necessary, depending on the temperature differential between the feed gas temp. & the ambient temp. Clients can heat trace & insulate the MRU inlet gas line & the MRU vessel itself. We recommend using insulation with a high R-Value with a thickness of at least 75mm. If the feed gas is wet & above ambient temperature at any time, then the feed line & MRU vessel should always be heat-traced & insulated.
One of our clients successfully installed & operated a mercury removal unit treating hot, water- saturated hydrogen gas in the Northeast USA (Maine), where it is very cold in the winter. Their vessel was steam-traced on the heads & vessel wall, and insulated over the steam-tracing. The system ran very well, even in the cold winter, and achieved high mercury removal efficiency. Clients can also achieve the same good results with hot-oil heat-tracing. Heat tracing the piping & vessel has an additional advantage during start up.
If Plant Operators begin the heat flow 48 hours before starting gas flow into the vessel, this will heat up the vessel and carbon, thus minimizing any condensation during start-up. Insulation alone cannot provide this advantage. Remember, with dew point gas, once the adsorbent is wet, it will not dry out, because the gas is already saturated.
Some Vendors have told us their mercury adsorbent works with high mercury removal efficiency even when it is wet. Should we believe this claim?
Short Answer: NO! Liquid Carry-over & Condensation-Protection are not just an issue with Mercury Removal Units. We have seen this same situation many times with molecular sieve drying units that had performance problems due to liquid carryover. When the Client finally installed a good Knock-Out Drum & a Filter-Separator (the Best Practice Liquid Control Equipment) upstream of the mol. sieve unit, the client then achieved excellent mol. sieve adsorption system performance. If a well-designed Knock-Out Drum and Filter-Separator are not installed upstream, then adsorption performance for mercury removal will not be as good as it can be with no liquids, in terms of both mercury removal efficiency and mercury capacity (bed life). THIS PRINCIPLE APPLIES TO ALL ADSORBENTS, NOT JUST MERCURY ADSORBENTS.
Metal-oxide mercury adsorbent manufacturers have claimed that they can perform well in a gas in the presence of liquids, but this claim has not proven out in actual practice. In fact, several such units have experienced severe pressure drop and the agglomeration of the adsorbent into large masses, even resulting in total cementation of the adsorbent into the vessel, requiring adsorbent removal with pick-axes. Even when exposed to large quantities of liquid carry-over, MERSORB® mercury adsorbent pellets will not break down physically and will not agglomerate.
How will carry-over of liquid amine or glycol affect an MRU?
ANY liquid allowed into a gas-phase adsorbent bed will reduce the gas-phase adsorption efficiency, regardless of whether the adsorbent is a mol. sieve or a mercury adsorbent.
How will hot feed gas (wet or dry) affect performance of the MRU?
In general, chemical reaction rates increase with temperature. This principle also applies to MERSORB® mercury adsorbents: higher temperatures provide faster chemisorption kinetics. Competitive sulphur-impregnated Hg adsorbents advise that THEIR Hg removal performance decreases as temperature is increased. The reason this counter-intuitive performance occurs with the competitive products, is because the competitive products begin to lose their sulphur at relatively low temperatures. MERSORB® mercury adsorbents are made by a different process & are quality control tested at 200°C.
Our product retains its sulphur at higher temperatures than the competitive products & therefore is able to take advantage of the faster reaction kinetics of the Hg in the gas with the elemental Sulphur on the adsorbent at elevated inlet gas temperatures. This performance has been demonstrated in a hot synthesis gas pilot plant with operating temperatures above 100°C. Because of its high surface area & high dispersion of sulphur, MERSORB® mercury adsorbent also performs better than competitive products at ambient temperatures.
GAS PHASE MERCURY REMOVAL UNITS (MRU)