Removal of black powder & other contaminants from furnace feeds
Black Powder is a common corrosion related contamination found in pipelines transporting LPG, LNG and hydrocarbon condensates. Studies1 of particle size distribution have shown that a majority of the contaminant is very fine and sub-micron in size. Previous studies2 have shown that they consist primarily of iron oxides. The presence of sour gas accelerates the formation of mixtures of iron sulfide and iron oxides. Black Powder is a global phenomenon and occurs in pipelines. Elevated temperatures appear to increase the formation of these solids. Additional non-corrosion related solids such as sand or silt may also be present in the gas stream. Characteristics of black powder solids are: • Very fine contaminants, forming agglomerates in micron size range – typically 1-10 micron • Easy to transport, difficult to capture • May be pyrophoric • Need for appropriate handling and disposal procedures “Pipeline rouge” is another term commonly used to describe corrosion products retrieved from pipelines, but the composition is similar to black powder, only the color is red instead of black. The predominant compound in pipeline samples is typically magnetite, which is black in color. Minor components such as greigite (Fe3S4), siderite (FeCO3), or hematite (Fe2O3) probably give pipeline rouge its color. In addition to these solids, the gases may pick up hydrocarbon and aqueous contamination from storage in the pipelines. They are present as fine droplets or aerosols in the gas phase. The presence of water in liquid phase promotes corrosion reactions. The presence of oxygen, hydrogen sulfide and carbon dioxide accelerate the corrosion reactions in the steel pipelines. Pigging operations have resulted in enormous amounts of black powder (500-5000 kg) being removed from scraped pipelines.
Impact of Black Powder The presence of black powder and aerosols in the incoming LNG/LPG to ethylene furnaces can have a detrimental effect on the performance of the furnaces. Many problems have been documented, leading to shortened furnace run lengths and expensive maintenance issues. These include: • Accelerated coking reactions in the tubes and fouling in the convection section • Formation of CO in cracked gas • Loss of furnace capacity during pigging operations of the pipelines • Severe corrosion resulting in frequent replacement of damaged furnace tubes In natural gas pipelines, the problems caused by black powder and water/hydrocarbon have been recently documented.3 Some of them include: • Process upsets such as off-spec product with high water in condensate/crude • Foaming in amine sweetening units and glycol dehydration units • Equipment failures leading to fouling and plugging of reboilers in the fractionation plants • Poor accuracy of meters/valves • Failure of critical instrumentation • Safety and environmental issues such as gas releases Measurements of Black Powder Isokinetic sampling1 offers the best practice to remove representative particles as they are in the gas stream under operating conditions. An in-line isokinetic probe combined with a disc membrane is used for gas sampling from the pipelines and the collected gas is analyzed for Particle Size Distribution. The equipment contains a probe that is inserted into the process pipeline at the appropriate location and has a flow meter and control valve to adjust the flow rate to maintain the same flow rate of the gas in the sampling apparatus as the main pipeline. The probe can be inserted at different lengths to sample the gas at different locations within the diameter of the pipe (near the walls or in the center). Particle Size Distribution (PSD) Process gas or liquid is passed through a test membrane rated at 0.1 micron in gas service to collect solids. The membrane can be evaluated manually using an optical light microscope for particle size analysis. It can also be automatically evaluated using an Energy Dispersive X-ray probe of a Scanning Electron Microscope (EDX-SEM) to scan the membrane. The particle sizes are measured to 1 micron. Total Suspended Solids (TSS) Pre-weighed 47 mm diameter test membranes rated at 0.1 micron in gas service are used to collect solids from the process pipeline. A sufficient amount of gas or liquid is passed through pre-weighed membranes to collect solids in the field and the membranes are then transported to the lab for final analysis. The TSS is reported in terms of ppmw or gram/MMSCF. Analysis of LPG Pipeline Deposits Deposits retrieved from Dow LPG and propane pipelines contain predominantly reduced iron oxide (magnetite) with minor amounts of other iron compounds like iron carbonate (siderite, FeCO3), and iron sulfide (greigite, Fe3S4). X-ray diffraction identified the major crystalline product as magnetite (Fe3O4) in all of the samples. Other crystalline phases in minor amounts are iron carbonate, iron oxide (hematite, Fe2O3) and magnetic iron sulfide or greigite (Fe3S4). For LPG or propane stored in a brine well, it is also common to find salt (NaCl) at minor levels, indicating carryover of brine from the well. Filtration/Separation Techniques Used Different solutions have been commercially devised in the natural gas industry and in the feeds to ethylene gas crackers to mitigate the impact of black powder and to remove the water/hydrocarbons from the gas. A brief description of these technologies is provided below. There are four (4) identified product platforms.
1. Depth and pleated cartridge filters to remove solid contaminants
2. Systems of cyclonic devices in combination with cartridge filters
3. Liquid/gas coalescers to remove contaminants such as water and hydrocarbons
4. Liquid/liquid coalescers to remove water/hydrocarbons assuming that the gas is in a supercritical state or liquefied single phase flow regime
Depth and Pleated Cartridge Filters to Remove Solid Contaminants A number of different filter materials are available. Cellulose, glass fiber and polypropylene are the most common types. It is important to capture black powder contaminants in finely rated “absolute” cartridge filters for maximum performance. An important distinction exists between how filters are rated, and the terms “nominal” and “absolute” which can give very different performance for a filter labeled with the same micron rating. Nominal filters are unreliable and are not subject to the same quality control. The consequence is that particles larger than the claimed removal rating pass through the filter. In some cases, nominal filters can collect contaminants and then as the pressure drop increases, release the trapped materials back into the stream. Absolute filters are tested by using particle counters to measure efficiency based on a distinct particle size cut off and manufactured to more demanding specifications. They are designed to not release any trapped contaminants up until the design terminal pressure drop.