Q & A > Question Details
I am working in DHDS. I would like to know the purpose of Carbon filter in Amine Recovery Unit. We use stripped water from Sour water stripping unit as wash water in DHDS over head coolers for dissolving ammonium salts. My query is if there are little amounts of ammonia and H2S in stripped water, and if we use the same stripped water in DHDS, will there be any problem in amine quality or will there be any effect in the quality of acid gas generated from ARU? We are facing the problem of increase in differential pressure across Carbon filter when we take stripped water in DHDS.
26/02/2010 A: Sanjay Rajora, Essar Oil Ltd, sanjay.rajora@essaroil.co.in
The carbon filter contains activated carbon bed. The main purpose of carbon filter in Amine Recovery Unit is to adsorb hydrocarbons. The problem of DP rise in carbon filter may be due to excess hydrocarbon carry over in amine.
23/02/2010 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Purpose of carbon filters in amine systems is twofold.
1) The main purpose of AC filter is to remove amine degardation products caused mainly by thermal decomposition of amine which increases the foaming tendencies of amine over time as surfactants build up. AC filter should have a well designed particulate filter (absolute rated filter with 99.98% removal at 10um) to protect AC bed from plugging as AC bed can also act as a deep bed filter too. (it's a poor and very expensive filter mind you but it will foul if subjected to solid contaminants in the amine feed without adequate protection). Benchmark life of AC filter should be 3 yrs+ between changeout of AC. There should also be an after-filter installed to prevent AC fines carryover intro amine circuit which can also prolong foaming episodes by stabilising foam.
2) Historically AC filters have also been used to remove hydrocarbons however AC filters are not very effective in the medium to long term as if subjected to anything other than traces of free liquid hydrocarbon, AC reaches its adsorbtion limit quite quickly and then stops removing HC and foaming often becomes worse than ever. Also AC filters are disarmed by some types of antifoaming agents commonly used which renders AC filter impotent. Modern thinking and experience in refineries, gas plants, and LNG plants is that it is generally far better to keep the liquid HCs out of the amine circuit in the first place by using properly sized high efficiency LG coalescers instead of or as well as KO pots/mesh pads on amine aborber sour gas inlet. By removing liquid hydrocarbons to <0.011 ppmw, amine foaming can be effectively tackled and also HSS precursor ingress reduced. In centralized amine unit with many absorbers where HCs may be coming in from a number of sources or liquid HC contaminated rich amine coming from LPG treater, PhaseSep Liquid Liquid coaleser installed on full flow rich amine has been found to remove HCs continuously and provide much better protection to the sulphur plant plus also provide much more stable amine plant operation. AC filter can be made virtually redundant in this case and possibly even eliminated altogether.
I suspect using sour water as wash water will be adding some HSS precursors like chlorides, sulphates, sulphides into amine system which will may increase HSS formation which will increase corrosion rates and create more corrosion products in the system. It is quite possibly this increase in corrosion products (FeS predominantly) that results in the increased rate of dP build up in your AC filter that you observed. How often do you have to clean the lean/rich exchanger, do you have tray fouling in either regen or absorner towers? What is your amine colour? (Note: best practice is to maintain HSS <1% at all times).
I'd suggest to make sure you have a proper pre-AC filter installed (plus a post AC filter if you haven't one). If you do have an existing pre-AC filter installed, it is not working properly and may be either bypassing, or the filter media being used is too course to properly protect the AC bed. (if it was working properly, AC dP would be stable) I'd also recommend to use a better quality water like demin water or process water and see if corrosion rates are the same. You should also monitor HSS % in your amine too if you haven't already done so. If HSS% is high, this can adversely effect corrosion rates in the amine circuit, plus tie up amine which may effect gas spec Can also lead to upsets in Amine units which can cause trisp to sulphur plants as well.
23/02/2010 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Additional information is needed as follows to be able to address the question:
# Does the DHDS use a two flash drum interface [CHPS & CLPS] or a four flash interface [HHPS & HLPS & CHPS & CLPS]?
# What is the concentration of ammonium bisulfide,aqueous in sour wash water decanted from the CHPS [via the CLPS] ?
# What is the concentration of soluble organics [full spectrum of heteroatom species containing organic oxygen and organic nitrogen] in the stripped sour water ?
Naturally, there are very limited quantitative methods availabe to determine total soluble organics in stripped sour water---but this is a critical question.
23/02/2010 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
The carbon filter in the amine circuit was probably installed to remove hydrocarbons from the amine to prevent foaming.
23/02/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
To answer the first question: Amines are not perfect in this application; small amounts decompose at these operating temperatures. We limit the reboiler steam temperature in order to minimize that decomposition. The amine decomposes into solids that can foul equipment, and acids that corrode equipment. Therefore filters are easily justified, as well as pH control and the use of alloys in trouble spots.
The second question involves the reuse of stripped sour water. Many plants use it for one purpose or another, most notably for desalter water. Some plants have experienced problems with reusing stripped sour water, and stop using it in their plant. Your correlation between its use as hydrotreating wash water and increased pressure drop buildup in amine regenerator filters is a new one to me, but others may have experienced that problem, too, in which case I hope you hear from them. If you are sure of your correlation, consider stopping its use in that service or live with shorter runs on the filter. I don't see this issue affecting the Claus unit.
23/02/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The term 'stripped sour water' does not adequately define the characteristics of that material. If it is poorly stripped (residual NH3 and H2S,) there are definitely other 'contaminants'. Is it full of dissolved solids and filterable particulates? Does it have heavy hydrocarbons that will precipitate?
Have you identified the material collecting on the carbon filter? Organic or inorganic? Have you contacted the process licensor and discussed this with them? What amine system is being used DEA, MDEA, or? Does the amine 'use' increase when this stripped sour water is used?
Personally I would use boiler feed water (BFW) for washing those overhead coolers to minimize the potential for introducing bad actors into the DHDS amine system.