Kuwait is located at the Northern tip of the Persian Gulf in the Middle East as shown below in Figure-1, and good known as an oil bring forthing state. Due to its location, Kuwait has an highly rough and dry environment, with comparatively high temperatures all twelvemonth unit of ammunition.
Crude oil Wellss are scattered Throughout Kuwait ‘s desert. The oil produced by the Wellss is gathered in Gathering Centers ( GCs ) to separate rough oil from of course accompanied natural gas and H2O. Once the separation procedure is concluding in the GCs, the petroleum oil and natural gas are transferred through separate theodolite lines TLs to shore located crude oil refineries.
The transferred petroleum oil and natural gas are considered “ rancid ” at this point, intending they contain high degrees of S and H sulphides ( H2S ) . Once the sulphides are removed from the petroleum oil and natural gas, the petroleum oil and natural gas is said to be “ sweet ” . H2S is extremely caustic and toxic, where it causes corrosion to TL ‘s pipes, valves, and adjustments, which leads to unfavourable leaks all along the TLs, which are a serious menace to employees and the public. Figure-2 shown in Appendix A ( Google Maps ) , shows that presently the TLs base on balls though residential countries, to reassign the oil merchandises from of oil field and GC locations to make the shore placed refineries.
The range of this survey is to urge a system design that removes hydrogen sulphide from gas watercourses, in a location closer to the oil Fieldss and GCs in Kuwait desert, to avoid reassigning it through residential countries, and to minimise corrosivity to TLs and equipment.
H2S – Health jobs
How people are exposed
Hydrogen Sulfide H2S is a of course happening gas mixed with natural gas, released upon exposure to atmospheric conditions. It is a colorless, extremely flammable and toxic gas that may be liquefied under force per unit area and is noticeable in low concentrations by an olfactory property resembling “ rotten eggs ” . Prolonged exposure inhibits and destroys the sense of odor, which eliminates a individual ‘s ability to observe the odor after certain degrees of exposure. This is inherently unsafe, since those at the most hazard of drawn-out exposure will be incognizant of state of affairss where they are continuously exposed to this gas. Relatively short exposure to high concentrations can be fatal. It is toxic by consumption and inspiration. Hydrogen Sulfide is heavier than air and tends to roll up in low topographic points such as cavities, sumps, trenches and flat countries with hapless airing ( KOC ) .
H2S has the ability to eat equipment, piping, valves, compressor, pumps and other ferric and non-ferrous stuffs as shown in Figure-3, for a pipe corroded by H2S. Exposure of welded stuff to H2S can do weld embitterment and subsequent dyer’s rocket checking taking to possible failure, particularly with the high temperatures in Kuwait desert ( KOC ) .
Fig. 3 – H2S pipe corrosion
Sour petroleum oils contain up to 4 % weight Sulfur, and the gas watercourse produced contain important measures of H2S. This extremely toxicant, caustic and odorous compound must usually be removed from the gas prior to further processing. Apart from H2S, Liquefied Petroleum Gas ( LPG ) propane and butane besides contain carbonyl sulphide and mercaptans, and these excessively may hold to be removed, because upon burning they are converted in SO2/SO3, which has a major environmental impact e.g. acerb Rain ( Mamrosh ) .
Specific procedures used normally today
Because of the great assortment of combination of contaminations in of course happening gasses ( H2S, carbonyl sulphides, mercaptans, CO2, etc. ) , these undertakings normally pose the most jobs to the procedure interior decorator. Presently, the natural gas contains more than 5 % H2S ( KOC ) , Where typical specifications for natural gas for domestic usage are 4 parts-per-million-volume ( ppmv ) H2S and about 100 ppmv for other sulfur constituents. The CO2 specification may be set by calorific value of merchandise, or by secondary processing, such as cryogenic workss. Complete liquefaction, for illustration, requires a CO2 specification of about 100 ppmv ( GPSA ) .
The CLAUS procedure is by far the most widely applied agencies of sulfur recovery, and is based on partial burning of H2S to SO2 at 1200-1400EsC and the 2nd measure CLAUS reaction of H2S and SO2 to organize elemental S in conformity with
Thermodynamic equilibrium bounds sulfur recovery to about 95 % , favored by lower operating temperatures ( OSHA ) . In order to accomplish a sensible velocity of reaction, the Claus reaction takes topographic point in a reactor over a Sodium Oxyde/Alumina accelerator.
The alleged “ tail-gas ” from a sulfur recovery unit is a mixture of S gases SO2, H2S, etc. staying after distilling the S. It is normally incinerated to SO2 and vented. More rigorous environmental demands have led to the development of several tail-gas treating procedures, which remove the staying sulfur compounds. The best known tail-gas treating procedures are Superclaus ( Comprimo ) and Scot ( Shell ) . The combined CLAUS and tail-gas treating procedure usually has a sulfur recovery of approximately 99 % ( Higman ) . Although most gas intervention procedures are accompanied by a sulfur recovery unit, but it is non within the range of this survey.
Air Absorption Scrubing
The remotion of the contaminations discussed above is about ever carried out by soaking up in re-generable dissolvers as shown in Figure-4.
Fig. 4 – Air Absorption Scrubing
The re-generable dissolvers can be classified as chemical, physical and mixtures of physical and chemical. The pick of solvent depends on force per unit area and type of provender gas, sum and combination of contaminations in the provender, and treated gas specification.
Another relevant factor in pick of dissolver is the composing of the sulfur-rich watercourse removed when the dissolver is regenerated, in instances in which a procedure for retrieving S from this watercourse is required ( Marmosh ) .
H2S is acidic in aqueous solution, and early in the development of gas-treating procedures a decrepit basic water-soluble dissolver was sought, which would respond reversibly to it ( Zare ) .
High force per unit area gas treating
A typical high force per unit area gas handling procedure shown in Figure-5 is ever accompanied with a sulfur recovery unit ( GPSA ) . In the gas handling unit, the H2S is removed by counter-currently reaching the gas with di-ethanolamine ( DEA ) solution in a column with trays or random wadding. The sum of solution and figure of trays or packing tallness is chosen to run into specification on H2S. The DEA solution go forthing the absorber is reduced in force per unit area to let dissolved entrained hydrocarbons to get away.
Fig. 5 – High Pressure Treatment
These gases are normally sent to the fuel gas system. After picking up heat from the hot regenerated dissolver, the DEA solution enters the regenerator, where it is contacted counter-currently with steam. The dissolver is raised to its boiling point of about 110EsC and stripped by the steam. The regenerated dissolver, after giving up heat to the laden dissolver, is cooled to about 40EsC before re-entering the absorber. Typically, the solvent stock list is circulated 50 times per hr between regenerator and absorber. A individual regenerator may function several gas absorbers and besides LPG extractors ( GPSA ) .
The steam in the H2S watercourse from the regenerator is mostly condensed by chilling to 40EsC and the H2O is returned to the top of the regenerator or sent to the rancid H2O stripper. The H2S gas is so fed to the sulfur recovery unit to take 99.9 % of the S.
Combined Chemical & A ; Physical Absorption
Chemical soaking up processes consume appreciable sums of steam, and in an attempt to cut down costs, there is a move towards higher concentrations of alkanolamines such as DEA and higher H2S burdens. This tends to travel with higher debasement and corrosion rates in the procedure equipment, so a via media must be found.
The solubility of H2S is much higher than that of methane, C monoxide and H in many liquids, methyl alcohol for illustration ( OSHA ) . Since the heat of solution is much lower than the heat of reaction in chemical dissolvers, desorption can be achieved by force per unit area decrease, perchance combined with moderate warming or inert gas denudation.
Such dissolvers are merely executable, nevertheless, when sufficiently high burdens of contaminations in the dissolver can be achieved, avoiding extra solvent demands. Such high burdens are possible when the partial force per unit area of the contamination is really high, typically above 10 saloon, or when infrigidation is used to chill the dissolver to increase solubility ( GPSA ) .
The application of other physical dissolvers for natural gas treating is limited to provenders with low concentrations of propane and heavier hydrocarbons, since the solubility of these constituents is excessively high.
The Sulfinol dissolver, developed in the early 1960ss, combines many of the attractive belongingss of physical and chemical dissolvers. It is a mixture of Sulfolane, an alkanolamine and H2O. It has been found to hold broad application in handling natural gases. Its chief characteristics are deep remotion of H2S, carbonyl sulphide, mercaptans and when required CO2. Operating costs are in general significantly lower than for a strictly chemical procedures.
It is found more practical to hold the scrubbers installed near or within the GCs, since it is the location where the natural gas is foremost encountered after go forthing the Wellss and after being separated from the petroleum oil. Besides, to avoid reassigning the rancid natural gas in TLs and through residential countries. Therefore, it is considered to put in multiple scouring cringles as a procedure unit within the GC ‘s locality.
The waste H2O from the H2S scrubbers will be transferred to the GC ‘s waste H2O installation, where the H2S waste should be transferred to a sulfur recovery unit such as the CLAUS procedure.
Alternate 1: Scrubing utilizing extra NaOH.
For really little capacity or infrequent use, acerb ( NaOH ) scrubbers may be designed to run utilizing a big extra sum of acerb. This design scheme assures the maximal H2S suspension at the cost of extra NaOH use. This unit would be designed every bit merely as possible ; it would typically dwell of a individual packed tower with a rush armored combat vehicle, a circulation pump and a heat money changer as shown in Figure-6.
Fig. 6 – Single cringle H2S scouring system
A big liquid recirculation flowrate is maintained to supply equal contact with the gas stage. Makeup acerb is added, and exhausted caustic is withdrawn continuously or as needed. Excess NaOH sums should be used for example, NaOH: H2S & gt ; 2:1.
When big surpluss of NaOH are used, the equilibrium partial force per unit area of the H2S solution is so low that it is typically negligible. Column design for this state of affairs is straightforward. Well known schemes for measuring the tallness of a transportation unit ( HTU ) of the column wadding can be used, such as from a wadding seller and HTU literature. For individual recirculation-Ioop systems, the extra acerb use will be the primary factor in finding the H2S remotion efficiency. Because of the high acerb ingestion and low value of the sulfidic caustic ( NaSH ) merchandise, this scrubber is limited to really low S throughputs.
Alternate 2: Scrubing while minimising extra NaOH.
When a big surplus of NaOH can-not be used for example, NaOH: H2S & lt ; 2:1 for economic or merchandise quality grounds, the chemical equilibrium of the system must be considered in the design computations, particularly when runing at higher temperatures. At a lower pH, the literature HTU informations may non be accurate. It must be recognized that the equilibrium partial force per unit area of H2S above the liquid stage represents the minimal H2S partial force per unit area that can be achieved in the treated gas. One design scheme for these instances is using two acerb recirculation cringles as shown in Figure-7.
Fig. 7 – Double cringle H2S scouring system
Fresh caustic is fed to a top cringle, which is used as a smoothing subdivision to maximise H2S remotion. The acerb floods from the top to the bottom subdivision. The majority of the H2S remotion is done in the bottom recirculation cringle, which operates at a lower pH. Double-loop systems are sometimes avoided due to greater complexness and cost, but this design can accomplish really low mercantile establishment H2S concentrations with less acerb ingestion than would be possible utilizing a single-loop system.
Options Analysis & A ; Selection
A comparing between the two options is done based on standards shown in subdivisions 4.1 – 4.4. Each standard will hold a weight factor assigned to it, where this factor will be multiplied by a graduated table. The graduated table will run between 0 and 10, where 0 will stand for “ really hapless ” , and “ first-class ” for 10. A mark stand foring the amount of all standards for each option, where the option with the highest mark will be selected.
The chief ground for executing this feasibleness survey is to cut down the wellness, assets, and environmental hazards associated with the H2S contamination attach toing the natural gas which passes through residential countries through the TLs. Therefore, a weight factor of 35 % ( 0.35 ) is assigned for the environmental effectivity standards.
Alternate 1 has a higher efficiency in taking H2S from the natural gas than that of Alternative 2, but Alternative 2 has less acerb waste that has to be dealt with further on, with a well high H2S remotion capableness. Therefore, Alternative 1 was assigned a mark 8, and 7 for Alternate 2.
Cost is given a weight factor of 20 % ( 0.2 ) , because this is a feasibleness survey although cost is really of import. Since this is merely a feasibleness survey, no elaborate cost interrupt down for the options was done. But a cost estimation can be determined by comparing the two options based on how much building stuff and clip will be invested in each, and how much caustic will be utilized.
Based on Figure-7 ( Alternative 2 ) shown above and by comparing it to Figure-6 ( Alternative 1 ) besides shown supra, it is obvious that Alternate 2 will necessitate more building stuff, therefore more building clip. Besides, Alternative 1 consumes more acerb. Keeping in head that acerb costs will be required continually one time the H2S scrubbers are operational. Therefore, Alternative 1 was assigned a mark of 7, and 5 for Alternate 2.
Operation is an of import standards, due to the fact that a GC operates around the hr, and the natural gas flow is uninterrupted. Operators must cognize that they can depend on a system to work continuously, without breaks. Based on that, a weight of 30 % ( 0.3 ) was assigned for the operation standards.
Alternate 1 lacks the capableness of working continuously, and can merely run on minimum flow of natural gas. Whereas Alternative 2 is capable of working expeditiously on uninterrupted bases. This is a ground for delegating a mark of 3 for Alternate 1 and an 9 for Alternate 2.
Maintenance relaxation was assigned a weight factor of 15 % ( 0.15 ) , because it affects the operation of the system. The more complex a system is, the longer and more hard it will be to keep it, which will take to break of the system operation.
Alternate 2 is evidently more complex than Alternate 1, which is a ground that Alternative 1 was assigned a mark of 7 and Alternative 2 was assigned a mark of 5.
Table 1 below shows the determination matrix built to set up an alternate choice. Alternate 2 has a higher mark than Alternative1, which lead to choosing Alternate 2.
Table 1 Design Decision Matrix
It is found that Alternate 2 is capable of taking H sulphide from gas watercourses for a crude oil installation in an unfastened rural country of Kuwait desert. Even though Alternate 2 with the dual cringle constellation has a higher cost and most complex for building, but it is found more acceptable by the design assigned standards. The sum of caustic used had considerable consequence on both environmental and cost effectivity. It is besides more dependable in respects to its operation, where it can run continuously. By implementing the dual cringle H2S scouring system, the H2S contaminant concentration will be minimized to the acceptable scopes that will non hold injury to the residential country population where the TLs base on balls through. It will besides protect the GCs equipment and TLs from corrosion caused by H2S.
A recommendation is made for put ining non less than 2 units of Alternate 2 with a penchant of holding 3 units, to see H2S removal efficiency and to hold a system redundancy. It is besides recommended to ab initio put in a pilot system within a individual GC, to measure the capableness of the installed system prior to put ining it in al GCs.