Read an Excerpt
Proceedings of the 3rd International Gas Processing Symposium5–7 March, 2012, Doha, Qatar
ElsevierCopyright © 2012 Elsevier B.V.
All right reserved.
Chapter OneExercising the Option of CO2 Slippage to Mitigate Acid Gas Flaring During SRU Expansion Bellow Failure
In the Claus process Sulphur Recovery Units, metallic bellow expansion joints in the tail gas line to Incinerator, seldom fail by developing crack on the bellow element due to various mechanical and metallurgical reasons.
Any such bellow failures are irreparable and it demands replacement of the entire expansion bellow assembly which is a long lead item. Though the Incinerator operates at a negative pressure, tail gas inlet line used to have a slight positive pressure proportionate to the volumetric flow rate of tail gas that will cause the harmful toxic tail gas to leak through the cracked bellow creating an HSE hazard. To prevent the above leakage, SRU unit will be forced to operate at reduced throughput rates by flaring Acid gas from the upstream Acid gas removal units until replacement of the bellow joint which will cause a serious impact on the environment and result in loss of sulphur production.
Exploiting the Selective absorption characteristic of absorbent (Methyl-Di- Ethanolamine) used in Acid gas removal units has been successfully undertaken in Qatar Petroleum's NGL-3 Plant in order to eliminate the prolonged, anti-environmental Acid gas flaring and to curtail the loss of sulphur recovery forced by the above situation. Detailed description of the attempts and actions taken towards the above task execution to minimize Acid Gas flaring without violating the AGRU product specs and without further deteriorating the leaking expansion bellow are covered in detail below with the details of benefits realized.
Keywords: Crack, Bellow Failure, Leak, CO2 Slippage, Selectivity, SRU Management.
1.1. Technical background
Qatar Petroleum's NGL-3 plant Gas Sweetening Facility treats 1020 mmscfd of Non-associated Natural gas containing approx. 0.6 % H2S and 2.4 % CO2 in two trains of Acid gas removal units (AGRU) using aqueous Methyl Di-Ethanolamine (MDEA) solution absorbent. Recovered Acid gas is processed in a single train Sulphur Recovery Unit (SRU) with two catalytic conversion stages.
The above SRU, designed on the basis of modified Claus process, is equipped with a tail gas Incinerator of natural draft, sub-atmospheric pressure type thermal incinerator with air flow controlled using burner air registers. Tail gas inlet line to the incinerator is installed with a Tied Universal type metallic bellow expansion joint to accommodate line expansion and movements. This expansion joint made out of Inconel-65 metallurgy was in operation since Year 2005 which subsequently during May-2011 had developed a crack on the metallic bellow element and started leaking toxic tail gas to atmosphere.
1.2. Tail gas line Bellow failure and Consequences
In general, weld repair of any cracked metallic bellow is not recommended, as it is not possible to restore the original bellow functionality after repair. That will further jeopardize its operational integrity. Therefore it is always considered safe to replace the entire expansion joint bellow assembly from the reliability point of view. Procuring a new bellow joint for replacement normally takes a minimum of 10 to 14 weeks subject to the supplier / manufacturer constraints. Shutting down SRU totally and flaring Acid gas for extended periods until the replacement of bellow is not permitted as it is anti- environmental.
Acid gas feed to SRU when reduced by 15% (from 17.05 mmscfd to 14.45 mmscfd) found creating less back pressure in the tail gas line that immediately led to temporary stoppage of leak. In order to avoid further degradation of leaking expansion bellow and to eliminate the likelihood of any toxic gas emission to atmosphere and untoward incidents, unit has been kept operating with reduced throughput rate. Sustained operation of SRU with reduced feed rate until replacement of bellow has negative consequences such as flaring partial amount of produced Acid gas to atmosphere which mounts to an estimated at 300 mmscf and loss of approximately 2600 tons of Sulphur product for the total estimated period required for new bellow procurement.
2. Methodology to minimize Acid Gas Flaring
2.1. Limitations restricting control of Acid gas loading
Total volume of Acid gas produced from Acid gas Removal unit is a function of total amount of H2S and CO2 absorbed into the amine stream towards achieving the treated gas specification requirements. To prevent or minimize Acid gas flaring without reducing the total Raw gas feed to the plant in the situation described above, controlling the acid gas loading of the amine in order to allow CO2 slippage in the treated gas has been chosen as a safe and economical option, provided the treated gas specification limits should not be violated.
Treated gas H2S content specification of 4 ppm vol. can never be violated whereas the CO2 content limit of 1% vol. can be marginally deviated without violating the specification limit of Ethane rich gas supplied to Ethylene crackers and Lean gas supplied to the Industrial consumers.
Presence of excessive CO2 in the sweetened feed gas to cryogenic section possibly leads to formation of Dry Ice, most probably over the trays below the feed tray of De- methanizer column where conditions are favorable for CO2 to reach stages of top equilibrium concentrations rather than at expander outlet conditions. Close monitoring of column differential pressure and other operating conditions is necessary for early identification of any onset of CO2 solidification.
2.2. Manipulating the Selectivity to control Acid gas loading
Acid gas loading of Amine is a function primarily based on two variables such as amine circulation rate and the contact time. As the inlet raw gas feed composition remains constant, total volume of acid gas picked up by the amine can easily be reduced by changing amine circulation rate. Whereas any amount of reduction in amine circulation rate has a direct negative impact on the treated gas H2S content and hence reducing amine circulation rate has been ruled out.
Manipulating with the selective absorption characteristic of MDEA is the only key parameter to be manipulated here for reducing the Acid gas pick up rate in order to achieve the above requirement. Being a tertiary amine, MDEA reacts slowly with CO2 and has only a moderate absorption rate. As the selectivity towards absorbing H2S is achieved by differences of reaction rates between H2S and CO2 with MDEA; reducing CO2 absorption can be accomplished by appropriately reducing the amine contact time which must be long enough to absorb almost all the H2S, but sufficiently short to remove only partial amount of CO2.
2.3. Increasing CO2 slippage-Implementation and Results
Amine absorbers in Acid gas Removal units have each 25 valve trays with multiple liquid feed provisions to Tray No. 2, 4, 6, 8, 10 and 12 and a dry tray at the top. Feed tray is chosen according to the required selective absorption performance.
At the time when SRU Incinerator bellow failure occurred, AGRU Train-1 & 2 were treating around 10,200 T/D of Sour Raw gas each with about 5,400 T/D of MDEA flow to Train-1 absorber fed over tray No.4 and 4,700 T/D of MDEA flow to Train-2 absorber fed over tray No.2 to meet the treated gas specification. This has generated an Acid gas feed flow of approximately 20,500 Sm3/h to SRU.
SRU throughput was reduced from 20,500 Sm3/h to 17,000 Sm3/h so as to reduce the amount of tail gas leaking from the cracked SRU incinerator bellow. This reduction has created sufficient drop in back pressure in the tail gas section which immediately led to stoppage of tail gas leak. Due to the above Acid gas feed reduction to SRU, AGRU start flaring around 3,500 Sm3/h of Acid Gas.
To reduce CO2 absorption and to increase CO2 slippage in the treated gas, MDEA feed nozzle to the absorber column was progressively switched to the adjacent lower ones so that the number of active trays will be systematically reduced to increase CO2 slippage in a controlled manner until reaching an optimum beneficial operating point
Initially, MDEA feed tray of AGRU Train-1 absorber which operates with comparatively higher MDEA circulation flow of 5,400 T/D due to pump mechanical limitation that led to increased CO2 absorption and obviously increased Acid gas flaring is switched from MDEA feed Tray No. 4 to the next lower MDEA feed Tray No. 6. Soon after the tray change over, the effect of Acid gas flaring reduction started to appear positively.
MDEA feed Tray of AGRU Train-2 absorber also switched from Tray No.2 to Tray No.6 and the effect of total 25% drop in flaring became apparent.
As the column conditions got stabilized and no further change observed in H2S and CO2 content of treated gas from both AGRUs, it has been decided to proceed further to switch MDEA feed trays for both the Trains from Tray No. 6 to Tray No.8. This action increased the CO2 slippage in both MDEA columns considerably and reduced Acid Gas flaring significantly by more than 80% (see stage 2 in Error! Reference source not found.).
During the mid day period, AGR columns experience significant increase in CO2 absorption due to the increase in MDEA temperature and this led to a marginal increase in flaring by approximately 3%.
After confirming by lab analysis that treated gas composition is well within the accepted specification limit, AGRU Train-1 feed tray was again changed over from Tray No.8 to Tray No.10 which almost led to total stoppage of Acid Gas flaring from AGRU. However, due to the effect of the increased inlet feed gas flow from Al-Shaheen wells which has a high CO2 content, slippage of CO2 in treated gas has increased together with marginal flaring due to the excess CO2 absorbed by the amine released in the acid gas.
These conditions were kept under close monitoring for two days and as part of this trial, periodical Lab analysis carried out for the treated gas and lean gas for H2S and CO2 content, results were found to be positive and product remained on-spec.
On the last day of this trial, plant conditions optimized and SRU throughput of Acid Gas gradually increased from 17,000 Sm3/h to around 18,000 Sm3/h as bellow leak remained under control and flaring of acid gas completely stopped.
Increasing the CO2 slippage in the Amine absorbers has reduced the CO2 content of Acid Gas considerably and stopped the anti-environmental Acid Gas flaring from AGRU. This achievement has saved and protected the environment from the impact of flaring around 300 mmscf of Acid Gas and therefore a loss of around 2610 tons of liquid sulphur product estimated to be 1.72 MM QR has been avoided.
Increasing CO2 slippage in absorber columns has a very minor contribution in change of lean gas composition, Only the CO2 content has increased from an average of 0.99% to around 1.4%. H2S content reached a maximum level of 2.0 ppm against 1.0 ppm at normal condition.
Flare Reduction Options and Simulation for the Qatari Oil and Gas Industry
Noora AlGhanim, Majeda Khraisheh, Farid Benyahia
Flaring and venting are two activities associated with oil and gas production. It is a combustion process used to dispose gases through vertical stacks or ground flares. Qatar is a world leader in gas and hydrocarbon processing. It has a world class Ethylene Cracker, Ras Laffan Olefins Company Limited (RLOC), of 1.3 million mtpa capacity. RLOC started up in April 15, 2010 and is operated by Qatar Chemical Company (QChem).
Qatar Ministry of Environment (MoE) cap targets flaring rate to less than 1% of inlet feed gas per year. As part of RLOC compliance with MoE regulations, a study was conducted, and tested at RLOC. The study is a comparison between the 'Normal' startup and shutdown procedures with another process called 'Methane Run', which was followed and suggest by RLOC shareholders from the U.S plants. This paper will present the tested case study for the purpose of flaring minimization at Ethylene plants during start up and shutdown and flaring reduction in Qatar and around the world. This study reveals that 'Methane Run' can significantly reduce flared volumes during plant startup and shutdown compared to the 'Normal' procedure. Additionally, 'Methane Run' can save two-week of the planned shutdown with a 93% reduction in cost compared to 'Normal' procedure. Furthermore, 'Methane Run' requires 75% less costs than the 'Normal' procedure, which makes it preferable by a new operated plant such as Qchem facility at Messiaed. On the top of its advantages, 'Methane Run' is environmentally friendly, such that it reduces the CO2 emissions by a minimum of 67% compared to the 'Normal' procedure.
Keywords: Ethylene, flare reduction, startup, shutdown, CO2 emissions.
Excerpted from Proceedings of the 3rd International Gas Processing Symposium Copyright © 2012 by Elsevier B.V.. Excerpted by permission of Elsevier. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.