Amine Plant Energy Requirements & Items impacting the SRU

Transcription

1 Amine Plant Energy Requirements & Items impacting the SRU 10 October 2016

2 AGRU energy needs Amine energy requirements Regeneration Processing effects Leanness required Determine required leanness Over stripping Energy sources In amine system Sulphur conversion waste heat loss due to AGRU Agenda SRU energy supply Waste heat Reactor Incinerator in excess of SRU/TGT needs, Is there more to save? Optimisation dependent on operating conditions All quoted figures are indicative 2

3 Amine closed system chemical reaction The amine solvent loading is set by temperature and partial pressure of H 2 S and CO 2 in a certain %wt amine solvent, determined by equilibrium, kinetics and the mass transfer between gas and liquid phase Equilibrium partial pressure of H 2 S Typical amine solvent Pressure (mbara) 2,000 1,500 1, Typical Solvent loading (mol/mol) Regenerator temperature, typical 115 C < T < 140 C Regenerator pressure, typical 1.5 < p < 2.5 bara Regeneration Operation: High temp, low pressure High solvent equilibrium partial pressure at solvent inlet, mass transfer acid component from solvent to gas phase Absorption Low temp, high - low pressure Low solvent equilibrium partial pressure at solvent inlet, mass transfer of the acid components from gas to solvent phase, kinetics role Absorber temperature, typical 30 C < T < 65 C Absorber pressure, typical 1.1 < p < 100 bara 3

4 Solvent regeneration heat Amine energy consumers Reboiler steam % of energy consumption in amine system Several components require energy (heat) o Desorption o Heating of solvent to regenerator bottom condition o Internal generation of steam in regenerator reboiler Heating medium: low pressure steam, also because of the constraint of thermal degradation of the amine at high temperature Solvent circulation pumps Remainder of total energy consumption depending on pressure differential and solvent properties Air cooling is a minor energy consumer compared to other energy consumers 4

5 Amine energy generation Equilibrium partial pressure of H 2 S Typical amine solvent 2,000 1,500 Amine type kj/kmol H 2 S kj/kmol CO 2 60%wt DGA %wt MDEA Pressure (mbara) 1, Solvent loading (mol/mol) 5

6 Absorber Heat (Enthalpy) Balance M feed, T feed, Cp feed Feed gas heat of absorption M sweet, Cp sweet, T sweet Sweet gas M lean, Cp lean, T lean Lean solvent M rich, Cp rich, T rich Heat of absorption (energy make) determined by: Solvent type o C p (for energy balance) Amine type Solvent %wt amine Type of treating o selectivity o other components removal Feed gas pressure Feed gas composition - C p Lean solvent temperature o prevention HC condensation criteria Feed gas acid content Sweet gas specification In principle independent of type of internals unless heat distributions is hampered The maximum temperature is usually above the bottom tray T rich > T sweet Rich solvent Heat of absorption is larger when the loading is smaller, thus not a constant! 6

7 Amine energy consumption for regeneration Equilibrium partial pressure of H 2 S Typical amine solvent Pressure (mbara) 2,000 1,500 1, Amine type kj/kg H 2 S kj/kg CO 2 60%wt DGA %wt MDEA Solvent loading (mol/mol) 7

8 Amine Regeneration (Heat) Enthalpy balance Rich solvent Two phase Acid gas/steam H 2 S Reflux All streams: M stream, T stream, Cp stream Generated steam LP steam Heat for regeneration determined by: Solvent type o C p (for energy balance) Amine type o Rich solvent loading and temperature after flash and L/R HE Solvent %wt amine o Solvent boiling temperature reboiler Type of treating o selectivity o other components removal Regenerator bottom pressure o Downstream unit pressure drop o Type of internals o Sweet gas specification Reflux temperature Lean solvent Stripped solvent Lean solvent Condensate Hydraulic constraint at regenerator inlet due to flashing and flow pattern may limit Overstripping causes corrosion/erosion 8

9 H r C p T P reaction heat specific heat temperature pressure Rich solvent Two phase Top steam S t Steam requirements regenerator Reflux The required solvent leanness is determined by the sweet gas specification S s = sensible heat heating solvent to bottom temperature S r = heat of reaction S s = C p T Steam S r = (CO 2 ) H r,co2 + (H 2 S) H r,h2s chemical desorption of H 2 S, CO 2, other Lean solvent CO 2, H 2 S S b Bottom steam Condensate LP steam + S t = top steam for sufficiently low acid partial pressure in the lean solvent at feed tray, minimum applied by process vendors S b = bottom steam (kg/m 3 ) = S t + S s + S r minimum applied by process vendors 9

10 The regenerator bottom temperature is determined by the amount of steam generated in the reboiler, which is a boiling aqueous amine at the bottom pressure: The more steam is generated, the higher the pressure drop ( P) across the regenerator trays, more gas phase The higher the pressure drop, the higher the regenerator bottom pressure (P 0 + P) The higher the bottom pressure, the higher the bottom temperature NB similar effect due to the stripping of acid gas in the reboiler is very small, because the solvent leanness should be met at the stripping stage Reboiler external steam regenerator steam Regenerator Bottom Temperature P 0 Rich solvent Two phase: Flashed H 2 S and CO 2 separates at feed inlet P 0 + ΔP Lean solvent Acid gas/steam H 2 S + CO 2 + steam Reflux Stripping stage LP steam Condensate 10

11 AGRU internal energy saving Inside AGRU Turbo expander SWEET GAS SRU FEED High pressure absorber only Not on one shaft with solvent pump FLASH GAS Saving on pump power dependent on pressure difference FEED GAS HEAT IN 11

12 AGRU internal energy saving Inside AGRU Lean/rich heat exchanger FEED GAS SWEET GAS FLASH GAS SRU FEED HEAT IN Optimisation limited by: - Regeneration needs (see leanness) - Type of heat exchanger - Use of low pressure steam limitation - Solvent loading (flashing in HE) - Fouling system - System configuration (vibration due to 2 phase flow in vertical regenerator inlet piping) 12

13 AGRU and SRU interfaces Outside AGRU affecting SRU AIR TAIL GAS ANALYSER SWEET GAS SRU FEED WASTE HEAT (Steam generation) FLASH GAS FEED GAS HEAT IN TO DEGASSER HYDROCARBON and CO 2 IN AMINE SOVENT H 2 S + R 3 N HS - + R 3 NH + H2S + ½O2 S + H 2 O 13

14 Entrained and soluble hydrocarbon route Hydrocarbon in acid gas AIR TAIL GAS ANALYSER SWEET GAS SRU FEED WASTE HEAT (Steam generation) FLASH GAS FEED GAS HEAT IN HYDROCARBON IN AMINE SOVENT When the color is more intensive, there is more entrained than soluble rated hydrocarbon in the stream TO DEGASSER Hydrocarbon combustion consumes energy in the SRU 14

15 SRU affected by hydrocarbon from amine unit Hydrocarbon content in SRU feed gas Preferably below 1%vol Maximum 5% vol Air consumption increase thus Reduction maximum capacity Therefore the overall Sulphur conversion Energy consumption increase This can be handled by Tail gas analyser when change is slow However Hydrocarbon composition fluctuation Black Sulphur Unburned liquid and heavy HC form soot on catalyst Can even block Sulphur rundown Fast composition changes cannot be controlled by analysers due to dead time 15

16 Co-absorption of CO 2 selective treating Ratio H 2 S CO 2 AIR TAIL GAS ANALYSER SWEET GAS SRU FEED WASTE HEAT (Steam generation) FLASH GAS FEED GAS HEAT IN TO DEGASSER HYDROCARBON IN AMINE SOVENT H 2 S + R 3 N HS - + R 3 NH + H2S + ½O2 S + H 2 O 16

17 CO 2 co-absorption determining parameters CO 2 co-absorption reduction design parameters based on: Chemical equilibrium differences H 2 S and CO 2 Meeting equilibrium would require infinite # trays/packing height Competition H 2 S and CO 2 Equilibrium condition dependent Reaction kinetics H 2 S has a very fast reaction at the gas phase CO 2 has a slow reaction in the liquid bulk, BUT CO 2 accelerator, even in minimum amount, destroys selectivity Gas/Liquid mass transfer Type of absorber internals gas/liquid interface, residence time 17

18 Reduction of CO 2 co-absorption H 2 S/CO 2 ratio Optimise DESIGN/DEBOTTLENECK based on realistic operating window, may be seasonally different Solvent selection and concentration Absorber temperature (different effect at high and low pressure) Flash vessel temperature and pressure Include sufficient on-line stream analysers for main components, include logic in control based on simulations based on operating experience Select the best practice internals to reduce entrainment (as with hydrocarbon) Optimise OPERATION within the equipment constraints in an existing plant Awareness of the contributing operational conditions is beyond pressure and temperature in an amine unit slow and smooth operational changes can be applied, opening on the run optimisation (based on modeling) Check if conditions change (new wells) and find new optimum process simulation may help, but mostly is not refined enough 18

19 AGRU operation trends affect energy demand Inside AGRU SWEET GAS SRU FEED Parameter trends in next slides FLASH GAS FEED GAS HEAT IN 19

20 Energy influencing trends Solvent variables %wt amine, heat of reaction Feed gas H 2 S and CO 2 Accelerator CO 2 removal Accelerator effectiveness Deeper spec H 2 S Deeper spec CO 2 Integration AGRU/TGT/AGE %wt amine, C p Mercaptan removal Type of amine, prim, sec, tert Hybrid solvent swap No trend 20

21 Energy influencing trends Operating variables Operating pressure absorber Trays Operating or packing pressure regenerator absorber Operating pressure absorber Lean solvent temperature Lean solvent temperature Lean solvent temperature Operating pressure regenerator Pressure drop regenerator Rich solvent inlet temperature regenerator, limit bottom Rich solvent inlet temperature Rich solvent inlet temperature regenerator, limit top AGE AGRU TGT Total removal, High pressure Selective H2S removal, Low pressure Selective removal, High pressure High loading regenerator, limit bottom Neutral Low loading External steam flow External steam temperature High Sulphur Recovery (TGT) Foaming tendency Retrograde HC condensation Solvent quality STEADY OPERATION Solvent rich loading Material integrity (corrosion) Regenerator overhead temperature Regenerator reflux temperature Trays Neutral Bottom limit Case dependent Risk of overstripping Risk of overstripping and degradation 21

22 Energy influencing trends Hardware Trays or packing absorber Trays or packing regenerator Isometrics, pressure drop Trays Trays Turbo expander Header system between processes Type of HE, pressure drop Trays or HE, approach Case dependent Effective mechanical filter Effective activated carbon filter + mechanical after filter More, other? Plant and condition specific Operational models can help explore options 22

23 Conclusion AGRU design, operation and installed hardware should take into account its impact on the SRU Amine units have many parameters to optimise its operation. Indicative trends are presented and can be used without violating Amine unit specification Integrated design and operation offer energy saving options Know your processes trends depend on conditions 23

24 Discussion Which do you use? 24

25 Thank you 25

26 References Reaction heat quotes averaged from Merkley, K.E., Christensen, J.J., Izatt, R.M., 1987, Enthalpies of Absorption of Carbon Dioxide in Aqueous Methyldiethanolamine Solutions, Thermochimica Acta, vol. 121, pp Kohl, A., Nielsen, R., 1997, Gas Purification, Gulf Publishing Company, Houston Texas. Versteeg, G.F., van Swaaij, W.P.M. 1988, 'Solubility and Diffusivity of Acid Gases (CO 2, N 2 O) in aqueous Alkanolamine solutions', Journal of Chemical engineering data, vol. 33, pp Oscarson, R.H., van Dam, R.H., Izatt, R.M., 1990, Enthalpies of Absorption of Hydrogen Sulfinolide in Aqueous Methyldiethanolamine Solutions, Thermochimica Acta, vol. 170, pp Oilfield Processing of Petroleum, Natural Gas, Volume 1 26