Common Causes of Catalyst Deactivation

Transcription

1 Common Causes of Catalyst Deactivation Differences of using Alumina versus Titania as Claus Catalyst and Tail Gas Catalyst carrier Mark van Hoeke, MSc, Dr. Bart Hereijgers Euro Support B.V., Amersfoort, The Netherlands

2 Outline Common Causes of Catalyst Deactivation Advantages of using Pure Titania as Tail Gas Catalyst Carrier

3 Euro Support Catalyst test-unit 4 identical stainless steel reactors used in parallel or in series

4 COMMON CAUSES OF CATALYST DEACTIVATION

5 Definition of a catalyst A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change

6 The performance of an Alumina catalyst predominantly depends on available surface area

7 Unfortunately catalytic activity will decrease over time due to various reasons

8 The main mechanisms of catalytic deactivation are through: 1. Porosity blocking access to active sites 2. Purity deactivation of active sites 3. Surface Area decrease in number of active sites

9 Reduced accessibility of surface area Porosity Liquid sulfur can fill up the pores of the catalyst when operating below sulfur dew point Prevention/Remedy: Operate around 15 o C above sulfur dew point Sulfur condensation in smallest pores Unavoidable

10 Reduced accessibility of surface area Porosity Soot formation Prevention: proper operation line burner. Ammonium Salts Prevention: ensure thermal stage temperature of > c for optimal ammonia destruction Coating by CarSul Prevention: ensure hydrocarbon destruction in thermal stage

11 Reduced accessibility of surface area Porosity Cracking of BTX in catalyst pores Prevention: thermal stage temperature of >1050 o C for hydrocarbon destruction

12 Poisoning by sulfation Purity Formed rapidly if the catalyst comes in contact with oxygen H 2 S/SO 2 ratio below 2 Presence of unreacted oxygen after direct fired reheaters More stable at lower temperature Therefore most common in the second and third Claus reactor Remedy: rejuvenation procedure

13 Purity Titania surface is far more resistant to sulfation Temperature programmed reduction of a sulfated titania and alumina catalyst with H 2 S H 2 S 1 The hydrolysis of Carbonyl Sulfide, Carbon Disulfide and Hydrogen Cyanide on Titania Catlayst. H.M. Huisman.1994

14 Loss in surface area by ageing Surface Area Hydrothermal aging Presence of steam in combination with higher temperatures and pressure results in loss of surface area Thermal aging Sintering of pores due to excessive temperature results in loss of surface area

15 Ageing of Titania catalyst Surface Area Specific Surface Area / m2.g Severe Ageing Time (h) Pure TiO2 Diluted TiO2

16 Limited Activity decline by ageing Pure TiO 2 CS 2 hydrolysis activity after hydrothermal aging, SV = 1832 h -1 Diluted TiO 2 CS 2 hydrolysis activity after hydrothermal aging, SV = 1832 h CS 2 conversion / % CS 2 conversion / % T-inlet / dgc T-inlet / dgc Fresh 16h severe ageing ES-Al2O3 Fresh Mild ageing 132h severe sgeing Lower SA but higher activity!

17 Purity and Poisoning Observed difference in activity not explained by surface area or porosity Fresh Pure TiO 2 Diluted TiO 2 Surface area (m 2 /g) Strength (N/mm) Pore volume (ml/g) TiO 2 (wt%) >99 86 Ca(SO 4 ) (wt%) 0 12 Density (kg/m 3 ) Incremental Pore Volume (ml/g) Mean Pore Diameter (nm) Pure Titania Diluted Titania Surface area and strength at the expense of purity and activity

18 Poisoning by (earth) alkali Purity CS2 conversion vs purity Activity at 300 C, GHSV = 1800/h, after Mild ageing CaO content NaO2 content CS2 conversion (%) CS2 conversion (%) Alkali content (XRF)/wt% (Earth)alkali impurities have detrimental effect on catalyst activity

19 ADVANTAGES OF USING PURE TITANIA AS TAIL GAS CATALYST CARRIER

20 Main reactions I SO 2 and S hydrogenation (CoMo) COS and CS 2 hydrolysis (support) CO conversion (CoMo) CO + H 2 O CO 2 + H 2 Water gas shift, H 2 production CO + H 2 S COS + H 2 COS production in sour gas shift CO + S COS COS production from sulfur

21 Titania based catalyst shows higher activity at lower temperatures than commercial leading Low Temperature Alumina based catalyst 100 COS conversion (%) Low T. Titania based Low T. Alumina based Hight T. Alumina based type 1 Hight T. Alumina based type Inlet Temperature (dgc)

22 Titania based catalyst shows higher activity after low temperature Insitu pre-sulfiding conditions 100 COS conversion (%) Low T. Titania based Low T. Alumina based Inlet Temperature (dgc)

23 Titania based catalyst shows a higher resistance to oxygen slip and easier resulfiding COS conversion % C 230 C 230 C + O2 230 C ex O2 230 C + O2-80 0:00:00 24:00:00 48:00:00 72:00:00 96:00:00 120:00:00 144:00:00 168:00:00 192:00:00 Time (hh:mm:ss) 230 C ex O2 TiO2 based commercial catalyst Leading Al2O3 based catalyst Test at Tin = 230 C Feed (%wet/%dry): H2S (1/1.28), SO2(0.5/0.64), COS and CS2 (0.025/0.032), H2 (1.5/1.92), CO (1.1/1.41), CO2 (16.7/21.4), H2O (22/), GHSV 1500 h-1. O2 (0.3/0.43 or 0).

24 Deactivation of Tailgas catalyst Commercial LT-TGTU catalyst activity fresh vs. after 2 years testing GHSV =1500/h, COS conversion (%) C 260 C 80.0 Conversion (%) TiO2- based, SOR TiO2- based 2 years Al2O3- based, SOR Al2O3- based, 2 years TiO2- based, SOR TiO2- based 2 years Al2O3- based, SOR Al2O3- based, 2 years

25 Conclusions Activity of Titania catalysts depends on more than just surface area Added Calcium Sulfate to enhance strength of Titania Catalyst has a negative impact on the catalytic activity Titania based Tail Gas Treating catalyst provides superior performance and operational benefits over Alumina based Tail Gas Treating catalyst

26 THANK YOU

27 BACKUP SLIDES

28 Dilution effect on performance? Pure TiO 2 catalyst 1000 L Diluted TiO 2 catalyst 1000 L 850 kg/m kg/m 3 >99% TiO 2 86% TiO 2 >842 kg Pure TiO kg Pure TiO kg CaSO kg additional material in reactor does not contribute anything to performance 28

29 Raw data activity measurement CS2/COS hydrolysis. Pure Titania Catalyst, after SEVERE ageing. 4-R-1 Feed gas (set points in mol% wet/mol% dry basis): H2S (8/10.67); SO2 (4.5/6); COS (0.5/0.67); CS2 (0.5/0.67); O2 (0.02/0.0267); H2O (25/-); N2 balance 1832 h h COS and CS2 conversion / % (320 C) (300 C) (280 C) (320 C) (300 C) (280 C) COS 30 CS Time on stream / hrs 29

30 Main reactions I Hydrogenation and shift reactions catalyzed by metal sulfides Claus and hydrolysis reactions catalyzed by support SO 2 and S conversion 2 H 2 S + SO 2 3/n S n +2H 2 O Claus reaction SO 2 + 3H 2 H 2 S + 2H 2 O SO 2 hydrogenation 3/n S n + H 2 H 2 S Sulfur hydrogenation CO conversion CO + H 2 O CO 2 + H 2 Water gas shift, H 2 production CO + H 2 S COS + H 2 COS production in sour gas shift CO + S COS COS production from sulfur

31 Main reactions II Hydrolysis (support) COS + H 2 O CO 2 + H 2 S COS removal CS 2 + 2H 2 O CO 2 + H 2 S CS 2 removal CS 2 hydrogenation CS 2 + 3H 2 CH 3 SH Mercaptan production CH 3 SH conversion CH 3 SH + H 2 CH 4 + H 2 S Mercaptan removal CH 3 SH + 3/nS n CS 2 + 2H 2 S Mercaptan removal CH 3 SH + SO 2 CS 2 + 2H 2 O Mercaptan removal

32 HT vs. LT High temperature (in)direct fired reheaters consumption of natural gas Conventional tail-gas catalyst HT-sulfiding of catalyst; Co x Mo y O z + H 2 + H 2 S CoMoS x + H 2 O Low temperature Steam reheaters, T in, max. = 240 o C Special catalyst In-situ or ex-situ presulfiding

33 Test conditions, evaluation High Temperature (HT) pre-sulfiding Heat up the catalyst to 375 C in 1 mol% H 2 S, 4 mol% H 2, N 2 balance, at a space velocity (GHSV) of 650 Nm 3 /m 3 /h. Keep the catalyst at 375 C for 16 hours. Cool to test temperature and switch to test gas

34 In-situ pre-sulfiding conditions for a Low Temperature TGT catalyst Heat up the catalyst to 130 C Feed Gas Composition: 1.5 mol% H 2 S, 6.0 mol% H mol% H 2 O, N 2 balance, GHSV = 450 Nm 3 /m 3 /h. Inlet Temperature (dg) ΔT = 300 o C, exotherm T in, max = 240 C, in plant additional ΔT from exothermal sulfiding reaction gives T bed = 300 C Test sequence: C Time (h)

35 Test conditions, evaluation Performance evaluation Start at 290 C inlet temperature in test gas. Feed gas (N 2 balance): 1 mol% H 2 S, 0.5 mol% SO 2, mol% COS, mol% CS 2, 1.5 mol% H 2, 1.1 mol% CO, 22 mol% H 2 O, 16.7 mol% CO 2, GHSV = 1500 Nm 3 /m 3 /h. Analyze dry feed gas and dry, sulfur free product gas. Temperature sequence: C

36 Assessing Catalyst Quality Low hydrogenation activity (CoMoS x ): CO shift reaction efficiency decreases COS emmision increases through COS formation from CO. Low hydrolysis activity (support): CS 2 hydrogenation competes with hydrolysis; Formation of CH 3 SH increases When catalytic activity is extremely low, CH 3 SH formation drops to zero as well!

37 Assessing Catalyst Deactivation CO conversions (CoMoS x ) CO + H 2 O CO 2 + H 2 Water gas shift (DECREASES) CO + H 2 S COS + H 2 Sour gas shift (INCREASES) CO + S COS COS production from S (INCREASES) Upon deactivation COS emissions will increase due to the reactions above. The COS that is formed in the lower layer of the bed cannot be hydrolysed anymore by the carrier. CS 2 hydrolysis-hydrogenation competition CS 2 + 2H 2 O CO 2 + H 2 S CS 2 Hydrolysis (DECREASES) CS 2 + 3H 2 CH 3 SH Mercaptan production (INCREASES) Upon deactivation CS 2 hydrogenation starts competing with the hydrolysis. This means mercaptans are formed that will contribute to emissions. When catalytic activity is extremely low, also CS 2 hydrogenation activity drops to zero.

38 Back up slide TITANIA AS CLAUS CATALYST P. D. Clark, N. I. Dowling and M. Huang, 2015 Ti 3+ cations under Claus process conditions -> Proton donor (Bronsted Acid) -> promotes the adsorption/activation of SO 2 and, hence, increases its Claus reaction rate. 38