Available online at www.sciencedirect.com
ScienceDirect Procedia Environmental Sciences 35 (2016) 610 – 617
International Conference on Solid Waste Management, 5IconSWM 2015
Minimization of Waste Spent Catalyst in Refineries Chiranjeevi T*, Pragya R, Gupta S., Gokak DT, Bhargava S * Corporate R&D Centre, Plot No. 2A, Udyogkendra, Surajpur, Greater Noida, India
Abstract Solid catalytic materials play major role in oil refining industry. There are various types of catalysts which are in use but the major ones are mainly hydroprocessing catalysts, Fluid catalytic cracking catalysts and reforming catalysts. During processing, catalysts gets contaminated with impurities viz coke, sulfur, vanadium and nickel in the crude oil feed and becomes deactivated over a period of time. Diesel hydrodesulfurization catalysts typically have life cycle of 3-4 years where as FCC catalysts gets lost to atmosphere on daily basis and is offloaded fortnightly / monthly based on activity. Once catalysts completes their life cycle they will be withdrawn from the process, at this stage, catalysts are considered “spent” and the heavy metals, coke, and other poisonous elements make them as hazardous waste. As per literature Dufresne estimated that the total quantity of spent hydroprocessing catalysts generated worldwide is in the range of 150,000 to 170,000 tons per year. Therefore, with anticipated 5% annual increase in catalyst consumption, the generation of spent hydroprocessing catalysts predicted to be 200,000 tons annually. The exact figures of spent FCC catalysts are not available but considering the short life span of the catalyst and 400 FCC units operating across the world volumes assumed to be very high. Significant increase of spent hydroprocessing catalysts volumes are mainly attributed to rapid growth in the distillate hydrotreating capacity to meet the increasing demand for ultra low sulfur transportation fuels, reduced cycle times due to in increased severity operations to meet stringent fuel specifications and demand of processing sour crudes based on economic criteria. The traditional way of disposing the spent catalysts is land filling which is not environmentally friendly and occasisionally it leads to ground water contamination. Various options to reduce the generation of hazardous waste spent catalyst are using highly active catalysts, regeneration of the catalysts and reuse of the spent catalysts in other processes. The present paper will discuss about alternate ways of using spent catalyst by taking case study of hydroprocessing catalysts and thereby minimizing the waste spent catalyst in refining industry. Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license Authors. Published by Elsevier © 2016 2016The TheAuthors. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 5IconSWM 2015. Peer-review under responsibility of the organizing committee of 5IconSWM 2015 Keywords: Waste minimization, Hydrotreating catalysts, Catalyst regeneration, CoMo or NiMo catalysts, Spent catalysts;
* Corresponding author. E-mail address:
[email protected]
1878-0296 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 5IconSWM 2015 doi:10.1016/j.proenv.2016.07.047
T. Chiranjeevi et al. / Procedia Environmental Sciences 35 (2016) 610 – 617
1.0 Introduction Varieties of catalysts are used in the petroleum refinery operations to improve the process efficiency. The catalysts often contain chemicals (e.g. metals, metal oxides, metal sulfides, inorganic support), which facilitate hydrocarbon transformations with high selectivity and permit the refiners to produce full range of clean transportation fuels and other chemicals with desired specifications from petroleum distillates and residues. The catalysts used in the refining processes deactivate with time due to structural changes, poisoning or deposition of extraneous materials like coke and metals. The volume of spent hydroprocessing or hydrotreating catalysts discarded as solid waste has increased significantly in recent years due to the following reasons: (i) A rapid growth in the distillates hydrotreating capacity to meet the increasing demand for ultra-low sulfur transportation fuels. (ii) Reduced cycle times due to higher severity operations in hydroprocessing units. (iii) A steady increase in the processing of heavier feed stocks. (iv) Rapid deactivation and unavailability of reactivation process for latest residue hydroprocessing catalysts [Silvy RP, 2004]. The total quantity of spent hydrotreating catalysts generated worldwide is in the range of 150,000–170,000 t/year [Dufresne,2007]. Disposal of spent catalysts requires compliance with stringent environmental regulations. Spent hydroprocessing catalysts have been classified as hazardous wastes by the environmental protection agency (EPA). As a result of the stringent environmental regulations on spent catalyst handling and disposal, research on the development of process for regeneration and reuse of waste hydrotreating catalysts has received considerable attention. Furimsky, 1996, reviewed the environmental, disposal and utilization aspects of spent refinery catalysts and the various options suggested (a) minimizing spent catalyst waste generation (b) utilization to produce new catalysts and other useful materials, (c) recycling through recovery of metals and (d) treatment of spent catalysts for safe disposal, are available to refiners to handle the spent catalyst problem. Traditionally catalyst activity was restored and reused by in-situ regeneration. In-situ regeneration includes coke burning step and activation treatments which were performed in the user’s reactor. During coke burning step other pollutants like nitrogen, sulfur (which is also part of active phase) are oxidized to CO, CO2, SOx and NOx. Activation of catalyst is done by sulfidation of metal oxides to metal sulfides which causes excess H2S gas handling at refinery site [Patrick, Sal Torrisi, 1999]. Current paper highlights the concept of waste minimization by regenerating and re-use of spent catalyst which is due for disposal by taking hydroteating catalyst as a case study. One commercial spent catalyst was regenerated in the lab and based on lab results, catalyst was recommended for ex-situ regeneration. Ex-situ regenerated catalyst properties were compared with lab regenerated catalyst and found that properties are having close match. The plant performance data of ex-situ generated catalyst was compared with catalyst performance before regeneration to assess the level of deactivation. 2.0 Experimental Details 2.1 Textural Properties Surface area and pore volume of the catalyst samples were determined using nitrogen adsorption-adsorption measurement technique by Autosorb -1MP (Quanta chrome, USA) at -196oC (ASTM No: 3663-99). Prior to the measurements, the sample (100 mg) was degassed at 3000C for 3 hours under vacuum (10-3torr). Surface area for the sample was estimated by analyzing the adsorption data in the relative pressure (P/P0) range of 0.05 to 0.3 using BET method. 2.2 Mechanical Properties Bulk density of the sample was estimated using Duel Auto Tap density meter supplied M/s Quantachrome, USA as per ASTM D 4164. Bulk crushing strength was measured by using test unit supplied by M/s.Vinci, France as per Shell Method, SMS 1471. Particle size of spent regenerated and fresh catalyst was carried out by sieve analysis and also by using standard vernier calliper equipment.
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2.3 Chemical Analysis Spent catalyst samples analyzed for V, Fe, Na, As and Cu metals by means of ICP (Inductively coupled plasma spectrometry) supplied by M/s Thermo Finigan. Metals from catalyst were extracted in solution by using microwave digestion technique before analyzing. The equipment is standardized for each metal against the known standards before analyzing the actual sample. Spent and regenerated catalysts were also analyzed for Carbon, Hydrogen, Nitrogen & Sulphur by using elemental analyzer, Model Flash EA 1112 supplied by M/s Thermofinnigan, Italy as per procedure described in ASTM D 5373. 2.4 Thermo Gravimetric Analysis (TGA) and Differential Scanning Calorimeter (DSC) TGA is used for finding out moisture content, loss on ignition (LOI), determination of volatile matters, and carbon oxidation behaviour of the catalyst. Typically about 10 mg of sample is heated from ambient to 1000oC @ 10oC per minute in air /inert gas (100 ml/min) using TA instruments (USA) DSC-TGA simultaneous unit and the weight loss /Heat flow was measured as a function of temperature. 2.5 Scanning Electron Microscopy and Energy Dispsersive X-Ray (SEM/EDX) The morphological characteristics of the regenerated and fresh catalyst samples were examined by scanning electron microscopy (SEM, TESCAN, Vega-LSU) equipped with X-ray microanalysis (OXFORD INCA PentaFETx3). Scanning electron microscope images were acquired at a magnification of 6.4 KX at 20 KV and WD of 9.8 with SE detector. Surface metal profiling was achieved using the energy dispersive X-ray (EDX) system at a WD of 23.00 and 15 KV. Pellet and powder samples were dispersed on carbon glued studs. 2.6 X-Ray Diffractometry Samples were analyzed by a Philips X'pert powder diffractometer system and employing silicon reflections as a reference. The X-ray pattern was recorded with CuKD radiation with a 0.04o step size and 3 seconds step time over the range 5o < 2T < 80o. 3.0 Results and Discussion One commercial HDS plant spent catalyst which is due for disposal was regenerated in the lab to assess the regenerability and reuse of the catalyst. Spent catalyst was analyzed for physico-chemical and mechanical properties and compared with fresh catalyst to study the extent of deactivation. Based on lab characterization data and economic evaluation spent catalyst Ex-Situ regeneration job was awarded to external party. Post regeneration catalyst sample was again analyzed for physico-chemical properties in lab to confirm the recovery of properties. Plant performance data of the regenerated catalyst and fresh catalyst are compared to evaluate the performance of the regenerated catalyst. 3.1 Characterization Spent catalyst sample collected was black in colour with fines (< 0.5wt %) in it. The colour of the sample changed to light green (similar to fresh catalyst) after coke burning. Textural properties like surface area and pore volume of fresh and lab regenerated catalyst samples are given in Table 1. Surface area of lab regenerated catalyst is 200 m2/g and fresh one is 211 m2/g. This shows that there is significant gain in SA after regeneration.
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T. Chiranjeevi et al. / Procedia Environmental Sciences 35 (2016) 610 – 617 Table 1: Physico-Mechanical Properties Catalyst Sl.
Property
Fresh
Lab Regenerated
Recovery
Textural properties 1
Surface Area (m2/g)
211
200
> 95%
2
Pore Volume (cc/g)
0.54
0.52
> 96%
0.79 0.70
0.75 1.00
100% 100%
Mechanical Properties 3
ABD (g/cc)
4
BCS (Mpa)
The loss in surface area is attributed to redistribution of pores or pore blockage. Lab regenerated catalyst restores its pore volume by about 96%. (0.52 cc/g regenerated vs 0.54 cc/g fresh). Nitrogen adsorption isotherms of fresh, spent and regenerated catalysts were presented in Figure.1, also show that regenerated and fresh catalysts are having similar N2 adsorption pattern where as coked one differs. Mechanical properties like ABD and Bulk Crushing Strength of fresh and regenerated samples were given in Table-1. Results show that mechanical strength of regenerated catalyst is retained after regeneration.
vo lu m e, cc (S T P )
400
Fresh
350
decoked
300
Spent
250 200 150 100 50 0 0.00E+00
2.00E-01
4.00E-01
6.00E-01
8.00E-01
1.00E+00
1.20E+00
P/Po
Fig. 1. Nitrogen Adsorption Isotherms of Fresh, Spent and Regenerated HDS Catalysts
3.2 Carbon, Hydrogen, Nitrogen and Sulfur (C, H, N,S) Analysis Elemental analysis viz. C, N, S and hydrogen results which were measured by the procedure explained in experimental section are presented in Table-2. Carbon on spent catalyst was 8.5 wt% which is due to coke deposition over period of time. 1.26 Wt% Hydrogen and 0.38 wt% nitrogen is due to left over high boiling hydrocarbon & nitrogen compounds. Sulfur on spent catalyst was 2.4 wt % which may be due to residual feed or product or may be due to metal sulfides. However, regenerated catalyst contain very less amount of carbon (< 0.1Wt %), Nitrogen (< 0.2Wt %),) and 0.1% Sulphur. The trace metals like Vanadium, Sodium, Iron, Arsenic and Copper which inhibit the catalytic activity are presented in Table-2. Results show that regenerated catalyst contains some amount of trace metals like, Fe, Na, V and which may not contribute significantly to activity loss because of low concentration [4].
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T. Chiranjeevi et al. / Procedia Environmental Sciences 35 (2016) 610 – 617 Table 2: CHNS and Metal Analysis Results Sample Name
Cu (ppm)
Fe (ppm)
Na (ppm)
V (ppm)
As (ppm)
Fresh
5.0
25
200
NIL
NA
Spent
7.0
528
603
258
17
Regenerated
28
220
NA
640 600 Elemental Analysis (wt %)
Carbon
Hydrogen
Nitro-gen
Sulp-hur
Spent
8.5
1.26
0.38
2.4
Regenerated
0.12
1.08
0.20
0.1
3.3 DSC-TGA Analysis Thermo gravimetric analysis results are presented in Figure.2. Weight loss observed in the range from 25°C 300°C for fresh and regenerated catalysts are 5.7 wt% & 6.4 Wt% respectively and the weight loss in range of 25°C to 1000°C for fresh and regenerated catalysts appear to be same. In case of spent catalyst 26% weight loss observed, it may be due to moisture, residual hydrocarbons, coke, sulphur and nitrogen etc. The weight loss in the range of 300-600°C may be due to burning of coke, sulphur & nitrogen compounds. Differential scanning calorimetric graph (Figure-3) shows two exothermic peaks due to coke oxidation at two different temperatures which represents soft and hard coke. (318°C & 466°C). Fresh
105
Spent
Weight loss, %
100
Decoked
95 90 85 80 75 70 0
200
400
600
800
1000
1200
Temperature, C
Fig. 2. Thermo Gravimetric Analysis Results
4
Spent
Heat flow, W/g
3.5
Fresh
3
Decoked
2.5 2 1.5 1 0.5 0 0
100
200
300
400
500
600
Temperature, C
Fig. 3. Differential Scanning Calorimetric Results
700
800
615
T. Chiranjeevi et al. / Procedia Environmental Sciences 35 (2016) 610 – 617
3.4 SEM/EDX/XRD Results Fresh and regenerated samples were analyzed by SEM/EDX. SEM analysis shows that there is no appreciable difference between fresh and regenerated catalysts morphology. EDX analysis of fresh and regenerated catalysts show that metals are uniformly distributed on surface of fresh catalyst where as slight deviation in metal distribution was observed in case of regenerated catalysts. Heterogeneity in metal concentration in case of regenerated catalyst may be due to ageing effect over a period of operation. Fresh and regenerated catalysts were analyzed for XRD with the aim of observing any sintering of active phase and subsequent crystallization of Mo or Ni separate metal phases. XRD results were presented in Figure 5. Results show that no peaks are observed due to crystal growth of Mo or Ni indicates that metals are dispersed well on the surface of alumina or presence of very small crystals (
