centration well above 3000 ppm. Once there is no more consumption of H 2 S, and the temperature is >300⁰C, the catalyst is deemed to be completely sulphided, and DMDS feed is stopped. The catalyst is sometimes soaked in recycle gas with high H 2 S concentrations at this high temperature for about an hour or more. The recycle gas is then partially purged**, fresh H 2 is brought in, a value of H 2 S >5000 ppm in the recycle gas is ascertained, and the unit is prepared to accept sour feed in the production mode. ** All venting and purges require the amines gas treating unit to be online. A RainerRakoczy,TechnologyAdvisorFuel, andMaximilian Dochnahl, Head of Modelling & On-site Technology, Clariant Catalysts A large number of catalysts for the petrochemical and pharmaceutical industries contain precious metals (PM), such as palladium, platinum, rhodium, ruthe- nium, and gold. These represent a high economic value even if only present in low percentage amounts. Precious metal reclaiming is, therefore, a common practice in the industry. Several specialised companies – typically locally active, rather than globally – have developed methods to recover a high percentage of those metals from spent catalysts. In general, the recy- cling company will credit the value of the recovered material to your account minus a fee for their service. Catalysts using base metals like copper, nickel, cobalt, iron, or molybdenum are used in even larger amounts than PM catalysts. They generally contain a much higher content of base metals than their PM counter- parts. The business case for recycling those metals is not as obvious as in the case of PM, and it depends on a number of factors such as the base metal content, over- all composition, metal price, and recovery efficiency. It needs to be determined case by case if reclaiming those metals is sensible. This will be done by the reclaimer, who will perform a detailed analysis weighing the influencing parameters upon receiving a representative sample of the spent catalyst. The reclaimer will share their findings, recommend a way to recycle the catalyst, explain the economics, and discuss the logistics with you. From a sustainability standpoint, it makes sense for most spent catalysts to reclaim the precious and base metals, as mining and refining those metals are resource and energy-intensive endeavours, which gen- erally result in a large carbon footprint. A Daniël Bruggeman, METCHEM, D.Bruggeman@ metchem.nl, Meritxell Vila, MERYT Catalysts & Innovation, mvila@meryt-chemical.com Catalysts are used on a large scale in refineries, petro - chemical plants, and chemical plants in general, as they are used in 90% of the chemical processes. The value of purchasing fresh catalysts is a very significant part of the production costs. Most of the catalysts are heteroge- neous containing metals, and when the catalyst reaches the end of its cycle life, it is time for recycling and metal recovery.
Typical reactions of DMDS with the catalysts are shown below:
MoO 3 + 4H 2 + CH 3 SSCH 3 MoS 2 + 3H 2 O + 2 CH 4 ΔH = -319 kJ.mol -1
9CoO + 13H 2 + 4 CH 3 SSCH 3 Co 9 S 8 + 9H 2 O + 8 CH 4 ΔH = -629 kJ.mol -1
3NiO + 4H 2 + CH 3 SSCH 3 Ni 3 S 2 + 3H 2 O + 2 CH 4 ΔH = -374 kJ.mol -1
WO 3 + 4H 2 + CH 3 SSCH 3 WS 2 + 3H 2 O + 2 CH 4 ΔH = -303 kJ.mol -1
All hydrotreating catalysts require a stoichiometric amount of sulphur to be activated. The process of cat- alyst activation involves both sulphiding coupled with controlled metals reduction. Knowing that the whole process is exothermic, the rate at which the sulphiding agent (DMDS) is introduced into the feed oil must be controlled. Both the sulphiding reaction and the reduc- tion reaction are accelerated by increasing temperature. One needs to be cognizant that there is a time delay between the increase in DMDS flow rate and the con - comitant increase in reactor outlet temperature. Below 240⁰C, the reduction reaction is not pro - nounced and thus, as we increase the sulphiding agent flow rate in the temperature range of 220-240⁰C, we are accelerating mainly the sulphiding reaction. For faster sulphiding, one needs to have higher H 2 S concentrations in the reactor while controlling the temperature. This can be done by the independent control of the liquid feed rate into the reactor if feasi- ble and the sulphiding agent flow rate into the liquid feed. Sulphiding is carried out in two phases: primary and secondary. Primary sulphiding is done at tem- peratures below 250⁰C, and secondary sulphiding is carried out from above 250⁰C to >300⁰C or start-of-run temperatures (SOR) (>300-350⁰C). Different sulphiding agents decompose differently to H 2 S as a function of temperature. DMDS is used extensively as a sulphiding agent because it starts to generate H 2 S at temperatures as low as 190⁰C and decomposes exclusively to H 2 S and methane at temperatures as low as 240⁰C. Maximising DMDS flow rate above 220⁰C, ascertaining a controlled exotherm by intermittent changes in DMDS flow rate while continuing oil feed, will allow for a quick break- through of H 2 S (3000 ppm in the recycle gas). At this point after breakthrough, DMDS feed needs to be con- tinued to establish an H 2 S concentration significantly higher than 3000 ppm in the recycle gas at a tempera- ture of about 240⁰C. A vent of the recycle gas can then be taken, to purge the CH 4 and some of the H 2 S**. Fresh H 2 is then introduced to restore at least 60% H 2 purity of the recycle gas (the minimum value prescribed by the catalyst manufacturer). The H 2 S concentration should still be significantly higher than 3000 ppm After H 2 S breakthrough, DMDS flow rate is adjusted to maintain H 2 S, typically in the 1.5-2.0% in the recy- cle gas, while the temperature is increased 15-20⁰C per hour (or the catalyst manufacturer’s prescribed increase). Remember, as the temperature is increased, the H 2 S is consumed to sulphide the catalyst. DMDS flow is continued at a rate that maintains the H 2 S con-
16 Catalysis 2022
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