Hydroprocessing in Aqueous PhaseEdward Furimsky*
IMAF Group 184 Marlborough Avenue Ottawa, Ontario, Canada K1N 8G4
ABSTRACT: A large consumption of H2 aects the overall economy of conventional hydroprocessing. The costs can bedecreased by using water as the source of active hydrogen. This can be achieved under subcritical and supercritical waterconditions providing that an active and stable catalyst is developed. Hydroprocessing in aqueous phase has been studied forpotential applications in upgrading of high oxygen content feeds and heavy petroleum feeds to liquid hydrocarbons. The feedswere tested at temperatures ranging from less than 200 to 500 C and total pressure from 1 to 30 MPa. These conditions coversubcritical and supercritical regions of water. Water takes part in hydroprocessing reactions as a free radical scavenger and ahydrogen donor. Hydrogen generated in situ via partial reforming and watergas shift reactions is more reactive than externalhydrogen. Catalyst development for hydroprocessing in aqueous phase has been receiving much attention. High performance wasobserved over the catalysts containing noble metals (Pt, Pd, Ru, and Rh) supported on various supports; however, theinformation on a long-term stability of these catalysts is limited.
Hydroprocessing (HPR) is the most important route forupgrading petroleum and nonpetroleum feeds to commercialfuels. It involves conversion of compounds containingcontaminants such as sulfur, nitrogen, and metals to hydro-carbons via reactions with hydrogen. In some liquids, a nalpolishing step is required to attain specication of trans-portation fuels and lubricants. For example, aromatics must beremoved by hydrogenation while straight chain hydrocarbonsby hydroisomerization. The cost of conventional HPR isaected by a large consumption of hydrogen. Water has beenidentied as an alternative source of hydrogen. Both subcriticaland supercritical water (SCW) conditions have been attractingattention.Water is an important constituent of the feeds produced via
hydrothermal liquefaction and pyrolysis of biomass. In this case,operating conditions and/or type of biomass dictate that liquidproducts are obtained in an aqueous medium. Separation of thewater-soluble components from the aqueous phase may bedicult and inecient. Therefore, conversion of polarcompounds to hydrocarbons directly in the same environmentmay be more advantageous.1 In such applications, HPR is themethod of interest. Once polar components in the feed areconverted to hydrocarbons, the separation of hydrophobicphase from the aqueous phase is simple. A similar approachmay be applied for upgrading of the aqueous phase separatedfrom the primary products obtained during FischerTropschsynthesis (FTS). This byproduct may contain up to 10 wt % ofdissolved oxygenates. Direct removal of these oxygenates fromaqueous phase via catalytic route has also been attractingattention.3,4
Depending on the method of production, petroleum crudesmay be obtained in the mixture with large quantities of water.This may be the case of heavy crudes produced during theenhanced-oil-recovery using steam ooding method and via hotwater separation process employed during the bitumenproduction from tar sands.5 A direct conversion of such crudes(without dewatering) via HPR may be a potential route for
primary upgrading. A unique case of an aqueous phase may beslurry bed hydrocracking (HCR) of heavy feeds. In this case, acatalyst dissolved in water is coslurried with feed beforeentering the reactor.The information on conventional HPR methods has been
extensively reviewed elsewhere.6 All reactions occurring inparallel during the HPR of conventional feeds, that is,hydrodesulfurization (HDS), hydrodenitrogenation (HDN),hydrodeoxygenation (HDO), hydrocracking (HCR), hydro-genation (HYD), hydroisomerization (HIS), hydrodemetalliza-tion (HDM), and hydrodeasphaltization (HDAs), have beendiscussed in details. In addition, the most important HPRreactions were reviewed separately (i.e., HDS,68 HDN,911
HDO,12,13 HYD,14 HIS,2 HCR,2,15 HDM15,16 and HDAs15,16).Some similarities in the mechanisms of these reactions in thepresence of water may be anticipated. However, rather thanrepeat this information here, the main focus of this review is onpotential role of water in modifying the mechanism of HPR.Also, the eect of water on operating parameters underaqueous conditions requires attention. Of particular signicanceare the eects of water on catalyst activity and stability. In thisregard, the advances in catalyst development for applications inaqueous phase are one of the objectives of this review.
2. PROPERTIES OF WATER
During the HPR in aqueous phase, water plays an importantrole as both solvent and reactant. Chemical and physicalproperties of water as well as their change with temperature andpressure were described in details elsewhere.17 This includedthe properties in subcritical and supercritical regions. Thus,under mild conditions, a direct involvement of water in HPRreactions may be much less evident. For HPR, the miscibilityand/or solubility of various feeds (including H2) in water as
Received: October 15, 2013Revised: October 30, 2013Accepted: November 21, 2013Published: November 21, 2013
2013 American Chemical Society 17695 dx.doi.org/10.1021/ie4034768 | Ind. Eng. Chem. Res. 2013, 52, 1769517713
well as diusivity and reactivity of water are of prime interests.For the purpose of this review, only a brief account of theseproperties is given in the following text. Thus, rather extensiveinformation on these and other aspects of water may be readilyaccessed in several books published elsewhere.1719
The original structure of liquid water, dominated byhydrogen bonds, is changing with increasing temperature.While critical temperature is being approached, an almostcomplete collapse of the hydrogen bond network occurs. As theresult of this change, polarity of water is signicantlydiminished. This was conrmed by a dramatic decrease indielectric constant.1719 Above the critical point, water behavesas a nonpolar medium, capable of dissolving organic substrates.In this regard, water is approaching properties of solvents suchas acetone, methanol, ethanol, etc. Then, the solubility ofvarious feeds (e.g., bio oils, petroleum residues, coal derivedliquids, etc.) in SCW is signicantly enhanced.19 From the HPRpoint of view, it is important that under super criticalconditions, gaseous H2 is completely miscible with SCW. Ahigh homogeneity of reaction streams attained in SCW isfavorable for the ecient transfer of hydrogen to reactantmolecules.Above the critical point, water behaves as a dense gas while
still retaining some characteristics (e.g., density) of liquid water.This behavior is the reason for a high diusivity and uniquetransportation properties of SCW. While increasing temper-ature from subcritical region toward critical temperature at 22MPa, the density of water abruptly decreases, for example, fromabout 0.6 g/mL at 350 C to less than 0.2 g/mL at 374 C.Although to a lesser extent, the SCW density further decreasedwith temperature increase above critical point temperature.This density decrease may be oset by increasing pressure.19
Some eect of density of subcritical water and SCW on HPRreactions may be anticipated. Then, if necessary, an optimalcombination of density with temperature and pressure may beestablished.It is obvious that the reactivity of water may change
dramatically as the consequence of hydrogen bonds networkcollapse. For example, while approaching 374 C, the pKw ofthe water dissociation equilibrium almost doubled.1719 Amuch higher concentration of H3O
+ and HO ions in SCWthan that in liquid water increases the chances for theinvolvement of these ions during HPR. For example, H3O
ions tend to add readily to heteroatoms such as S, N, and O.6,9
An interaction of HO ions with carbons, particularly thoseattached to heteroatoms, may be anticipated.20 These factsincrease the probability of an ionic mechanism as part of theoverall mechanism during the HPR in aqueous phase. Ionicreactions are favored by high density of water. This may beachieved under subcritical conditions; however, under super-critical conditions, high pressures are needed to get densitiessuitable for ionic chemistry.
3. PROPERTIES OF FEEDS
In this section, attention is being paid to those feeds that havebeen included in the studies on HPR in aqueous phase. In thisregard, model compounds alone and/or mixtures of variousmodel compounds used to study HPR under conventionalconditions have been also studied under aqueous phaseconditions. This is illustrated on several examples presentedin this review. In the case of real feeds, the focus is on thosefeeds which are being produced in an aqueous environment.
The objective of research in this eld was a direct upgrading ofsuch complex mixtures without pretreatment.The most typical source of high water content feeds is the
conversion of biomass (both by pyrolysis and hydrothermaltreatment) always to a high water content biocrude. Detailedaccounts of the conversion of an aquatic biomass viahydrothermal liquefaction and gasication, both in the presenceand absence of catalysts was given by Yeh et al.21 and Savage.22
An example of the biocrude (primary liquids) from pyrolysisand liquefaction of lignocellulosic biomass is shown in Table1.23 At least two stages may be needed to upgrade such feeds to
hydrocarbons via HPR route.24 Chemical compositions of thesebiocrudes and corresponding products were discussed in detailselsewhere.13 Unless an extensive dewatering was conducted, ahigh water content in the biocrude obtained from algae biomassand municipal solid wastes using similar methods may beanticipated.12 Interes