COMBINATION GAS DISENGAGING
DOWNCOMER-REJUVENATION TUBE FOR
IN-SITU SLURRY CATALYST
BACKGROUND OF THE DISCLOSURE
1. Field of the Invention
The invention relates to a process and means for rejuvenating catalyst particles in-situ in a slurry. More particularly, the invention relates to a process and means for rejuvenating solid catalyst particles in-situ in a three phase, FischerTropsch type hydrocarbon synthesis slurry comprising said particles and gas bubbles in a hydrocarbon slurry liquid, using a combination gas disengaging downcomer and rejuvenation tube.
2. Background of the Invention
Slurry hydrocarbon synthesis (HCS) processes are known. In a slurry HCS process a synthesis gas (syngas) comprising a mixture of H2 and CO is bubbled up as a third phase through a slurry in a reactor in which the slurry liquid comprises hydrocarbon products of the synthesis reaction and the dispersed, suspended solids comprise a suitable Fischer-Tropsch type hydrocarbon synthesis catalyst. Reactors which contain such a three phase slurry are sometimes referred to as "bubble columns", as is disclosed in U.S. Pat. No. 5,348,982. Irrespective of whether the slurry reactor is operated as a dispersed or slumped bed, the mixing conditions in the slurry will typically be somewhere between the two theoretical conditions of plug flow and back mixed. One or more gas disengaging downcomers may be used in maintaining catalyst distribution as is disclosed in U.S. Pat. No. 5,382,748. Syngas made from hydrocarbon feedstocks which contain nitrogen (i.e., natural gas) or nitrogen containing compounds (i.e., resids, coal, shale, coke, tar sands, etc.) invariably contains HCN and NH3 which contaminate the reactive slurry and rapidly, but reversibly, deactivate the catalyst. Certain oxygenates and carbonaceous compounds formed in the slurry as by-products of the HCS reaction can also cause rapid deactivation. Deactivation of such catalysts by these species is reversible and catalytic activity is restored (the catalyst rejuvenated) by contacting the deactivated catalyst with hydrogen. The activity of the HCS catalyst in the reactive slurry may be intermittently or continuously rejuvenated by contacting the slurry with hydrogen or a hydrogen containing rejuvenation gas to form a rejuvenated catalyst slurry as is disclosed, for example, in U.S. Pat. Nos. 5,260,239 and 5,268,344. In these patents the slurry, containing gas bubbles, is rejuvenated by circulating it through either a rejuvenation tube immersed in the slurry or an external rejuvenation vessel. The rejuvenation gas also acts as a lift gas to circulate slurry through the tube. It has now been found that the presence of CO hinders catalyst rejuvenation until the CO is consumed. This limits the overall efficiency of the rejuvenation process and wastes CO and H2. It would be an improvement to the art if these gas bubbles could be removed from the slurry before it contacts the rejuvenation gas. It would be a still further improvement if the slurry circulation through the tube could be achieved with less rejuvenation gas.
SUMMARY OF THE INVENTION
The invention relates to a process and means for rejuvenating solid catalyst particles in a three phase hydrocarbon synthesis (HCS) slurry comprising gas bubbles and catalyst particles dispersed in a hydrocarbon slurry liquid, in which gas bubbles are removed from the slurry prior to rejuvena
tion and a downcomer provides a hydrostatic head to the rejuvenation zone to achieve the desired slurry circulation with less lift gas. Briefly, the process comprises passing a portion of slurry, from a slurry body in which at least a
5 portion of the catalyst is reversibly deactivated, through a gas disengaging zone to remove gas bubbles and into a downcomer which feeds the gas reduced slurry into a catalyst rejuvenation zone in which it contacts gas which rejuvenates the catalyst. This may be accomplished by a
10 combination gas disengaging downcomer and rejuvenation tube immersed in the slurry body. The open top of the rejuvenation tube may be outside the slurry body. The catalyst rejuvenation is done either continuously or intermittently, as desired, with the slurry reactor either
15 operating and producing hydrocarbon products, or with it off-line. The gas bubbles comprise valuable unreacted synthesis gas (syngas) which interferes with the catalyst rejuvenation and would otherwise be wasted, along with gas products of the HCS reaction. The slurry liquid comprises
20 hydrocarbon products of the HCS reaction which are liquid at the reaction conditions. The gas injected into the rejuvenation zone rejuvenates the catalyst in the slurry and, at the same time, serves as a lift gas to insure circulation of the slurry through the rejuvenation zone. The gas reduced slurry
25 is denser than that comprising the slurry body and the slurry mixed with rejuvenating lift gas in the rejuvenating zone. This enables the downcomer to apply a hydrostatic pressure to the bottom of the rejuvenation zone which is determined in part by the difference in densities of the gas reduced slurry
30 in the downcomer, the slurry body, the gas filled slurry in the rejuvenation zone, and the vertical distance between the top of the downcomer and the gas injection point in the rejuvenation zone. This means that less lift gas is required to maintain a given slurry circulation rate through the rejuve
35 nation zone. The rejuvenation gas, which also serves as a lift gas, will comprise hydrogen in the case of rejuvenating a slurry HCS catalyst. It may be all hydrogen or hydrogen mixed with one or more other gasses as explained in detail below.
40 The combination gas disengaging downcomer and rejuvenation zone may simply be a generally "J" shaped, elongated hollow conduit open at both ends, with the lower leg forming the downcomer portion which terminates at its upper end in an open gas disengaging cup immersed, and
45 upwardly opening, in the slurry body. The longer leg of the J comprises the rejuvenation zone and has means for injecting gas into its interior near the bottom, with the open end or top opening into the top of the slurry body in which it is immersed, or at a location out of the slurry. Catalyst reju
50 venation is accomplished within the slurry either in the HCS reactor or reaction zone, or in an outboard or separate catalyst rejuvenation zone or reactor, as is disclosed in U.S. Pat. No. 5,260,239. However, in at least one embodiment it is preferred that the catalyst be rejuvenated within the slurry
55 in the reaction zone. When the practice of the invention is performed in the slurry in an HCS reactor or reaction zone, the HCS reaction is not disturbed, as the gas disengaging, downcomer and catalyst rejuvenation zones, while immersed in the slurry, are separate from it. Further, the
60 rejuvenation process produces an offgas which may contain catalyst deactivating species which will recontaminate the slurry body if this gas is passed back into the slurry body. In this case it is preferred to pass the rejuvenated slurry containing the offgas from the rejuvenating zone to a gas
65 disengaging and separating zone to separate and remove the gas from the slurry, before passing the rejuvenated slurry back into the slurry body.