Process for regenerating an impure catalyst comprising sulphuric acid deposited on silica

The invention concerns a process for regenerating an impure catalyst comprising sulphuric acid deposited on silica. It is characterized in that the catalyst is calcined at least once to eliminate organic hydrocarbon substances contained in the catalyst, in particular as a result of its use in aliphatic alkylation reactions.

The present invention concerns a novel process for eliminating organic 
hydrocarbon substances contained in an impure catalyst comprising 
sulphuric acid and silica which has been contaminated during chemical 
reactions in which the catalyst has participated. It also concerns the 
treatment of the gaseous effluent resulting from elimination of the 
organic hydrocarbon substances. 
The present invention particularly concerns a process for the treatment of 
a used catalyst comprising silica and sulphuric acid-resulting from the 
catalytic alkylation of isobutane and/or isopentane by means of an olefin 
to produce at least one product from the group constituted by 
dimethylbutanes, trimethylpentanes, trimethylhexanes and 
trimethylheptanes, for example. 
A variety of chemical and petrochemical processes use sulphuric acid as a 
catalyst. This acid is generally recycled for as long as the concentration 
of impurities, in particular organic impurities, allows this. The 
sulphuric acid which is purged is thus relatively concentrated in organic 
material which and is sent to a reprocessing plant. The only industrial 
reprocessing process for this discharged sulphuric acid is a process 
during which the acid is transformed into sulphur dioxide SO.sub.2 by 
combustion, followed by transformation into sulphur trioxide (SO.sub.3) 
which is then transformed back into sulphuric acid by absorption in water. 
Examples of processes which produce impure sulphuric acid, often called 
sulphuric sludge, are the synthesis of alcohols from ethylene hydrocarbons 
(in particular the synthesis of ethanol from ethylene, isopropanol from 
propylene and 2-butanol from a mixture of 1-butene and 2-butene), the 
alkylation of isobutane by olefins such as propylene or butenes and 
purification of hydrocarbons in refining operations. 
These processes, in particular alkylation, produce relatively small amounts 
of used acid which are currently considered to be too small to be treated 
on site. The product is thus sent to sulphuric acid production plants 
where it is reprocessed into pure sulphuric acid and returned to the 
alkylation unit. Frequently, the treatment units for sulphuric sludge are 
a long way from the site where the sludge is produced, entraining numerous 
risks as regards transport, generally by road, of a product which is as 
dangerous and polluting as sulphuric sludge or the pure acid. This 
treatment comprises a first combustion step which transforms the acid into 
SO.sub.2. In addition, the pure acid obtained is frequently more expensive 
than the fresh acid and the reprocessing plant will only reprocess if the 
supplier of the sludge takes back the pure acid, implying extra, non 
negligible costs for the alkylation plant. 
A number of purification processes for used acid have been described. 
United States patent U.S. Pat. No. 3 652 708 describes a method for 
reducing the concentration of hydrocarbons in a residual acid by treatment 
with an excess of olefin before sending it to a combustion plant. This 
does not completely eliminate the problems mentioned above. The process 
described in European patent EP-B-0 052 548 uses nitric acid as an 
oxidizing agent for the organic hydrocarbon substances. This process 
involves treating the gases formed in a unit for eliminating nitrogen 
oxides, which represents an important drawback. The article by D E 
Shenfel'd et. al., in Zhurnal Prikaladnoi Khimii, Vol 61, No 7, pp 
1550-1553, Jul. 1988, describes a treatment process for a used acid by 
decomposing the acid in two steps. During the first step, carried out at a 
temperature of between 50.degree. C. and 270.degree. C., a solid black 
residue is formed which resembles coke. This residue, with a weight which 
substantially corresponds to the carbon content of the initial used acid, 
is then oxidized in the presence of a stream of air at a temperature of 
more than 400.degree. C. During the first step, conversion of the organic 
hydrocarbon raw materials present in the initial used acid, measured from 
the quantity of oxides of carbon formed by reaction with the sulphuric 
acid, is about 12%. The process described in this document has the major 
drawback of producing a solid carbon-containing residue which requires 
oxidation in air at a very high temperature. French patent application 
registration number 92/02072 describes agents with an oxidizing power 
which is greater than that of the sulphuric acid, which are introduced 
into the used acid to be treated. Examples of such oxidizing agents are 
H.sub.2 O.sub.2, H.sub.2 SO.sub.5 and H.sub.2 S.sub.2 O.sub.8. The use of 
these agents makes the process complex and expensive. 
The process of the present invention eliminates the problems associated 
with the techniques used in the processes of the prior art and provides a 
solution which can be readily installed at the location where the 
sulphuric sludge is produced, in particular an alkylation unit. This novel 
process is of particular interest for an isobutane alkylation process 
using olefins and a catalyst constituted by silica impregnated with 
sulphuric acid such as that described in French patent application 
registration number 91/13303. 
The process of the present invention concerns a process for the elimination 
of organic hydrocarbon substances contained in this type of catalyst, 
i.e., comprising sulphuric acid on silica, said impure sulphuric acid at 
this stage generally containing about 50% to about 99.5% by weight of 
sulphuric acid and at least 0.1% by weight, expressed as the number of 
carbon atoms, of organic substances in their free or combined forms. 
As indicated above, the silica impregnated with impure sulphuric acid may 
be an alkylation catalyst as described in French patent application 
registration number 91/13303 which has been purged from the alkylation 
unit for reprocessing. 
By way of information, the average diameter of the catalyst particles is 
generally between 0.1 and 200 microns (1 micron=10.sup.-6 m). 
The treatment process for the impure acid contained in the pores of the 
silica comprises one or two steps. Depending on the chemical composition 
of the impure sulphuric acid to be treated, it can be effected in a single 
step. 
In the first step, the catalyst, constituted by the silica impregnated with 
the impure acid, is calcined in a gas stream, for example a gas containing 
molecular oxygen, for example air or pure oxygen, at a flow rate of 
between 0.05 and 10 l/h/g of material to be treated, preferably between 
0.1 and 5 l/h/g, at a temperature of between 100.degree. C. and 
400.degree. C., preferably between 100.degree. C. and 350.degree. C., and 
more preferably between 170.degree. C. and 330.degree. C. The duration of 
the treatment is advantageously between a few minutes (for example, 3 
minutes) and 8 hours. 
In a second, optional, step, the solid obtained at the end of the first 
step is calcined in a gas stream, for example a gas containing molecular 
oxygen, for example air or pure oxygen, at a flow rate of between 0.05 and 
10 l/h/g of material to be treated, at a temperature of between 
400.degree. C. and 600.degree. C., preferably between 450.degree. C. and 
550.degree. C., to eliminate the hydrocarbon deposits still present in the 
silica at the end of the first step. 
The first and second calcining steps produce a gaseous phase containing the 
products formed by oxidation of the hydrocarbon compounds initially 
present in the impure sulphuric acid and by oxidation of sulphur dioxide 
SO.sub.2, and a condensable liquid phase constituted by purified sulphuric 
acid. 
We have discovered that, surprisingly, a large portion of the sulphuric 
acid contained in the silica is recovered in the first step: the sulphuric 
acid is then condensed from the vapor leaving the tube reactor in which 
the first calcining step of the invention is carried out. The condensed 
sulphuric acid is sufficiently pure for it to be used for the manufacture 
of an aliphatic alkylation catalyst, after mixing with an oleum such that 
the water content of the sulphuric acid solution thus prepared is less 
than about 2% by weight and after reimpregnation into the silica. In 
addition, the calcined silica leaving the second step of the process of 
the invention can be impregnated again with sulphuric acid to produce a 
new alkylation catalyst. 
In the process of the invention, the gases formed during the second 
calcining step, in particular during the first part of the second step, 
are frequently not discharged directly into the atmosphere both in order 
to make use of the products they contain and because of legislation 
concerning environmental protection. These gases are most often reduced to 
transform the major portion of the oxides of sulphur therein into sulphur. 
When the process of the present invention is used in a refinery, it 
integrates easily into the refinery and the sulphur dioxide formed can be 
sent to a CLAUS unit (fume treatment) which is nearly always present in 
this environment, in which it is reduced to sulphur and may then be 
transformed back into sulphur trioxide by oxidation. 
The following example illustrates the invention without limiting its scope.

EXAMPLE 
Treatment of a used catalyst constituted by silica impregnated with 
sulphuric acid which has been tested in a pilot unit for alkylation of 
isobutane with olefins. 
Composition of Fresh Alkylation Catalyst 
The fresh alkylation catalyst was constituted by 41.7% by weight of a 
silica with an average particle diameter of 115 .mu.m, and 58.3% by weight 
of anhydrous sulphuric acid. 
Use of Catalyst for Isobutane Alkylation 
A tube reactor with a diameter of 7 cm and a height of 15 cm was used to 
alkylate isobutane with 1-butene. The reactor, which contained 200 g of 
catalyst with the composition given above, was supplied at the bottom with 
a liquid phase with a linear velocity in the reactor of 0.36 cm/sec to 
ensure fluidization of the catalyst. 
In order to simulate recycling of the isobutane from the isobutane/alkylate 
separation zone to the reactor inlet, a liquid mixture of isobutane and 
1-butene containing 8.3% by weight of 1-butene was used as a feed. 
The volume flow rate of the feed was 102 ml/h. 
The major portion of the liquid effluent leaving the reactor was recycled 
to the reactor inlet after mixing with the feed. The recycle volume flow 
rate was 50 l/h. A liquid phase containing isobutane, unconverted 1-butene 
and the alkylate product was continuously extracted from the unit at a 
rate of close to 100 ml/h. 
The temperature in the reactor was maintained at -3.degree. C., and the 
pressure was 5 bar. 
After 15 days of testing, the catalyst was discharged for regeneration 
using the process described in the present invention. 
Regeneration of Used Catalyst 
200 g of used catalyst in the form of a dry powder was calcined in a stream 
of 1.9 l/h/g of dry air, at a temperature of 290.degree. C. for 5 hours. 
The gases were cooled and the condensed liquid was collected in a flask. 
After calcining, a liquid fraction was recovered which had a weight of 
87.54 g, along with a dry powder with a weight of 84.7 g. The weight of 
the non condensed gases was 27.76 g. 
The recovered liquid contained about 93% by weight sulphuric acid, and the 
carbon-containing residue on the silica (about 1.1% by weight of carbon) 
could be completely eliminated by supplemental calcining in air at 
550.degree. C. for 4 hours. 
The process of the invention recovered about 75% of the sulphuric acid 
initially present in the catalyst.