Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Organic component
Reexamination Certificate
2001-04-27
2002-12-10
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Organic component
C585S809000, C585S810000, C585S843000, C585S844000, C423S345000
Reexamination Certificate
active
06491888
ABSTRACT:
This invention relates to a method of reducing nitrogen oxide emissions in processes involving selective olefin recovery from a mixture of gases generated in petrochemical processing.
In petrochemical processing such as eg an ethylene cracker, olefins are made by steam cracking hydrocarbon feedstocks. Such a process results in the formation of a mixture of olefins and paraffins such as eg ethylene, ethane, propylene and propane and is contaminated further with by-products which include hydrogen, diolefins and acetylenic compounds. Thus, the so-called ‘Selective Olefin Recovery’ (hereafter “SOR”) processes have been devised to selectively recover olefins such as ethylene and propylene from the steam cracked products. The SOR process is based on the formation of a complex between the desired olefins and silver ions at relatively high gas pressures and relatively low temperatures. This complex formation is reversible in that when the pressure is reduced and the temperature is increased, the complex breaks down into the component olefin and silver ions. The SOR process is normally carried out by contacting the mixture of gases from the cracking process containing the olefins with an aqueous solution of silver nitrate (e.g. 2-6 M silver nitrate) in a vertical absorption tower at about 7 to 24 barg at and at 10-40° C. solution inlet temperature. A solution rich in the complex is withdrawn from the bottom of the absorption tower and fed into a lower pressure vessel which is maintained at a relatively higher temperature (e.g 60-110° C.) where the complex is broken down and the complexed olefin released and recovered. The remaining solution lean in olefins is cooled and recycled back to the absorption tower. A known SOR process is described in WO 98/25871.
It is well known, however, that the SOR process involving the use of silver nitrate solution is not straightforward. For instance, it is also known that hydrogen can lead to the decomposition of silver salt solutions to produce elemental silver. The high sensitivity of the silver nitrate solution to low levels of hydrogen has been established experimentally. Silver nitrate solution has been shown to remove free hydrogen down to levels of less than 10 ppm by bubbling gases containing various levels of hydrogen through silver nitrate solutions in glassware. The lowest concentration of hydrogen used corresponded to 10 ppm at 9 barg pressure. The reaction between hydrogen and silver nitrate can be represented by the following equation:
H
2
+2AgNO
3
2Ag+2HNO
3
(I)
Thus, it is clear that this reaction in which nitric acid is generated will lead to an increase in the acidity of the solution. For instance, the pH value of the silver nitrate solution continuously falls from the initial value of 4.5 to about 3.5 in about 2-3 weeks in a pilot reactor. Moreover, the precipitation of silver from the silver nitrate solution means that the silver nitrate solution becomes depleted in the silver ions necessary to complex the desired olefins, reducing the efficiency of the SOR process. Moreover, traces of methyl acetylene and acetylene in the gaseous cracked products fed to the absorber will also react with the silver nitrate solution to increase the acidity.
We have found that the above chemical reaction with hydrogen reaches equilibrium provided it is operated at a sufficiently high concentration of HNO
3
, thereby preventing the deposition of silver. However, in order to achieve the desired concentration of acid, an increased level of nitric acid has to be used which in turn leads to an increase in the evolution of undesirable oxides of nitrogen (hereafter “NO
x
”) in the olefin product gas. For instance, the level of acidity required to prevent silver deposition was determined to be of the order of 20 wt % by determining the level of nitric acid required to redissolve precipitated silver. At this acidity level, the NO
x
impurity level is above the permitted limit of 1 ppm for NO
x
in eg the ethylene product. As a rough guide, the relative concentrations of NO
x
in a given silver nitrate solution can be summarised as follows:
pH of 5 Molar
Silver Nitrate
NO
x
Level in Absorber
NO
x
Level in Product
Solution
Offgas (@ 30° C.)
Offgas (@ 90° C.)
4.0
2 parts per billion
4 parts per billion
1.0
2 parts per billion
1333 parts per billion
It has now been found that the problem of controlling the pH of the silver nitrate solution in order to minimise NO
x
emissions can be reduced or eliminated by contacting the silver nitrate solution with silver oxide. This not only neutralises the acidity of the silver nitrate solution, but also replaces the silver ions lost by silver deposition by producing silver nitrate as a salt of neutralisation.
Accordingly, the present invention is a process for the selective recovery of a olefins from a mixture of gases, said process comprising:
a. bringing a gaseous mixture comprising olefins and hydrogen into contact with silver nitrate solution whereby the olefins are absorbed into the silver nitrate solution as a complex;
b. separating the solution comprising the complexed olefins from the non-absorbed gases;
C. depressurising and heating the olefin complex solution from (b) so as to release the olefins from the complex and regenerate the silver nitrate solution;
d. passing said regenerated silver nitrate solution through a bed comprising silver oxide so as to maintain the pH value of the silver nitrate at between 3 and 6; and
e. recycling the silver nitrate solution regenerated in (d) to step (a).
The reaction of step a) may be carried out in an absorption tower. Under the reaction conditions which are usually of relatively high pressure and relatively low temperature, the olefins are absorbed into the silver nitrate solution as a complex. The hydrogen present in the gaseous mixture also reacts with silver nitrate to produce silver metal and nitric acid, which lowers the pH of the resulting solution.
Step a) may be carried out at pressures of about 5 to 40 barg, preferably, 7 to 24 barg.
Step a) may be carried out at temperatures of about 5 to 50° C., preferably, 15-30° C.
Preferably, the silver nitrate solution employed has a concentration of 1 to 10 M, preferably, 2 to 6 M, for example 5 M.
In step b), the solution comprising the complexed olefins is separated from the non-absorbed gases, and then depressurised and heated in step c). Step c) releases the olefins from the complex, and regenerates the silver nitrate solution. The regenerated silver nitrate solution is of an increased acidity compared with the original silver nitrate solution, and contains particulate silver as a result of the reaction between silver nitrate and hydrogen. The high concentrations of acid may result in high NO
x
values, and furthermore the formation of silver metal depletes the concentration of silver ions in the regenerated solution, thereby reducing its ability to complex olefins. As will be explained below, these difficulties may be alleviated by the use of silver oxide.
Optionally, the particulate silver suspended in the regenerated silver nitrate solution may be recovered by passing the regenerated silver nitrate solution through a filtering aid which is capable of retaining the particulate silver. This step is preferably carried out before passing the regenerated silver nitrate solution through a bed comprising silver oxide (i.e. step d). It is possible, however, for such a filtering step to be carried out at any point during the process of the present invention.
In step d), the regenerated silver nitrate solution is passed through a bed comprising silver oxide. The silver oxide neutralises the acidity of the silver nitrate solution by a reaction represented by the following equations (IIA and IIB):
Ag
2
O+H
2
O→2AgOH
AgOH+HNO
3
→AgNO
3
+H
2
O
The reaction not only neutralises the nitric acid, but also produces silver nitrate. This silver nitrate is a source of silver ions which can be used to replace the silver ions lost, for example, in the reaction between
Bell Peter Simpson
Coker Eric Nicholas
Small Karen
BP Chemicals Limited
Finnegan, Henderson Farabow, Garrett and Dunner L.L.P.
Langel Wayne A.
Medina Maribel
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