Gas separation: processes – Liquid contacting – And recycle or reuse of contact liquid for further contact
Reexamination Certificate
2000-11-30
2002-01-08
Simmons, David A. (Department: 1724)
Gas separation: processes
Liquid contacting
And recycle or reuse of contact liquid for further contact
C095S205000, C095S214000, C095S230000, C423S210000
Reexamination Certificate
active
06336958
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for purifying gas loaded with dust, a purified gas being obtained.
There are many different ways of processing waste. Processing by combustion is currently widely used. Other possibilities are pyrolysis and gasification, both of which can also be followed by a (s)melting step. In the case of pyrolysis, the waste is heated under (virtually) oxygen-free conditions at temperatures of between 350° C. and 700° C., gases and a coke-like residue being produced. The higher molecular weight constituents can be removed from the resulting gas by cracking at temperatures of about 1100° C. A mixture of CO, CO
2
, H
2
O and H
2
is produced on cracking. In the case of gasification (,) more oxygen is added and usually temperatures of up to 900° C. are used. Predominantly CO is produced and little coke-like residue forms. The said residues also contain the inorganic constituents from the waste. A (s)melting step at temperatures of about 1400° C. can convert these residues into a usable slag and a fly ash which contains the volatile metals/metal compounds. In addition to the said constituents of the gas, the gas will also contain virtually all halides, mainly in their direct acid form. The pyrolysis and gasification processes yield, as product gas, a fuel gas that still has a calorific value. The same applies to the off-gas from the (s)melting step, at least if the latter is operated under reducing conditions.
In both pyrolysis gas and smelter gas the solids are generally present in two different types: the directly entrained original or slightly melted material (the so-called “entrainment”) and the fly ash that is produced from the vaporised metals or metal compounds present in the gas phase. The entrainment contains substances such as lime and silica and is coarser than the fly ash.
In the existing processes for purifying smelter gas and pyrolysis gas the gas is usually fed successively through a gas condenser, a cyclone and/or bag filter to capture solids and through a gas scrubber to remove the acid constituents from the gas. Such processes have the disadvantage that the fly ash obtained frequently still contains acid constituents adsorbed thereon, as a result of which further processing and/or use of the fly ash is rendered more difficult. Moreover, the fly ash is mixed with the entrained material (the entrainment). Furthermore, in the existing processes the fly ash is usually obtained in wet form, as a result of which an additional drying step is necessary.
In connection with the above reference can be made to DE 42 37 228 A1. In this literature reference (,) medical waste is treated together with laundry/dry cleaning waste products in a pyrolysis installation with an after-burner. After first being used to generate steam, the gas obtained from the after-burner is fed into a gas scrubber in which the gas is cooled to below the dew point and freed from HCl constituents. Water loaded with waste substances is thus obtained. After reheating the scrubbed gas to 100-120° C. it is fed to a dust collector such as a bag filter and then to a coke filter. The filter cakes, salts and spent coke materials obtained with this method are finally dumped in a special dump.
A further publication cited is U.S. Pat. No. 4,301,128, in which a somewhat complicated method for the selective purification of gases, in particular exhaust gases containing both solid and gaseous impurities, is described. According to this known method, such a gas is
first passed through a separator for solid impurities, for example an electrostatic precipitator, in which about 90% of the fly ash is collected;
the partially purified gas is then fed to a spray dryer, in which the gas is treated with an absorbent, for example a calcium hydroxide solution in water, to remove the gaseous impurities; and
is finally passed through a third separator, such as a bag filter, by means of which the remaining quantity of solid impurities is removed from the gas stream, so that ultimately a gas free from solid and gaseous impurities is obtained.
The final citation U.S. Pat. No. 3,668,833 relates to a method for purifying off-gases, which method is made up of a large number of steps, specifically steps (e)-(j) in claim
1
:
(e) passing the off-gases through a coarse filter system;
(f) after-burning the particles and gases in the off-gas issuing from (e);
(g) collecting particles from said off-gas with the aid of an electrostatic collector;
(h) scrubbing the off-gas with water of an alkaline pH to remove the acid gases;
(i) subsequently scrubbing the off-gas with neutral water; and
(j) passing the off-gas ultimately obtained through a second filter system.
The aim of the present invention is to provide a method which can be carried out in a simple manner, which overcomes the above disadvantages and which therefore yields a fly ash (i) which contains no or hardly any adsorbed acid constituents, as a result of which it is easier to process or to re-use, (ii) which contains no or hardly any entrainment, as a result of which the concentration of valuable metals in the fly ash is higher, and (iii) which can be obtained dry, so that an additional drying step can be dispensed with and a saving in transport costs for transport to a processing company can be achieved as a consequence of the difference in weight between dry and wet fly ash.
SUMMARY OF THE INVENTION
The above objectives are achieved by first subjecting the gas loaded with dust to wet gas scrubbing and only thereafter to a dust-removing treatment.
The invention therefore relates to a method for purifying gas loaded with dust particles of various sizes, which method comprises the following steps:
(a) the gas loaded with dust particles is subjected to wet gas scrubbing at or above the dew point, during which operation any acid gaseous components present in the gas are absorbed and the major proportion of coarse dust particles having a size of ≧50 &mgr;m is removed by a scrubbing liquid, after which
(b) the remaining dust particles present in the gas are collected at or above the dew point in an installation for removing dust from gas, purified gas being obtained.
The method according to the invention is in particular very suitable for purifying gases which are relatively heavily loaded with dust, the fly ash preferably being recovered in dry form. The gas loaded with dust, which can be purified by the method according to the invention, is therefore also preferably a smelter gas obtained from the pyrometallurgical treatment of metal-containing waste materials, a pyrolysis gas obtained from the pyrolysis of waste or a mixture of the two. However, other gases loaded with dust, such as flue gas and heating gas, can also be purified using the method according to the invention.
In step (a) of the method according to the invention the acid or acid-forming constituents which are present in the gas, such as HF, HBr and HCl, are removed by means of wet (acid) gas scrubbing. A treatment of this type and equipment suitable for this are known per se. Step (a) of the method according to the invention can therefore also be carried out in manners known per se. Preferably, however, wet scrubbing is carried out in a gas scrubber in which a circulating salt solution, which is kept at a pH of between 3 and 11 and preferably of between 5 and 9, is used as scrubbing liquid. The circulating salt solution is circulated by pumping and is kept at the correct pH by adding a base, for example sodium hydroxide solution or potassium hydroxide solution. The acid constituents present in the gas loaded with dust are absorbed by the salt solution and removed from the wet gas scrubber via a salt solution stream in order to be fed—if necessary after adding a little base—back to the top of the gas scrubber, after which spraying is usually carried out. By this means the acid constituents can be absorbed well by the salt solution.
Advantageously 75% or more, preferably 85% or more, of the entrainment (particle size ≧50 &mgr;m, preferably ≧100 &mgr;m) (lime,
Hazewinkel Jacob Hendrik Obbo
Lotens Jan Pieter
Van Der Giesen Abraham Barend
Droan B.V.
Prince Fred
Simmons David A.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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