Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-12-22
2001-04-10
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S252000, C204S254000, C204S255000, C204S257000, C204S258000, C204S263000, C204S266000, C204S279000
Reexamination Certificate
active
06214181
ABSTRACT:
DESCRIPTION OF THE PRIOR ART
Today chlorine and caustic soda are industrially produced in plants based on the mercury cathode, diaphragm or ion-exchange membrane technologies. While the first two technologies are considered fully developed and only marginal improvements may be foreseen, the third one, much more recent, is the only one used in, grass-roots plants and is under continuous evolution. The modifications made in the last times are substantially directed to obtain lower energy consumption, reduced investment costs, and to solve typical problems affecting this technology, such as:
U.S. Pat. No. 4,340,452 describes an internal structure of the electrolyzer, the so-called “zero gap” configuration, wherein the anodes and the cathode, separated by an ion exchange membrane, are pressed to each other. In this way the anode-cathode gap, which directly influences the energy consumption, is represented by the membrane only. This results is obtained by resorting to an expensive electrode structure (flexible mesh and resilient metal mattress).
U.S. Pat. No. 4,655,886 discloses a membrane having microporous hydrophilic films applied to both surfaces thereof, which prevent the gas bubbles (hydrogen on the cathode side and chlorine on the anode side) from sticking to the membrane. In this way all the membrane surface is kept in contact with the electrolytes, thus avoiding dangerous current concentrations which would increase the energy consumption.
U.S. Pat. No. 4,488,946 discloses a structure of the elements provided with projections obtained by hot or cold forming. The electrodes are connected to said projections without any spacer interposed inbetween. The use of spacers, described for example in U.S. Pat. No. 4,111,779 involves an additional complex production step which makes the structure more expensive. The concept disclosed in U.S. Pat. No. 4,488,946 of eliminating the spacers is found also in U.S. Pat. No. 5,314,591.
The structure described in U.S. Pat. Nos. 4,488,946 and 5,314,591 involves however the possibility that the electrodes connected to the projections may cause the formation of occluded areas wherein gas pockets could accumulate and hinder current flow and damage the membranes. Further, the elements provided with projects as described in U.S. Pat. Nos. 4,488,946 and 5,314,591 hinder the electrolyte circulation and in particular the internal mixing.
Various solutions have been suggested:
U.S. Pat. No. 4,294,671 describes an electrode made of a thick mesh having large openings, cold pressed to form dimples. The dimples are the points where the screen is fixed to the projections of the elements. Subsequently on said screen an additional fine screen provided with an electrocatalytic coating is applied to form the electrode. The production, i.e. pressing and connection, is automated and therefore the cost increase is given only by the fine screen.
U.S. Pat. No. 5,372,692 teaches the introduction of spacers to be applied on the upper part of the projections of the element wall. This procedure may be automated and is less expensive than the one disclosed in U.S. Pat. No. 4,111,779 but still remains very complicated and delicate due to the need of a correct positioning of a high number of spacers whereon the electrode is subsequently fixed.
The second problem, that is insufficient internal mixing of the electrolytes is solved in U.S. Pat. No. 5,314,591 by the introduction of a lower distributor, an upper collector and an offset positioning of the projections. This solution is certainly very delicate as the occlusion of even a few holes in the distributors and collectors could lead to important variations of the electrolyte concentration which, even if localized, certainly would damage the ion exchange membranes. Further the solution described in U.S. Pat. No. 5,314,591 can ensure homogeneity of the electrolyte concentration in a horizontal plane (that is along a plane perpendicular to the upward motion), but certainly is totally ineffective as to the concentration in the vertical direction. To keep said concentrations within acceptable limits for the membranes, large electrolyte flows are necessary, which means external pumps of large dimensions with the consequent increased energy consumption. It must be considered that the same applies to temperature. Today these considerations regarding concentration and temperature gradients are more important than in the past with the modern commercial membranes which are characterized by low ohmic drops and are thus capable of decreasing the operating voltage of the electrolyzers and therefore the specific energy consumption. These membranes are particularly sensitive to impurities in the electrolytes, as well as to concentration and temperature gradients. Under this point of view, in conclusion, the devices described in U.S. Pat. No. 5,314,591 cannot be considered as particularly efficient.
An alternative solution consists in ensuring a very high flow rate by means of gas disengagers positioned above the electrolyzer and connected to the electrolyte inlet by means of downcomers (“Modern Chlor-Alkali Technology”, Vol. 5, Society of Chemical Industry, Elsevier 1992, page 93).
This system is very efficient but involves additional costs and in particular large dimensions of the electrolyzer-gas disengager-downcomers assembly, which are often incompatible with the available room in the plants.
An alternative system is illustrated in U.S. Pat. No. 4,557,816 wherein the elements are provided with an internal downcomer connected to a lower distributor. This device represents a partial solution of the problem of homogenizing the electrolytes as the limited cross section of the gas-free liquid flow does not permit a high recirculation speed.
A further delicate problem to be faced is the discharge of the gas-electrolyte mixture from the electrolyzer elements. An improper geometry causes pressure pulsations and consequently vibrations and abrasion of the delicate membrane. U.S. Pat. No. 5,242,564 solves this problems by means of a double discharge duct which, if suitably designed, discharges the electrolytes and the gases as separate phases. This solutions obviously involves higher production costs and a higher number of delicate items which could be the source of defects, such as the elements/discharge ducts welding area.
U.S. Pat. No. 4,839,012 is not directed to solving the problem of pressure pulsations caused by a single outlet duct positioned in the upper side of the elements but rather dampening their transmission inside the elements, to the membranes. This result is obtained by the positioning inside the elements of a perforated tube. The holes, having a suitable diameter, dampen the pressure pulsations generated in the areas close to the outlet ducts.
A further solution is represented by the downcoming discharge duct described in “Modern Chlor-Alkali Technology”, Vol. 4, Society of Chemical Industry, Elsevier 1990, page 171. In this case a single downcoming duct, either external or inside the elements, collects at the same time gas and electrolytes without causing internal pressure pulsations. In fact, in the absence of an uprising vertical path, in the electrolyte there is no separation of gas bubbles, varying as to the dimensions and number with time (first cause of the problem) but rather a downcoming motion of the liquid along the walls of the downcoming duct and an undisturbed gas flow in the central section of the duct not occupied by the liquid. These devices, however, perform satisfactorily only when the upper side of the downcoming duct is continuously and uniformly fed by a gas-free electrolyte and a gas phase entrapping only small drops of liquid. Therefore the gas-electrolyte mixture produced by the electrodes must be efficiently separated in the upper portion of the elements before being fed to the downcoming ducts.
DESCRIPTION OF THE INVENTION
The present invention discloses a new design of elements for ion exchange membrane electrolyzers for the electrolysis of brine to produce chlorine, hydrogen and caustic soda. Thi
Iacopetti Luciano
Marzupio Maurizio
Bell Bruce F.
Bierman, Muserlian and Lucas
De Nora S.p.A.
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