EP1343579A2 - Dual section system for the discharge of bi-phase gas-liquid mixtures - Google Patents

Abstract

The invention describes the design of a discharge duct for bi-phase gas-liquid mixtures in electrochemical reactors, which provides high stability of the outflow in a large variety of process parameters. The system is characterised by a vertical duct (1) wherein the gas and liquid are conveyed at the same time downward and the geometry of the inlet area, which foresees the concurrent presence of two inlet flow sections (3, 4) with different orientations and profiles, is suitable for avoiding both the formation of a discharge pulsating regimen, and the consequent fluctuations of the internal pressure of the electrochemical reactor.

Description

DUAL SECTION SYSTEM FOR THE DISCHARGE OF BI-PHASE GAS- LIQUID MIXTURES

DESCRIPTION OF THE INVENTION The present invention relates to a dual section system for the discharge of biphase gas-liquid mixtures.

It is known, in the field of industrial electrochemistry, the use of electrochemical reactors at the inside of which gas is evolved from a liquid electrolyte, at it happens, for example, in the case of the electrolysis of aqueous solutons or in the anode compartment of cells for electrometallurgical applications. Also known are different electrochemical applications which use thin separators, for example ion exchange membranes or semi-permeable diaphragms, generally for dividing the anode compartment from the cathode compartment. An example of such applications is given by membrane or diaphragm electrolysers for chlor-alkali production from alkali chloride solutions. The following description will make reference, for simplicity sake, to membrane sodium chloride electrolyzers for the production of chlorine and caustic soda, as typical examples of electrochemical reactors wherein at least a gaseous product is evolved, in this case chlorine, from an aqueous solution, in this case from a sodium chloride brine, and wherein an element, in this case a cation exchange membrane, is present which is affected in a particularly disadvantageous way by the pressure fluctuations inside the reactor. However, it will be evident to the expert of the art that the invention disclosed in the present description may be applied also to decrease the pressure fluctuations inside any reactor where a gaseous product is evolved inside a liquid phase. Different solutions have been adopted in the prior art to face the problem of the discharge of produced gas-electrolyte mixture from the elements of a membrane electrolyser, with solutions of increasing sophistication as the improvement of the production technologies made thinner separator available, capable of granting a remarkable decrease of the resistive penalties connected to the electrolysis process, but also with the drawback of an ever lower mechanical resistance. It must be taken into account that the pursue of decreasing resistive penalties had the aim of operating electrolysers at always higher current density, with the consequent increase of the evolved gas volume and of turbulence and pressure fluctuation phenomena. At the same time, the problem of the resistive penalties was faced also by decreasing the distance between the cation-exchange membrane and the electrodes whereon the gas evolution reactions takes place, up to providing for constructive solutions which foresee a direct contact between the electrode and the membrane, as described in US patent 4,340,452. A first solution to improve the discharge of the biphase gas-electrolyte mixture, capable of decreasing the pressure pulsations which cause vibrations and thus abrasion of the membranes, is disclosed in US patent 5,242,564 which foresees a double duct for the upward flow which permits to release the gases and electrolytes from the electrolyser as separate phases. This solution, however, involves high production costs and risks as concerns the reactor reliability, due to multiplying of delicate parts such as weldings between cell elements and discharge ducts, which involve long manual operations and are sites of possible defects. Further, the efficiency of this solution is anyway limited to relatively high current densities, beyond which the speed of the upward flow and its gaseous content do not permit a sufficient dampening of the fluctuations.

An interesting alternative to face the problem is given by the discharge of biphase mixtures through an outlet duct as described for example in "Modern Chlor-alkali Technology", Vol. 4, Society of Chemical Industry, Elsevier 1990, page 171. In this case, a single downcoming duct collects at the same time gas and electrolytes generating pressure fluctuations of small entity. In the absence of an uprising flow, in fact, no separate gas bubbles are formed in the electrolyte, which may vary as to the dimension and number with time, a phenomenon which is, generally, the main cause of the problem, but rather a downcoming motion of the liquid takes place as a falling film along the walls and an undisturbed gas flow in the empty central section of the duct. These devices, however, have a good performance only when the upper part of the downcoming duct is fed uniformly with time by a gas-phase entrapping only a few drops of liquid, and by a liquid electrolyte phase substantially free from gas bubbles. It is therefore necessary to have a very efficient separation of the gas-electrolyte mixture produced on the electrode in the upper part of the cell elements, before gas and electrolyte are fed to the downcoming ducts, a task which is rather difficult when operating at high current density. An improved design directed to facilitate said separation is described in International patent application WO 98/55670. According to this design, the separation of the electrolyte from the gas bubbles is achieved by collecting the latter in a channel on the upper part of the cell delimited by the cell wall and by a suitably dimensioned baffle, perforated or anyway provided with suitable perforations or holes. The produced gas and the gas-disengaged liquid are then fed, through the holes of the baffle, into the discharge downcoming pipe. The downcoming pipe illustrated in WO 98/55670 is a duct with a slanted inlet section. It is specified that a suitable tailoring of the pipe, in terms of diameter and shape of the inlet section is decisive to maintain a stable separate flow of gas and liquid. The condition for achieving discharge stability is the one that allows the liquid to flow without occluding the internal section of the pipe, so that a certain internal portion of the same be continuously available for the gas, while the liquid, due to this surface tension, flows along the internal walls. However, also this solution, although valid, grants per se only a partial dampening of the pressure fluctuations. Operating the cells with thin membranes at high current densities, for example above 4 kA/m², some reliability problems caused by the rubbing of the membranes due to residual pressure fluctuations are still detectable. Further, the dimensioning of the discharge pipes and of the other cell elements according to the described ducts results rather rigid or penalising towards possible variations of the operating conditions. For example, in situations where the electric energy cost is remarkably different between night and day, or among different time bands, it may be convenient to operate the electrolysers at different conditions in order to meet the need for products minimising the energy costs; a typical situation, for example, is the one foreseeing operation at high current density during the night and at a reduced current density during the day. It is however difficult to dimension the cell according to the teachings of the prior art in order to obtain a sufficient disengagement of the bi-phase gas-electrolyte mixture at remarkably different current densities. Pressure instability during operation is generally negative for the membrane which separates the two electrode compartments: it is known that the membrane chlor-alkali electrolysers for the production of chlorine and caustic soda typically operate with a process pressure differential between the two compartments of 30-35 mbar in order to permit the complete and rigid abutting of the membrane onto one of the two electrodes, typically the anode. The presence of pressure fluctuations of an amplitude comparable to that of the process pressure differential could thus lead, at least instantaneously, to a annulment or even reversal of the differential with the consequent displacement of the membrane during operation. The membrane, subject to stress cycles, undergoes in this case the quickest erosion action the amplest pressure fluctuations are and the worst the surface conditions of the electrodes (erosion, welding defects, etc.) and of the membranes themselves become, for example due to the presence of plies generated by an imperfect assembling or anomalies of operation. It is an object of the present invention to provide a discharge system for bi- phase gas-liquid mixtures from electrochemical reactors capable of overcoming the prior art inconveniences.

In particular, it is an object of the present invention to provide a discharge system for bi-phase mixtures from electrochemical reactors provided with a downcoming duct or pipe having an inlet capable to carry out or complete the disengagement of the mixtures in order to abate in a remarkable way the pressure fluctuations caused by the discharge.

Under another aspect, it is an object of the present invention to provide a design for a discharge pipe for gas-liquid mixtures characterised by high stability of the outflow in a large variety of operating conditions. The invention concerns a vertical pipe wherein gas and liquid are conveyed at the same time downwards, comprising an inlet area characterised by the concurrent presence of a dual inlet section, with different orientations and profiles. In particular, the upper inlet section is positioned on top of the pipe and has a sub-horizontal orientation, with a suitable inclination in order to deflect the gaseous part of the mixture. The lower section, has the function of conveying the liquid portion of the mixture in order not to flood the discharge pipe but rather to generate a falling film along the walls of the dicharge pipe. The lower section has a substantially vertical orientation, for example with a semi- elliptic, parabolic or hyperbolic, or a polygonal profile consisting of a suitable number of planar sections, for example rectangular, close to each other: generally this profile may be visualised as the ideal penetration of a prism or cylindrical body or sphere into the vertical pipe. The two inlet sections are preferably separated by a portion of the pipe wall capable of preventing the liquid from overflowing from the level of the lower flow section and invading the upper section. This efficiently avoids that the liquid occupies entirely, even temporarily and for small tracts, the pipe section, ensuring the separate flow of the two phases. The pipe is made of any type of duct or cylindrical body or with different geometries, for example the pipe of WO 98/55670. The discharge system of the invention may be applied to any kind of reactor, for example an electrochemical reactor, wherein a gaseous product is evolved within a liquid phase, and it is particularly advantageous in those cases where the pressure fluctuations inside the reactor must be minimized. The discharge system of the invention is particularly advantageous for the discharge of the reaction products and the electrolyte from diaphragm or membrane electrolysers. In the case it is used in membrane electrolytic cells, the discharge system of the invention is particularly advantageous in combination with the cell design described in WO 98 / 55670, wherein the bi-phase mixture is previously gas-disengaged in the upper portion of the cell in a system provided with a baffle with suitable apertures and is fed in the discharge pipe provided with the inlet according to the present invention, where the disengagement of the two phases is completed.

The present invention is schematised in figures 1 and 2, wherein : fig. 1 shows a possible embodiment and fig. 2 shows the embodiment of fig. 1 according to a point of view rotated by 90°.

Figure 1 shows a side view of a possible embodiment of the discharge system of the invention, which comprises a discharge pipe (1) provided with an inlet (2) having a generic form of a whistle, characterised by the concurrent presence of at least two inlet sections, in particular at least an upper flow section (3) having a sub-horizontal orientation, with a suitable inclination to deflect the gaseous portion of the mixture, and at least a lower flow section (4), preferably obtained on the wall of the pipe (1), having substantially a vertical orientation. In the preferred embodiment of fig. 1 , the two inlet flow sections (3) and (4) are separated by a portion of the wall (5) of the pipe (1), to prevent the liquid from overflowing from the lower flow section (4) into the upper flow section (3). Figure 2 shows a front-view of the same apparatus, rotated by 90° with respect to figure 1. In this case a lower flow section (4) having a semi-elliptic profile (6) is illustrated, however it is evident that other types of profiles may be adopted as well, such as for example a profile with planar sections close to each other, preferably rectangular, without departing from the scope of the invention. The fundamental principle on which the invention is based is the formation of separate flow sections for the discharge of the two phases by exploiting their different densities: a lower flow section with a mainly vertical orientation for the heavier fluid (liquid) and an upper flow section having a sub-horizontal orientation for the lighter fluid (gas).

The liquid naturally tends by gravity to overflow from the lower flow section and its higher density hinders the rising to the upper flow section directed to the discharge of gas, conversely the low density of the gas creates a floating force over the heavier phase which prevents its sinking in the lower section. The continuous area of wall (5) ha a two-fold function: it prevents that periodical level variations (waves), generated on the liquid phase by the high turbulence of the system, overflow by inertia up into the upper flow section and it further physically separates the points of flow direction changes of the two phases, permitting to collect the gaseous phase in parallel to the pipe axis ahead of the inlet of the liquid phase and thus favouring the flow of the latter as a falling film along the internal wall of the pipe. The preferred elliptic form of the lower flow section (4) has the propriety of increasing the discharge section occupied by the liquid phase with a quadratic progression with respect to the level of the liquid itself. This permits to maintain a constant inlet speed in the liquid phase in the pipe in a wide range of operating conditions and in the presence of continuous level variations caused by the waves.

The preferred inclined plane of the upper flow section (3) acts as a baffle for the gaseous phase which must turn at 90° its flow lines to enter the discharge section, decreasing or eliminating the vortexes connected to said direction change. It is an object of the present invention to limit the flow instability due to the inlet of the two separate phases through the same discharge pipe. The generation of

the pressure fluctuation is in fact strictly connected to the formation of a liquid slag which periodically invades the section available for the gas flow inside the pipe close to the inlet, narrowing or even completely filling the same. In such conditions the liquid slag proceeds in the pipe with a speed comparable to that of the gaseous phase. The higher viscosity and density (typically three magnitude orders) of the liquid involves a high energy expenditure which is supplied by the pressurisation of gas inside the electrolysis compartment, therefore a rapid pressure transient takes place, defined as "fluctuation". The high speed due to the transient prevents separation of the phases and the slag survives all the length of the discharge pipe.

A typical situation for the formation of liquid slags is the concurrence of two phases conveyed towards a common discharge section, as it may happen in the presence in particular the presence of waves in the liquid phase leading to a periodical submerging of the outlet level, thus inhibiting the gas outflow.

The pressure fluctuation causes in turn the variation of the level of the liquid (waves) and therefore the phenomenon is self-sustaining. The presence of a preferential inlet for each phase prevents or remarkably limits the generation of instability. EXAMPLE

Before applying the invention on site, inside an industrial electrolyzer, a series of tests were carried out in laboratory using a model with a 1 :1 scale, produced in a transparent material in order to completely visualise the phenomenon. During the tests, liquid flow (water) and a gas flow (air) were suitably adjusted in order to perfectly simulate the real conditions of an electrolyser operating at various current densities.

Some tests were first carried out at atmospheric pressure by using the device of WO 98/55670, consisting of a vertical pipe with a single section inlet with a diameter of 32 millimeters, and an orientation inclined by 45 degrees with respect to the horizontal plane. The amplitude of the pressure fluctuation in the compartment was then measured simulating the gas flow and liquid flow at different current densities. In particular the pressure fluctuation amplitude resulted 8 mbar at 3 kA/m² and 18 mbar at 6 kA/m². Said tests were subsequently repeated under identical conditions but modifying the discharge pipe in the inlet area according to the invention. In particular an inlet in the form of a whistle was adopted, as illustrated in figs. 1 and 2, provided with an upper flow section inclined by 45° with respect to the horizontal plane, and a lower flow section with a rhomboidal profile, 28 millimeters high, separated from the upper flow section by a vertical wall 30 millimeters high. The device was oriented in order to feed the bi-phase mixture in an orthogonal direction with respect to the flow direction entering into lower vertical section of the pipe. The pressure fluctuation amplitude resulted 3 mbar at 3 kA/m² and 11 mbar at 6 kA/m². By rotating the discharge pipe with respect to its axis, in order to feed the bi- phase mixture according to different incidence angles with respect to the lower flow section of the pipe, that is parallel to the direction of the flow entering into said section, slightly higher pressure fluctuations were detected with respect to the above mentioned values. In a membrane electrolyzer produced according to the description of WO 98/55670 sodium chloride electrolysis was carried out at 210 g/l, by regulating the produced caustic soda concentration around an average value of 32% by weight. The membrane was a N2010WX type, produced by DuPont de Nemours (U.S.A.), having a thickness of 170 micrometers. A pressure differential of 30 mbar was maintained between the two compartments, operating at a relative pressure of 216 mbar in the cathode compartment and 186 mbar in the anode compartment respectively. The discharge from both the anodic and the cathodic compartments was effected through the device of WO 98/55670, using a vertical pipe having a single inlet, with a diameter of 32 millimeters, and inclined by 30 degrees with respect to the horizontal plane. The amplitude of the pressure fluctuation in the two compartments was then measured at different current densities.

In particular, the pressure fluctuation amplitude on the cathode side, where hydrogen and caustic soda are discharged to be sent downstream to the production facilities, resulted 31 mbar at 3 kA/m² and 32 mbar at 5 kA/m².

These characterisations were repeated in quite analogous conditions, but modifying the discharge pipe in the inlet area according to the invention. In particular an inlet in the form of a whistle was adopted, as illustrated in figs. 1 and 2, provided with an upper flow section inclined by 30° with respect to the horizontal plane, and a lower flow section with a semi-elliptic profile, 40 millimeters high, separated from the upper flow section by a vertical wall 5 millimeters high.

The device was oriented in order to feed the bi-phase mixture in an orthogonal direction with respect to the direction of the flow entering into the lower vertical flow section of the pipe. By maintaining a pressure differential of 30 mbar in both compartments, the pressure fluctuation amplitude measured on the cathode side was 12 and 24 mbar, at 3 and 5 kA/m² respectively.

Claims

1. A dual section system for the discharge of a bi-phase gas-liquid mixture from an electrochemical reactor comprising at least one vertical pipe for the downward flow of the bi-phase mixture, said at least one vertical pipe provided with an inlet area comprising at least one upper flow section and at least one lower flow section, said at least one upper flow section oriented in a sub- horizontal direction and said at least one lower flow section oriented in a substantially vertical section.

2. The system of claim 1 , characterised in that said at least one upper flow section and said at least one lower flow section are separated by a least a wall of said at least one vertical pipe.

3. The system of claims 1 or 2, characterised in that said at least one upper flow section is inclined with a degree comprised between 0° and 60° with respect to the horizontal plane in order to deflect the gaseous portion of the bi- phase mixture.

4. The system of the preceding claims, characterised in that lower cross- section is orthogonal with respect to the direction of the flow of the bi-phase mixture.

5. The system of claims 1 or 2, characterised in that said at least one lower flow section has a polygonal, semi-elliptic, parabolic or hyperbolic profile, generated by the ideal penetration of a prism or cylindrical body or sphere into the surface of at least one vertical pipe.

6. The system of claims 1 or 2, characterised in that said at least one lower flow section has a profile made of a series of planar sections close to each

other.

7. The system of claim 6, characterised in that said sections are rectangular.

8. An electrolytic cell comprising an anode compartment, a cathode compartment, and at least one dual section system of the preceding claims in at least one of said anode and cathode compartments.

9. The cell of claim 8, further comprising at least one ion exchange membrane.

10. The cell of claim 9, further comprising an apparatus for the separation of at least one gas from at least one liquid electrolyte comprising at least one channel in the upper part of the cell delimited by at least one baffle provided with apertures for feeding said at least one vertical pipe.

11. A method for discharging a bi-phase gas-liquid mixture from an electrochemical reactor comprising outflowing said bi-phase mixture in said at least one vertical pipe of said dual section system of claims 1 to 7.

12. The method of claim 11 characterised in that the gas phase of said biphase mixture is mainly fed through said upper flow section of the inlet area of said vertical pipe, and the liquid phase of the bi-phase mixture is mainly fed through the lower flow section of the inlet area of said vertical pipe.

13. The method of claims 11 or 12 characterised in that said reactor is an electrolysis cell.

14. The method of claim 13 characterised in that said electrolysis cell comprises at least one ion exchange membrane.

15. A discharge system for a bi-phase mixture from an electrochemical reactor which comprises the characteristic elements illustrated in the description and in the figures.