Method and device for the continuous neutralization of hydrochloric acid

Abstract

Disclosed is a method and a device for the continuous neutralization of hydrochloric acid at an industrial scale.

Claims

1. A process for the multistage continuous neutralization of hydrochloric acid having an HCl concentration of at least 10% by weight and a volume flow of at least 1 m.sup.3/h, to a target pH in the range from 3 to 9, which comprises the following steps: A) introducing the hydrochloric acid to be neutralized and a proportion of 95%, of a stoichiometrically required amount of alkali metal hydroxide in a first stage into a volume flow of neutralized hydrochloric acid, wherein the volume flow of neutralized hydrochloric acid is recirculated and cooled from a second stage, subsequent mixing of the neutralized hydrochloric acid, the alkali metal hydroxide and the volume flow to form a primary reaction mixture and reaction of the primary reaction mixture in a neutralization and residence zone, where the pH of a stream flowing from the first stage has a pH of greater than 1 and the volume flow of the hydrochloric acid recirculated and cooled from the second stage corresponds to at least three times the hydrochloric acid introduced to be neutralized in the first stage, B) transferring the stream flowing from the first stage into a neutralization zone of the second stage, further setting of the pH of a secondary reaction mixture formed in the second stage to a value of greater than pH 3, cooling a second volume flow exiting the second stage (7) forming a cooled second volume flow and recirculating the cooled second volume flow by a secondary substream (7′) of a secondary circuit to the neutralization zone of the second stage and by a primary substream to the first stage and, wherein said further setting of the pH is set by addition of alkali metal hydroxide or hydrochloric acid to the recirculated cooled volume flow of the secondary substream, where the ratio of the second volume flow exiting the second stage (7) to the volume flow of the secondary substream is at least 10:1, and C) introducing a further substream of the secondary reaction mixture of the second stage (10) into a neutralization zone of a third stage forming a tertiary reaction mixture, further setting of the pH value of the tertiary reaction mixture in the third stage to a pH in the range from pH 3 to pH 9 by means of addition of alkali metal hydroxide or hydrochloric acid to join a recirculated cooled volume flow of the third substream, cooling a third volume flow exiting the third stage (11) forming a cooled third volume flow, and recirculating the cooled third volume flow by a third substream (11′) of a third circuit to the neutralization zone of the third stage and by a further substream (13) to a final quality control unit (14) comprising a temperature and pH monitoring stage, and is taken off as a product stream (15) if the cooled third volume flow satisfies a quality criteria in the monitoring stage, or otherwise is taken off as a recirculated stream (16) to the third stage.

2. The process as claimed in claim 1, wherein the alkali metal hydroxide is sodium hydroxide.

3. The process as claimed in claim 1, wherein an average residence time of the primary reaction mixture in the first stage is from 20 seconds to 3 minutes.

4. The process as claimed in claim 1, wherein an average residence time of the secondary reaction mixture in the second stage is from 15 to 100 minutes.

5. The process as claimed in claim 1, wherein an average residence time of the tertiary reaction mixture in the third stage is from 45 to 250 minutes.

6. The process as claimed in claim 1, wherein, independent of one another, the temperature of the primary reaction mixture exiting from the first stage is set to a value in the range from 45° C. to 80° C., the temperature of the secondary reaction mixture is set to a value in the range from 40° C. to 75° C., and the temperature of the tertiary reaction mixture exiting from the third stage is set to a value in the range from 15° C. to 55° C.

7. The process as claimed in claim 1, wherein the primary reaction mixture is formed in a static mixer, where the static mixer has a mixing quality of at least 98%.

8. The process as claimed in claim 1, wherein a buffer volume in the range of +/−20% is provided in the neutralization zone of the second stage and in the neutralization zone of the third stage.

Description

(1) The invention will be illustrated below with the aid of the figures and the examples, which, however, do not represent any restriction of the invention.

(2) The figures show:

(3) FIG. 1 a schematic view of a three-stage neutralization of hydrochloric acid

(4) FIG. 2 a schematic view of details of the first neutralization stage

(5) FIG. 3 a schematic view of details of the second neutralization stage

(6) FIG. 4 a schematic view of details of the third neutralization stage

(7) In the figures, the reference symbols have the following meanings: 1 first neutralization stage 2 second neutralization stage 3 third neutralization stage 4 hydrochloric acid to be neutralized 4′ hydrochloric acid stream for setting of the target pH in 2.sup.nd and 3.sup.rd stage 5 sodium hydroxide 5′ sodium hydroxide stream for setting of the target pH in 2.sup.nd and 3.sup.rd stage 6 reaction mixture exiting from the first stage 7 main stream from the second neutralization stage 7′ smaller secondary circuit of partially neutralized hydrochloric acid from main stream 7 7″ metered addition of sodium hydroxide into secondary circuit of the second neutralization stage 7′″ metered addition of hydrochloric acid into secondary circuit of the second neutralization stage 8 cooling of the first and second neutralization stages 9 recirculation of cooled reaction mixture of the second stage 10 reaction mixture exiting from the second stage 11 reaction mixture exiting from the third stage 11′ smaller secondary circuit of neutralized hydrochloric acid of stream 11 11″ metered addition of sodium hydroxide into secondary circuit of the third neutralization stage 11′″ metered addition of hydrochloric acid into secondary circuit of the third neutralization stage 12 cooling of the third neutralization stage 13 cooled reaction mixture of the third stage 14 monitoring of the release criteria or quality of the product stream 15 discharged product stream from the neutralization in the quality window 16 recirculated reaction mixture outside the quality window 17a residence and neutralization zone after the first stage 17b neutralization zone of the second stage 17c neutralization zone of the third stage 18a (primary) reaction mixture of the first stage 18b (secondary) reaction mixture of the second stage 18c (tertiary) reaction mixture of the third stage 20 static mixer of the first stage 21 mixing nozzles of the second stage 22 mixing nozzles of the third stage F1 flow measurement for hydrochloric acid inflow into first neutralization stage F2 flow measurement for hydrochloric acid inflow into second neutralization stage F3 flow measurement for hydrochloric acid inflow into third neutralization stage F4 flow measurement of sodium hydroxide inflow into first neutralization stage F5 flow measurement of sodium hydroxide inflow into second neutralization stage F6 flow measurement of sodium hydroxide inflow into third neutralization stage K1 regulating device for hydrochloric acid inflow into first neutralization stage K2 regulating device for hydrochloric acid inflow into second neutralization stage K3 regulating device for hydrochloric acid inflow into third neutralization stage K4 regulating valve pair for sodium hydroxide inflow into first neutralization stage K5 regulating valve pair for sodium hydroxide inflow into second neutralization stage K6 regulating valve pair for sodium hydroxide inflow into third neutralization stage P1 inlet pressure measurement for hydrochloric acid P2 inlet pressure measurement for sodium hydroxide PH1 pH measurement after first neutralization stage for pH regulation PH2 pH measurement after second neutralization stage for pH regulation PH3 pH measurement after third neutralization stage for pH regulation PH4 monitoring of target pH T1 temperature measurement after first neutralization stage for coaling water regulation T2 temperature measurement after third neutralization stage for cooling water regulation T3 monitoring of target temperature

EXAMPLES

Example 1

(8) After starting up the cooling and pumping circuits and also activating the starting material supply, the neutralization plant is ready for operation. An operating pressure of 6.3 bar/0.63 MPa prevails at the measurement position P2 for the 32% strength sodium hydroxide used and an operating pressure of 5.4 bar/0.54 MPa prevails at the measurement position P1 for the 31% strength hydrochloric acid to be neutralized. An intended value for the hydrochloric acid stream to be neutralized is entered into the process control system by the operating employee. A hydrochloric acid inflow stream 4 having a volume flow of 30.0 m.sup.3/h is introduced into the recycle stream of cooled reaction mixture from the second stage 9. Via a ratio regulator F1 and F2 and via the pH regulation of the first stage PH1, an amount of 28.5 m.sup.3/h of the stream from 9 and 4 is metered into the first neutralization stage K4 via the regulating valve pair of the sodium hydroxide feed. Each total volume stream of 179 m.sup.3/h subsequently went into a static mixer 20 which represents the mixing device of the first neutralization stage 1. After passage through intensive homogenization, the temperature T1 for regulating the cooling water flow for cooling of the first and second stage 8 is measured downstream of the static mixer. A temperature of 65.4° C. is established at this position. The primary reaction mixture 18a subsequently goes into a residence and neutralization zone 17a which is realized by means of a vessel through which flow occurs and is provided for further reaction of each reaction mixture in order subsequently to guarantee a reliable pH measurement PH1 for regulating the amount of sodium hydroxide 5 introduced. A pH of 1.6 is established in the reaction mixture exiting from the first stage 6.

(9) In the next step, the still acidic salt brine goes into the second stage of the neutralization plant 2 which is operated at ambient pressure. The residence time thereof is ensured by means of an atmospheric-pressure reaction vessel located in a high position (not shown) by a till level of 58.3%, corresponding to about 30 m.sup.3, being established via a free overflow in normal operation. By means of installation of the mixing nozzles 21, the resulting turbulence in the reaction stage 2 is utilized for mixing. In addition, the mixing nozzles 21 draw in art about four-fold stream of each reaction mixture 18b from the surrounding vessel volume. Two small mixing nozzles are oriented tangentially to the bottom and a large jet mixer acts centrally and obliquely upward and thus ensures thorough mixing in the volume. This mixing principle is employed analogously in the third neutralization stage 3. From this stage, a main stream 7 of the reaction mixture 18b is taken off and passed to cooling 8. Here, a large part of the heat of neutralization of the first and second stages is transferred to the cooling water. In the process, the cooling water of the cooling facility 8 heats up from 14.7° C. to 24.5° C. The main part of the brine outlet which has been precooled in this way is conveyed in the form of the recycle stream 9 in an amount of 120 m.sup.3/h to upstream of the first stage of the neutralization 1. A smaller substream 7′ of this brine operates the mixing nozzles 21 in a secondary circuit. After setting the pH regulation PH2, 120.0 l/h of alkali are metered at the introduction position 7″ into this stream and 0.7 l/h of acid are metered in at the introduction position 7′. The back-coupling of the metering device K5 is again effected by means of a flow meter F5. During transport of the secondary stream 7′ through the mixing nozzles 21, homogenization of the reaction mixture 18b in the neutralization zone 17h takes place and a pH of 9.2, which is measured by means of the pH measurement PH2 in the outflow stream 7 from this neutralization stage 2 to the cooling 8, is established.

(10) The reaction mixture 18b goes in the form of the stream 10 from the second stage 2 via an overflow into the third stage of the neutralization 3, which owing to the 3-fold volume capacity realizes a significantly higher residence time. This volume ratio of the volume of the second stage to the volume of the third stage is designed to avoid resonant oscillation in the regulation circuits and thus avoid an associated resonance catastrophe. From this third stage, a main stream 11 of the reaction mixture 18c is likewise taken off and passed to cooling 12. In this process step, the heat of neutralization of the third stage is transferred to the cooling water. In the process, the cooling water of the cooling facility 12 heats up from 14.7° C. to 29° C. In return, the reaction mixture 18c cools down from 36.5° C. to 29° C. A cooling water volume stream of 466 m.sup.3/h is required for cooling in the first, second and third stages (8 and 12). After cooling, the secondary circuit 11′ operates the mixing nozzles 22 of the third neutralization stage 3. After setting the pH regulation PH3, 28.0 l/h of alkali were metered into this for stream at the introduction position 11″ and 34.0 l/h of acid were metered in at the introduction position 11′″. When the secondary stream 11′ is conveyed through the mixing nozzles 22, homogenization of the reaction mixture 18c in the neutralization zone 17c occurs and a of 8.6, which is measured in the stream 13 by means of the pH measurement PH3, is established. To secure the measurement of the output pH from the third stage and for availability requirements which the instrumentation has to meet, this pH measurement is configured in triplicate (redundant). As a function of the fill level in the third reaction stage 3, the reaction mixture 18c is discharged via a fill level regulator LI with adherence to the release criteria pH (measurement PH3 and PH4) and temperature (measurement T3) in process step 14. 58.7 m.sup.3/h are discharged at a constant fill level of 60.8%. When the limit values for the parameters of the resulting brine are exceeded in stream 13, the output is interrupted and the volume stream 13 is conveyed in the form of the stream 16 back to the third neutralization stage 3. Thus, in the first step, brine can be buffered for a short time in the second and third reaction stage (2 and 3). For this purpose, each of the two stages is operated about ¾ full in normal operation. In the second step, if further adjustment of the circuit operation of the third stage is not successful, the inflowing stream of acid and thus alkali into the first stage is gradually decreased (see concept of load reduction).

(11) Details of the Design of the Metering Devices:

(12) The alkali for the first stage 1 is metered from the network via two parallel valves K4 which have a gradated valve size (kvs value). The fine valve is regulated directly and has a maximum throughput which is a factor of 10 lower than the coarser valve. The latter is regulated more slowly from the manipulated variable of the small valve so that no resonance occurs between the valves. When the smaller valve reaches its maximum opening during running-up of production over a ramp, the coarser valve is open slightly. As a result, the smaller valve can close somewhat again. The larger valve is actuated sufficiently frequently for the required target pH to be attained. Likewise, the coarser valve closes stepwise when the fine valve threatens to close. Rapid and also precise regulation of the alkali stream can be achieved in this way. In the transition region to the first opening of the coarser valve, a hysteresis is run through because the valves no longer meter linearly in the boundary region. Thus, the small valve runs in this region through the total setting range, while at higher volume flows it should remain at from 20 to 80% manipulated variable.

(13) This basic principle described here for the example of the first stage is also implemented in the second stage 2 at the sodium hydroxide introduction K5 and in the third stage 3 at the sodium hydroxide introduction K6. The second stage and the third stage attempt primarily to regulate the prescribed pH values. While the bandwidth for the third stage is determined by the output limits, the first and second stages can be prescribed as a function of the power of the regulating means. Since the expected metered streams of sodium hydroxide in the third stage are very small, metering is effected via a valve and in parallel via a displacement pump.

(14) Due to the high accuracy requirements which the metering has to meet because of the desired pH values of the stages, overswing is possible. Although acidic solution again flows out from the preceding stages after correction of the regulation, an additional introduction of acid at the two stages 2 and 3 was realized because of the sometimes long residence times. The simple regulating valve is again guided by the following flow meter.

(15) Details of the Design of the Regulating Concept:

(16) The comprehensive process concept described here is based on a regulating concept, which is characterized by measurement of many process parameters such as incoming volume flows, entry pressures and also temperature, fill level and pH per reaction stage and also monitoring of the cooling water temperature, and allows firstly fully automated operation of the plant by means of intelligent process control and secondly a particular variation of the process parameters of the inflowing media (concentration, pressure and amount) to which the overall system automatically adjusts. Direct intervention by the operator after start-up of the plant is not necessary in normal operation. Thus, a regulating circuit for the pH, via which the amounts of required neutralizing agent are determined and set at the metering valves is employed in each stage. Accordingly, a constant pH is aimed at in each stage and with increasing number of reaction stages approaches the target pH.

(17) The integrated concept for load regulation makes it possible to run the neutralization plant with efficient utilization and considerably simplifies operation of the plant by personnel. The load regulation carries out an automatic reduction of the load in order to keep critical process parameters below their limit values. In this way, the capacity of the neutralization plant can be matched optimally and economically to the required neutralization capacity. The neutralization plant is operated automatically at maximum load while adhering to the prescribed limit values for the process parameters of the product solution and optimal utilization of the plant capacity at maximum throughput. In this load regulation taking into account the regulation of load-dependent process parameters, the setting of a constant intended load value for the HCl stream (4) to be neutralized by the plant operator is combined with an automatic load change effected by that in the process control system if process parameters approach their upper limit value. This concept is suitable for applications in which the load has an inverse effect on the process parameters, i.e. an increase/decrease in the load leads to a rise/reduction in the process parameters. In normal operation, i.e. when the process parameters taken into account (T1, PH1, PH2, T2, PH3, T3, PH4 and also further quality parameters (turbidity and conductivity)) are below their limit value, the load prescribed by the plant operator is run. The fact that the critical process parameters are in the subcritical region tells the plant operator that the set value of the load can be increased by intervention of the plant operator. When the respective process parameter approaches its upper limit value, an automatic load change is carried out by the process control system in order to keep the deviating process parameter within its threshold value. The automatic load change is carried out by means of regulating circuits (e.g. PID, MPC) provided for the respective process parameter. For this purpose, in each case superposed master regulating circuits are provided for the prescribed load and the process parameters, which have the task of regulating the respective process parameter to its set value using the load as manipulated variable. The manipulated variables of the superposed master regulating circuits are in each case intended values for the subordinate regulating circuit (slave) which intervenes in the process via an actuator (e.g. valve K1) so that, at the prevailing pressure conditions and the given valve properties, the flow required by the slave regulator is established.

(18) A further instrumentational optimization of the process is provided by the automatic cooling water regulation. The process temperatures in the first, second and third neutralization stages are measured and when the set values are exceeded the cooling water stream is automatically increased by means of actuators in the form of regulating valves. Here, for the temperature measurement of the first neutralization stage (T1), there is a regulating circuit for cooling water amount of the cooling facility of the first and second stage (8). Furthermore, the temperature measurement of the third neutralization stage (T2) acts by means of direct regulation on a regulating valve of the cooling facility of the third stage (12). As a result, the system reacts to load changes or temperature fluctuations and avoids direct intervention of the automatic load reduction when a process parameter is exceeded (see previous paragraph).

(19) The pH regulations of the individual stages are implemented according to the recommendation of the literature in the form of a “feed-forward” regulation (cf. LIPTAK, Bela G.: Instrument Engineers Handbook, 4.sup.th edition, 2005, p. 2044 ff). Thus, not only the local volume and the local pH are utilized, but the respective inflowing solution of the previous stage is also taken into account. For the first stage, the introduced volume streams flow in together. In addition, the supply pressures, which have been recognized as main malfunction parameters, have also been taken into account. For the second stage, the required amount of alkali is from the recirculation stream plus the streams metered into the first stage and the pH. For the third stream, this calculation is carried out from the previously introduced volume streams and the pH at the outlet from the second stage, which thus represents the content of the reaction mixture 18c.

(20) The neutralization plant is configured according to the safety requirements for the chemicals used. Here, compatible materials of construction are employed and appropriate safety concepts are provided for severe deviation of process parameters. In addition, the plant is a closed plant in which acidic waste air streams which occur are sent in a targeted manner into an existing exhaust air treatment plant.