Method for flexibly controlling the use of hydrochloric acid from chemical production

Abstract

The invention relates to a method for flexibly controlling the use of hydrochloric acid having an HCl concentration of at least 10 wt %, in particular at a volume flow rate of at least 1 m.sup.3/h, obtained from a continuous chemical production process (A). In the method, purified hydrochloric acid (54) from a hydrochloric acid store (E) is optionally fed to a dispatch station (H), an HCl electrolysis station (F) and a chloralkali electrolysis station (L), which are consumption points for the hydrochloric acid, or to a neutralisation station (G) in that if one or more of said consumption points (H, F, L) is not available or if there are bottlenecks at the consumption points (H, F, L), the hydrochloric acid (54) is fed to the neutralisation station (G) and neutralised with concentrated alkali solution (55), in particular with concentrated sodium hydroxide solution, and the resulting salt solution (56) is fed either to the chloralkali process station (L) or to a disposal station (M).

Claims

1. A method for flexibly controlling the use of hydrochloric acid having an HCl concentration of at least 10% by weight, obtained from a continuous chemical production process (A), wherein gaseous hydrogen chloride from the continuous chemical production process is converted with water in an HCl-absorption station to hydrochloric acid, the hydrochloric acid is purified in a hydrochloric acid purification station and purified hydrochloric acid is delivered to a hydrochloric acid storage facility (E), the purified hydrochloric acid is supplied from the hydrochloric acid storage facility (E), selectively to a transport station (H), an HCl electrolysis station (F) or a chloralkali electrolysis station (L), which are offtake stations for the purified hydrochloric acid, or to a neutralization station (G), where on failure of one or more of the offtake stations (H, F, L) or in the case of offtake bottlenecks at the offtake stations (H, F, L), the hydrochloric acid is fed to the neutralization station (G) and neutralized with concentrated alkali metal hydroxide, forming a salt solution optionally fed to the chloralkali electrolysis station (L) or a disposal station (M).

2. The method as claimed in claim 1, wherein the chemical production process (A) is a production process for the chlorination of organic compounds, for phosgene production and phosgenation, a process for producing polycarbonate, or a process for incinerating chlorine-containing wastes.

3. The method as claimed in claim 1, wherein the purified hydrochloric acid is converted in the HCl electrolysis station (F) forming chlorine, and the chlorine is recycled to the chemical production process (A).

4. The method as claimed in claim 1, wherein alkali metal chloride, optionally obtained from the neutralization station (G), and hydrochloric acid is converted in the chloralkali electrolysis station (L), and the chlorine is recycled to the chemical production process (A).

5. The method as claimed in claim 1, wherein a multistage neutralization process is employed in the neutralization station (G).

6. The method as claimed in claim 1, wherein the multistage continuous neutralization (G) of hydrochloric acid having an HCl concentration of at least 10% by weight and having a volume flow of at least 1 m.sup.3/h, is carried out to a target pH in the range from 3 to 9, by means of 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 (5) in a first stage into a volume flow of neutralized hydrochloric acid, wherein the volume flow of neutralized hydrochloric acid which is recirculated and cooled, from the 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 resident zone, where the pH of a stream coming from the first stage has a pH of greater than 1 and the volume flow which is 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 the 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, 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 of 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.

7. The method as claimed in claim 6, wherein the alkali metal hydroxide is sodium hydroxide.

8. The method as claimed in claim 7, wherein an average residence time of the primary reaction mixture in the first stage is from 20 seconds to 3 minutes.

9. The method as claimed in claim 6, wherein an average residence time of the secondary reaction mixture in the second stage is from 15 to 100 minutes.

10. The method as claimed in claim 6, wherein an average residence time of the tertiary reaction mixture in the third stage is from 45 to 250 minutes.

11. The method as claimed in claim 6, wherein, independent of one another, the temperature of the primary reaction mixture 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 from of the third stage is set to a value in the range from 15° C. to 55° C.

12. The method as claimed in claim 6, wherein the primary reaction mixture is formed in a static mixer, where the static mixer has a mixing quality of at least 98%.

13. The method as claimed in claim 6, wherein a buffer volume in the range of +/−20% is provided in each of the neutralization zone of the second stage and the neutralization zone of the third stage.

14. The method as claimed in claim 6, wherein the mixing of the secondary reaction mixture with the mixture of the tertiary reaction mixture are carried out independently of one another using stirring tools in the neutralization zone by means of mixing nozzles which are provided in the entry region of the feed conduits for the substreams into the neutralization zone of the third stage.

15. A method for flexibly controlling the use of hydrochloric acid having an HCl concentration of at least 10% by weight, obtained from a continuous chemical production process (A), wherein gaseous hydrogen chloride from the continuous chemical production process is converted with water in an HCl-absorption station to hydrochloric acid, the hydrochloric acid is purified in a hydrochloric acid purification station and purified hydrochloric acid is delivered to a hydrochloric acid storage facility (E), the purified hydrochloric acid is supplied from the hydrochloric acid storage facility (E)selectively to a transport station (H), an HCl electrolysis station (F) or a chloralkali electrolysis station (L), which are offtake stations for the purified hydrochloric acid, or to a neutralization station (G), where on failure of one or more of the offtake stations (H, F, L) or in the case of offtake bottlenecks at the offtake stations (H, F, L), the hydrochloric acid is fed to the neutralization station (G) employing a multistage neutralization process, and neutralized with concentrated alkali metal hydroxide, forming a salt solution optionally fed to the chloralkali electrolysis station (L) or a disposal station (M), wherein the multistage continuous neutralization (G) of hydrochloric acid having an HCl concentration of at least 10% by weight and having a volume flow of at least 1 m.sup.3/h, is carried out to a target pH in the range from 3 to 9, by means of 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 (5) 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 the 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 resident zone, where the pH of a stream coming from the first stage has a pH of greater than 1 and the volume flow which is 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 the 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, 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.

Description

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

(2) The figures show:

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

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

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

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

(7) FIG. 5 a schematic view of the total process for hydrochloric acid management

(8) FIG. 6 a schematic view of a particular embodiment of hydrochloric acid management

(9) In the figures, the reference symbols have the following meanings

(10) 1 first neutralization stage

(11) 2 second neutralization stage

(12) 3 third neutralization stage

(13) 4 hydrochloric acid to the neutralized

(14) 4′ hydrochloric acid stream for setting the target pH in 2.sup.nd and 3.sup.rd stage

(15) 5 sodium hydroxide

(16) 5′ sodium hydroxide stream for setting the target pH in 2.sup.nd and 3.sup.rd stage

(17) 6 reaction mixture exiting from the first stage

(18) 7 main stream from the second neutralization stage

(19) 7′ smaller secondary circuit of partially neutralized hydrochloric acid from main stream 7

(20) 7″ introduction of sodium hydroxide into the secondary circuit of the second neutralization stage

(21) 7′″ introduction of hydrochloric acid into the secondary circuit of the second neutralization stage

(22) 8 cooling of the first and second neutralization stage

(23) 9 recirculation of cooled reaction mixture of the second stage

(24) 10 reaction mixture exiting from the second stage

(25) 11 reaction mixture exiting from the third stage

(26) 11′ smaller secondary circuit of neutralized hydrochloric acid from stream 11

(27) 11″ introduction of sodium hydroxide into secondary circuit of the third neutralization stage

(28) 11′″ introduction of hydrochloric acid into secondary circuit of the third neutralization stage

(29) 12 cooling of the third neutralization stage

(30) 13 cooled reaction mixture of the third stage

(31) 14 monitoring of the release criteria or quality of the product stream

(32) 15 discharged product stream from the neutralization in the quality window

(33) 16 recirculated reaction mixture outside the quality window

(34) 17a residence time and neutralization zone downstream of the first stage

(35) 17b neutralization zone of the second stage

(36) 17c neutralization zone of the third stage

(37) 18a (primary) reaction mixture of the first stage

(38) 18b (secondary) reaction mixture of the second stage

(39) 18c (tertiary) reaction mixture of the third stage

(40) 20 static mixer of the first stage

(41) 21 mixing nozzles of the second stage

(42) 22 mixing nozzles of the third stage

(43) F1 flow measurement of hydrochloric acid feed to first neutralization stage

(44) F2 flow measurement of hydrochloric acid feed to second neutralization stage

(45) F3 flow measurement of hydrochloric acid feed to third neutralization stage

(46) F4 flow measurement for sodium hydroxide feed to first neutralization stage

(47) F5 flow measurement for sodium hydroxide feed to second neutralization stage

(48) F6 flow measurement for sodium hydroxide feed to third neutralization stage

(49) K1 regulating device for hydrochloric acid feed to first neutralization stage

(50) K2 regulating device for hydrochloric acid feed to second neutralization stage

(51) K3 regulating device for hydrochloric acid feed to third neutralization stage

(52) K4 regulating valve pair for sodium hydroxide feed to first neutralization stage

(53) K5 regulating valve pair for sodium hydroxide feed to second neutralization stage

(54) K6 regulating valve pair for sodium hydroxide feed to third neutralization stage

(55) P1 inflow pressure measurement for hydrochloric acid

(56) P2 inflow pressure measurement for sodium hydroxide

(57) PH1 pH measurement after first neutralization stage for pH regulation

(58) PH2 pH measurement after second neutralization stage for pH regulation

(59) PH3 pH measurement after third neutralization stage for pH regulation

(60) PH4 monitoring of target pH

(61) T1 temperature measurement after first neutralization stage for cooling water regulation

(62) T2 temperature measurement after third neutralization stage for cooling water regulation

(63) T3 monitoring of target temperature

(64) A chemical production (isocyanate production)

(65) E hydrochloric acid storage facility

(66) F HCl electrolysis station

(67) G neutralization station

(68) L chloralkali electrolysis station

(69) M disposal station

(70) 40, 41 hydrogen chloride

(71) 42, 43 purified hydrogen chloride

(72) 44, 49 hydrochloric acid, crude

(73) 48 hydrochloric acid, 20% by weight

(74) 54, 54a, 54b, 54c purified hydrochloric acid

(75) 55 cone, sodium hydroxide

(76) 56, 56a salt solution

(77) 57, 58 chlorine

(78) 59 hydrogen gas

(79) 60 hydrochloric acid, unpurified

(80) 61 water

EXAMPLES

(81) The following examples illustrate the flexible hydrochloric acid management.

Example 1

Description of a Multistage Neutralization

(82) The integrated system of hydrochloric acid management at a production site is shown schematically in FIG. 5.

(83) A chemical production plant A forms HCl gas which is fed to station B for gas purification, in particular by means of distillation, freezing-out of impurities and purification over activated carbon (gas stream 40). The purified hydrogen chloride gas 42 is fed to a station C for hydrogen chloride absorption, which operates by means of steam stripping. Part of the purified hydrogen chloride gas 43 is fed to a plant K for catalytic gas-phase oxidation, in which hydrogen chloride is reacted with oxygen in the presence of catalysts containing ruthenium compounds at elevated temperature to form chlorine 58a and water or hydrochloric acid 49. The chlorine gas 58a is recirculated to the production A for reaction.

(84) If desired, an HCl gas stream 41 can also be fed directly to the station C for hydrogen chloride absorption.

(85) Hydrochloric acid 44 from the HCl absorption C and optionally excess hydrochloric acid 49 from the HCl gas-phase oxidation K are fed to a station D for purification of hydrochloric acid, which removes further impurities, for example by means of activated carbon. The purified hydrochloric acid is conveyed further to a hydrochloric acid tank E as hydrochloric acid storage facility. When the capacity of the hydrochloric acid tank E is exceeded, hydrochloric acid 54 is conveyed either from the hydrochloric acid tank E or directly from the station D for purification of hydrochloric acid to the neutralization station C (not shown) and neutralized there by means of concentrated sodium hydroxide 55. The salt solution 56 formed here can be fed preferably to a plant L for electrolysis of sodium chloride or in a stream 56a to a disposal station M. In the NaCl electrolysis station L, part of the hydrochloric acid 54c from the storage facility E can be used for acidifying the sodium chloride solution. The chlorine 57 formed in the electrolysis L is passed to reuse in the chemical production A.

(86) A further output station for the hydrochloric acid 54b is the transport station H in which the hydrochloric acid is loaded onto either road transport vehicles (stream 43), rail tank cars (stream 44) or ships (stream 45).

(87) Here too, a hydrochloric acid electrolysis station F in which part of the purified hydrochloric acid 54a from the hydrochloric acid storage facility E is converted into chlorine gas 58, optionally hydrogen 59 and depleted hydrochloric acid 48 is provided at the production site. The chlorine gas 58 is if required recirculated to the chemical production A, the hydrogen 59 is utilized thermally or in another way and the depleted hydrochloric acid 48 is fed to the HCl absorption C.

(88) The neutralization station U is taken into operation when the capacity of the hydrochloric acid storage facility E is exhausted and the other recycling possibilities hydrochloric acid electrolysis F, sodium chloride electrolysis L and sales or transport H are not possible for various reasons.

(89) After start-up of the cooling and pump circuits and also activation of the starting material supply, the neutralization plant is ready for operation (FIG. 1). 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 gauge/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 set in the process control system by the operating personnel. A hydrochloric acid feed stream 4 having a volume flow of 30.0 m.sup.3/h is introduced into the recycle stream of cooled reaction mixture of the second stage 9. According to a ratio regulation F1 and F2 and the pH regulation of the first stage PH1, an amount of 28.5 m.sup.3/h is metered via the regulating valve pair of the sodium hydroxide feed of the first neutralization stage K4 into the stream from 9 and 4. 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 is measured downstream of the static mixer in order to regulate the cooling water flow for the cooling of the first and second stage 8. 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 has been realized by means of a vessel through which flow occurs and has been 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 fed in. A pH of 1.6 is established in the reaction mixture exiting from the first stage 6.

(90) In the next step, the still acidic salt brine goes into the second stage, which is operated at ambient pressure, of the neutralization plant 2. The residence time thereof is ensured by means of a reaction vessel (not shown) which is operated at atmospheric pressure and is located in a high position by a fill level of 58.3%, corresponding to about 30 m.sup.3, being established by means of a free overflow in normal operation. As a result of installation of the mixing nozzles 21, the turbulence arising in the reaction stage 2 is utilized for mixing. In addition, the mixing nozzles 21 also draw in an 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 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 major part of the heat of neutralization of the first and second stage is transferred to the cooling water. In the process, the cooling water of the cooling 8 heats up from 14.7° C. to 24.5° C. The major part of the brine outflow which has been precooled in this way is conveyed in the form of a recycle stream 9 having a flow of 120 m.sup.3/h to a point upstream of the first stage of the neutralization 1. A smaller substream 7′ of this brine drives, in a secondary circuit, the mixing nozzles 21. According to the pH regulation PH2, 120.0 l/h of alkali are metered into this stream at the introduction position 7″ and 0.7 l/h of acid are metered in at the introduction position 7′″. The backcoupling of the metering device K5 is again effected through a flow meter F5. When the secondary stream 7′ is conveyed through the mixing nozzles 21, the reaction mixture 18b is homogenized in the neutralization zone 17b and a pH of 9.2, measured in the outflow 7 from this neutralization stage 2 for the cooling 8 by means of the pH measurement PH2, is established.

(91) 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 capacity realizes a significantly longer residence time. This volume ratio of the volume of the second stage to the volume of the third stage was designed to avoid resonant oscillation of the regulations and an associated resonance catastrophe. From this third stage, a main stream 11 of the reaction mixture 18c is likewise taken off and fed to the cooling 12. In that 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 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 flow of 466 m.sup.3/h is required for cooling of the first, second and third stages (8 and 12). After cooling, the secondary circuit 11′ drives the mixing nozzles 22 of the third neutralization stage 3, According to the pH regulation PH3, 28.0 l/h of alkali are metered into this stream at the introduction position 11″ and 34.0 l/h of acid are metered in at the introduction position 11′″. When the secondary stream 11′ is conveyed through the mixing nozzles 22, the reaction mixture 18e is homogenized in the neutralization zone 17c and a pH of 8.6, 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 reasons of availability of the instrumentation, this pH measurement was triplicated (redundant). As a function of the fill level in the third stage 3, the reaction mixture 18c is discharged via a fill level regulator L1 in process step 14 when the release criteria pH (measurement PH3 and PH4) and temperature (measurement T3) are satisfied, 58.7 m.sup.3/h are discharged at a constant fill level of 60.8%. When the limit values for the parameters for the resulting brine in stream 13 are exceeded, the discharge 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, both reaction stages are operated only about ¾ full in normal operation. In the second step, if further regulation of the circuit operation of the third stage is not successful, the feed stream of acid and thus alkali into the first stage is gradually decreased (see load reduction concept).

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

(93) The alkali for the first stage 1 is taken from the network and metered in 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 that of the coarser valve. The latter is regulated more slowly by the manipulated variable of the small valve, so that no resonance between the valves occurs. When the smaller valve reaches its maximum opening when production is increased over a ramp, the coarser valve is opened slightly. As a result, the smaller valve can close somewhat again. This actuation of the larger valve occurs repeatedly until the required target pH is reached. Likewise, the coarser valve closes stepwise when the fine valve threatens to close. Rapid and precise regulation of the stream of alkali can be achieved in this way. In the transition to the first opening of the coarser valve, a hysteresis is passed through because the valves no longer meter linearly in the boundary region. Thus, the small valve in this region goes through the entire setting range, while at higher volume flows it should remain at from 20 to 80% manipulated variable.

(94) This basic principle described here for the example of the first stage is also implemented analogously in the second stages 2 in the sodium hydroxide introduction K5 and the third stage 3 in the sodium hydroxide introduction K6. The second and third stages attempt primarily to regulate to the prescribed pH values. While the bandwidth for the third stage is predetermined by the release limits, the first and second stages can be prescribed according to the performance of the regulations. Since the expected streams of sodium hydroxide introduced in the third stage are very small, metering is carried out via a valve and in parallel via a displacement pump.

(95) According to the high accuracy requirements which the metering has to meet due to the desired pH values of the stages, overswing is possible. Although, after correction of the regulation, acidic solution continues to flow from the preceding stages, introduction of acid in both stages 2 and 3 has been additionally realized because of the somewhat long residence times. The simple regulating valve is again guided via the subsequent flow meter.

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

(97) The comprehensive method concept described here is based on a regulating concept which is characterized by measurement of many process parameters such as inflow volume streams, inflow 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 via an 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 reacts automatically. Direct intervention of the operator after start-up of the plant is not necessary in normal operation. Thus, a regulating circuit for the pH, which determines the amounts of neutralizing agent required and sets these at the metering valves, is used in each stage. Accordingly, a constant pH which with increasing reaction stage number approaches the target pH is aimed at in each stage.

(98) The integrated concept for load regulation makes it possible to run the neutralization plant at an efficient load and considerably simplifies operation of the plant by personnel. The load regulation carries out an automatic reduction in the load in order to keep critical process parameters within their limit values. Thus, the capacity of the neutralization plant can be matched optimally and economically to the required neutralization capacity. Here, the neutralization plant is run automatically operated under maximum load while adhering to the prescribed limit values for the process parameters of the product solution and optimal usage 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 desired load value for the HCl stream (4) to be neutralized by the plant operator is combined with an automatic load change via that in the process control system in the case of process parameters approaching 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/lowering of 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 operated. The fact that the critical process parameters are in the noncritical range indicates to the plant operator that the set value of the load can be increased by intervention by the plant operator. When the respective process parameter approaches its upper limit value, an automatic load change is brought about by the process control system in order to keep the deviating process parameter within its threshold value. The automatic load change is carried out via regulating circuits (e.g. PID, MPC) provided for the respective process parameter. For this purpose, superposed master regulating circuits are provided in each case for the load setting and the process parameters; these have the object of regulating the respective process parameter by means of the load as manipulated variable to its intended value. The manipulated variables of the master regulating circuits are in each case intended values for the subordinate regulating circuit (slave) which intervenes via an actuator (e.g. valve K1) in the process so that, at the prevailing pressure conditions and the given valve properties, the stream required by the slave regulator is established.

(99) A further instrumentational optimization of the process conditions is the automatic cooling water regulation. The process temperatures of the first neutralization stage and also second and third neutralization stages are measured and when the intended values are exceeded, the cooling water flow is automatically increased by means of actuators in the form of regulating valves. Here, there is a regulating circuit for amount of cooling water to the cooling device of the first and second stage (8) for the temperature measurement of the first neutralization stage (T1). Furthermore, the temperature measurement of the third neutralization stage (T2) acts by means of direct regulation on a regulating valve of the cooling device of the third stage (12), In this way, 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).

(100) The pH regulations of the individual stages are, as recommended in the literature, implemented 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 preceding stage is taken into account. In the case of the first stage, the introduced volume streams flow in together. In addition, the supply pressures, which have been recognized as main malfunction parameters, are also taken into account. For the second stage, the required amount of alkali is from the circulation stream plus the streams introduced into the first stage and the pH. For the third stream, this calculation is carried out from the previously introduced volume flows and the pH at the outlet from the second stage, which thus represents the content of the reaction mixture 18c.

(101) The neutralization plant is designed in accordance with the safety requirements for the chemicals used. Here, compatible materials of construction are employed and appropriate safety concepts for severe deviations of process parameters are provided. In addition, the plant is a closed plant if the acidic waste air streams occurring are introduced in a targeted manner to an existing exhaust air treatment plant.

(102) The concentration of the NaCl-containing solutions formed is 20-25% by weight.

(103) The NaCl-containing stream 15 obtained from the neutralization G can be fed to a chloralkali electrolysis L or a disposal station M.

Example 2

Hydrochloric Acid Management in an Integrated Isocyanate Plant (FIG. 6)

(104) The following example describes the hydrochloric acid management in a production process (A) for producing isocyanates which has a capacity of 100 000 t/h of tolylene diisocyanate (TDI) and is operated at a load of 11.5 t/h of TDI and produces 9.353 t/h of HCl gas as by-product.

(105) A hydrochloric acid electrolysis F in the form of a hydrochloric acid diaphragm electrolysis plant F having an uptake capacity of 3 t/h of HCl (100%), a transport station H for filling rail tank cars and road transport vehicles 43 are present on the site. The integrated plant also encompasses a sodium chloride electrolysis L which has a capacity of 300 000 till of chlorine and can thus take up 0.829 t/h of hydrochloric acid, calculated as 100% HCl. Also in the integrated plant there is a neutralization plant G which can process an amount of hydrochloric acid of 10 t/h, calculated as 100% HCl. Hydrogen chloride 41 which is obtained in the isocyanate production A is in normal operation fed to an HCl gas absorption C. The depleted hydrochloric acid 48 from the hydrochloric acid electrolysis F, which has an amount of 20.36 t/h and a concentration of 20% by weight, and also 15.04 t/h of water 61 are introduced as absorption medium here. Impurities are taken off together with water and hydrogen chloride from the absorption unit C. Here, 0.4 t/h of a 30% strength hydrochloric acid 60 contaminated with organics is taken off and disposed of. An amount of 44.353 t/h of a 30% strength hydrochloric acid 44 is discharged from the HCl absorption unit C.

(106) The hydrochloric acid 44 is fed to a purification station D. In the purification station D, the hydrochloric acid 44 is treated further with activated carbon in order to remove residues of impurities.

(107) The purified hydrochloric acid is fed to a hydrochloric acid storage facility E. From the storage facility E, 23.27 t/h of the 30% strength hydrochloric acid 54a are taken off and fed to an HCl diaphragm electrolysis F. Here, 2.91 t/h of chlorine 58 is produced from the hydrochloric acid fed in and the depleted hydrochloric acid 48, 20.36 t/h, having a concentration of 20% by weight is fed to the HCl absorption C.

(108) From the hydrochloric acid storage facility E, the chloralkali electrolysis L present on the site is supplied with 2.769 t/h of hydrochloric acid 54c for acidification of the brine. Furthermore, 18.314 t/h of 30% strength hydrochloric acid 54b are passed to sales. For this purpose, tank cars 43 are filled.

(109) Since no disposal by means of road transport vehicles is possible at the weekend, the 18.314 t/h of 30% strength hydrochloric acid are stored in the storage facility E. After the weekend, the storage facility E has been filled with 879 t of hydrochloric acid which at the beginning of the week is taken off either by means of road transport vehicles or by means of rail tank cars.

(110) If the hydrochloric acid cannot be sold or taken off, the storage facility would, at a storage capacity of 1000 t, be full after 54.6 hours and the production A of TDI would have to be shut down, with the adverse economic effects. This state can occur, for example, at long weekends or over Christmas or Easter.

(111) In this case, a proportion of 10 t/h of the hydrochloric acid 54 from the storage facility E is fed to a neutralization station G and neutralized there with concentrated sodium hydroxide 55 as described in Example 1. 10.2739 t/h of 32% strength by weight sodium hydroxide 55 are used here. 20.2739 t/h of sodium chloride solution 56 having a concentration of 23.7% by weight of sodium chloride are formed. This sodium chloride solution 56 is fed to the NaCl electrolysis L From the NaCl electrolysis 6.187 t/h of chlorine 57 are always recirculated, independently of the introduction of sodium chloride solution 56 from the neutralization G, to the chemical process A, the TDI production.