PROCESS AND APPARATUS FOR NEUTRALISATION OF ACID STREAMS

Abstract: A process for the neutralisation of an acid stream by (a) feeding the acid stream and calcium hydroxide through a first reaction zone, preferably a plug flow reactor (1) to a second reaction zone, preferably a stirred tank reactor (12) so that the calcium hydroxide partially neutralises the acid stream during feed of the stream to the second reaction zone, the second reaction zone being provided with means for measuring the pH of the solution within the second reaction zone (14) and means for feeding aqueous base to the second reaction zone. The apparatus comprises a plug flow reactor (1) provided with means for feeding the acid stream (4) and calcium hydroxide stream (8) thereto under ratio feed-forward control (7), and a stirred tank reactor (12) provided with means for measuring the pH of the solution therein (14), an aqueous base feed (15) and feedback control means (17) for controlling the addition of aqueous base thereto.

Description: PROCESS AND APPARATUS FOR NEUTRALISATION OF ACID STREAMS. This invention relates to a process for the neutralisation of aqueous acid streams with calcium hydroxide, and in particular to a process of neutralising a hydrochloric acid stream with calcium hydroxide. The invention relates in a further aspect to an apparatus in which the process of the invention may be effected. Hydrochloric acid is produced on a wide scale around the world, indeed it is produced as a by-product in very many industrial processes. In particular it is produced as a by-product in the reaction of chlorine containing compounds with hydrogen fluoride in which fluorine is exchanged for chlorine, many of which are practised around the world. Although hydrochloric acid may be a commercial product in itself, it is not practical to store large quantities for lengthy periods of time and consequently, if an immediate demand cannot be found, then it is necessary to dispose of the hydrochloric acid. Disposal of the acid involves first neutralising the acid stream followed by discharge of the neutralised effluent. In the past, the neutralisation of such acid streams has posed less of a problem than recently due to the rather less stringent environmental controls on the pH of the effluent discharge that have prevailed hitherto. However, the pH control requirements on the discharge of such neutralised acid streams are becoming increasingly stringent and pose particular problems for the control of pH during the neutralisation of the acid streams. Acid streams are typically neutralised with a basic reagent containing sodium hydroxide or calcium hydroxide. Each of these reagents offers certain advantages and disadvantages. Thus, whilst sodium hydroxide is convenient to use due to its high solubility in water which allows it to be employed in the form of an aqueous solution, it is significantly more expensive than calcium hydroxide and poses problems of both pH measurement, due to the presence of sodium ions which have a deleterious effect on the operation of pH meters, and control, due to the characteristic titration curve which shows a very sharp change in pH over a very small change in sodium hydroxide addition in the neutral region. On the other hand, whilst being cheap, the use of calcium (hydr)oxide has posed various problems associated with the low degree of solubility of calcium hydroxide in water such that calcium hydroxide is predominantly present in solid form when added to the acid stream. In particular it poses control problems in ensuring that the calcium hydroxide has sufficiently reacted with the acid so that downstream carryover of solid reagent and subsequent reaction does not cause pH increase after the neutralisation process. The process of the present invention is based upon the use of calcium hydroxide as the neutralising agent. In the past, control of the neutralisation reaction has been achieved in dilute systems where the acid concentration is usually about 5-102. However, the neutralisation of dilute acid systems necessitates the use of very large vessels thereby increasing the overall cost of the process. It has also been proposed, in the belief that alkaline conditions promote the rate of dissolution of calcium hydroxide in water, and therefore in order to achieve greater control of the neutralisation process, to neutralise the acid stream by first adding sufficient calcium hydroxide to result in an alkaline solution, i.e. overshoot of the neutral point and then to lower the pH towards neutral by further addition of acid. However, this approach increases the deposition of solids, which is more common under alkaline conditions, and which may have deleterious effects on the performance of pH meters used to ensure that accurate monitoring of the neutralisation process is achieved. These known processes are typically carried out using a series of continuous stirred tank reactor vessels (CSTR), each tank being provided with feedback control by which the measured pH of the solution within the tank controls the addition of often both acid and alkali in order to ensure that the pH is correctly adjusted in each vessel. According to the present invention there is provided a process for the neutralisation of an acid stream which process comprises feeding the acid stream and calcium hydroxide through a first reaction zone to a second reaction zone so that the calcium hydroxide partially neutralises the acid stream during feed of the stream to the second reaction zone, the second reaction zone being provided with means for measuring the pH of the solution within the second reaction zone and means for feeding aqueous base to the second reaction zone. The first reaction zone may comprise a conduit to which the acid stream and a calcium hydroxide stream are fed, and the acid and calcium hydroxide streams may be combined to flow through the conduit co-currently to the second reaction zone. The acid streams which are neutralised by the process of the invention need not be diluted prior to neutralisation and the acid streams will usually therefore be concentrated streams, for example acid streams with an acid concentration of at least 102 by weight, and especially acid concentrations of at least 252 by weight. Acid streams having a concentration of up to 352 or 402 by weight may be neutralised with satisfactory control by the process of the invention. The acid stream is preferably that of an acid, the calcium salt of which is soluble in aqueous solution. Thus, the acid stream will usually be a hydrochloric acid or nitric acid stream, although small amounts of other acids, for example sulphuric and hydrofluoric acid may also be present. The calcium hydroxide fed to the first reaction zone is preferably in the form of an aqueous slurry of calcium hydroxide. The aqueous slurry of calcium hydroxide is preferably prepared from powdered hydrated lime in order to reduce the amount of heat liberated during the slurrying process and thus the temperature of the reagent. The proportion of solids in the slurry may vary within a wide range but preferably the calcium hydroxide slurry contains from about 102 solids by weight to about 402 solids by weight, especially from about 152 by weight to about 252 by weight. The calcium hydroxide component of the slurry preferably comprises calcium hydroxide solids particles with a mean particle size in the range from about 1 micron to about 10 micron with less than about 0.12 of the particles having a size larger than about 250 microns, in order to promote rapid reaction of the calcium hydroxide with the acid. Preferably the first reaction zone comprises a Plug Flow reactor. The second reaction zone is preferably a Stirred Tank Reactor. Plug Flow and Stirred Tank Reactors are conventional within the art and any design of these reactors may be employed in the process of the invention. The reactors are preferably constructed of materials resistant to the chemical streams with which they are to be contacted. Thus, the reactors may be constructed using rubber, per-fluorinated copolymers, for example polytetrafluoroethylene , or from metal alloy systems also having resistance to the chemical streams within them, for example suitable Hastelloy or Inconel alloys. The reactors may also comprise for example steel, having an inner lining of these materials. In practice we prefer to use rubber coated carbon steel. We have found that the provision of a first reaction zone comprising a plug flow reactor which serves to maximise the extent of reaction for any given reactor volume, allows the most efficient reaction of the calcium hydroxide and thus reduces the reactor volume required for sufficient conversions of the calcium hydroxide thus reducing the tendency for any carry-over of unreacted calcium hydroxide from the neutralisation process. The accelerated reaction of calcium hydroxide which we have found under acid conditions is further enhanced by the use of the plug flow reactor. The plug flow reactor employed in the process preferably comprises a static mixer into which the acid and calcium hydroxide streams are fed followed by a length of pipe sufficient to allow adequate reaction of the calcium hydroxide. The optimum capacity of the plug flow reaction zone depends upon the properties of the acid stream and of the calcium hydroxide, in particular the particle size distribution of the calcium hydroxide reagent. Furthermore, may commercially available forms of calcium hydroxide may contain calcium carbonate impurities which may yield carbon dioxide during the process. The presence of such carbon dioxide may also influence the desired capacity of the plug flow reactor. However, we generally prefer to employ sufficient length of plug flow reactor to allow a residence time within the reaction zone of at least 30 seconds, preferably at least 35 seconds and especially at least 40 seconds. Preferably, the acid stream and calcium hydroxide are fed to the first reactor zone under ratio feed-forward control in order to control the degree of neutralisation which is effected in the first reaction zone. Ratio feed forward control of the streams may be achieved by monitoring, for example, the density or conductivity of the acid stream whereby to obtain the acid concentration of the acid stream and by monitoring the flow of the stream. The flow of calcium hydroxide slurry theoretically required to achieve a required first degree of neutralisation (as described hereinafter) in the first reaction zone may then be calculated and that flow of calcium hydroxide may be fed to the first reaction zone. The required degree of neutralisation in the first reaction zone may vary within a wide range but preferably sufficient calcium hydroxide is fed to the first reaction zone to effect as large a proportion of the total neutralisation as possible, thus reducing to a minimum the amount of further aqueous base required to be added to the second reaction zone whilst allowing a substantial acid concentration to be maintained throughout the first reaction zone and minimising the chance of carry-over of calcium hydroxide and hence pH overshoot following the neutralisation. Preferably the amount of calcium hydroxide fed to the first reaction zone is sufficient to effect at least 802 of the neutralisation, more preferably at least 902, and especially about 952 of the neutralisation. Preferably the ratio of the flow of calcium hydroxide slurry and acid to the first reaction zone is also adjusted by a feedback control loop from the proportion of aqueous base added to the second reaction zone whereby to compensate any variations in the concentration of the calcium hydroxide feed to the first reaction zone and so as to maintain the desired proportion of calcium hydroxide being added to the first reaction zone. The product of the first reaction zone is then fed to a second reaction zone which comprises a Stirred Tank Reactor, preferably a Continuous Stirred Tank Reactor (CSTR), to which further aqueous base is added, preferably under feedback control from means for monitoring the pH of the solution within the CSTR, in an amount sufficient to achieve a second required degree of neutralisation. The amount of aqueous base necessary to achieve the required second degree of neutralisation will depend upon the amount of calcium hydroxide fed to the first reaction zone, but in order that a reliable reading may be taken from the pH meter, the amount of aqueous base added is preferably such as to result in a solution which may be recovered from the second reaction zone having a pH of at least 1, preferably at least 1.5. Further, in order to minimise any carry-over of calcium hydroxide or other aqueous base in the solution recovered from the second reaction zone, the amount of aqueous base added may be such as to produce a solution having a pH of not greater than 4, preferably not greater than 3. The aqueous base which is added to the second reaction zone is preferably calcium hydroxide since the use of calcium hydroxide in the second reaction zone as well as the first reaction zone promotes the buffering effect previously described. However, other aqueous bases may be employed if desired, for example sodium hydroxide. According to a preferred embodiment of the invention there is provided a process for the neutralisation of an acid stream which comprises the steps of (a) feeding to a first reaction zone under ratio-feed-forward control the acid stream to be neutralised and sufficient calcium hydroxide whereby to perform a first required degree of neutralisation (as hereinbefore described), wherein said first reaction zone comprises a plug flow reactor, (b) feeding the product from step (a) to a second reaction zone comprising a stirred tank reactor provided with means for measuring the pH of the solution within the tank and feeding calcium hydroxide thereto, wherein a feedback control loop from the pH measuring means controls the addition of sufficient calcium hydroxide to the tank to perform a second required degree of neutralisation and (c) recovering a solution from the second reaction zone having a pH of from about 1 to about 3. The solution recovered from step (c) may then be passed to a third reaction zone in which fine adjustment of the solution to a pH of between 6 and 9 is carried out by the addition of small amounts of an aqueous base under feedback control. The aqueous base employed may be calcium hydroxide, or it may be another aqueous base, for example sodium hydroxide. We have found that achieving the required pH control in the third reaction zone in order to meet discharge requirements is facilitated by a strong buffering effect (reduced sensitivity of pH to concentration variations) observed when employing calcium hydroxide as the neutralisation reagent in the first reaction zone in which the major part of the neutralisation is effected, and preferably also as the aqueous base which is added to the second reaction zone. This buffering effect may be exploited in the third reaction zone by the use of calcium or sodium hydroxide. We particularly prefer to employ sodium hydroxide since only very small amounts are added to achieve the required pH adjustment, and controlled addition of such small amounts is facilitated by the use of an aqueous solution of a base (sodium hydroxide) rather than an aqueous slurry (calcium hydroxide ) . During the process of the invention, there is an increase in the temperature of the solution flowing through the system due to the highly exothermic reactions taking place and the absence of substantial dilution. Indeed, the temperature of the solutions flowing through the system, may rise to a temperature of about 80°C. In order to maintain the full benefits of reliable control and minimal plant provided by carrying out the neutralisation at high concentrations, the pH in the third reaction zone is preferably controlled to a value we have determined by investigation of the chemical characteristics of the calcium hydroxide/acid system, to give a pH on solids removal and cooling which is within the regulatory pH limits allowed for effluent discharge. Consequently, cooling may be carried out ' on the final neutralised stream without the need for subsequent pH adjustment on the cooled stream. and without the need for pre-cooling of the acid stream and/or the calcium hydroxide stream, all of which would require the use of equipment constructed from exotic materials having resistance to the acid media being cooled within them. Cooling may be carried out, for example by evaporative cooling. Our investigations have shown that the pH of the final neutralised solution recovered from the third reaction zone should be lower than that desired for the cooled solution to be discharged due to our observed decrease in pH with increasing temperature. Preferably, the pH sensing means employed in the third reaction zone is pre-set to compensate for the effect of the temperature reduction (cooling) on the pH . The pH of the pre-cooled solution may therefore be from about 6.5 to about 7.5, such that after cooling, the neutralised stream to be discharged has a pH within the range from about 7.5 to about 8.5. According to a further aspect of the invention there is provided an apparatus for the neutralisation of an acid stream with calcium hydroxide which comprises a first plug flow reaction zone provided with means for feeding the acid stream and calcium hydroxide thereto under ratio feed-forward control, and a second reaction zone comprising a continuous stirred tank reactor provided with means for measuring the pH of the solution therein, an aqueous base feed and feedback control means for controlling the addition of aqueous base thereto. Preferred embodiments and further features of the apparatus of the second aspect of the invention are as described in relation to the first process aspect of the invention. The invention is illustrated but not limited by the following figure which is a schematic flow diagram of an apparatus in which the process of the invention may be operated. In the figure, a plug flow reactor 1 comprises a static mixer 2 and a pipeline 3 constructed from rubber-lined carbon steel. A hydrochloric acid feed line 4 to the static mixer 2 is provided with a conductivity meter 5 and flow meter 6. The outlets from the conductivity and flow meters are connected to a ratio feed-forward control unit 7. A calcium hydroxide feed line 8 to the static mixer 2 is provided with flow meter 9 and valve 10. Flow meter 9 and valve 10 are connected to the outlet from the ratio feed-forward control unit 7. Outlet 11 from the pipeline 3 enters a Continuous Stirred Tank Reactor (CSTR) 12 which is provided with an agitator 13, pH meter 14 and calcium hydroxide feed line 15. Calcium hydroxide feed line 15 is provided with a valve 16 controlled by feedback circuitry 17 connected to the pH meter 14. Outlet 18 from the CSTR 12 is connected to inlet 19 of a Continuous Stirred Tank Reactor (CSTR) 20 which is provided with an agitator 21, pH meter 22 and sodium hydroxide feed line 23. Sodium hydroxide feed line 23 is provided with a valve 24 controlled by feedback circuitry 25 connected to the pH meter 22. Outlet 26 from the CSTR 20 is connected to downstream cooling and solids removal equipment (not shown). In operation of the apparatus, the hydrochloric acid feed stream is fed through line 4, conductivity meter 5 and flow meter 6 to the static mixer 2. The conductivity and flow measurements are fed to the ratio feed-forward control unit 7 which performs a mathematical ratio function based on the theoretically required amount of calcium hydroxide required to neutralise an acid stream having the measured flow and concentration and controls the valve 10 and flow meter 9 to provide a flow of calcium hydroxide through line 8 to the static mixer 2 which is sufficient to provide 952 of the theoretical amount of calcium hydroxide required to perform the neutralisation. The calcium hydroxide and hydrochloric acid streams are mixed in the static mixer 2 and flow through the pipeline 3 which is of sufficient capacity to allow a 40 second residence time within the pipe. The mixed stream then enters the CSTR 12 where it is mixed by agitator 13. The pH of the solution within the CSTR is continuously monitored by pH meter 14 and the addition of calcium hydroxide to the CSTR is controlled by feedback circuitry 17 from pH meter 14 to valve 16, to produce a solution in the CSTR with a pH of about 2. The solution flows under gravity from outlet 18 of CSTR 12 through inlet 19 of, and into CSTR 20. The pH of the solution in the CSTR 20 is then adjusted to pH 7.5 by the addition of sodium hydroxide through line 23 under the control of feedback circuitry 25 provided from pH meter 22 to valve 24 on the sodium hydroxide feed line 23. The neutralised stream from outlet 26 then passes to downstream processes in which residual solids are removed and the stream is cooled, with a consequent increase in pH to about pH 8. The invention is further illustrated by the following example which was conducted on a laboratory apparatus described below. The apparatus comprised a 6" spiral helix plastic mixer (supplied by RS Components Ltd, Ref: 503/385) set inside a glass tubing of length 15cm and diameter 10mm, and provided with hydrochloric acid and calcium hydroxide feed inlets, each having a peristaltic pump. A two metre length of rubber tubing was attached in fluid flow communication with the exit from the glass tubing, the glass and rubber tubing together providing the plug flow reaction zone and the rubber tubing having provided therein a vent for carbon dioxide generated during the neutralisation reaction. The outlet from the rubber tubing fed into a 50ml vessel provided with a pH feedback control system comprising a pH meter set at pH 1, and electronic circuitry connected to the calcium hydroxide feed line to control the flow rate of calcium hydroxide into the glass tubing (this arrangement providing a small-scale, short-lifetime equivalent to a ratio-feed-forward control of the calcium hydroxide feed from the acid flow) . Below the 50ml vessel, there is provided a stirred tank of capacity 3.5 litres provided with a pH feedback control system comprising a pH meter set at pH 2 and a calcium hydroxide feed line. The pH control system controls the feed of calcium hydroxide to this stirred tank to achieve a measured pH of 2 for the solution in the tank. The outlet from the tank is connected to a second stirred tank of capacity 3.25 litres and provided with a sodium hydroxide feed line and pH feedback control system set at pH 6.1. The pH control system controls the feed of sodium hydroxide to this stirred tank to achieve a measured pH of 6.1 for the solution in the tank. The outlet from this second stirred tank is connected to a third vessel provided with a pH meter and in which the pH of the product neutralised solution is measured. In a continuous run over a 5 hour period, a 332 w/w solution of hydrochloric acid was fed to the glass tubing at a flow rate of 3.6 kg/hour together with 10.6 kg/hour of a 102 w/w lime slurry, corresponding to 852 of the theoretically calculated amount of lime required to achieve the neutralisation and the flow rates being such that a residence time of 40 seconds was provided in the plug flow reaction zone . The process stream flowed continuously into the 50ml vessel in which the pH of the solution therein was measured in order to control the flow rate of the calcium hydroxide to the glass tubing to achieve a pH of 1 for the solution in this vessel. The overflow from the vessel flowed into the first stirred tank beneath the vessel and in which the solution had a residence time of 15 minutes and to which further (about l.lkg/hour) calcium hydroxide was added under feedback control from the pH meter in order to achieve a measured pH for the solution exiting the tank of pH 2. The solution exiting the first tank flowed continuously into the second stirred tank in which the solution had a residence time of 15 minutes and to which sodium hydroxide was added under feedback control from the pH meter in order to achieve a measured pH for the solution exiting the tank of pH 6.1. The solution exiting the second stirred tank flowed into a third tank in which the pH of the product neutralised stream was measured. Over the 5 hour period, a neutralised stream was produced in which the pH of the product varied between 6.9 and 7.4. It will be readily apparent from the foregoing description and example that a concentrated acid stream may be neutralised without first diluting the stream, thus directly reducing the required reactor volume and associated reactor costs, whilst providing the required control to meet the increasingly stringent requirements of environmental legislation on the pH control permitted for the neutralised discharge. The rate of reaction of calcium hydroxide in the aqueous solution is in fact promoted by acid conditions and this effect may be exploited to further decrease the required reactor volumes and costs. Moreover when employing calcium hydroxide derived from typical high calcium lime, significant advantage is obtained by a buffering effect which allows effective and reliable control of pH to neutrality.

Claims: CLAIMS. 1. A process for the neutralisation of an acid stream which process comprises feeding the acid stream ad calcium hydroxide through a first reaction zone to a second reaction zone so that the calcium hydroxide partially neutralises the acid stream during feed of the stream to the second reacti-on zone, the second reaction zone being provided with means for measuring 10 the pH of the solution within the second reaction zone and means for feeding aqueous base to the second reaction zone . 2. A process as claimed in claim 1 in which the first J5 reaction zone comprises a conduit through which the calcium hydroxide and acid stream flow co-currently. 3. A process as claimed in claim 1 in which the first reaction zone comprises a plug flow reactor. 20 4. A process as claimed in any one of claims 1 to 3 in which the second reaction zone comprises a stirred tank reactor. 25 5. A process as claimed in any one of claims 1 to 4 in which the calcium hydroxide fed to the first reaction zone comprises an aqueous slurry of calcium hydroxide . 30 6. A process as claimed in any one of claims 1 to 5 in which the acid stream has an acid concentration greater than 102 by weight. 7. A process as claimed in any one of claims 1 to 6 in which the acid stream comprises a hydrochloric or nitric acid stream. ^ 8. A process as claimed in claim 3 in which the plug flow reactor comprises a static mixer followed by sufficient length of pipe to allow a residence time of at least 30 seconds. 9. A process as claimed in any one of claims 1 to 3 in which the acid and calcium hydroxide streams are fed to the first reaction zone under ratio-feed-forward control. 10. A process as claimed in claim 9 in which the flow and concentration of the acid stream are monitored and sufficient calcium hydroxide is fed to the first reaction zone in order to achieve at least 802 of the neutralisation. 11. A process as claimed in any one of claims 1 to 10 in which sufficient aqueous base is added to the second reaction zone under feedback control such that the product solution in the second reaction zone has a pH of at least 1. 12. A process as claimed in claim 11 in which the aqueous base is calcium hydroxide. 13. A process for the neutralisation of an acid stream which comprises the steps of (a) feeding to a first reaction zone under ratio-feed-forward control the acid stream to be neutralised and sufficient calcium hydroxide whereby to perform a first required degree of neutralisation as hereinbefore described), wherein said first reaction zone comprises a plug flow reactor, (b) feeding the product from step (a) to a second reaction zone comprising a stirred tank reactor provided with means for measuring the pH of the solution within the tank and feeding calcium hydroxide thereto, wherein a feedback control loop from the pH measuring means controls the addition of sufficient calcium hydroxide to the tank to perform a second required degree of neutralisation and (c) recovering a solution from the second reaction zone having a pH of from about 1 to about 3. 14. A process as claimed in claim 13 in which the product of step (c) is passed to a third reaction zone to which aqueous base is added under feedback control to produce a solution having a pH from about 6 to about 9. 15. A process as claimed in claim 14 in which the solution produced has a pH from about 6.5 to about 7.5. 16. A process as claimed in claim 15 in which the solution having a pH from about 6.5 to about 7.5 is then cooled to ambient temperature such that the solution has a pH of from about 7.5 to about 8.5. 17. An apparatus for the neutralisation of an acid stream with calcium hydroxide which comprises a first plug flow reactor provided with means for feeding the acid stream and calcium hydroxide thereto under ratio feed-forward control, and a second reaction zone comprising a stirred tank reactor provided with means for measuring the pH of the solution therein, an aqueous base feed and feedback control means for controlling the addition of aqueous base thereto

Inventors: ANNELLS DAVID HUGH, HARKIN JOHN JOSEPH, WALSH STEPHEN PATRICK KIERAN, ANNELLS, DAVID, HUGH, HARKIN, JOHN, JOSEPH, WALSH, STEPHEN, PATRICK, KIERAN
Applicants: ANNELLS DAVID HUGH, HARKIN JOHN JOSEPH, ICI PLC, WALSH STEPHEN PATRICK KIERAN, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1993
Main classification: C02F1
IPC codes: A62D3, A62D101, C02F1