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The material system hydrogen chloride/water displays a maximum azeotrope at a boiling temperature of 108.6°C, for a system pressure of 1 bar and a HCl concentration of 20.2wt%.
If the acid concentration is lower than the azeotrope mixture, the acid can be concentrated only up to the azeotropic point. Further concentration needs special procedures.
Basically, two technologies may be considered:
- Dual-pressure technology (variation of system pressure)
- Extractive rectification technology (addition of an extractive agent)
Both technologies can also be used for the production of HCl gas.
Dual-pressure Technology
The location of the azeotrope point is contingent to pressure. Therefore, it is possible to achieve full separation due to rectification by employing two different pressures. The principle of dual-pressure technology is presented in illustration 1. At first, the mixture is treated by rectification at low pressure (vacuum/standard pressure), so that a mixture of azeotrope composition is produced. Then the mixture is further rectified in a second column at high pressure (standard pressure/over-pressure). An azeotrope mixture results at the bottom outlet in the second column, yet whose concentration is lower than the azeotrope composition of the first column due to the system pressure. Thus, the bottom product of the second column may be recycled back to the first column. Concentrated hydrochloric acid of the desired concentration level is produced at the head of the second column. The composition of the azeotrope mixture water/hydrochloric acid versus system pressure is presented in illustration 2. The pressure range from 100 mbar to about 3 bar is technically interesting. Yet within this pressure range, composition change to the azeotropic point is marginal so that the dual-pressure technology is generally considered to be less economical in that pressure range.
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Illustr. 1: Dual-pressure Technology
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<<< Illustr. 2: Composition of the Azeotrope Mixture at Different Pressures |
Extractive Rectification Technology
A more economical technology is the extractive rectification technology, as presented in illustration 3. Employing this technology, whereby the azeotrope point is to be suppressed as best as possible, increases the relative volatility of HCl. The relative volatility may be impacted by change of the activity coefficients. This is achieved by addition of a third ingredient, the extractive agent, which displays strong hygroscopic properties. Hydrochloric acid of high concentration or optionally HCl gas may be produced at the head of the extractive rectification column upon choice of an appropriate extraction agent. In the column the extraction agent is diluted with water, which is extracted in a selective fashion from the inflowing mixture. Subsequently, the extractive agent is recycled and newly feed into the extractive rectification process.
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<<< Illustr. 3: Extractive Rectification Technology |
An appropriate extractive agent must display the following physical properties:
The agent:
- must increase the relative volatility of HCl towards water;
- should be soluble with the components of the feed acid throughout the whole concentration range;
- should display a high boiling point relative to the components of the feed acid, and;
- should, finally, lend to separation under economic conditions for energy consumption in order to regenerate from water.
Sulphuric acid, and aqueous solutions with metal chlorides, such as MgCl2 and CaCl2 are appropriate extraction agents for the concentrationing of hydrochloric acid, for example. Generally, concentrated CaCl2 solutions are deployed due to availability reasons, and in order to be able to combine in a sensible fashion throughout the different process steps of the technology for the recycling, concentration, and cleaning of hydrochloric acid.
The phase equilibrium of the material system CaCl2/HCl/H2O is of decisive importance for the design of the extractive rectification technology. One must especially consider that the components do not behave in an ideal fashion within the fluid phase, and display electrolytic behavior. In order to reduce the thermal load of the column material in the best lowest fashion, extractive rectification procedures are generally performed by employing low vacuum status, thus enabling to base the design of the phase equilibrium as presented in illustration 4.
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<<< Illustr. 4: Phase Equilibrium for HCl/Water upon Addition of CaCl2 at 200 mbar(a) |
Illustration 4 displays an equilibrium concentration for HCl in steam phase versus HCl concentration in water for 200 mbar(a). The concentration of CaCl2 in fluid serves as parameter for the displayed diagram. In order to enhance HCl at the column head and to prevent eventual HCl loss via the column bottom, one must chose the CaCl2 concentration level in fluid in a magnitude that suppresses the azeotrope point, and that delivers a greatest most possible gradient rise of the equilibrium curve for low content of HCl. This requirement meets for a CaCl2 concentration of 50wt% for fluid.
It is of decisive importance that the inflow contains the least most possible water content for the extractive rectification process in order to enhance the effectivity of the rectification process. This is achieved in a combined technology process by pre-concentration of the absorbed acid. Independent of the absorption concentration, the hydrochloric acid is forwarded to the extractive rectification process on an azeotropic level. Thus, a lower amount of extraction agent is sufficient due to the higher HCl concentration level of the feed flow. In the course, the circulation for extraction agent may be designed smaller, compared to contemporary technologies for the recycling of HCl from smoke gases. Illustration 5 displays such a combined technology, which is comprised of preliminary concentration, removal of bromide, removal of HF, and extractive rectification processing with an extraction agent circulation.
Illustr. 5: Combined Technology for Concentration and Cleaning of Hydrochloric Acid.
Combination of Extractive Rectification and Dual-Pressure Technology
For certain extraction agents favorable process conditions may be achieved for extractive rectification by combining the recycling process of the extraction agent with the dual-pressure technology.
Ammonium chloride is an example of an extractive agent that bestly lends itself to such a technology. Illustration 6 displays the according process in a triangular diagram. A distillation borderline exists for the three-material mixture, beginning at ammonium chloride and forwarding to the maximum azeotrope of HCl-H2O, which presents a barrier for the rectification of the three-material mixture. The location of the distillation borderline is also contingent to pressure, just as the azeotrope point of the dual material mixture. Illustration 6 displays the distillation borderlines for 150 mbar, 1 bar, and 4.5 bars.
Illustr. 6: Borderlines for the Combination of Extractive Rectification and Dual-Pressure Technologies
Hydrochloric acid displaying almost azeotrope composition is deployed as feed flow F. This is mixed with a aqueous ammonium chloride solution S2 of approx. 25wt% NH4Cl. The mixture point M1 lies in the distillation field, which may allow separation of water as top product for pressures below approx. 2 bars. This separation is preferably performed in a vacuum column K1.
The bottom product S1 practically lies on the borderline for separation at chosen system pressure of, for example, 150 mbar. If the system pressure is increased to 4.5 bars, the point S1 then lies in the distillation field, where HCl may be separated as top product. This separation is performed in a pressure column K2. 35wt% HCl is recovered as head product D2, whereas the bottom product S2 is recycled into the process as extractive agent.
Due to the combination of extractive rectification and dual-pressure technology one achieves that the extractive agent amount is minimized, and to furthermore retain sufficient distance from the solubility borderline throughout the whole extractive agent circle. It is furthermore possible to heat the vacuum column K1 by using the vapour of pressure column K2 due to the deviant pressure level of the columns. The circulating amount of extractive agent is also significantly reduced, compared to pure dual-pressure technology, so that a very economically attractive technology unfolds, in total.
Illustr. 7: Combined Technology for Concentration of Hydrochloric Acid.
Illustration 7 displays the flow chart of such a combined technology for the concentration of hydrochloric acid. The deployed raw acid is pre-heated in the feed pre-heater W4 due to the concentrated hydrochloric acid; it is then mixed with the extractive agent, and then relaxed in the flash container B1. The resulting vapour is separated from the fluid in flash container B1, and feed into the middle of the vacuum column K1. The fluid from flash container B1 is also fed into the vacuum column K1 via an appropriate distributor. The mixture is rectified in vacuum column K1 so that basically water results in the column head and a mixture unfolds in the column bottom, whose composition almost matches to such found at a distillation borderline. This mixture is removed from the bottom of the vacuum column K1 via the help of pump P1, and fed into the pressure column K2.
The bottom mixture of the vacuum column is further rectified in the pressure column K2. A mixture now results as bottom product of pressure column K2 that matches to such found at the distillation borderline, and whose HCl concentration level is yet less than that of column K1 based on its system pressure. Thus, the bottom product of the pressure column K2 may therefore be recycled back to the column K1. For this, the bottom product of column K2 is relaxed in the flash container B1 together with the feed flow. As an extractive agent, the deployed ammonium chloride displays a high boiling point relative to the components of the feed acid; it then remains in the described fluid circulation. At the same time, employing ammonium chloride increases the relative volatility of HCl against water, so that the circulating volume of the described fluid circulation is minimized. In order to remove the contamination brought in by the feed flow, a partial flow is extracted from the bottom of the pressure column K2 and an according amount of extractive agent is re-fed.
Concentrated hydrochloric acid of the desired concentration is produced at the head of the column K2. Vapour of the column K2 is condensed in the heat exchanger W1, which also serves as bottom heater of vacuum column K1. Achieving this heat integration minimizes energy deployment and thus operational costs of the plant. Hydrochloric acid of over-azeotrope concentration, which has condensed in heat exchanger W1, is collected in the distillation vessel B2, whereby a partial flow is re-fed to the top of pressure column K2 via hydrochloric acid pump P2. The main amount of the condensed hydrochloric acid is removed from distillation vessel B2 via the help of hydrochloric acid pump P2, and cooled down in feed pre-heater W4 against the existing feed flow, so that a cooled over-azeotrope hydrochloric acid flows as product from the concentration plant. Bottom heater W3, heated via saturated steam performs the heating of pressure column K2.
The vapour of vacuum column K1 is condensed in condenser W2, and the condensate is collected in vessel B3. A partial condensate flow from vessel B3 is re-fed to the head of pressure column K1. Vacuum generation is performed via the help of fluid injection pump V1. Vapour condensate of vacuum column K1 is used as injection fluid, which is taken from container B3. Injection fluid is driven in the circulation via the help of circulation pump P3. The temperature of the fluid circulation is held constant via the heat exchanger W6. The under-cooled condensate now exits the deposit container B4, which is equipped with a floating switch, into an open overflow.
Generally, the above problem does not limit itself to sole absorption, extractive rectification, or cleaning of hydrochloric acid. Holistic solutions are frequently required, whereby e.g. an exhaust flow is simultaneously cleaned, and concentrated and cleaned hydrochloric acid is recycled. QVF possesses the know-how enabling to offer an appropriate combination of the presented technology steps, especially holistically and economical solutions.
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