For the synthesis of gaseous hydrogen chloride from the elements chlorine and hydrogen, the reaction mixture is combusted in a substantially hollow, cylindrical furnace, in the course of which considerable quantities of heat are produced. The hydrogen chloride gas produced is cooled and generally absorbed by water in an absorber that follows. The furnace used for performing the synthesis is usually made of graphite.

Several methods are known for recovering and utilising the heat liberated in the hydrogen chloride synthesis, for example the indirect cooling of the hydrogen chloride gas in heat exchange with a water circulation loop, as disclosed in German Patent No. DE-PS 857 343.

The temperature increase limited to approximately 55 °C substantially restricts the utility of the heat.

In another method as described in German Patent No. DE-PS 33 13 761, it is known to cool hydrogen chloride gas in a first heat exchanger to approximately 200 °C with water carried in a circulation loop, to extract some of the absorption heat in a second heat exchange, and to effect flash evaporation of at least some of the heated water in heat exchange with the first. In this process, steam is produced at a temperature of approxi- mately 138° C and a vapour pressure of approximately 350 kPa which can be used industri- ally for instance as process steam.

German patent No. DE-PS 38 07 264 discloses a method and apparatus for generating saturated steam by the combustion of chlorine and hydrogen to produce hydrogen chloride gas. The apparatus includes an elongated synthesis furnace having an upper segment, a lower segment and a middle segment having a wall in the form of a heat exchanger. A heat transfer medium is circulated between the heat exchanger and a steam generator while heating the heat transfer medium to between 170 ° and 230 °C by heat exchange between

the heat of the combustion and the heat transfer medium. Saturated steam is generated from water at a pressure of at least 700 kPa in the steam generator.

The object of the present invention is to increase the production capacity of HCI per unit.

Another object is to recover produced energy efficiently at a higher temperature and pressure level than possible earlier. It is also an object of the invention to find more durable materials of construction which will result in less maintenance costs and longer life.

These and other objects of the invention are obtained with the method and apparatus as disclosed below, and the invention is further defined and characterised in the accompany- ing patent claims.

The invention thus concerns a method for the synthesis of HCI with energy recovery, where hydrogen and chlorine is supplied to a cooled synthesis furnace with a burner section and the produced HCI gas is absorbed in water. The cooling water for the synthesis furnace is used for production of steam. The energy is recovered as steam at a pressure above 2.8 MPa. It is preferred that the HCI gas is further cooled in a gas cooler before the gas is absorbed in water. Preferably a gas cooler of the convective type is used where the HCI gas flows through the tube side of the gas cooler. It is also preferred that the cooling water for the gas cooler as well as the burner section is integrated in the steam generating system.

The invention also includes an apparatus for the synthesis of HCI with energy recovery comprising a burner section, a combustion chamber, an absorber and a steam generator, wherein a gas cooler is arranged between the combustion chamber and the absorber.

Preferably a gas cooler of the convective type is used where the HCI gas flows through the tube side of the gas cooler. The wall of the combustion chamber constitutes a heat exchanger made up of vertical tubes connected with bars preferably fixed off the centerline between the tubes. It is preferred to use construction materials of alloy with a high Ni content.

In existing technology the apparatus is essentially made of graphite or steel, and this leads to limitations concerning the quality of recovered energy, i. e. vapour temperature and

pressure. Furthermore, the method of producing the graphite elements sets a limitation with regard to the maximum size of the separate units of the furnace that can be manufactured.

According to the present invention, it is possible to build essentially larger furnaces with a heat recovery at a much higher pressure level and with improved efficiency with regard to energy recovery. This is obtained by replacing graphite as a construction material with a metal alloy rich in nickel, and at the same time cooling the produced hydrogen chloride gas to a lower temperature than used in existing technology.

The invention will be further described with reference to the Figures 1-2, where Figure 1 shows a HCl synthesis reactor with integrated heat recovery system.

Figure 2A shows a wall of the combustion chamber with center welded bars.

Figure 2B shows a wall of the combustion chamber with off-center arrangement of flat bars.

As shown in figure 1, in the synthesis process hydrogen and chlorine are fed to a burner section 1, situated at the lower part of the furnace 2, from a controlled pressure source and at controlled rates. Above the burner the gases are ignited and burn to produce hydrogen chloride gas. The hydrogen chloride gas at a flame temperature of above 2000 °C flows through the combustion chamber 3 to a gas cooler 4 and into an absorber 5, where it is absorbed in water or dilute acid 6 to form hydrochloric acid at the required strength. By installing a gas cooler 4 after the furnace, the heat load for the following absorber is reduced and the size of this unit can be reduced accordingly. The product acid 7 flows into a tank at atmospheric pressure (not shown). The residual unabsorbed gas 8 is fed to a scrubber counter-current to the absorption water (not shown). The gas cooler 4 used is preferably of a convective type, and the hydrogen chloride gas should flow through the tube side of the cooler.

The heat generated in the synthesis process is removed by circulating pressurized water and recovered as energy in steam at high temperature and pressure. Water is circulated between the heat exchanger forming the wall of the combustion chamber and a steam generator (steam drum) 9 where steam is produced. The cooling water for the gas cooler 4 is also integrated in the steam generating system, thereby giving increased energy recovery. In addition, heat is also recovered from the burner section 1 as an integrated part of the steam generation system.

The wall of the combustion chamber 3 forms a heat exchanger and is made up of vertical tubes 10, for circulation of water, that are interconnected by flat bar strips 11. A conven- tional arrangement is to have the flat bars center to center between the tubes. This is illus- trated in Figure 2A. Due to varying stress in the bar and tube, there will be axial strain in the middle of the bar and the inner part of the tubes, and stress at the outer side of the tubes.

An alternative construction is shown in Figure 2 B where the bars are welded off the centerline between the pipes. The avantage of the latter arrangement is higher mechanical strength of the wall due to lower heat stress at a given heat load. Thereby the temperature differences in bars and pipes are reduced, and the heat recovered by the cooling medium will increase. In addition, the manufacture is less complicated, since the tube wall needs welding on only one side, in contrast to the center arrangement where welding on both sides is required.

At a given set of design parameters the combustion chamber can withstand a 25 kPa (g) inner reactor pressure when flat bars are welded centric to the tubes. With the flat bars welded off-centre, the construction can withstand 50 kPa (g) inner pressure due to less thermal stress on the flat bars. By putting buck stays at two meters distance around the reactor, the explosion pressure can be 2 barg in the reactor without serious damages to the construction.

The above conditions have been simulated by FVM (Finite Volume Method) calculations using the FLUENT software (available from Fluent Inc., Lebanon, NH03766-1442, USA), and further supported by mechanical stress calculations for the two wall configurations as shown in Figure 2A and 2B respectively.

The construction material for the apparatus is important. In HCI gas (no condensation) it is possible to use carbon steel (boiler grade) up to a material temperature of about 150-160 °C with a life expectancy of the material of 2-3 years. For applications above 200 °C, specifi- cally above 250 °C (which is required for production of >2.8MPa steam), materials with a high Ni content are required. Possible material can be Incoloy 0 alloy 800/825, Inconel O alloy 600 and various Hastelloy O alloy qualities.

The Incoloy O alloys may be borderline with respect to economic feasibility (cost of installation/life expectancy) due to their relatively low Ni content of 30-40 %. The Inconel 0 alloy is considered more optimal, giving an extended life time due to a Ni content of 60 %. The Hastelloy O materials are considerably more costly than the Inconel @ alloy due to a high Mo content without giving an improved performance in HCI gas service.

In the Examples below typical steam production performance and corresponding HCI synthesis reactor capacity is given for existing technologies as well as the present invention.

Example 1 (not according to the invention) Ref. : German patent No.: 3 313 761 Steam production: 1.8 t/hr at 350 kPa HCI synthesis reactor capacity: 41.2 t/d (100% HCI basis) giving a specific steam produc- tion of 1.05 t/t HCI produced, available at a condensation temperature of 138 °C.

Example 2 (not according to the invention) Ref.: SGL Technik GmbH (Sigri) Steam production: 4.1 t/hr at 800kPa HCI synthesis capacity: 151.5 t/d (100 % HCI basis) giving a specific steam production of 0.65 t/t HCI produced, availabble at a condensation temperature of 170 °C.

Example3(accordingtotheinvention) Steam production: 11.4 t/hr at 3100 kPa HCI synthesis reactor capacity: 275 t/d (100 % HCI basis) giving a specific steam produc- tion of 1.0 t/t HCI produced, available at a condensation temperature of 235 °C.

HCI synthesis reactors accordingly to the present invention can be anticipated to have the same or better heat recovery performance for daily production rates of at least 500 tld of HCI (100 % basics)