AU1842899A - Reactor for hydrogen cyanide gas production - Google Patents

Description

-1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant/s: African Oxygen Limited Actual Inventor/s: Cornelius Johannes Welgemoed; Gideon Francois Van Staden Address for Service: BALDWIN SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 Invention Title: "REACTOR FOR HYDROGEN CYANIDE GAS PRODUCTION" The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 20960.00 -2- BACKGROUND TO THE INVENTION THIS invention relates to a reactor which can be used, for example, for the production of hydrogen cyanide gas.

The advantages of on-site hydrogen cyanide gas (HCN) production for use in 5 metals recovery processes are discussed in South African patent no. 95/0256.

Hydrogen cyanide gas is produced by reacting, endothermically, a saturated hydrocarbon, typically methane, and ammonia.

The most common production process is the Andrussow process in which air is added to methane and ammonia and this is reacted over a platinum based catalyst. The quantity of air added is such that the partial combustion of reactants provides sufficient energy for the heat of reaction required, as well as to preheat the reactants to the required operating temperature, typically 850° C.

-3- Another process for producing hydrogen cyanide gas is to use the Degussa process, which they call the BMA process. In this process, the energy required is supplied by a fired furnace, with the reactants contained in platinum lined refractory tubes within the furnace- Degussa have also patented an electrical version of their radiantly heated tube reactor, but this has not been commercially exploited.

Yet another process for producing hydrogen cyanide gas is the Shawinigan process. In this process, the energy required is provided by electrical energy via a fluidised bed of carbon particles. The absence of a catalyst in this process means that the reaction must be carried out at a very high temperature, at around 1450'C.

Although the Andrussow process is the simplest, and therefore the most common, it suffers from the disadvantage of producing a relatively low concentration hydrogen cyanide gas stream. Also, it only works effectively with methane as the hydrogen source. Although the other processes produce :hydrogen cyanide gas in higher concentrations, and also operate relatively efficiently with other saturated hydrocarbons as the carbon source for the reaction, they are relatively complex processes which are expensive to carry out.

It is an object of the invention to provide an alternative reactor to those used in the above processes.

I

SUMMARY OF THE INVENTION According to the invention, a reactor comprises: a reactor body defining at least one conduit into which reactants can be introduced, said at least one conduit having a ratio of length to diameter or width thereof which is less than 10:1; and induction heating means arranged to heat said at least one conduit to a temperature sufficient to cause a reaction between the reactants.

Preferably, the conduits each have a ratio of length to diameter which is less than 8:1.

The reactor body may comprise a solid block in which the conduits are formed, the block being formed from a material having an electrical resistance suitable for use as a susceptor of the induction heating means.

The block is preferably formed from graphite.

The block may be disc shaped or toroidal and the conduits are preferably arranged circumferentially about the block, with longitudinal axes of the conduits extending substantially parallel with the longitudinal axis of the block.

S

S. S

S

.5SS

S.

S.

Each conduit may have a sleeve or insert made of alumina.

Preferably, the reactor body defines an annular slot having a ratio of length to width which is less than 5:1. This annular slot may be formed by a reactor body which comprises a solid cylinder disposed concentrically within a hollow cylinder.

The solid cylinder and the hollow cylinder are preferably formed from graphite.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic sectional side view of a first embodiment of a reactor according to the invention; Figure 2 is a section on the line 2-2 in Figure 1; Figure 3 is an enlargement of a portion of the reactor body shown in Figure 2, indicating the spacing between adjacent conduits in the reactor body; Figure 4 is a schematic block diagram of a control system for the reactor; Figure 5 is a schematic representation of a first embodiment of a plant incorporating the reactor of the invention; -6- Figure 6 is a schematic sectional side view of a second embodiment of a reactor according to the invention; Figure 7 is a section on the line 6-6 in Figure 6; Figure 8 is an enlargement of a portion of the reactor body shown in Figure 7; and Figure 9 is a schematic representation of a second embodiment of a plant incorporating the reactor of the invention.

.9 *9 DESCRIPTION OF EMBODIMENTS The prototype reactor of the invention utilises electrical energy induced in the S. 10 reactor core to produce hydrogen cyanide gas. Use is thus made of indirect heat, generated by passing an electrical current through a relatively poorly conductive material, i.e. a material with sufficient electrical resistance, to generate sufficient heat to promote a reaction between the reactants to produce a concentrated stream of hydrogen cyanide. Because of the reactivity and toxicity of the cyanide ion, it is safer to entrain the hydrogen cyanide produced in a water or an alkaline stream. Typical alkaline streams employed are aqueous solutions of calcium oxide (CaO) or sodium hydroxide (NAOH) or the gas may be absorbed or used directly on site where it is required. The hydrogen cyanide can then be removed from the gas stream exiting the tube in the reactor by absorption into water or an aqueous alkaline solution. The hydrogen gas that is -7co-produced with the hydrogen cyanide remains and is available for use as a fuel or for other purposes.

A first embodiment of the reactor is illustrated in Figures 1 to 3 (not to scale).

The heart of the reactor 8 is a core 10 comprising a solid toroidal block of graphite, which functions as a susceptor in an induction heating arrangement.

The core has a central axis X-X as shown in Figure 1. Drilled through the core parallel with the axids X-X and disposed concentrically in two rows about the axis are conduits 12, each of which effectively serves as a reaction vessel.

Figure 3 indicates the spacing between the adjacent conduits and the inner and outer edges of the core .4.:.Graphite is used for the core/susceptor as it has ideal electrical resistance a OV properties (resistivity approximately 10 1 .ifl) and good thermal properties (thermal conductivity 100-150 W/mK) for the intended application. It also has adequate mechanical strength and is very machinable.

feel 15 In the prototype reactor, the conduits have a diameter of 5 0mm and a length of 0. 400mm, that is, a length to diameter ratio of 8:1. It is preferred that the length of the conduits should be no more than 10 timnes, and preferably no more than 8 tunes, their diameter. This enables a compact core to be used and enhances the efficiency of the reactor.

-8- Each of the conduits 12 is provided with a sleeve or insert 14 of alumina (Al0 3 which fits snugly within the conduit and which has an internal diameter of 40mm. The alumina inserts protect the graphite core from attack by NH3. A disadvantage of the use of the inserts is a degree of heat transfer loss across the inserts, resulting in somewhat higher core temperatures. For this reason, inter alia, it may in some cases be preferred to omit the sleeve or liner, with resultant cost savings and higher efficiency.

The conduits 12 are spaced apart evenly for even gas distribution through the core and for efficient take-up of the heat energy released in the core.

10 Above the core is an insulating disc 16 of alumina which is used to prevent excessive heat loss from the core in operation.

The reactor has a furnace chamber 18 above the core 10 which is refractory lined, having a lid 20, a side wall 22 and a base 24, all formed of alumina. A central inlet 26 is formed in the lid 20 and feeds reactants into the furnace 15 chamber 18. Below the core 10 is a lower reactor chamber 28 formed by a refractory support 30 on which the core rests. Below the support 30 is a cooled stainless steel conduit 32 which converges into a central outlet 34 at the bottom of the rector, for conveying the reacted gases out of the reactor. The cooled conduit counteracts the breakdown of hydrogen cyanide, and stainless steel is used as it is a poor susceptor for induction and resists H 2 embrittlement.

-9- Surrounding the reactor is an insulating layer 36 which is in turn encased within an outer refractory shell 38. Disposed circumferentially around the shell 38, at a height corresponding to the position of the core 10, is an induction coil which is connected to a power supply. Finally, an outer pressure-resistant shell 42 of non-metallic material such as glass fibre and resin surrounds the reactor shell, with an air gap 44 between the reactor shell and the pressure vessel. The outer shell has a mild steel lid 82. The air gap facilitates forced air cooling.

The outer shell 42 is provided, inter alia, as a safety measure against gas leaks.

As illustrated in Figure 2, the insulating layer 36 comprises a 40mm thick zone 10 of carbon black powder 46 surrounded by a 40mm thick layer of alumina wool 48. The space at the centre of the core 10 is also filled with carbon black powder Figure 4 shows the control system for the reactor. The reactor is shown surrounded by the induction coil 40, which in the prototype was a water cooled copper coil with ten turns. A power supply 52 operates at 1kHz and can apply a current of 400A to the coil 40 at an operating voltage of approximately 500V.

The operating frequency, voltage and current are optimised in conjunction with the geometry of the core 10 and the corresponding geometry of the coil 40 to generate the required heat within the core. The power supply 52 is controlled by a PLC controller 54, which monitors the power generated by the power supply and a temperature signal generated by a temperature measurement circuit 56. The power and temperature measurement signals, together with the output of a gas flow volume measurement circuit 58 are fed to a summamtion block 60 and thence to the PLC controller 54. The controller 54 also receives alarm signals from an absorption system alarm circuit 62 and a cooling system alarm circuit 64, and operates to shut down the reactor in the event of cooling system failure.

Figure 5 shows a plant incorporating the reactor of the invention. The inlet of the reactor 8 is connected to a propane storage tank 66 and an ammonia storage tank 68 via respective valves. An oil cooling circuit 70 including a pump 72 cools the lower portion 32 of the reactor and a pipe conveys the hydrogen cyanide gas stream from the outlet of the reactor to two absorbers 74 and 76, connected in series.

The primary absorber 74 consists of three plates with a plurality of 10Mmn slots :formed therein. A stream of sodium hydroxide solution is circulated through the absorber. The hydrogen cyauide gas stream enters at the bottom of the absorber and passes, in a counter current arrangement through the sodium hydroxide solution. On contact with the hydrogen cyanide gas stream, a sodium cyanide solution is produced by the reaction of the hydrogen cyanide contained in the gas stream and the sodium hydroxide. The absorber is placed above a circulation tank 78 to allow the solution to be circulated until a 33% by mass of cyanide solution has been produced, which is pumped to storage. Sufficient sodium hydroxide solution can be stored in the circulation -11 tank for up to one hundred hours of continuous operation of the plant before the hydroxide'is converted to cyanide and must be replaced with a fresh batch.

The secondary absorber 76 is of identical design and is provided to allow continuous operation with only one absorber on-line when the saturated batch is discharged and a fresh batch of sodium hydroxide is required.

A second, preferred embodiment of the reactor is illustrated in Figures 6 to. 8 wherein common parts have common reference numerals. In place of the conduits 12 of the first embodiment, the second embodiment has a single annular slot 84 parallel with the axis X-X of the core. This annular slot 84 acts 10 as the reaction vessel. The reactor body preferably comprises two solid components, one a solid cylinder 96 and the other a hollow cylinder 86 disposed concentrically about the solid cylinder 96. Both cylinders are formed from solid graphite.

In the prototype reactor, the solid inner cylinder 96 has a diameter of 300mm, 15 while the hollow outer cylinder 86 has a radial thickness of 100mm and an outer diameter of 700mm. Both cylinders have a length of 500mm. Thus the annular slot 84 has a radial width of 100mm and a length of 500mm, that is, a length to width ratio of 5:1. It is preferred that the length of the slot 84 should be no more than 5 times its radial width. This enables a compact core to be used and enhances the efficiency of the reactor. Furthermore, the dimensions of -12the cylinders 86 and 96 allow an even gas distribution through the slot 84 and an efficient take up of the heat energy released in the hollow cylinder 86.

The hollow cylinder 86 is isolated with a layer of carbon black powder 36, which, in the prototype, has an approximate width of In this embodiment, the reactants are fed into a central inlet 88 in the base of the reactor. The reactants pass from there into a gas distributor 90, which is a ollow cylinder with two rows of holes 92. Graphite chips 94 are packed at the .bottom of the reactor which, together with the holes 92, ensure an even gas flow into the annular slot 84. Above the gas distributor 90 is the solid graphite 10 cylinder 96.

Above the graphite cylinder 96 and the annular slot 84 is a conical graphite conduit 98 which converges into a central outlet 100 at the top of the reactor, *for conveying the reacted gases out of the reactor.

Figure 9 shows a plant incorporating the reactor of the second embodiment of S. 15 the invention. The inlet of the reactor 88 is connected to a propane storage tank 66 and an ammonia storage tank 68 via respective valves. A pipe conveys the hydrogen cyanide gas stream from the outlet 100 of the reactor to two absorbers 74 and 76, connected in series. These absorbers work in the same manner as the absorbers of the embodiment described above and illustrated in Figure 5. A difference between this plant and the plant illustrated in Figure 5 is that the gas -13is cooled by water jacketed exit pipes only, whereas an oil cooling circuit is used in the first embodiment together with water jacketed pipes.

A cycle of the prototype reactor of the second embodiment would typically start with a batch of Caustic Soda of 32% m/m strength, 26 nm'/h of ammonia, 7,55 nm'/h of propane and a power setting of 180kW on the induction unit. The graphite core would reach an operating temperature of 1600 0 C. After approximately 87 hours of continuois operation, the batch would reach near saturation, being sodium cyanide strength of 31,4 m/m NaCN. This would be transferred to the cyanide storage tank. The data for a typical run is as 10 follows: NaCN produced 4,11 tons (as 100%) Ammonia consumed 1,72 tons Propane consumed 1,29 tons Power consumed 15660kWh Carbon collected at bag filter: 50 kg An advantage of the present invention is that, although similar efficiencies may be achievable using the Shawinigan or Degussa reactors described above, these efficiencies are easily achievable with the present invention commercially on a small scale, namely 80 tons per month or less.

The geometry of the reactor 10 in both of the above embodiments ensures efficient utilisation of the heat generated in the core, and allows the reactor to -14be used without a platinum catalyst to generate hydrogen cyanide gas. The use of a core having conduits therein with a length to diameter ratio of 10:1 or less, or a core having a singular annular slot of length to width ratio of 5:1 or less, is an important factor in obtaining the desired degree of efficiency. The described induction heating arrangement, with the induction coil 40 being spaced apart from the graphite core itself, with insulating material between the coil and the core, also enhances the efficiency of the reactor. This enables the required heat, in the region of 1600 0 C, to be generated within a narrow zone in the central portion of the conduits or in the annular slot.

10 Although the reactor has been described in the context of the production of S" hydrogen cyanide gas, it will be appreciated that the reactor can be used for other reactions, especially endothermic gas phase reactions. For example, the reactor could be used in the destroying of chlorofluorocarbons at high :temperatures.

o *0

Claims (13)

1. A reactor comprising: a reactor body defining at least one conduit into which reactants can be introduced, said at least one conduit having a ratio of length to diameter or width thereof which is less than 10:1; and induction heating means arranged to heat said at least one conduit to a temperature sufficient to cause a reaction between the reactants.

2. A reactor according to claim I wherein the reactor body defines a plurality of conduits, each having a ratio of length to diameter which is 10 less than 10:1.

3. A reactor according to claim 2 wherein the ratio of length to diameter of the conduits is less than 8:1.

4. A reactor according to claim 2 or claim 3 wherein the reactor body comprises a solid block in which the conduits are formed, the block being formed from a material having an electrical resistance suitable for use as a susceptor of the induction heating means.

A reactor according to claim 4 wherein the block is formed from graphite. -16-

6. A reactor according to any one of claims 2 to 5 wherein the block is disc shaped or toroidal and the conduits are arranged circumferentially about the block, with longitudinal axes of the conduits extending substantially parallel with the longitudinal axis of the block.

7. A reactor according to any one of claims 2 to 6 wherein each conduit has a sleeve or insert.

8. A reactor according to claim 7 wherein the sleeve or insert is made of alumina.

9. A reactor according to claim 1 wherein the reactor body defines an annular 10 slot having a ratio of length to width which is less than 5:1. o*

10. A reactor according to claim 9 wherein the reactor body comprises a solid material having an electrical resistance suitable for use as a susceptor of the induction heating means-

11. A reactor according to claim 10 wherein the reactor body comprises a solid cylinder disposed concentrically within a hollow cylinder.

12. A reactor according to claim 11 wherein the solid cylinder and the hollow cylinder are formed from graphite. 17

13. A reactor substantially as herein described and illustrated with reference to Figures 1 to 3 or 6 to 8 of the accompanying drawings. DATED this 25th Day of February, 1999 AFRICAN~ OXYGEN LIMITED Attorney: PAUL G HARRISON Fellow Institute Of Patent Attorneys of Australia of BALDWIN SHELSTON WATERS 0 p. to.0 t. 0 .00. *0 00