US2394164A - Noncorrosive media for heating transfer - Google Patents

Description

Feb. 5, 1946. c; G. GERHOLD NONCORROSIVB EDIA FOR HEATING TRANSFER Filed Sept. 30, 1944 Patented Feb. 1946 NONCOBROSIVE MEDIA FOR HEATING TRANSFER Clarence G. Gerhold, Chicago, IIL, assigner to Universal i] Products Company, Chicago, Ill., a corporation oi' Delaware Application September 30, 1944, Serial No. 556,621

22 Claims.

This invention relates to improvements in the manufacture of fused salt mixtures and to improved methods and means for handling such mixtures to prevent corrosion of the equipment with which they are brought in contact.

Fused metal salts, particularly mixed metal halides, are well suited for use as heat conductive media because of their low melting points, low vapor pressures, extreme stabilities, high heat transfer coeflicients and high heat capacity per unit volume. A great deal of diiliculty has been experienced, however, in using such mixtures due to corrosion of the equipment with which they are in contact. The present invention is directed towards means of eliminating this corrosion.

The corrosion in such systems falls into the two general types, namely replacement corrosion and electrolytic corrosion.

Replacement corrosion occurs when fused metal salts'are employed having cations whose position in the electromotive series is lower than that of the metal of the walls of the vessel. In these cases a substitution occurs in which the metal from the walls displaces the cations in the salt mixture lower in the electromotive series 25 thereby effecting corrosion of the walls. For example, if one of the components of the salt mixture were cuprous chloride the salt would attack a steel vessel resulting in deposition of the copper and the substitution of iron ions in the salts in place of the copper ions. This replacement reaction may be set up as follows:

If the cations are higher in the electromotive series than the metal of the walls of the vessel, for example iron, their replacement reaction Fe+M+++ Fe++++M cannot proceed unless energy in the form of electromotive force is applied. The magnitude of the necessary EMF decreases with the proximity of the cations to iron in the electromotive series. This latter type of corrosion is usually spoken of as electrolytic corrosion and is the more viscous of the two types in respect to the deterioration of the walls of the chamber. It has 'been previously proposed to add iron salts to the salt mixtures to prevent corrosion of the vessel, the thought being that the presence of iron cations in the salt mixture would in accordance with the law of mass action suppress the tendency of the iron in the vessel walls to go into solution. However, I have discovered that the rate of corrosion of iron vessels which are contacted with fused salt mixtures is directly ture. Although the exact reason for this is not known it can probably be explained as follows: As previously stated, when using salts having metal cations higher in the electromotive series than the metals of the walls of a vessel as heat carrying media, it is necessary that some electromotive force be applied before any displacement of the metal cations of the salt mixture by the metal of the vessel walls occurs. It is also true that the potential necessary decreases as the respective positions of the metals in the electromotive series approach one another. Therefore, presence of cations which are similar to the metal .of the wall provides a setup in which any slight potential would be suiiicient to effect displacement and the attending corrosion. Further, with increases in the concentration of the iron cations, excessive amounts of corrosion will result as a result of the increase in current density. I have discovered that by maintaining the iron cation concentration in the fused salt mixture below about 300 to 400 parts per million and preferably below about 200 parts per million, the rate of corrosion is very low and the equipment can be used for long extended periods of operation without replacement. For example, in a casein which an iron vessel contains a bath of very pure sodium aluminum chloride mixture, that is a mixture in 0 which the cations in the bath are all considerably higher in the electromotive series than the walls of the vessel, the ow of current can only occur by corrosion of the iron and corresponding depositions of aluminum or sodium. Since this 5 requires a definite and fairly high potential no corrosion would be encountered in such a system until this potential were reached, Theoretically speaking, therefore, if a very pure salt were maintained no diilculties with corrosion would result. However, as a practical measure it is impossible in the ordinary operation to maintain the degree of purity necessary to avoid corrosion.

In equipment of commercial size ordinarily small amounts of iron are introduced into the salt mixture by scaling or perhaps by replacement corrosion which oecurs if small amounts of water are present in the system forming HCl. Since hydrogen can be replaced vby iron ions small amounts of iron chloride are formed in the salt mixture. As the amount of iron ions increases due to the accumulation in the salt mixture a point is reached where a very low electromotive force will cause substantial electrolytic corrosion.

In vessels of commercial size this driving potenrelated to the presence of iron cations in the mixtial may be set up due to temperature gradients ,ws-arisenor due to contacts with dissimilar metal. As a result of the presence of the iron cations in the salt mixtures these low potentials obtained by temperature differences in the vessel which would ordinarily be insufficient to cause excessive electrolytic corrosion may because of the low potential now necessary effect corrosion at a substantial rate and subsequently result in failure of the vessel.

I have discovered a method for effecting continuous utilization of heat carrying media by maintaining the purity of said media at a level such that no accumulation of iron cations occurs thereby substantially preventing the previously described electrolytic corrosion.

I have found that if the salt solutions are kept in contact with a metal which is higher in the electromotive series than any of the components of the vessel walls and which preferably corresponds to the cation in the salt mixture lowest in the electromotive series, the impurities in the salt solution are removed thereby preventing the accumulation of these impurities in the salt solution and eliminating or substantially reducing any electrolytic corrosion. The metal introduced into the salt solution may be in a finely divided state such as metal powder or when desired may be in the form of a plate electrode.. In the latter case it is preferable that the electrode is electrically insulated from the walls of the containing vessel to prevent the metal from going into solutions and plating on the walls of said vessel.

Salt mixtures which may be employed in the process of the present invention comprise such metal salt mixtures as potassiumA zinc chloride, lithium aluminum chloride, sodium aluminum chloride, mixtures of sodium aluminum chloride and potassium aluminum chloride, sodium nitrate and sodium nitrite and similar materials. The preferred salt mixtures are those comprising mixtures of the halides of the alkali metals and aluminum halides and particularly the chlorides such as sodium chloride and aluminum chloride.

In selecting the metals to be employed to replace the impurities accumulating in the salt mix- 45 ture it is important that the metal chosen will not upon replacement change the composition of the salt mixture appreciably in order for the characteristics of the salt mixture, such as heat capacity, etc., to remain substantially constant 5 throughout the operation. Further, it should be noted that in some instances alloys of metals may be used instead of a single metal to effect the removal of impurities.

In a cyclic system in which the fused metal 55 salts are passed through a heating means and then inA indirect heat exchange with materials undergoing reaction such as hydrocarbon conversion reactions, for example, dehydrogenation,

catalytic cracking, catalytic reforming, catalytic 00 aromatization, etc., the process of the present invention may be used to great advantage. The

I advantages will be more clearly set forth hereinafter in the specification and description of a catalytic dehydrogenation process to produce butadiene utilizing a fused salt mixture as a heat carrying media.

In one embodiment'my invention comprises a' system for effecting heat transfer employing a fused salt mixture in indirect heat exchange with the materials to be heated, said mixture being characterizedin that the cations areof metals which are higher in the electromotive series than the metal of the walls of the vessel, which comprises passing said heat carrying medium through a heating zone and subsequently through a zone in which a portion of the heat contained in said medium is removed by indirect heat exchange and.

stituents of said metal salt mixture and preferably with a metal corresponding to cations of the metal lowest in the electromotive series.

The following is a description of a diagrammatic sketch of a process for the conversion of normal butane to butadiene employing a heat carrying medium consisting of a mixture of sodium and aluminum chlorides. Although the description is limited to a butadiene process, it is not intended that the general broad scope of the invention be so limited since the invention is applicable to various processes for the conversion of organic compounds, particularly hydrocarbons employing some means of transferring heat from a furnace or other heating means to a reaction zone.

For simplification, the diagrammatic sketch includes only elements necessary for the explanation of the invention. In the present description, a two-reactor system is employed, the catalyst in one of the reactors being utilized for the conversion operation, while the catalyst ,in the other reactor is simultaneously undergoing regeneration by the oxidation of the carbonaceous deposits laid down upon the catalyst during aprevious conversion operation. By alternately contacting the catalyst with the reactants and regenerating medium, a substantially constant over-all operation is obtained.

Referring to the drawing, normal butane is introduced in the system through line I containing valve 2, commingled with recycled normal butane and butylenes obtained as hereinafter set 40 forth, and introduced into pump 3 which discharges through line 4 containing valve 5 into heating coil 6 disposed within furnace 1 wherein the combined feed is raised to a temperature sufficient to compensate for heat losses by radiation, convection, etc. and still maintain the desired temperature in catalyst tube I2. The heated combined feed leaves furnace 1 through line 8 and is directed to line 9 containing valve l0 through line II into catalyst tube I2 wherein it is contacted with a catalyst capable of eiecting dehydrogenation of the normal butane-butylene'mixture to a substantial yield of butadiene.

Any suitable catalyst capable of accelerating dehydrogenation may be employed in reactor tube I2 such as, for example, the oxides of the lefthand columns of Groups V and VI of the Periodic Table supported on refractory materials such as silica, alumina, particularly an alumina having the characteristics of Activated Alumina of commerce, magnesia, titania, etc. The preferred catalyst is one comprising chromium oxide supported on Activated Alumina of commerce. 'Ihe temperatures employed in catalyst tube I2 may be within the range of about 950 to 1200 F. and will ordinarily be in the range of about 1000" to 1150 for the conversion of a butane-butylene mixture to butadiene. However, due to the endothermicity of the reaction, it is necessary to provide some means of maintaining a substantially constant temperature in the reaction tube. If this is not done, a considerable temperature differential will result between the inlet temperature and outlet temperature, thereby affecting the conversion and eilils so operated that it ciency of the operation. 'I'he temperature is maintained in this system by indirectheat exchange with a solution of fused salts passed through jacket I3 in contact with the outer surface of tube I2. The molten salt is passed through line 2'I containing valve 28 from salt tank 26 and is pumped by pump 29 through line 30 containing valve 3I through switch valve 32 which can direct the flow into the annular space between tube I2 and jacket I3 or tube 2| and jacket 22 through lines 33 or G9, respectively.

In the present description, since the catalyst in tube I2 is being employed for the endcthermic dehydrogenation reaction While the catalyst in tube 2l is being regenerated, the flow of heated salt is directed through switch valve 32 through line 33 into annular space between tube I2 and jacket I3. The salt passes upwardly through the annular space in contact with the outer surface of reactor tube I2 and leaves the heating zone through line 49, through switch valve 50 which directs the iiow through line 5I through heating coil 52 disposed within furnace 53 wherein the salt is raised to the desired temperature necessary to compensate f r all heat losses in the system and still maintain the desired temperature in the heating jacket I3. The salt leaves furnace 53 through line 54, valve 55 and is directed into salt tank 26 wherein it is contacted with bar aluminum which reduces any iron ions present in the system because of the presence of small amounts of impurities such as HC1, and said iron is deposited as a sludge in the bottom of salt tank 26, The sludge is periodically removed from tank 28 through line 58 containing valve 5T. Instead of using bars of aluminum in the salt tank, finely divided aliuminum may be introduced into the circulatory .system and thesludge accumulated in a trap of some sort and removed from the system by this method.

As previously stated in the butane dehydrogenation process, at least two reactors are employed with the ow of hydrocarbon and regenerating gas being alternated between the reactors to give a more continuous operation. In this description, it has beenassumed that the catalyst in reactor 2I is undergoing regeneration. Since this reaction is highly exothermic, it is necessary to provide some means for removing the complished by stream of fused salt in indirect heat exchange with the catalyst reactor during the regeneration operation to remove the excess heat. The system employed is substantially the same as that described previously for the addition of heat to reactor I2 with the exception that furnace 53 is replaced by cooler 82. The flow of the system is as follows: fused salt is pumped from tank 34 through suction line 35 containing valve 36 into pump 31 .which discharges through line 38, valve 39 and switch valve 32 which directs the iiow through the annular space between jacket 22 and reaction tube 2|. The cool salt passes upwardly through said annular space, thereby absorbing excess heat during its passage and is removed through line l0 into switch valve 50 which directs the flow through line $0, valve BI, into cooler 62 wherein its temperature is reduced sufciently to facilitate its use as a cooling medium. The cooled salt still in liquid form is passed -covery system wherein through line 83 containing valve 34 into tank 34 wherein it is contacted with bar aluminum to reduce the iron cations present, thereby forming a sludge and maintaining a salt in a substantially pure state. The sludge is removed through line 58 containing valve 59.

The regenerating gas usually comprising a combustion gas containing regulated amounts of oxygen is introduced into the system through line `II and may be directed through either lines 46 or 4l containing valves 45 and 48, respectively, depending upon which reactor tube is being regenerated. As previously stated, while the catalyst in reactor tube 2I is in the process of regeneration the ilow 1s through line 4I containing valve '48 into line 20 through the catalyst bed in reactor 2l and the carbonaceous material from the catalyst in tube 2I removed by the passage of the regenerating gas through the catalyst bed. Ordinarily, 1t is necessary that the regenerating gas be heated to a temperature of about 600 to 800 F. to initiate combustion. However, to simplify the explanation of the drawing, no heating means is shown. The regenerating gases are removed from tube 2I through line -23 and directed through line 43 containing valve 44 into line 42 and may be vented to the atmosphere or may be recycled through the reactor after introduction of additional oxygen-containing gases and the excess vented to maintain a constant pressure throughout the regeneration cycle.

The products of the dehydrogenation reaction are removed through line I`I into a suitable rethe butadiene is separated from the remaining hydrocarbons by any of the well known means such as azeotropic distillation, formation of chemical compounds, etc. and the remaining material comprising normal butane and butylenes introduced through line 65 and valve S8 into line I wherein it is commingled with the fresh butane charge to form the com` bined feed to the operation.

In the above description of the drawing, the catalyst reactors have been represented as single tubes disposed in the heating jacket. However, `for commercial operation, a number of tubes in two or more banks connected by a common inlet manifold are disposed withina common heating or cooling jacket with the conversion or regeneration reaction being conducted simultaneously in all the tubes ineither bank.

The following example presents a comparison of the data obtained in a circulatory salt system in which a fused salt of sodium and aluminum chloride was employed as a heat carrying medium. These data were obtained by circulating a salt mixture through stainless steel equipment for a period of 132 hours.

Rate of penetration No alumi` Aluminum p'gegt present Temp.. oF.

ture of 1100-1150 F. tor a period of 49 days. The following data were obtained:

Rate of Mean penetration Weight diameter Days on test Inches Inches/year one of said cations.

2. In a heat transfer process wherein a molten metal salt mixture is passed through a confined least one metal corresponding to one of said cations.

than the metals of the Walls of said confined ture with a metal corresponding to the cation constituent of said mixture which is lowest in the electromotive series.

4. The method of claim 2 further characterized 'in that said molten metal salt mixture comprises a molten mixture of metal halides.

5. The method of claim 2 further characterized in that said molten metal salt mixture comprises a molten mixture of metal chlorides.

7. In a heat transfer process wherein a molten sodium aluminum chloride mixture is passed through a confined zone in indirect heat exchange .relationship with another fluid, the walls with aluminum.

8. In a heat transfer process wherein a molten sodium aluminum chloride mixture is passed through a confined zone in indirect heat exlower in electromotive series than aluminum, the method which comprises passing said molten metal salt mixture together with finely divided aluminum through a heating zone and subsequently through a Zone in which a portion of the heat contained in said molten salt mixture is removed by indirect heat exchange.

9. In a heat transfer process wherein a molten metal salt mixture is passed through a confined zone having walls composed predominantly of passing said heat carrying medium through a heating zone and subsequently through a zone in which a portion of the heat contained in said medium is removed by indirect heat exchange and maintaining the iron cation concentration in said molten salt mixture below about 400 parts per million by contacting said molten salt mixture with a metal corresponding to one of said cations.

10. The process of claim 8 further characterized in that said molten metal salt mixture comprises a mixture of metal chlorides.

11. In a heat transfer process wherein a molten sodium aluminum chloride mixture is passed Athrough a confined zone having walls composed 12. In a heat transfer process wherein a molten sodium aluminum chloride mixture is passed through' a zone in which a, portion of the heat contained in said mixture is removed by indirect heat exchange and maintaining the iron cation concentration in said molten salt mixture below about 400 parts per million by passing finely divided aluminum through said zones in admixture with the molten salt mixture.

13. A method for preventing corrosion of the walls of a retaining zone, said walls being composed predominantly of iron and contacted with a molten metal salt mixture characterized in that the cations of the salts thereof are of metals higher in the electromotive series than iron. which comprises maintaining the iron cation concentration in said molten salt mixture below about 400 parts by weight per million by contacting said mixture with a metal corresponding to one o! said cations.

14. The method for preventing corrosion of claim 13 further characterized in that said molen metal salt mixture comprises a mixture of metal halides.

15. The method for preventing corrosion o! claim 13 further characterized in that said molten metal salt mixture comprises a mixture of aluminum chloride and at least one alkali metal chloride.

16. The method for preventing corrosion of claim 13 further characterized in that said molten metal salt mixture comprises sodium aluminum chloride.

17. In a heat transfer process wherein a molten mixture of metal salts is employed as an indirect heat exchange medium, the method which comprises maintaining a source of supply of said molten mixture of metal salts, cyclically circulating molten salt mixture between said source of supply and an indirect heat exchange zone defined by walls of a metal lower in the electromotive series than the metals of said salts, whereby cations'of the metal of said walls tend to accumulate in the circulating molten mixture, contacting the circulating salt mixture with a metal higher in the electromotive series than the metal of said walls to reduce said cations, and separating the thus reduced cations from the circulating molten salt mixture.

18. The method as defined in claim 17 further characterized in that said metal higher in the electromotive series than the metal of said walls corresponds to the metal of one of said salts.

19. The method as defined in claim 17 further characterized in that said metal higher in the electromotive series than the metal of said walls corresponds to the metal of said salts which is lowest in the electromotive series.

20. A heat transfer method which comprises cyclically circulating molten sodium aluminum chloride through and between a heating zone and an indirect heat exchange zone defined by walls composed predominantly of iron, whereby iron cations tend to accumulate in the circulating molten chloride, contacting the circulating molten chloride with metallic aluminum to reduce said cations and thereby deposit iron from the molten chloride, and separating the deposited iron from the circulating molten chloride.

21. A heat transfer method which comprises cyclically circulating molten sodium aluminum chloride through and between a heating zone and an indirect heat exchange zone defined by walls composed predominantly of iron, whereby iron cations tend to accumulate in the circulating molten chloride, contacting the circulating molten chloride with metallic aluminum in a separating zone between the heating zone and the heat ex change zone te reduce said cations and thereby deposit iron in the separating zone, and withdrawing the deposited iron from the separating zone.

22. I'he method as defined in claim 20 further characterized in that said metallic aluminum is circulated in finely divided form through said zones in admixture with the molten sodium aluminum chloride. l CLARENCE G. GERHOLD.

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