Method for catalytic alkane degradation

Abstract

Method for converting alkanes dissolved in the heat transfer fluid in the heat transfer fluid circuit of a heat pump into non-flammable, water-soluble compounds, wherein at least one corrosion inhibitor is added to the heat transfer fluid, at least one water-soluble oxygen carrier is added to the heat transfer fluid, and at least one catalyst in the heat transfer fluid circuit oxidizes the dissolved alkane into a liquid oxidation product.

Description

[0001] The invention relates to a method for removing flammable gases containing alkanes from a heating circuit or brine cooling circuit of a heat pump system. It is known that heating circuits occasionally need to be vented because air can accumulate in the system. This usually occurs due to leaks at elevated points in the heating circuit, where a leak combined with negative pressure leads to air being drawn into the water circuit. In some cases, the air is dissolved in the top-up water and is released when it is heated. The same applies to brine-to-water split systems where brine is present in the heating circuit. US 2018 / 347860 A1 discloses a method for providing a heat pump with a heat transfer fluid circuit containing a heat transfer fluid. Furthermore, the heat pump contains alkanes as additives.

[0002] On the other hand, flammable refrigerants are now used in heat pumps and in refrigeration and freezing systems. These have the advantage of not harming the climate or the ozone layer if accidentally released. However, such accidental releases should be avoided whenever possible due to their flammability. In refrigeration circuits using such working fluids, these unintentional releases can occur via the heat exchangers, which act as condensers and evaporators and are connected to the heating or cooling brine circuit via their exchange surfaces. Unlike conventional gas boilers, the working fluid in the refrigeration circuit is under a higher pressure than the heat transfer fluid in the heating or cooling brine circuit; therefore, in the event of a leak, it could easily enter the lower-pressure heat transfer circuit.

[0003] Such leaks can also cause problems in heat pumps used as air conditioning systems in vehicles. US 2003 / 0056529A1 describes a vehicle air conditioning system that operates with carbon dioxide or propane. In this system, outside air is passed over a heat exchanger and mixed with the air from the passenger compartment. If the heat exchanger is corroded, there is a risk that the working fluid will enter the passenger compartment, which must be prevented or at least mitigated.

[0004] To prevent all of this as much as possible, conventional technology uses expensive double-walled heat exchangers. Besides the high cost, this leads to efficiency losses because the materials, such as stainless steel, are poor conductors of heat, and the thin air gap between the heat exchanger surfaces acts like insulation. In practical terms, this means that higher temperature differences must be set in the heat exchangers, which reduces the efficiency of heat pumps.

[0005] Gaseous components that enter the heating circuit through leaks can be removed using gas separators, but dissolved and very fine-bubbled gas components remain. These can cause problems during temperature changes by outgassing at unfavorable points in the water or brine circuit when their solubility changes due to temperature fluctuations. This applies particularly to the now commonly used propane in refrigerant R290, but also to isobutane in refrigerant R600a, n-butane in R600, propylene in R1270, and other alkanes.

[0006] Dissolved or very fine-bubbled gas components of alkanes must be eliminated from the heating circuit. This elimination occurs through the oxidation of the dissolved alkane, requiring an oxygen carrier or oxidizing agent and a suitable catalyst. However, common oxygen carriers, such as hydrogen peroxide, are themselves flammable and also aggressive to the typical components of a water or brine circuit. This also applies to almost all other common oxidizing agents; these also oxidize the pipe material of heating circuits and lead to rusting.

[0007] To prevent rusting, corrosion inhibitors are occasionally added to heating circuits, for example, a sodium molybdate solution available under the trade name Fernox. The use of triazine carboxylic acids is also possible, as described in EP 46139 B1. Such corrosion inhibitors form a passive layer on the pipe material and fittings, which prevents further rusting, even though the corrosion inhibitors themselves are partially oxygen carriers. US 3,133,028 A teaches the use of water-soluble chromates as corrosion inhibitors and their addition to the heat transfer fluid of a heat transfer fluid circuit.

[0008] In heating systems, which typically consist of water, it is sufficient to protect the pipes and fittings against oxidation. However, in brine-based heat pump systems that extract heat from the outside or are used as air conditioners in summer, the brine components themselves must also be protected from oxidation. Neither the oxygen carrier used may directly attack these brine components, nor may the added catalyst cause the brine to oxidize. Antifreeze agents such as ethylene glycol or propylene glycol are most commonly used as brine, especially the latter because it is non-toxic.

[0009] US 2010 / 0181524 A1 proposes the use of radical scavengers to stabilize working fluids. These scavengers include acids, oxygen radical scavengers, polymerization inhibitors, or combinations thereof. However, the aim is to stabilize working fluids composed of hydrofluoroolefins and / or hydrochlorofluoroolefins. The proposed stabilizers or radical scavengers include 1,2-epoxybutane, glycidyl methyl ether, d,1-limonene oxide, 1,2-epoxy-2,2-methylpropane, nitromethane, methyl styrene, isoprene, phenol, and hydroquinone. Hydrazine is suggested as a corrosion inhibitor. However, these working fluids are so different from the brines used in heating or cooling circuits that their use or transferability is not possible.

[0010] Oxalic acid, acetic acid, lactic acid, and formic acid are known oxidation products of propylene glycol, particularly when the system is connected to a solar thermal system that is occasionally exposed to high temperatures. This regularly occurs in midsummer when the absorbed heat is not dissipated, the system is temporarily out of operation, and the brine heats up considerably. These reaction products are highly corrosive; therefore, the use of an oxygen carrier in the system is generally avoided. However, antifreeze cannot be omitted due to the risk of damage from freezing in winter; often, the antifreeze concentration in the brine exceeds 50%.

[0011] This presents a dilemma. On the one hand, dissolved alkanes should be oxidized, but on the other hand, this oxidation should not lead to alkanoic acids, but only to alkanols. The brine should also not decompose into alkanoic acids. Surprisingly, it was found that some of the known corrosion inhibitors also act catalytically with regard to the oxidation of dissolved alkanes, simultaneously serving as oxygen carriers, and do not attack the brine, but rather protect it.

[0012] The invention therefore aims to achieve a heterogeneous catalytic reaction between a corrosion inhibitor serving as an oxygen carrier and a dissolved alkane in the heating or brine circuit of a heat pump. The oxidation reaction is to be carried out until the respective alkanol is formed, since the alkenes initially formed in the heating or brine circuit are just as detrimental as the starting alkane. This means that sufficient oxygen must be provided for the desired reaction, which affects the amount of oxygen required.

[0013] While the amount of corrosion inhibitor to be added is, as usual, determined by the inner surface area of ​​the pipe system, for the reaction with an alkane, at least two oxygen atoms must be provided for each alkane molecule. Further oxidation to carbon dioxide is harmless, however. The oxygen carrier can therefore be added generously if corrosion of the pipes and fittings can be reliably prevented.

[0014] To determine the minimum quantity, it must be established whether an alkane has entered the heating or brine circuit due to a leak. However, this is difficult if an oxygen carrier and a catalyst are already present in the heating or brine circuit and would immediately react any alkane entering due to a leak. A leak can also be detected in the gas phase and, if this gas phase is separated in a gas separator, can be identified in the gas phase.

[0015] From this alkane concentration in the separated gas phase, the alkane concentration in the heating or brine circuit can be determined using known solubility data. Provided the leakage is small and remains small, the oxygen carrier can be added accordingly. Ideally, this happens automatically, but manual addition is also possible.

[0016] Provided that the oxygen carrier and the catalyst are identical to the corrosion inhibitor, the corrosion inhibitor can be generously applied and will then already be present in the heating or brine circuit before a leak occurs.

[0017] The object of the invention is therefore to convert working fluid components or refrigerant components containing alkanes, which are dissolved or very fine-bubbled in the heat transfer fluid, into a harmless form.

[0018] The invention solves the problem by a corresponding method according to claim 1. This is a method for converting alkanes dissolved in an aqueous heat transfer fluid into non-flammable, water-soluble compounds, wherein at least one corrosion inhibitor is added to the heat transfer fluid, at least one water-soluble oxygen carrier is added to the heat transfer fluid, and at least one catalyst in the heat transfer fluid circuit oxidizes the dissolved alkane into a liquid oxidation product.

[0019] In one embodiment of the process, a gas phase is separated from the heat transfer fluid and the alkane concentration within it is determined. If a concentration is measured that indicates a leak, the required amount of an oxygen carrier is calculated based on the alkane concentration and this amount is added to the heat transfer fluid circuit.

[0020] In a further embodiment of the process, it is provided that the corrosion inhibitor, the oxygen carrier and the catalyst are the same substance, preferably sodium molybdate or lithium chromate or mixtures thereof.

[0021] It is not necessary for the oxidation to proceed completely to carbon dioxide, as all partial oxidation products such as alkanols, alkanals and alkanones are acceptable for achieving the desired conversion goal, but not alkanoic acids.

[0022] The invention is illustrated below by an example. Here, the alkane is propane, which forms the main component of the refrigerant R290. Sodium molybdate is used as an oxygen carrier, catalyst, and corrosion inhibitor. First, the entire heating and brine circuit is coated with a layer on the inside, which is achieved by dosing sodium molybdate, for example in the form of Fernox solution, into the circuit.

[0023] The gas separator is then checked for the presence of flammable gas in the separated gas; this can be done using a standard gas detector. As soon as the gas detector is triggered, the concentration and quantity of propane are determined. Based on the solubility of propane in the brine at the given temperature, the likely amount of propane dissolved in the brine is estimated, and a corresponding amount of sodium molybdate is added.

[0024] While the initial amount of sodium molybdate depends on the size of the heating and brine circuit, the amount to be added is determined by the stoichiometric requirement. Lithium chromate can be used instead of sodium molybdate in the same way, and parallel use is also possible.

[0025] This method is particularly suitable when the usual double-walled heat exchangers, which act as evaporators and condensers in contact with the working fluid, can be replaced by simpler designs that simultaneously allow for higher overall system efficiency, as smaller temperature differences are required for heat transfer. Because such leaks, as long as they are small, do not cause any problems and no longer pose a safety risk, it is possible to continue operating the heat pump system and the heat transfer fluid circuit for a long time after a leak has been detected, and necessary repairs can be postponed until the next scheduled maintenance.

Claims

1. Method for converting alkanes dissolved in an aqueous heat transfer fluid in the heat transfer fluid circuit of a heat pump into non-flammable, water-soluble compounds, characterized in that - at least one corrosion inhibitor is added to the heat transfer fluid, - at least one water-soluble oxidizer is also added to the heat transfer fluid, and - in the heat transfer fluid circuit, at least one catalyst oxidizes the dissolved alkane into a liquid oxidation product.

2. Method according to claim 1, characterized in that a gas phase is separated from the heat transfer fluid and the alkane concentration is determined therein.

3. Method according to claim 2, characterized in that the required quantity of an oxidizer is determined on the basis of the alkane concentration and this quantity is metered into the heat transfer fluid circuit.

4. Method according to any one of claims 1 to 3, characterized in that the corrosion inhibitor, the oxidizer and the catalyst are the same substance.

5. Method according to any one of claims 1 to 4, characterized in that sodium molybdate is added as a corrosion inhibitor, oxidizer and catalyst.

6. Method according to any one of claims 1 to 4, characterized in that lithium chromate is added as a corrosion inhibitor, oxidizer and catalyst.