EP3936790A1 - Catalytic alkane degradation - Google Patents

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 also added to the heat transfer fluid, and at least one catalyst in the heat transfer fluid circuit converts the dissolved alkane into a liquid oxidation product oxidized.

Description

The invention relates to the removal of flammable gases containing alkane in a heating circuit or cooling brine circuit of a heat pump system. On the one hand, it is known that heating circuits occasionally have to be vented because air can collect in the system. This usually happens due to leaks at elevated points in the heating circuit, where a leak in connection with negative pressure leads to air being sucked into the water circuit. In some cases, it is also air that is dissolved in top-up water and is released when it is heated. The same applies to brine-split systems in which there is brine in the heating circuit.

On the other hand, flammable refrigerants are now used in heat pumps and in refrigeration and freezing systems, which have the advantage of not damaging the climate or the ozone layer if accidentally released. However, due to their flammability, such an accidental release should be avoided as far as possible. In refrigeration circuits in which such working fluids are used, such unintentional releases can happen via the heat exchangers, which are used as condensers and evaporators and which are connected to the heating circuit or cooling brine circuit via their exchange surfaces. In contrast to conventional gas combustion boilers, the working fluid in the refrigeration circuit is under a higher pressure than the heat transfer fluid in the heating circuit or cooling brine circuit, so it could easily get into the heat transfer circuit, which is under lower pressure, in the event of leaks.

Such leaks can also lead to problems in heat pumps, which are used for air conditioning in vehicles. the U.S. 2003/0056 529 A1 describes a vehicle air conditioning system that runs on carbon dioxide or propane. Outside air is routed past 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 leak into the passenger compartment, which must be prevented or at least mitigated.

In order to prevent all this as much as possible, expensive double-walled heat exchangers are used in the conventional art. In addition to the high price, this use leads to a loss of efficiency, since the materials, such as stainless steel, conduct heat poorly and the thin air gap between the heat exchanger surfaces acts as insulation. In practice, this means that higher temperature differences have to be set in the heat exchangers, which reduces the efficiency of heat pumps.

Such gaseous components that get into the heating circuit via leaks can be separated using gas separators, but dissolved and very fine-bubble gas components remain. These can cause problems when the temperature changes by outgassing at unfavorable points in the water or brine circuit if their solubility changes due to temperature. This applies above all to the propane in the refrigerant R290, which is now frequently used, but also isobutane in the refrigerant R600a, n-butane in R600, propylene in R1270 and other alkanes.

Such dissolved or very fine-bubble gas components from alkanes should be eliminated from the heating circuit. This elimination takes place through oxidation of the dissolved alkane, for which an oxygen carrier or oxidizing agent and a suitable catalyst must be provided. However, common oxygen carriers, such as hydrogen peroxide, are themselves combustible and also aggressive to the usual installations of a water or brine circuit. This also applies to almost all other common oxidizing agents, these also oxidize the pipe material for heating circuits and lead to rusting.

To prevent rusting, corrosion inhibitors are sometimes added to heating circuits, for example with a solution of sodium molybdate available under the Fernox trade name. The use of triazine carboxylic acids is possible, as in the EP 46139 B1 is described. Such corrosion inhibitors form a passive layer on the pipe material and the fittings, which prevents further rusting, although the corrosion inhibitors are sometimes themselves oxygen carriers.

In the case of heating circuits, which usually consist of water, it is sufficient to protect the pipes and fittings against oxidation. However, in the brine circuits of heat pumps that gain heat from outside or that are operated as air conditioning systems in summer, the brine components must also be protected from oxidation. The oxygen carrier used must not attack these brine components directly, nor must the added catalyst lead to oxidation of the brine. Anti-freeze agents such as ethylene glycol or propylene glycol are mostly used as brines, especially the latter because it is non-toxic.

the U.S. 2010/0181524 A1 proposes radical scavengers for the stabilization of working fluids, which are, for example, acids, oxygen radical scavengers, polymerization inhibitors or combinations thereof. However, this is intended to stabilize working fluids made from hydrofluoroolefins and/or hydrochlorofluoroolefins. 1,2-epoxybutane, glycidyl methyl ether, d,1-limonene oxide, 1,2-epoxy-2,2-methylpropane, nitromethane, methylstyrene, isoprene, phenol hydroquinone are suggested as stabilizers or radical scavengers, and hydrazine is suggested as a corrosion inhibitor. However, these working materials are so different from the brines of a heating or cooling circuit that they cannot be used or transferred.

Oxalic acid, acetic acid, lactic acid and formic acid have become known as the oxidation products of propylene glycol, for example when the circuit is connected to a solar system with flow through which is occasionally exposed to high temperatures. This occurs regularly in midsummer when the heat absorbed is not taken off and the circuit is temporarily not in operation and the brine heats up significantly. These reaction products are very corrosive, so it is usually avoided to provide an oxygen carrier in the circuit. However, the antifreeze cannot be dispensed with because of the risk of damage due to frost in winter, the proportion of antifreeze in the brine is often even more than 50%.

This creates 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 have a catalytic effect with regard to the oxidation of dissolved alkanes, and at the same time serve as oxygen carriers, but do not attack the brines but protect them.

The invention therefore strives for 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 should be carried out up to the respective alkanol, since the alkenes that form first in the heating or brine circuit are just as unfavorable as the starting alkane. This means that enough oxygen must be provided for the desired reaction, which affects the amount of oxygen to be provided.

While the amount of corrosion inhibitors to be added is, as usual, dimensioned according to the inner surface of the pipe system, the reaction with an alkane, at least two oxygen atoms are additionally provided for each alkane molecule. However, further oxidation to carbon dioxide is harmless. The oxygen carrier can therefore be added generously if the corrosion of the pipes and fittings can be reliably prevented.

To determine the minimum quantity, it must be determined whether an alkane has entered the heating or brine circuit due to a leak. However, this is difficult if there is already an oxygen carrier and a catalyst in the heating or brine circuit and immediately convert any alkane that occurs due to a leak. However, a leak can also be seen in the gas phase and can be determined in the gas phase if this gas phase is separated in a gas separator.

This alkane concentration in the separated gas phase can be used to draw conclusions about the alkane concentration using known solubility data in the heating or brine circuit. If the leakage is small and remains small, the oxygen carrier can be replenished accordingly. Ideally, this happens automatically, but it is also possible to top up manually.

If the oxygen carrier and the catalyst are identical to the corrosion inhibitor, the corrosion inhibitor can be added generously and is then already in the heating or brine circuit before a leak occurs.

The object of the invention is therefore to convert working fluid components or refrigerant components containing alkanes that are dissolved or have very fine bubbles in the heat transfer fluid into a harmless form.

• dem Wärmeträgerfluid mindestens ein Korrosionsinhibitor zugegeben wird,
• dem Wärmeträgerfluid ferner mindestens ein wasserlöslicher Sauerstoffträger zugegeben wird, und
• im Wärmeträgerfluidkreislauf mindestens ein Katalysator das gelöste Alkan in ein flüssiges Oxidationsprodukt oxidiert.

The invention solves the problem by a corresponding method for converting alkanes dissolved in heat transfer fluid into non-flammable, water-soluble compounds

• at least one corrosion inhibitor is added to the heat transfer fluid,
• at least one water-soluble oxygen carrier is also 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.

In one embodiment of the method, it is provided that a gas phase is separated from the heat transfer fluid and the alkane concentration therein is determined. If a concentration is measured, which indicates a leak, the required quantity of an oxygen carrier is determined on the basis of the alkane concentration and this quantity is metered into the heat transfer fluid circuit.

In a further embodiment of the method, 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.

In this case, it is not necessary for the oxidation to proceed completely to carbon dioxide, since all partial oxidation products such as alkanols, alkanals and alkanones are also acceptable for achieving the desired conversion goal, but not alkanoic acids.

The invention is illustrated below using an example. Here, the alkane is propane, which is 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 covered with a layer on the inside, which through Dosing of sodium molybdate, for example in the form of Fernox solution, takes place in the circuit.

Then it is measured at the gas separator whether there is flammable gas in the separated gas. This can be done with a commercially available gas detector. As soon as the gas alarm goes off, the concentration of propane and the amount of propane are determined. From the solubility of the propane in the brine at the prevailing temperature, the amount of propane probably dissolved in the brine is deduced and a corresponding amount of sodium molybdate is replenished.

While the originally submitted amount of sodium molybdate depends on the size of that heating and brine circuit, the amount to be replenished results from the stoichiometric requirement. Lithium chromate can be used in the same way instead of sodium molybdate, and parallel use is also possible.

The method is particularly suitable in the event that the usual double-walled heat exchangers, which are in contact with the working fluid as evaporators and condensers, can be replaced by simple designs that also allow a higher efficiency of the entire system, since lower temperature differences are required for heat transfer . Since such leaks, as long as they are small, do not cause any impairments and no longer pose a safety risk, it is possible to continue to operate the heat pump system and the heat transfer medium circuit for a long time after a leak has been detected and to postpone repairs that are due until the next regular maintenance.

Claims (6)

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, characterized in that - at least one corrosion inhibitor is added to the heat transfer fluid, - At least one water-soluble oxygen carrier 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. Method according to Claim 1, characterized in that a gas phase is separated from the heat transfer fluid and the alkane concentration therein is determined. Method according to Claim 2, characterized in that the required quantity of an oxygen carrier is determined on the basis of the alkane concentration and this quantity is metered into the heat transfer fluid circuit. Process according to any one of Claims 1 to 3, characterized in that the corrosion inhibitor, the oxidizer and the catalyst are the same substance. Process according to one of Claims 1 to 4, characterized in that sodium molybdate is added as a corrosion inhibitor, oxygen carrier and catalyst. Process according to one of Claims 1 to 4, characterized in that lithium chromate is added as a corrosion inhibitor, oxygen carrier and catalyst.