Energy usage in exothermic reactions.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to the use of energy from an exothermic reaction in a formaldehyde plant.

2. Description of related prior art

Formaldehyde gas is usually produced by oxidisation of methanol with oxygen from the atmosphere in the presence of iron molybdate or silver. The formaldehyde gas is absorbed in water and this solution is called formalin. The formalin plant operates at a pressure above the atmospheric pressure to increase the production rate of the plant. This pressurisation is achieved by a pressurisation fan or compressor commonly known as a blower. The energy consumption of this blower is very much depending on desired increase in pressure. At the levels below 0.3 Bar in increased pressure the amount of energy can be said to be moderate. In cases where the desired pressure increase is higher or much higher, say in the range above 0.3 Bar, the amounts of energy to the blower will become a great economical factor.

It is known from GB l 154058 to enter cold air which is compressed in a compressor. The air is then preheated in a co-current heat exchanger and separated in two air streams. The smaller air stream is then passed into an evaporator into which methanol is also fed. The methanol is evaporated in evaporator by means of steam drawn from a heat exchanger. The gaseous mixture of methanol and air is then merged with the main stream of the air and fed into a reactor. The temperature in the reactor is regulated in the usual way by a heat transfer medium and a heat exchanger. The surplus heat is taken out in the vapour state. It is possible to vary the steam pressure up to an overpressure of 40 kg/cm2. The warm reaction gases from the reactor are partly cooled in the heat exchanger which is producing steam. The reaction gases are finally cooled in a co-current heat exchanger and are then passed into an absorption system where they are absorbed in, for example, water.

The waste gas from the process is to some extent re-circulated while the remaining portion is combusted.

Accordingly the exothermic reactions in the formaldehyde plant, including the combustion process, generates great amounts of excess heat. This heat can be used for

producing rather large amount of high pressure steam. It is known to use this steam for heating in for example industrial processes.

It would be advantageous if the pressure from the exothermic energy from the plant could be used more directly in the process thereby reducing the need of an external energy source.

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SUMMARY OF THE INVENTION

The object of the present invention is to radically reduce the need of an external power source for achieving pressurisation of the process. Accordingly the invention relates to a process as well as an apparatus for the manufacturing of formaldehyde where the formaldehyde is manufactured in a reactor and wherein exothermic energy is a result of the process. The reactor is pressurised through means of a compressor and exothermic energy from the process is used for feeding energy to the compressor, hereby allowing increase in pressure in the reactor with a minimum of required power input from external source.

DETAILED DESCRIPTION OF THE INVENTION.

The reactor is pressurised through means of a compressor and that the exothermic energy from the process is used for feeding energy to the compressor, hereby allowing increase in pressure in the reactor with a minimum of required power input from external source. According to a preferred embodiment of the invention the process further comprises the step combustion of waste gases, that said waste gas combustion results in exothermic energy which is used for feeding energy to the compressor. The exothermic reaction results in an increase in volume flow and said volume flow increase is used for powering a turbine.

According to one embodiment of the invention the turbine powers a generator. This generator may then power the compressor. The turbine velocity may be regulated by means of a bypass stream. Alternatively the turbine velocity is regulated by means of a throttle valve. The power from the generator to the compressor can then be guided through any known means.

According to another embodiment of the invention the turbine powers the compressor via an axle. The turbine velocity is then regulated by means of a bypass stream. Alternatively the turbine velocity is regulated by means of a throttle valve. The

turbine velocity may also be regulated by means of a throttle valve and a bypass stream.

Even though the volume flow increase resulting from the exothermic energy is used within the process according to the present invention it is, according to an embodiment of the invention an object that the exothermic energy is further used for generating steam.

The invention also relates to an apparatus for manufacturing formaldehyde wherein the formaldehyde is manufactured in a reactor and wherein exothermic energy is a result of the process. The reactor is pressurised through means of a compressor and the exothermic energy from the process is used for feeding energy to the compressor, hereby allowing increase in pressure in the reactor with a minimum of required power input from external source.

According to a preferred embodiment of the invention the apparatus further comprises a device for combustion of waste gases and that said waste gas combustion contributes to the exothermic energy which is used for feeding energy to the compressor. In an even more preferred embodiment of the invention the device for combustion of waste gas provides the exothermic energy used for feeding energy to the compressor. The exothermic reaction results in an increase in volume flow and said volume flow increase is used for powering a turbine.

According to one embodiment of the invention the turbine powers a generator. This generator may then power the compressor. The turbine velocity may be regulated by means of a bypass stream. Alternatively the turbine velocity is regulated by means of a throttle valve. The power from the generator to the compressor can then be guided through any known means.

According to another embodiment of the invention the turbine powers the compressor via an axle. The turbine velocity is then regulated by means of a bypass stream. Alternatively the turbine velocity is regulated by means of a throttle valve. The turbine velocity may also be regulated by means of a throttle valve and a bypass stream.

Even though some of the volume flow increase resulting from the exothermic energy is used within the process according to the present invention it is, according to

an embodiment of the invention an object that the exothermic energy is further used for generating steam.

According to the present invention the power from the turbine feeds the compressor either direct via an axle or via a generator/motor assembly. In either case it may be necessary to guide the velocity of the turbine in order to protect it from reaching critical velocity. This can be achieved through any known means of which the following, non-limiting examples are given;

-It is possible to guide the tangential velocity of the turbine via a throttle and/or bypass assembly. A bypass assembly is then suitably arranged in an array so that both the turbine and the compressor are provided with bypass streams. This will facilitate start up and shut down of the process.

-It is also possible to use a generator as a break to continuously guide the tangential velocity of the turbine. The power generated may of course be used in steps both inside and outside the process. A resistor battery may be used as a backup in cases where energy from the generator of different reasons is higher than the rate of consumption.

-It is alternatively possible to utilise a mechanical break to protect the turbine from exceeding safe tangential velocity. This should, however, be regarded as a safety device and is not well suited for continuous operation.

-It is also possible to utilise any combination of the guiding methods mentioned above.

In an embodiment example of the process the formaldehyde reactor is provided with a re-circulation blower so that a semi-closed loop is formed. It is here advantageous to arrange a blow off valve after the circulation blower in order to facilitate the start up process. Also a throttle in the circulation stream is advantageous. The main outputs from the reactor are formaldehyde and waste gas. Portions of the waste gas is part of

the re-circulation while the remaining portion of the waste gas is feed to an emission control system.

Fresh air is feed into the re-circulation loop, preferably before the circulation blower. The fresh air is feed through means of a compressor according to the present invention. This compressor pressurises the process. In order to facilitate start-up and shutdown of the process this compressor is provided with a bypass valve allowing the compressor to obtain proper working speed in a controlled manner. As said above the compressor is powered directly or indirectly by means of a turbine. The gas flow feeding this turbine is obtained by means of exothermic energy at one or several steps of the process. It is advantageous to obtain this energy from the emission control system. The outlet from the turbine is then preferably feed through a steam generator where further amounts of energy can be claimed. Also the steam generator may be provided with a bypass valve suitably guided by the temperature on the pressure side of the turbine. The turbine is suitably provided with a bypass valve which is used during start up an shut down. This bypass valve may also be used for regulating the tangential velocity of the turbine. The bypass valve may be guided by the pressure in the formaldehyde reactor circulation loop. A fresh air flow may further be taken from the pressure side of the compressor and fed via a valve, guided by the Q2 levels after the compressor in the formaldehyde reactor circulation loop, and into the turbine bypass valve.

The invention is not limited to the embodiments shown since it can be varied in different ways within the scope of the invention.