A PROCESS FOR THE ELIMINATION OF VOLATILE ORGANIC COMPOUNDS AND HAZARDOUS AIR POLLUTANTS IN AMMONIA PLANTS

Abstract

In a process for the elimination of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) formed as by-products in the shift section (SS) of an ammonia plant, a carbon dioxide (C0.sub.2) stream from a vent line, which is arranged downstream from the shift section and the C0.sub.2 removal section, is recycled to the primary reformer (TR) of the ammonia plant. This way, the oxygenates contained in the carbon dioxide vent will be decomposed in the primary reformer burners, and the total emission of VOCs and HAPs will be considerably reduced.

Claims

1. A process for the elimination of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) formed as by-products in the shift section of an ammonia plant, wherein a carbon dioxide (CO.sub.2) stream from a vent line, which is arranged downstream from the shift section and the CO.sub.2 removal section, is recycled to the primary reformer of the ammonia plant.

2. The process according to claim 1, wherein the CO.sub.2 stream is recycled to the fuel system of the primary reformer.

3. The process according to claim 1, wherein the CO.sub.2 stream is recycled to the combustion air system of the primary reformer.

4. The process according to claim 1, wherein the CO.sub.2 stream is recycled to the combustion chamber of the primary reformer.

5. The process according to claim 2, wherein the CO.sub.2 stream is recycled to the combustion air system of the primary reformer.

6. The process according to claim 2, wherein the CO.sub.2 stream is recycled to the combustion chamber of the primary reformer.

Description

[0009] The present invention relates to a process for the elimination of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) formed as by-products in the shift section of an ammonia plant, wherein a carbon dioxide (CO.sub.2) stream from a vent line, which is arranged downstream from the shift section and the CO.sub.2 removal section, is recycled to the primary reformer of the ammonia plant.

[0010] Thus, the present invention relates to a process for the elimination of emissions of volatile organic compounds and hazardous air pollutants from a carbon dioxide (CO.sub.2) vent in an ammonia plant. The processes carried out in an ammonia plant i.a. comprise feeding fuel to a tubular reforming section, passing the effluent from the tubular reforming section to a secondary reformer and then to the shift section, and passing the effluent from the shift section to a CO.sub.2 removal unit, where the CO.sub.2 is separated from the syngas. This CO.sub.2, or at least part of it, is vented to the atmosphere.

[0011] The synthesis gas generation part of an ammonia plant roughly consists of a desulfurisation section, such as a hydrodesulfurisation (HDS) section (necessary in order to avoid poisoning of the catalyst in the downstream steam reformer), a reforming section, a shift section, a carbon dioxide removal unit, a methanator and an ammonia synthesis unit. The reforming section can for example be based on a tubular reformer preceded by a pre-reformer. The pre-reformer is used for low temperature steam reforming of a hydrocarbon feed such as natural gas. It provides complete conversion of higher hydrocarbons and removal of sulfur, and it is also protecting the downstream catalyst.

[0012] The pre-reformer is placed upstream from the tubular reforming unit. In order to obtain the required steam-tocarbon ratio, the feed is mixed with process steam before entering the pre-reformer. In the pre-reformer, all higher hydrocarbons are converted into a mixture of carbon oxides, hydrogen and methane.

[0013] In an ammonia plant the carbon monoxide conversion unit is located downstream from the secondary reformer.

[0014] The purpose of the shift section is to maximise the hydrogen output and reduce the carbon monoxide level in the synthesis gas.

[0015] In an ammonia plant, the shift section normally consists of a high temperature shift (HTS) reactor followed by a low temperature shift (LTS) reactor. The shift section may optionally consist of a medium temperature shift (MTS) reactor followed by a low temperature shift (LTS) reactor. To ensure that the synthesis gas in an ammonia plant being fed to the ammonia synthesis loop is free from carbon oxides, it is passed through a methanator, which will convert any traces of carbon dioxide and unconverted carbon monoxide from the shift section into methane.

[0016] The performance of the shift unit strongly affects the overall energy efficiency of the ammonia plant, because unconverted carbon monoxide will consume hydrogen and form methane (CH.sub.4) in the methanator, thereby reducing the feed and increasing the inert gas level in the synthesis loop.

[0017] In the following, the invention will be explained in detail with reference to the figures, which show the parts of an ammonia plant which are relevant in connection with the invention. A feed stream f from a pre-reformer is led through a tubular reformer TR to which fuel and optionally combustion air CA is also supplied. The flue gas from the tubular reformer is sent via a waste heat recovery section (WHS) to the stack.

[0018] The effluent from the tubular reformer is routed to a secondary reformer SR and shift section SS for cooling and separation. Then carbon dioxide is separated from the stream, which then consists of syngas to be routed to a methanator (not shown). After venting off some of the separated CO.sub.2, the rest is routed to e.g. urea production.

[0019] According to the prior art illustrated in FIG. 1, the CO.sub.2 vent, which typically contains approximately 300 ppm methanol, 5 ppm dimethyl ether, 50 ppm methyl formate and 15 ppm acetaldehyde, is washed by scrubbing it with a liquid, preferably water, in a wash system WS. However, as already mentioned, the pollutant compounds in the CO.sub.2 vent are in general much too volatile to be sufficiently absorbed in the scrubber liquid. More specifically, the gas discharged to the atmosphere after the scrubbing process still contains around 15 ppm methanol, 5 ppm dimethyl ether, 40 ppm methyl formate and 15 ppm acetaldehyde.

[0020] In the process according to the invention for elimination of emissions of volatile organic compounds (VOCs) from the carbon dioxide (CO.sub.2) vent in ammonia plants, the CO.sub.2 vent gas is instead routed to the tubular (primary) reformer (FIG. 2), especially to the combustion chamber of the primary reformer. Preferably the CO.sub.2 vent is routed to the fuel system of the primary reformer (FIG. 3) and most preferably to the combustion air system (FIG. 4).

[0021] By recycling the carbon dioxide stream from the vent line to the primary reformer of the ammonia plant, the oxygenates contained in the carbon dioxide vent will be decomposed in the primary reformer burners, and the total emission of VOCs and HAPs will be considerably reduced compared to the scrubber solution of the prior art. In addition, the need for expensive equipment, which is necessary in relation to the scrubber solution and the treatment of the waste liquid stream, will be eliminated.