Vent line for use in ammonia and hydrogen plants

Abstract

In an ammonia or hydrogen plant comprising a desulfurisation section, a reforming section and a shift section, where the shift section comprises a low temperature shift converter and a medium temperature shift converter, a vent line is arranged downstream from the low temperature shift converter and the medium temperature shift converter in order to allow the shift converters to be re-heated with process gas at a low pressure (typically 3-7 bar). This way condensation of water vapour in the process gas is avoided. By applying this vent line it becomes possible to save significant time, more specifically 8-24 hours, for restarting the production after temporary shut-down thereof, because a heat-up of the LTS/MTS converter in circulating nitrogen is avoided.

Claims

1. An ammonia or hydrogen plant including a synthesis gas generation part comprising a reforming section, a shift section, a carbon dioxide removal unit and a methanator, said shift section comprising a low temperature shift (LTS) converter and/or a medium temperature shift (MTS) converter, and further including a vent line arranged downstream from the shift section comprising the LTS converter and/or the MTS converter, said vent line allowing the shift converter(s) to be re-heated with process gas at a pressure of 3 to 7 bar, and an inlet pressure reducing valve placed at the inlet of the shift section and a vent valve placed in the vent line downstream from the shift section, said valves facilitating the pressure control.

2. An ammonia or hydrogen plant according to claim 1, wherein the reduced pressure is established by means of an inlet pressure reducing valve and a vent valve, said valves normally being closed and the synthesis gas being led to the MTS and/or LTS section through a normally open valve.

3. A process of using the vent line in the plant of claim 1, comprising opening a normally closed valve of the vent line and conveying process gas at a pressure of between 3-7 bar from said vent line to said shift section, upstream of the low temperature shift (LTS) converter and/or a medium temperature shift (MTS) converter, to restart the plant after temporary shut-down thereof, where the catalyst has only cooled slightly down, to a temperature of 140-250.

Description

(1) Examples of the dew point of the synthesis gas at reduced pressure are indicated in Table 2:

(2) TABLE-US-00002 TABLE 2 synthesis gas pressure synthesis gas dew point 27 kg/cm.sup.2 .Math. g 177 C. 5 kg/cm.sup.2 .Math. g 121 C.

(3) The FIGURE illustrates both the closest prior art and the vent line according to the present invention. The desulfurised hydrocarbon feed (HC) is led to the reforming section (R) together with process steam (s). Optionally an oxidant (ox), which can be air, enriched air or pure oxygen, is led to the reforming section too. From the reforming section the synthesis gas product is sent to the MTS and/or LTS section, optionally after passing a high temperature shift (HTS) reactor.

(4) In the prior art, a nitrogen start-up circulation is established around the MTS and/or LTS section to be used after shut-down of the plant. This circulation of N.sub.2 (shown as a dotted arrow) to heat the MTS and/or LTS catalyst can be avoided by applying the vent line according to the invention. This way the MTS and/or LTS converter(s) is/are reheated in synthesis gas at a reduced pressure (typically 3-7 bar), whereby condensation of water vapour in the process gas is avoided. As mentioned, the use of a vent line is relevant after a trip (possibly unplanned) of the plant or converter of a relatively short duration, in which case the catalyst has only been slightly cooled, i.e. to 140-250 C.

(5) If the hydrocarbon feed gas is submitted at a normal operating pressure, then water condensation will occur, and water condensation will damage the LTS and/or MTS catalyst.

(6) The reduced pressure needed when the synthesis gas stream enters into the MTS and/or LTS catalyst is established by means of an inlet pressure reducing valve (prv) and a vent valve (vv). These valves are normally closed, and the synthesis gas is led to the MTS and/or LTS section through the normally open valve (nov). When applying the vent line, the normally open valve (nov) is closed and the inlet pressure reducing valve (prv) and the vent valve (vv) are opened. This way the pressure control is facilitated, thereby controlling the dew point in the MTS and/or LTS catalyst.

(7) By applying the vent line according to the invention, it is possible to save significant time, more specifically 8-24 hours, for re-starting the production after temporary shut-down thereof. This is because heating up the LTS and/or MTS converter in circulating nitrogen is avoided.