High Temperature Shift Converter

In 2001 Hyprotech and Synetix announced an ammonia plant simulation that can be used for modeling, on-line monitoring and optimization of the plant. The simulation includes Synetix reactor models, customized thermodynamic data and information to simulate the performance of a range of catalysts. The reactor models in the simulation include Primary and Secondary Reformers, High Temperature Shift converter, Low Temperature Shift Converter, Methanator and Ammonia Synthesis Converter80. 

The compressed gas is shifted in a high temperature shift converter and passed through a Rectisol CO 2 removal system. In this way, a clean stoichiometric gas is obtained, suitable for compression and injection into a standard ICI LP methanol loop. Byproduct tars are fed to a partial oxidation gasifier, adding to the synthesis gas supply. 

In the heterogeneous model for the high temperature shift converter, the effectiveness factor accounts for the external and the intraparticle mass and heat transfer resistances (e.g. Satterfield and Roberts, 1968 Hutchings and Carberry, 1966 Petersen et ai, 1970 Chu and Hougen, 1972) and is multiplied by the rate of reaction at bulk conditions to get the actual rate of reaction. 

The nuKlclling, siimilution and optimization of the low temperature shift converter is very similar lo the high temperature shift converter (Alhabdan, 1990) c.u cpl for the conslants of the rate equations. 

Methanol is produced as a by-product in the shift converters within the hydrogen plant. In High Temperature Shift Converters (HTSC), the formation of methanol is an equilibrium reaction similar to that of ammonia formation in the reformer. The equilibrium reaction for methanol is given below. 

Reforming with a stoichiometric amount of process air using high preheat temperatures for process feed, process air, and combustion air. Low methane leakage. As a special feature, natural gas feed is preheated downstream of the high temperature shift converter, and reformer feed is preheated downstream of the process gas boiler after the secondary reformer. 

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

Downstream of the reformer the CO is converted into hydrogen by two subsequent water-gas shift sections a high-temperature shift (HTS) followed by a low-temperature shift (LTS). This is done because the equilibrium of the WGS reaction lies at the product side at lower temperatures (around 200 °C), but the reaction kinetics are faster at increasing temperature. Therefore, to reach high CO conversions, most of the CO is converted in a HTS section and the remainder is converted within a LTS section. 

A review of conventional hydrogen production via steam reforming is useful to appreciate the advantages of the POLYBED PSA system. The conventional system consists of a feed desulfurizer, reforming furnace, high-temperature and low-temperature shift converters, C02 removal system and a methanator (see Figure 2). 

Some shift converters have high- and low-temperature sections, the high-temperature section converting most of the carbon monoxide to carbon dioxide. Cooling to 38°C is followed by carbon dioxide absorption with monoethanolamine (HOCH2CH2NH2). The carbon dioxide (an important by-product) is desorbed by heating the monoethanolamine and reversing this reaction. 

A consequence of the steam reformation process and the subsequent clean up steps of high and low temperature shift converters and a selective oxidizer (called the prox unit) the typical levels of CO at the inlet stream of a PEMFC s are expected to be in the range of 50 and 100 ppm, higher levels in the range of 500 to... 

Generally, in a conventional WGS system a two-step shift is used to obtain high CO conversion rates. In the first high-temperature shift reactor the major part of the CO is converted at high activity, whereas in the second shift reactor the rest of the CO (closely up to the thermodynamic equilibrium) is converted at low temperature and also low activity. Steam to carbon monoxide ratios above the stoichiometric ratio (higher than 2) are generally being used to attain the desired carbon monoxide conversion, but also to suppress carbon formation on certain catalysts. 

The conventional ammonia production line consists of seven gas-solid catalytic reactors, namely desulfurization unit, primary reformer, secondary reformer, high temperature shift, low temperature shift, methanator and finally the ammonia converter. In addition the production line includes an absorption-stripping unit for the removal of CO2 from the gas stream leaving the low temperature shift converter. The ammonia converter is certainly the heart of the process with all the other units serving to prepare the gases for the ammonia synthesis reaction which takes place over an iron promoted catalyst under conditions of high temperature and pressure. 

The flow diagram for a steam methane reformer is illustrated in Figure 3, This is a conventional reformer designed to maximize the production of hydrogen, A plant designed for production of syngas or carbon monoxide would not include the high and low temperature shift converters and the... 

Steam is added to the desulfurized feed to achieve the specified steam/ carbon ratio and the mixture is further preheated before entering the primary reformer. Methane is converted to hydrogen and carbon oxides in the primary reformer. The gas is cooled to about 340-455°C (645-850 F) and then enters the high temperature shift reactor [1]. 

Depending on the temperatures at which the carbon monoxide is shifted, another distinction is made between high-temperature shift conversion (300-500 °C) and low-temperature shift conversion (180-280°C). Low- temperature shift conversion is, however, normally used only if the residual CO content in the converted gas has to be very low. As this is not the case fcx methanol production, and as there is no reason to put up with the high vulnerability to sulfur of the copper catalysts used for low-temperature conversion nor their considerable cost, the following description will be limited to high-temperature conversion. 

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