The effect of the cold tube effect on synthetic reactions

Due to the cold gas flowing inside the cold tubes of the Three J Towers, the temperature outside the walls of these cold tubes is lower than that of the catalyst bed, which prevents the catalyst surrounding the surface of the cold tubes from functioning properly. Thus affecting the rate of the synthesis reaction. Furthermore, there is generally a phenomenon of gas bypassing outside the cold tube walls, so the temperature difference within the same plane as the cold tube layer is greater than that in the insulated layer. The cold tube effect also has a significant impact on catalyst reduction; therefore, we usually turn off the cold tube connections during reduction to reduce this effect.

The cold tubes installed within the catalyst layer have a temperature of the cold fluid flowing through them that is lower than the temperature of the catalyst bed – usually lower than the activation temperature of the catalyst. As a result, the catalyst surrounding these cold tubes becomes over-cooled and loses its activity or is unable to function properly； Furthermore, due to the larger pores in the catalyst layer on the surface of the cold tube, the flow resistance is low, resulting in higher gas permeability at the tube wall. This leads to an uneven distribution of airflow across the cross-section of the catalyst layer; in particular, large radial temperature differences cause uneven reduction of the catalyst layer, increasing the chances of repeated oxidation and reduction. Additionally, incomplete reduction of the catalyst in certain areas affects its reduction activity. All these factors contribute to the low ammonia conversion efficiency in the internal components of cold-tube synthesis towers.

The cold tube effect affects the reduction of the catalyst and its proper performance in terms of activity. Since the cold tubes occupy part of the bed space, they increase the resistance of the tower. The cold pipe has a complex structure and numerous connection points with various components, which makes it prone to thermal stress and subsequent damage to its internal parts. (Since the temperature of the cold fluid inside the cold tube is lower than the temperature of the catalyst bed, and generally also lower than the activation temperature of the catalyst, this causes the catalyst surrounding the surface of the cold tube to become over-cooled, resulting in a loss of activity or an inability to function properly.) Furthermore, due to the high porosity of the catalyst layer on the surface of the cold tube, the flow resistance is low, resulting in higher gas flow at the tube wall. This leads to an uneven distribution of airflow across the cross-section of the catalyst layer, thereby increasing the radial temperature difference. During catalyst reduction, the radial airflow distribution is uneven; in particular, large radial temperature differences lead to varying degrees of reduction within the same catalyst layer, increasing the chances of repeated oxidation-reduction cycles. Moreover, incomplete reduction of the catalyst in certain areas affects its reduction activity. These factors constitute the fundamental reasons for the low net ammonia value of cold-tube internals. The reduction in the reactor’s reaction efficiency caused by these various disadvantages resulting from cold tubes is known as the “cold tube effect”. ）