May I ask, is the fluid in a tubular reactor moving at a constant speed? Or in other words, does the plug flow maintain a constant velocity?

In tubular reactors used for continuous chemical processes, an uneven velocity distribution always exists across a cross-section perpendicular to the direction of material flow. The unevenness of this velocity distribution, along with radial and axial mixing, poses many difficulties in the design calculations of the reactor. To this end, an ideal flow has been conceived – one in which it is assumed that there is a strictly uniform velocity distribution within the reactor, with no mixing in the axial direction. This type of flow is known as plug flow, or ideal flow, or piston flow. This is an idealized flow that does not exist in reality; it is studied as a typical flow model. The flow conditions in an actual reactor can only approach this ideal flow to varying degrees. In a tubular reactor, when the length of the tube is much greater than its diameter, flow approaches this ideal state; such a reactor is commonly referred to as an ideal tubular reactor. An ideal tubular reactor has the following characteristics: (1) Under normal conditions, it operates in a continuous, steady-state manner, with the material concentration at each radial section of the reactor varying only with axial position and not with time; (2) The concentrations at different axial positions within the reactor are not necessarily equal, and the reaction rate varies with spatial axial position; (3) Due to a strictly uniform velocity distribution in the radial direction, that is, no concentration variations exist in the radial direction, the variation in the reaction rate with spatial position is limited to the axial direction only. It can be seen that the flow velocity inside the reactor is not uniform in reality; what the original poster referred to as plug flow is the same as ideal flow. There’s no need to discuss it separately

The concentrations at different axial positions within the reactor are not necessarily equal; the reaction rate varies with the axial position in space. When the flow velocity is very high (the limit value being the speed of light), the concentration remains constant, and the reaction rate does not change with the axial position in space. So, can it be said that the reaction rate changes with the flow rate?