A modular manifold arrangement for distributing a flow of a gas in a reactor. The manifold arrangement includes a tank or a container, within the walls of which a bed for a granulated catalyst or substance, with which the gas is to react, is arranged. The bed is at least partially surrounded by gas-permeable gratings. In order to provide elements that may pass through a narrow manhole and from which it is possible to construct passages for providing a uniform flow through the reactor bed, the manifold arrangement includes an elongate tube. The tube is closed at one end and open for inlet of gas at the other end. The tube further has a gas-impermeable rear sidewall with a shape corresponding to the walls of the tank and a front side which is a gas-permeable grating having a sheet or plate provided therein with a large number of nozzles.