Vapor and liquid flow concurrently down through a vertical vessel. A horizontal scale collection and predistribution tray is located in the vessel to remove solid contaminants and to redistribute the liquid to a fine distribution tray. The scale collection and predistribution tray consists of a tray plate with a scale collection zone where the solid contaminants settle and deposit. In one embodiment, an upstanding permeable wall forms the scale collection zone, and liquid is filtered as it flows through the permeable wall, leaving the solid contaminants trapped upstream from the permeable wall. The predistribution tray has a rim provided with a slotted weir. Liquid from the scale collection zone forms a liquid level in a trough located between the permeable wall and the weir. Due to the uniform liquid level in the trough, liquid flow rates through the slots in the weir are nearly equal. Because of the polygonal shape of the tray, the liquid exits the slots in a direction along lanes defined between distribution units on the fine distribution tray, and the amount of liquid entering the vapor inlets of the distribution units is therefore small. Vapor by-passes the scale collection and predistribution tray through the area between the reactor wall and the permeable wall, and through the area between the reactor wall and the weir to the fine distribution tray. The scale collection and predistribution tray protects the fine distribution tray and the catalyst bed from fouling, predistributes liquid to the fine distribution tray to minimize level gradients on this tray, and reduces flow velocities to ensure calm flow conditions on the fine distribution tray.