ALK Electrolysis BOP Process: A Technical Guide

Introduction#

In an alkaline (ALK) electrolysis system, the balance-of-plant (BOP) units determine overall efficiency, safety, and hydrogen quality. The BOP comprises gas-liquid separation, alkali circulation and cooling, and purification. These auxiliary systems not only maintain stable electrolysis but also account for a sizable share of plant parasitic load. Understanding core process flows and tuning key parameters is essential for operators and process engineers.

This guide focuses on practical process design and control strategies. It examines each BOP subsystem, suggests setpoints, and explains how to reduce self-consumption without compromising product purity or system stability.

Gas-Liquid Separation: Process Principles and Parameters#

Each electrolyzer cell stack discharges a two-phase mixture of hydrogen or oxygen with entrained alkaline electrolyte. After leaving the cell, the mixture enters dedicated separators. The separation relies on gravity settling and residence time, with electrolyte collected at the bottom and gas leaving from the top. For an ALK system operating near 80 deg C and 1.6 MPa gauge pressure, the residence time in the primary separator should be 6 to 8 minutes to achieve gas carry-under of less than 0.5 percent.

Set the electrolyte level at 40 to 60 percent of separator height. Use differential pressure transmitters and level control valves to maintain stable liquid levels. At the gas outlet, install a demister to remove caustic aerosol. For a 100 Nm3/h electrolyzer, recommend keeping the gas line temperature between 70 and 90 deg C before the cooler to prevent premature condensation.

Alkali Circulation and Cooling: Closed-Loop Control#

The circulating alkali serves three functions: ionic conductor, heat transfer medium, and product gas carrier. A typical electrolyte formulation is 25 to 30 wt.% KOH. The circulation pump should deliver a stack flow equivalent to 1.5 to 2.5 times the stoichiometric flow required for electrolysis, or about 2 to 4 m3/h per 100 kW of electrode area. A circulation rate that is too low reduces ionic transport and creates local hotspots; too high a flow promotes leakage and excessive pump power.

Use a plate heat exchanger downstream of each separator to control the electrolyte temperature entering the stack. Keep the stack inlet temperature at 80 +/- 3 deg C, because higher temperatures increase conductivity but shorten membrane or diaphragm life. Maintain the differential pressure between the hydrogen side and oxygen side below 0.05 MPa. Install a bypass with a three-way control valve around the heat exchanger for rapid temperature correction and to avoid thermal shock to the stack.

Purification Unit: Deoxidation and PSA Dewatering#

Raw hydrogen leaving the separator contains saturated water vapor and small amounts of oxygen due to diffusion through the diaphragm. To remove oxygen, the hydrogen passes through a catalytic deoxidizer. At an operating temperature of 150 to 200 deg C, hydrogen and oxygen combine over a palladium catalyst, reducing oxygen concentration to below 5 ppm. Keep the oxygen content at the reactor inlet below 2 vol.% to avoid excessive exothermic temperature excursion.

The downstream PSA dries and purifies the hydrogen. The typical adsorption pressure equals the electrolyzer discharge pressure, normally 1.2 to 1.6 MPa. A four-tower PSA cycle with pressure equalization achieves recovery above 80 percent. Set the purge ratio to 10 to 15 percent of the dry gas flow; this minimizes purge