Experimental and Numerical Investigations of Salt Separation from Biomass at Hydrothermal Conditions Exploiting Aqueous Solutions of Sodium Sulfate and Pure Water as Model Systems

Abstract

Using concentrated solar energy to power a hydrothermal biomass gasi- fication process (HTG process) requires a thermal energy storage (TES) to compensate for the inherent intermittence of solar irradiation. The energy transfer from the TES to the HTG process is accomplished via a heat-transfer fluid (HTF) passing through a heat exchanger (HX) incor- porated into the salt separator of the HTG process. The HX performance determines the temperature profile inside the salt separator, thereby in- fluencing the removal of the salts from the feedstock. The first part of this work compares the performances of three HX types based on exploiting fluidized beds, porous media, and axially-finned tubes. The effect of the HX configuration on the temperature profile inside a labe- scale dip-tube salt separator (945 mm tall, 12 mm internal and 50 mm ex- ternal diameter) is assessed through computational fluid dynamics (CFD) simulations considering pure water as the model feed to the separator. All considered HX types could provide the desired temperature profile within the separator. However, the estimate for the power required to pump the HTF through the fluidized bed HXs is roughly two orders of magnitude higher than those for the axially-finned tubular and porous-media HXs. The second part of this thesis investigates the effect of the temperature and velocity fields inside the salt separator on type-2 salt deposition. Experimentally the effect of the centerline temperature profile inside the salt separator, the diameter of the dip tube, and the feed flow-rate on salt deposition is investigated employing a lab-scale dip-tube salt separator (444 mm tall, 12 mm internal and 30 mm external diameter). Aqueous Na2SO4 solution is used as model feed. Effective salt separation leading to an enrichment of the salt at the bottom outlet of the salt separator could not be achieved. Salt deposition eventually causing plugging of the separator occurred in all experiments. It is found that the centerline temperature profile controls the location of the salt plug. Increasing the temperatures at the dip-tube exit, increasing the dip-tube diameter at constant feed flow rate, as well as reducing the feed flow-rate at constant dip-tube diameter reduces the amount of deposited salt. Based on the experimental results, an empirical correlation is developed to estimate the mount of salt accumulation in the separator. This correlation allows for a fast screening of different salt separator configurations and operating conditions in terms of expected amount of deposited salt. Additionally, a riser-tube salt separator is tested for the first time with salt solution exhibiting type-2 phase behavior. Under the investigated conditions, it is shown that the riser-tube separator actually achieves an enrichment of salts at the bottom outlet but it also plugs faster than the dip-tube separators. Complementary to the experiments, the steady-state velocity and tem- perature fields of pure water flowing though the dip-tube separator config- urations under the operating conditions employed in the salt-separation experiments are numerically simulated using CFD. It is found that the location of the recirculating flow pattern inherent to the dip-tube salt separator configuration is controlled by the temperature profile in the salt separator. Stokes number distributions, computed based on the flow field of water and typical diameters of precipitated Na 2 SO 4 particles, in- dicate that salt particles that are formed inside the separator deposit from the recirculating flow pattern. A correlation between a volume-averaged Stokes number and the experimentally determined amount of deposited salt is established, allowing for a comparison of different salt separator configurations and operating conditions in terms of amount of deposited salt based on the flow field of water. Finally, a method for online detection and monitoring the growth of the deposits is presented, which is indispensable for scheduling the timely removal of the salt deposits.