Ammonia Recycle Percolation (ARP)
In the ammonia recycle percolation (ARP) pretreatment process a 5-15% aqueous ammonia solution is percolated through the biomass in a flow through column reactor at moderate temperatures around 150-180°C and at a reasonable flow rate giving residence time of 10-90 min. After the percolation, ammonia is recycled or recovered [84-87]. During this process most of the hemicellulose is hydrolyzed, with a concomitant decrease in cellulose crystallinity, and the lignin is removed from the biomass. When compared with AFEX pretreatment the advantages of ammonia recycle percolation (ARP) are its ability to take away the majority of the lignin (75-85%) and solubilization of more than half of the hemicellulose (50-60%), while maintaining high cellulose content [88]. This is due to the selectivity of ammonia and its reactivity towards the breakdown of lignin by ammonolysis, concurrently solubilizing hemicellulose. At the end of the ammonia recycle percolation (ARP), the solid material left consist of short-chained cellulosic material containing a high amount of glu — can [88]. Some of the common literature examples of ARP are in corn stover [84, 89-91], switchgrass [92] and poplar [93, 94].
Ammonia recycle percolation has been applied primarily to herbaceous biomass materials with high lignin removal. For example, 60-80% delignification has been achieved for corn stover and 65-85% delignification for switchgrass [95]. Another advantage of this process is that washout is not needed as the production of inhibitors is low [96]. In an attempt to minimize the costs and optimize the process, and also establish a continuous process by reducing the liquid ammonia throughput, Tae et al. developed the low liquid ammonia recycle percolation (LLARP) process [91]. In comparison to the common ammonia recycle percolation (ARP) process, this process significantly reduces the liquid throughput to one reactor void volume in packed bed (2.0-4.7 mL of liquid/g of corn stover) and, thus, is termed low-liquid ARP (LLARP). In addition to attaining short residence time and reduced energy input, this process achieved 59-70% of lignin removal and 48-57% of xylan retention. With optimum operation of the LLARP to corn stover, enzymatic digestibilities of 95, 90, and 86% were achieved with enzyme loadings 60, 15, and 7.5 filter paper units/g of glucan, respectively. Furthermore, in simultaneous saccharification and fermentation test of the LLARP samples using Saccharomyces cerevisiae (NREL-D5A), an ethanol yield of 84% of the theoretical maximum was achieved with 6% (w/v) glucan loading [91].
High liquid loadings and energy costs are the main disadvantages associated with the ARP process, which are still some major concerns that need to be addressed before the ammonia recycle percolation method is proven to be economical in large-scale ethanol plants.