Understanding activated carbon reactivation and low - temperature regeneration technology

Activated carbon is one of the mainstay technologies for the decolorization of raw sugar to produce refined sugar products. This paper discusses issues relevant to the application of granular activated carbon in the sugar refining industries. The topics covered include evaluating the performance of recycled adsorbents, the evolution of activated carbon properties during initial activation and reactivation, and a review of analytical methods for measuring adsorption properties. The second half of the paper explores a new lower temperature process being developed for the regeneration of spent sugar decolorization activated carbon. In particular, the features and performance of this regeneration approach are evaluated against “high-temperature reactivation”, which is the dominant practice in the sugar industry today. The paper concludes with a discussion of the energy requirements and capital costs of low-temperature regeneration and high-temperature reactivation. tion into the internal surfaces of the adsorbent, but the color bodies remain chemically unchanged. While the effect of the adsorbent on the sugar solutions is conceptually straightforward, in that unwanted compounds are removed from the aqueous sucrose solution, the impacts and consequences of the adsorption step on the adsorbent are considerably more complex. Once the adsorbent has accumulated its capacity for adsorbable compounds, it is termed “spent” and must be replaced with fresh adsorbent. Exiting the adsorption process, after sweetening-off, any spent adsorbent consists mainly of the original adsorbent and the accumulated color bodies. One option for spent adsorbent is disposal after a single adsorption cycle. This is the typical industrial practice when using powdered activated carbon. While powdered activated carbon is probably the least expensive adsorbent to purchase on a unit-weight basis, disposal after a single decolorizing cycle generally results in higher operating costs than other adsorption media that can be regenerated and reused. Furthermore, the spent powdered media with color bodies represents a continuing environmental concern, since the color bodies are subject to biological degradation, which can lead to odor issues and water contamination concerns. For granular activated carbon and ion exchange resins, regeneration and reuse is the industrial norm. It should be noted that regeneration speaks to the impact on the adsorbent – recovering some or all of the adsorption capacity – and does not specify the impact on the previously adsorbed color bodies. In the case of ion exchange resins, the regeneration process desorbs the color bodies into another aqueous solution – and the disposal of this “spent regenerant” solution must be addressed. Depending on the local circumstances, the outlet may be a POTW (Publicly Owned Treatment Works), a dedicated on-site treatment capability or discharge untreated into receiving waters. While the last practice may be exceptionally cost-effective, the deterioration of the local waterways is often undeniable and unacceptable over the long term. When granular activated carbon is utilized for color removal, the subsequent regeneration process typically results in the destruction of the color bodies. The most prevalent industrial practice is termed “high-temperature reactivation” and is usually performed in multiple hearth furnaces or rotary kilns. The process is called “reactivation” because the regeneration conditions mirror the original conditions utilized during the manufacture of the virgin activated carbon in the “activation” step. Another option for the regeneration of spent granular activated carbon is termed “low-temperature regeneration” also referred to as “CarbOxLT” in the literature. This approach is a recently developed option that appears to have significant advantages over traditional “high-temperature reactivation” and these features will be explored later in this paper. In order to explore the technical and operational considerations involved in the regeneration of spent granular activated carbon, it is necessary to develop a framework for evaluating the performance of a regenerable and reusable adsorbent. After establishing this framework for the current practice in the real world application of sugar decolorization, the performance advantages of low-temperature regeneration over high-temperature reactivation will be discussed. Understanding and evaluating the performance of recycled adsorbents Before delving into the specific differences in the regeneration options, it is useful to understand the interactions that occur between the effectiveness of the regeneration process and the make up rate of virgin activated carbon. The current industrial practice is to add virgin carbon to “make up” the amount of carbon in circulation to a constant volume, in order to keep the adsorbers full during the decolorization process. Hence, there is a tendency to consider the rate of 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 0.00% 1.00% 2.00% 3.00% 4.00% 5.00% 6.00% 7.00% 8.00% 9.00% 10.00% Capacity loss per cycle P o o l c a p a c it y a s % o f V ir g in G A C a d s o rp ti o n