ANS:C - multiple effect evaporator.

multiple effect evaporators followed by a crystallizer. Explanation: Black liquor, a byproduct of the papermaking process, typically contains a high concentration of dissolved solids, including lignin and other organic compounds. To recover these solids and concentrate them for further processing or disposal, the black liquor is often subjected to an evaporation process. The most common method for concentrating black liquor is using multiple effect evaporators followed by a crystallizer. Here's how the process typically works:

1. Multiple Effect Evaporators: Multiple effect evaporators utilize the heat from steam to evaporate water from the black liquor. The evaporated water vapor from the first effect is used to heat the second effect, and so on, thereby increasing energy efficiency. As the black liquor moves through each effect, it becomes increasingly concentrated.
2. Crystallizer: After the black liquor has been concentrated to a certain extent in the evaporators, it may then be sent to a crystallizer. In the crystallizer, further concentration and cooling of the liquor can lead to the precipitation of solid materials, such as lignin and other organic compounds, forming crystals. These crystals can be separated from the remaining liquid phase for further processing or disposal.

By using multiple effect evaporators followed by a crystallizer, the concentration of solids in the black liquor can be maximized efficiently, allowing for the recovery of valuable materials and the proper treatment of waste products in the paper manufacturing process.