So, your challenge is, with a PoD after July 21, 1830, to make what was OTL's Belgian Congo/Congo Free State a possession of the Kingdom of the Netherlands. Belgium must remain an indepedent nation.
EDIT: The Dutch must also keep the East Indies as well.
There are a couple of problems with a Dutch Congo. First of all the Dutch stopped caring about Africa in the 19th century. They actualy still had some colonies in Africa (Dutch Ghana), but sold them to the British. That leads to the next problem, if the Dutch want an African colony, the most likely choice would be an increased Dutch Gold Coast, not Congo. Besides that there is the danger of overstretching. The Netherlands is only a small country; it can only have a limited amount of colonies, before Dutch resources are spread too thin, including most importantly Dutch administrators and colonists, but also money (the Netherlands was pretty poor during the 19th century).
Too solve these problems I propose this scenario:
he Netherlands does better than OTL during the Belgian revolt and manages to keep Antwerp and occupies more of Limburg (and Luxemburg?). So in the peace the Netherlands keeps Antwerp and western/OTL Belgian Limburg (and Luxemburg?). Because of the important Antwerp port and the coalmines of Limburg the Dutch economical situations is better than OTL (also the Netherlands have a slightly larger population). In the late 19th century the Netherlands decide not to sell the Dutch Gold Coast to Britain (this also leads to a British Aceh, which was part of the same deal). Because of the larger population, stronger industrial and economic positio and lack of Aceh (the most troublesome part of the Dutch East Indies), the Netherlands is less overstretched than OTL (although OTL was still within the Dutch limits). Although the Dutch still have the Gold coast in Africa, they don't realy care a lot fo it, it is scattered between various British outposts and does not make a lot of money. Anyway the scramble of Africa happens and the Netherlands is invited. The problem of the Congo comes up. None of the great powers wants any of the other great powers to have it. Belgium, lacking the port of Antwerp, is not interested either. The options left are Portugal and the Netherlands. The Netherlands offers to trade their, rather worthless, Gold coast outpost with Britain for the Congo. The British accept the offer. The Germans, who have a good relationship ith the Netherlands, accepts it and France agrees too. TheCongo is smaller than OTL Belgian Congo as parts of it end up British, German or French.