Sickle cell disease is a genetic red blood cell disorder characterized by vaso-occlusion from sickling of red blood cells, that can lead to pain or organ complications such as acute chest syndrome. Sickle cell disease is associated with low amounts of nitric oxide, a compound important for dilating the blood vessel wall. Citrulline is a substance that is known to increase nitric oxide. The goal of this Phase I study are to find the highest safe dose of continuous IV citrulline that can be given to individuals with sickle cell disease experiencing a sickle cell pain crisis or acute chest syndrome without causing severe side effects.