Hangers Cleaners is trying to edge out dry cleaners with a new eco-friendly cleaning system that uses biodegradable soap dissolved in liquified carbon dioxide, a substance considered so safe it is used to inject the fizz into soft drinks. Traditional dry cleaners rely on an industrial solvent, perchloroethylene (perc), which is believed to cause cancer and other health problems and which is a major source of groundwater pollution. Hangers, a burgeoning chain, has reported that repeat business at its first stores has been high and that some 40 franchises are in the works. Still, the company’s growth has been slower than expected, in part because its cleaning system, while easy to operate, is large and pricey — $150,000 a unit, compared to $60,000 for a top-of-the-line perc system.