Vacuum pumps are used in freeze drying (lyophilization) to remove the water or other solvent from the product being dried. The process begins by freezing the product, then placing it under vacuum. As the vacuum is applied, the frozen water or solvent sublimates, or turns from a solid directly into a gas, leaving behind only the dried product. The vacuum pump is used to maintain the vacuum throughout the process, which can take several days or even weeks to complete. The vacuum pump is also used to pull the water vapor out of the drying chamber, where it can be collected or recycled.