Fuel Treatment Plant
Although Gas giants are characterized by being made primarily from hydrogen, they vary in the makeup of their outer atmospheres. Most typically, a starship skimming fuel will obtain a number of unwanted chemicals including helium, ammonia, methane, and assorted other hydrocarbons resulting in unrefined fuel. Likewise, fuel skimmed from an ocean will contain a variety of salts and dissolved solids. A fuel treatment plant removes all of these leaving behind chemically pure hydrogen (H2) in liquid form.
In most cases the filtered chemicals are dumped overboard, but some (such as the oxygen from water) may be retained.
Shipboard Fuel Labs
Larger ships typically employ a very large fuel treatment plant along with a fuel lab, while smaller ships use a much smaller, automated device.
The Vland system has no gas giants, and neither did Tauri, the first system the Vilani explored with stl drives. So the Vilani did not encounter a gas giant until they arrived at Kirma around -9700. The question of who first skimmed a gas giant for hydrogen is lost to antiquity, but Kirma was the likely site. By the time the Jump Drive was developed in -9235, the concept of skimming hydrogen was well-established as a means to refuel vessels.
The link between unrefined fuel and misjump was quickly discovered by the Pre-Imperial Vilani, and their second generation of starships came equipped with a fuel treatment system to liquefy and purify both water and gas giant atmospheres. By the time of the establishment of the Ziru Sirka, these had been standardized throughout Vilani space.
As each later race developed the Jump Drive they learned of the necessity for purified hydrogen by trial and error.
Drive Malfunction & Fuel Correlation
One of the two primary factors influencing starship malfunction are unrefined fuel and lack of maintenance. Military Ships tend to be built with more capable and tougher reactors capable of handling either unrefined or refined fuel. Civilian Ships are more often built with cheaper reactors that handle refined fuel well, but have a relatively high failure rate when using unrefined fuel. The odds go up for a misjump when unrefined fuel is used within an economy civilian reactor. This forces most civilian ship captains to spend more time refining unrefined fuel supplies. On-ship fuel refining equipment can vary greatly in cost and capability.
Fuel can quickly add up costwise and the cheaper fuel, Unrefined Fuel, adds extra risk, danger of drive malfunction, and misjump to a voyage. For this reason, many ship captains prefer the more time-consuming, but less expensive option of Wilderness Refueling, which is to say the stopping to collect water off a planet or from icy asteroids (...which can be refined into hydrogen fuels), or the collection of raw fuel substances from a gas giant. Such ships employ Fuel Scoops and a Fuel Treatment Plant. The ability to collect fuel on the fly is also of great value to Exploration Vessels and Scout Vessels. A military implication of this fact is that large areas (...rifts) without significant planets or fuel sources become strategic blocks to starship and warship movement. Jump Bridges of various types are one of the few solutions to this problem and they often come at great expense of resources, funds, time, and effort.