The battle to keep a highly toxic chemical from exploding took a step backward Saturday.

The temperature in a critically failing tank filled with a highly toxic chemical and at risk of exploding in Orange County is rising, not falling, officials said.

By Saturday morning, the temperature in the pressurized tank at an aerospace firm in Garden Grove was 90 degrees, up from 77 degrees a day earlier. Temperatures are increasing about a degree per hour, Craig Covey, an Orange County Fire Authority division chief, said Saturday morning.

But with the chemical crisis in its third day, new details are offering more insight into how it could be resolved, though no one is sure when.

The question, experts say, is whether officials can somehow deal with the dangerous chemicals in a way that does not end in a blast or the type of spill that causes environmental degradation.

Trying to cool damaged tank

An increase in temperature raises the risk of an explosion.

The boiling point of the chemical inside the tank, methyl methacrylate, is 101 degrees Celsius. The temperature gauge on the tank detects temperatures only up to 100 degrees. Officials haven’t disclosed at what particular temperature would they think that an explosion is imminent.

It’s not necessarily the case that the tank would explode at 101 degrees, said Elias Picazo, assistant professor of chemistry at USC.

“It depends on the integrity of the tank, and the composition of the tank, and the pressure capacity of the tank,” Picazo said. “But, yes, above 100, the pressure starts to increase dramatically, because the liquid phase becomes gas phase, and gas takes up any space available. It’ll take up more volume and become highly pressurized.”

What does it mean that the temperature is rising?

The fact is that the temperature in the tank indicates that the liquid MMA molecules — monomers, essentially a bunch of single molecules — are reacting with one another to form polymers, forming a solid, according to Picazo. “The reaction is releasing heat. That’s going to initiate more reaction to happen, so it might even cascade.”

The big fear is producing what’s known as “thermal runaway reactions.”

Covey has said that if the temperature in the tank exceeds a certain threshold “we know the tank is going into thermal runaway, and we’re going to pull everybody out of the area, make sure it’s safe, and let the tank do what it’s going to do.”

The fact that some liquid in the tank is reacting to become a solid is probably what happened to gum up the valve leading into the tank. The primary solution to resolve this crisis would’ve been to pump a neutralizing agent into the problem tank, quenching it and making it no longer explosive.

But that valve is clogged up, and so there is no way to get the neutralizing agent inside the tank. Nor is there a way to slowly drain the tank of the MMA toxic chemical.

Keeping the tank as cool as possible could be a workable approach to prevent an explosion.

How are officials gaming out scenarios?

There remains the possibility that the tank will still explode or rupture in a massive leak that could send the chemical to foul up waterways and the ocean. Officials have marked a huge evacuation zone — anywhere from about 1 to 3 miles from the tank — affecting an estimated 40,000 residents covering portions of the cities of Garden Grove, Anaheim, Buena Park, Cypress, Stanton and Westminster.

It is clear that the spraying of water on the tank is helping.

Even though the temperature is rising inside the tank, not putting any cool water on it at all would have allowed the tank’s interior temperature to rise far more quickly, Picazo said.

So the main solution right now is for crews to do their best to keep the tank as cool as possible — and buy time.

How keeping the tank cool could avoid an explosion

Continuing to pour cool water on the tank could allow the liquid chemical inside to cure at a slower rate — becoming a solid at a slower speed — and reduce the buildup of pressure inside the tank, Covey said.

“Like an ice cube that freezes from the outside in — this stuff cures, it heats up and cures from the outside in. While it’s doing that process, it’s building that pressure,” Covey said.

The tank has some capacity to hold in some pressure. There is empty space between the surface level of the MMA chemical to the ceiling of the tank.

“We’re hoping that that space can absorb a slower cure rate and not over-pressure and blow up,” Covey said.

In other words, continuing to cool the tank could slow down the chemical reaction occurring inside in a way that avoids an explosion.

Picazo agreed.

“One of the best-case scenarios is to let the [MMA] monomers react, but you do it in a controlled way,” he said.

“Maybe if it’s slow enough, you can form solid within the tank and cause the monomers, the reactive monomers, to stay apart from one another.

“If they don’t come into contact, therefore they cannot react,” Picazo said. “You need contact for reactivity, and you can’t have contact if you have solid state.

At that point, “then you can start to think about other solutions of how to quench the unreacted starting material.”

Can the worse-case scenario be prevented?

Firefighters said they’re hopeful they can prevent an explosion.

“We’re optimistic,” Covey said. “We’re bringing people in from all over the country, talking to people all over the place, trying to come up with additional options.

“Letting this thing just fail and blow up is unacceptable to us.”

Why crews erroneously thought temperatures were cooling inside the tank

Officials on Friday had thought spraying water was actually cooling down the problem tank — and not merely reducing the speed at which the temperature was increasing.

On Friday evening, Picazo said drone thermometers indicated the tank was at 61 degrees, and the goal was to get the tank down to 50 degrees, which would be its “happy place.”

But as it turns out, the drone thermometers were detecting the temperatures only on the outside of the problem tank, not its inside.

Officials discovered the error of their assumptions when a crew of workers returned overnight to the problem tank, which has an estimated 7,000 gallons of MMA in it. Adjacent to the problem tank is a second tank, which has 15,000 gallons of chemicals in it, but is not at immediate risk of failure.

Nonetheless, officials wanted to inject a neutralizing agent into that second tank, so that if the primary failing tank explodes, it doesn’t cause an even greater blast by igniting the second tank. So there was an overnight operation of chemists and first responders sent in to try to get that done, which was attempted even though it put them “in harm’s way,” Covey said.

When they arrived, they were able to again manually read the internal temperature gauge of the failing tank. (That gauge isn’t visible unless someone is there to read it; it’s covered by the cooling sprays of water and cannot be seen from a distance, nor by putting a drone with a camera near it, Covey said.)

And that’s when the crew realized that the tank’s internal temperature was at 90 degrees, and that relying on drones to estimate the temperature from afar showed only the temperature of the outside of the tank, not the inside.

Staff writers Hailey Branson-Potts, Hannah Fry and Eric Licas contributed to this report.