THE OXYGEN CONUNDRUM

Oxygen is available in plenty in the atmosphere. It doesn’t have to be extracted from deep mines or manufactured in factories. It simply has to be pulled out of thin air, literally and figuratively, either by liquefaction or concentration. Then why is there such a serious and seemingly insurmountable shortage of this lifesaving gas in our hospitals? The COVID crisis has exposed the ugly underbelly of our hospitals, both government and private. The government, as usual, has become the target of all curses and insults by the media and ignorant public, which finds it easy to criticise the government for after that all other responsibilities are shrugged and forgotten.

Even large hospitals are seen to be using banks of steel cylinders, heavy and unwieldy, which are connected to the hospitals’ oxygen manifold and serve individual patients. There is no reason they shouldn’t have installed liquid oxygen storage facilities given their massive requirements. Smaller hospitals place the cylinder at the bedside of the patient that need to be replaced every few hours. Typical steel cylinders for medical use in India come in two sizes – 10 litre and 40 litre. They are made from seamless tubes of expensive CrMo alloy steel since the gas in them is stored at 150 bars or higher pressure. These 10 litre and 40 litre cylinders may weigh up to 18 and 40kg and accommodate only 1500 and 6000 litres of free oxygen respectively. So, an 18 kg cylinder delivers only about 2kg oxygen and the larger one contains about 8kg. Besides, as the pressure in the cylinder drops the rate of delivery drops as well.

Now, imagine the logistic nightmare our medical oxygen supplies are burdened with - expensive steel cylinders which need to be manufactured at great cost and the volumetric and weight inefficiencies of the delivery system. A 20kg cylinder gives only 2kg oxygen and a 50kg cylinder gives just about 8kg after which they have to be sent back for refills. Liquid oxygen, on the other hand, converts to over 750 litres of usable free gas for each kg of liquid. Let’s compare the logistics:

Delivery System	Total Weight including Oxygen	Deliverable Oxygen	Weight Efficiency of Supply Chain
40 litre cylinder	40+8 = 48 kg	8 kg	125 lt/kg
10 litre cylinder	18+2 = 20 kg	2 kg	75 lt/kg
Liquid Oxygen	1 kg pure oxygen plus a proportionate weight of the cold supply chain.	750 lt	500-750 lt /kg

                 There is a small weight penalty for liquid oxygen too since it is delivered by cryogenic lorries.

I am not even mentioning the cost of expensive cylinders, which is actually an unnecessary burden on the national economy. Besides, cylinders have to be handled, when empty and carried back to refillers. With liquid supplies there will be no handling of heavy cylinders by hospital staff, who could be utilized elsewhere more fruitfully. Besides, industrial cylinders repurposed for medical supplies, as is being done now, should ideally be flushed and purged of impurities to match the “drug standards” of medical oxygen.

Considering that almost all oxygen, industrial or medical, is obtained cryogenically by liquifying atmospheric air to below minus 183 deg C and then regasified to fill the steel cylinders, it is easy to skip the bottling activity and deliver liquid oxygen directly to hospitals to be gasified on the spot. Indeed most metal industries and factories, which use oxygen, have an onsite storage of liquid oxygen. There is no reason all large hospitals, which have been in business for decades, shouldn’t have such a storage. Indeed, it is a criminal neglect. Liquid oxygen is delivered by road lorries in a cryogenic supply chain from the oxygen liquefaction plants to the end users. Modern cryogenic storage tanks have such efficient insulation that they are often built in the open facing direct sun, yet lose very little oxygen from the tanks. Liquid oxygen, as supplied to hospitals, will also be inherently more pure for the simple reason that impurities have different temperatures at which they become liquid. 

There are two ways in which liquid Oxygen can be introduced in hospitals:

1. Large hospitals should install multiple cryogenic ground tanks of capacities ranging from 20 to 40 tonnes. They will be refilled by liquid oxygen lorries periodically. This is also the way most industries procure and store oxygen. 

2. There is a need to design smaller cryogenic tanks, not unlike large thermos flasks,  that can be delivered prefilled with liquid oxygen from oxygen plants to smaller hospitals just the way we get Liquefied Petroleum Gas in our homes. These tanks will be swapped with empty ones and each hospital can even have a few spare prefilled tanks for handling contingencies, temporary shortages, or surge in demands. This can also be an interim arrangement for large hospitals till their larger tanks are built.

Most hospitals are already pipelined for handling such supplies. Cryogenic tanks can, in most cases, be accommodated in the same or less space that banks of oxygen cylinders take up today. Liquid oxygen is stored at much lower pressures than compressed gaseous oxygen and is actually safer in handling and storage.

There will be a need to build a fleet of cryogenic tanker lorries exclusively catering to medical needs since purity levels of medical oxygen are more stringent than industrial oxygen. In fact medical oxygen is legally a drug and must meet those requitements. 

An adult breathes in about 10,000 - 11,000 litres of air (20% Oxygen) in 24 hours. He will breathe the same volume of oxygen (99%) in that period. Forty and ten litre cylinders will last only 13-14 hours and 3-4 hours respectively. Hence there is a need to create an uninterrupted supply of oxygen, which can be ensured only with liquid storage.

A ten-tonne lorry that carries, say 200 large cylinders, delivers only 1600 kg of oxygen. A ten-tonne cryogenic lorry, on the other hand delivers six times as much. The liquid supply chain removes eighty lorries for every hundred from our roads as well.

To summarize, there is an immediate need to promote, rather mandate, storage of liquid oxygen in our hospitals forthwith. There is no shortage of oxygen even today. It is the cumbersome supply chain that has let us down. The solution is easy and is staring in our faces.

Let's do it.

 

Lessons from the Texas Blackout of Feb 2021

The unprecedented, though not unforeseen, blackout in Texas has a lesson for energy planners in general and mechanical and electrical engineers in particular.

Texas is a relatively warm state of the USA. But, an unusual snow storm and resultant low temperatures caused the electricity demand to rise beyond previously known peaks. Unlike colder states, where building heating systems are gas based, Texans use electricity since the heating demand is low and sporadic. A grid collapse can happen even with a 5% overload. In practice the grid managers go for a cyclic or rolling blackouts - area wise or time wise - to prevent damage to the grid and equipment. That is what was done, and it was done rather well, just a few minutes before the grid collapsed. The rolling blackouts disfavoured poor areas since they didn’t have high priority establishments like hospitals amidst them.

Texas takes pride in having its own independent grid rather than be connected to the Eastern or Western grids. If they were connected, the state could have procured power from a larger grid to tide over the crisis. They couldn’t do that. 

The gas and oil equipment as well as windmills, which were designed for a warmer environment “froze”, more specifically the lubricants in them froze or became so thick and viscous that they simply shut down. The gas wells themselves “froze” thus reducing the gas output to nearly half of the regular levels.

Solar energy, a matter of pride for Texans, failed too since there was no sun and hence no output.

The overzealous alternate and renewable energy proponents had clearly not foreseen this. Renewables and alternate energy form a substantial part of the energy mix in the state. That a reliable baseload plan is unavoidable is a major lesson for all energy planners. Baseload comes primarily from coal, oil, gas, hydro, or nuclear sources, like it or not.

A permanent solution may take years coming.