Thanks for the great question! There are really 2 questions here, so I'll try to answer them separately.
First question: What are the safest ways to extract? When it comes to extraction, safety is an important issue and has multiple areas to consider. The list below represents some of the major areas that need to be addressed with the popular solvents being used in the cannabis industry today:
Design - The extraction equipment needs to be designed to handle the solvent being used
Materials of Construction- Stainless steel materials for food/consumed oil applications
Electrical for Flammable Solvents - Class 1, Division 1 (explosion proof) electrical components for compressed flammable gasses, Class 1, Division 2 for ethanol/alcohol
Electrical, Non Flammable Solvents - NEMA 4x wash down electrical enclosures
Pressure Rating - usually 300 psi for hydrocarbons, 2000 or 5000psi for CO2.
Overpressure Protection - non-isolable relief valves set to 110% of maximum allowable working pressure
Purpose - The equipment needs to be constructed for its intended use
Food grade - welds in contact with extracted material should be ground flush and polished
Accessibility for Cleaning - vessels and piping should be accessible from both ends to allow proper cleaning
Storage tanks - should be stainless steel to prevent corrosion
Facility - In addition to the equipment considerations, the facility must also be appropriate for the extraction solvent
Compressed Flammable Gasses - Class 1, Division 1 facility. This includes electrical fixtures, and also monitoring and evacuation equipment in the event there is a release of flammable gas into the area around the equipment.
Ethanol/Alcohol - vent hood or equivalent walk in vent area
CO2 - asphyxiation hazard. Monitoring and audible alarm to warn of leaks.
Human Consumption - The solvent should be safe for consumption by humans
Compressed gas - GRAS for use as a propellant, states differ on safe residual solvent levels
Ethanol - GRAS for food products, states differ on safe residual solvent levels
So the answer to the question about safety really doesn't have anything to do with the solvent, rather the equipment chosen and the facility where the extraction is performed determine safety. The solvents commonly used in cannabis extractions today all have pros and cons, and all can be operated safely as long as proper guidelines and regulations are followed.
I addressed the safety question in the first part of the answer, in the second part I'll address the efficiency question:
What is the most effective way to extract CBD-rich cannabis oil?
A major problem facing the cannabis industry today is a lack of commonly accepted standards - as evidenced by the question referring to "CBD-rich". Does "CBD-rich" mean 40% CBD? 99% CBD? And CBD in what form, CBD, CBD-A or some combination? There are groups that are working towards creating standards, such as
ASTM, but they have not been widely accepted yet. Without standards, quality becomes difficult to determine because the only standard is personal subjectivity.
That being said, there are some generalizations about extraction methods that can be made. Keep in mind -
every extraction method has pros and cons. Each method will shine in certain applications, and perform poorly in other. No method is great at everything.
Selective and tunable for different molecular weights
No residual solvents - great for vape pens
Cold extractions and separations are good for temperature sensitive extractions (i.e. terpene preservation)
Minimal facility safety requirements/costs - just signs and CO2 monitor
Automation is easy and available
Equipment is expensive
Manually operated systems are complex to learn
Extraction rates are slow on less expensive equipment (competitive rates for more expensive equipment)
Less expensive equipment than CO2
Fast extraction rates
Great for dabbable products like shatter, honeycomb, crumble
High potency levels
Facility is very expensive - Class 1, Division 1 requirements
Automation is difficult and expensive due to C1D1 requirement
Heavily scrutinized by local regulators and inspectors
Not selective or tunable
Scaling challenges from limits on amount of hydrocarbon that can be on site (150#)
Residual solvent testing required (adds additional time and expense)
Very inexpensive equipment
Facility is less expensive than hydrocarbons
Less power intensive than CO2
Very fast extractions - great for distillate products
Not selective or tunable
Will freely extract chlorophyll
Requires significant levels of secondary processing