About Solvent Recovery
So, you’ve decided to shop for a solvent recovery unit, only to find that the salesmen and literature speak a language that is very similar to yours, but very undecipherable. The purpose of this document is to explain how a solvent recovery system works, what features to look for, and which highly vaunted specifications are totally useless, besides being misleading.
By the time you get through reading this, you should be able to do the following:
- Know what to expect from a solvent recovery system in real terms.
- Assess your own needs.
- Be prepared for the “hidden costs” of some stills.
- Know what basic features you require in your system.
- Know what convenience features you require in your system.
One thing to remember is that if a huge price differential exists between two quotes that seemingly have the same specifications, they are most likely not equivalent. Very small passages of text can correspond to very large differences in price. It is often helpful to create your own specification list to assure that the selected manufacturers are quoting the same specification. By making special note of all exceptions, you will be able to see where the price differentials really exist.
‘The fundamental differences between still costs are normally found in the heating method, control automation, convenience and safety features. In general, you will find that more expensive units will have circulation hot oil heating systems, complete automation via multiport PLC or computer, and a full complement of safety and convenience features, including but not limited to multipoint temperature control and vacuum operation. Be aware that different manufacturers will quote these different ways, with some packages logically being lumped together with certain features that you may not want or need.
Distillation is older than history itself, and occurs naturally in the atmosphere every day. The popularity of solvent recovery and the increase in the use of stills can be linked to government regulations and insurance costs where worker exposure is involved. Developments in automation hardware and software make it possible to offer safe, manageable stills in size ranges useful to industry for solvent recovery.
It is very important to know what to expect from your solvent recovery system. First, figure out how many gallons or drums you require the still to recover per time period, such as per day, week, or month. Then reduce this to a simple gallon per hour figure. Next, check the literature of the competitive units to see that they provide throughputs in this range. It makes manufacturers nervous to quote throughputs on a broad range of solvents. They generally quote rates based upon some very low boiling, still friendly solvent such as MEK or Toluene. Chances are, you are not processing this type of material, or if you are, it is mixed with a veritable smorgasbord of other materials including water. A reputable manufacturer will be more than happy to run a laboratory evaluation on a production still with material that you provide out of your current waste recycling system.
WHAT DIFFERENCES WILL I FIND IN COMPETITIVE EQUIPMENT?
It is important to note the throughput of a still is not based 100 percent on pure physical size. In general, throughput rate is a ratio of specific heat to vapor space and condenser efficiency. Just because one manufacturer specifies a still with a 300 gallon vessel and the next specifies a volume capacity half as large, don’t assume that the smaller unit will produce less than the larger one. If the smaller of the two units boasts a throughput rate that is equal to the larger, it is likely that it has a different heating system, more accurate controls, and a more thoughtfully designed vapor path.
Heating systems fall into 2 categories -- convection and forced circulation systems. As a general rule, forced circulation systems are more efficient, control tighter tolerances, and offer more and better convenience options than convection heated stills. It is far more likely that you will encounter a convection system on either a very small still (below 20 gallons where the heating system makes little difference) or on a larger still that has been “built to a price.” A circulating hot oil system offers the following benefits over convection systems: 1. faster heat-up 2. Higher energy input 3. Less thermal oil used 4. No hot spots, which can cause control problems or worse 5. Allows use of heat exchanger for “quick cool-down” 6. Reduced chance of temperature overshoot
Most of the benefits of the circulating system are because it allows a system which can transfer very high absolute amounts of BTU’s from the heating element to the process fluid without having a high thermal mass and its associated high hysteresis. In other words, the circulating system heats up quicker, cools down quicker, and controls to a tighter tolerance than a system that has much more oil in the system, therefore a higher thermal mass.
CONVECTION HEATING involves a calrod-type immersion heater in the bottom of a large vessel filled with thermal oil, which forms the jacket for the still vessel. No pump is used to circulate the oil, so the bulk of the oil is heated by thermal conduction, or is displaced in small eddy currents caused by the heater. The type of heating causes hot spots in the still chamber, and unusually long heat-up and cool-down periods. This is especially notable as the volume of oil in the system is increased.
Because convection heating is so slow in heating and cooling; overall process efficiency is compromised. Note that the bottom of the still vessel becomes a hot-spot, causing the process to be hard to control.
In general, the volume of oil is kept to an absolute minimum to decrease thermal mass. The thermal oil is non-carbonizing and requires replacement twice a year in all units, no matter who the manufacturer is. Most circulation systems are self-draining, but a few are not. All properly designed stills also have an expansion tank to compensate for the oil’s thermal space requirements.
In a circulated hot oil heating system, the heating oil in the vessel is pumped from the still vessel jacket to a small heating vessel with a calrod [immersion] heating unit in it. Heat-up time is determined by the volume of oil in the heating vessel rather than the volume in the jacket proper. Circulation of the oil eliminates hot-spots in the jacket, which can cause numerous process problems. The system also allows for the quick cooling of the jacket, which allows the dumping of still bottoms sooner and at lower temperature. The smaller volume of oil being heated at any one time also allows much closer control of overall jacket temperature, (typically within 1 degree F, +-].
In addition to the type of heating system used, you will also want to look for and determine the usefulness of the following features. Each of these can contribute to the end cost of your still, but any of them might save you thousands of dollars over the useful life of the still, depending on the material that you are recovering, the composition of the still bottom, and how far to dry you wish to take your sludge.
WHAT BASIC FEATURES SHOULD I LOOK FOR?
1. Materials of Construction: Some stills are built completely of T304 or T316 Stainless steel. It is important to make sure that your still has this feature if you use hygroscopic solvents (most are) or if you could conceivably mix your solvent borne and waterborne recovery systems. Water and acid distillation by-products can eat through carbon steel still quickly. Make sure to check the entire solvent stream, including piping, valves, condenser, scraper shaft and mounting arms. 2. Sturdy and Easily Adjusted Scraper System: the best material for the purpose of building scraper blades is aluminum bronze. Unalloyed bronze is too soft and wears quickly when scraping over a stainless steel cone. In addition, make sure that the scraper blades can be removed without completely disassembling the still. They must disassemble easily from the inspection hatch. 3. Get the most Efficient Condenser Possible: Many stills use old technology shell—and-tube condensers. Flat plate heat exchangers offer many times greater cooling surface area, making them more efficient, and a higher velocity gas and liquid path, which keeps them cleaner for all around superior operation. 4. Mechanical Seals: If you specify a hot oil circulation heating system, make sure the manufacturer fits the system with a pump equipped with mechanical seals. A packed pump will not survive in an environment that routinely reaches 500 degrees F. Oil leaks at this temperature are dangerous. 5. Make Sure You Can See: Most still manufacturers do not place a high priority on observing the process, so they provide a single sight glass. Imagine being perched on a ladder trying to look through a single sight glass while aiming a flash light at the liquid surface. The best possible setup is 2 sight glasses, one to look through and one that has an XP light mounted. Some premium units include a clean solvent flush that spray cleans the sight glasses. 6. RTD’s: Platinum RTD’s (resistance thermal devices) are the preferred way to tell temperature in a still. They are very accurate in all ambient conditions, and do not drift or need calibration like thermocouples. They need no associated circuitry to amplify or buffer their signal, and are, for all practical purposes, indestructible.
WHAT CONVENIENCE FEATURES ARE AVAILABLE?
1. Auto-Cool Down: This feature is essential in increasing still throughput and limiting worker exposure. The sludge is cooled from boiling temperature by a heat exchanger mounted in the circulation heating loop. Most manufacturers discharge still bottoms at a dangerously high temperature into external agitated sludge tanks for cooling, if they offer a discharge option at all. 2. Vacuum System: In addition to a vacuum depressing the boiling point of solvents to make their recovery more efficient, it also allows the loading of the still without an additional pump for loading. The vacuum pump literally sucks feed material into the still. 3. Sludge temperature monitoring: Improves the overall efficiency of the still, and is not offered by many manufacturers, because they can monitor the whole process more safely by reading the vapor temperature. A good still should monitor both. Monitoring sludge temperature allows you to use only the energy you need to get the job done. 4. Programmable Stepped Heat Application: Allows the control computer to apply only the heat needed to raise the temperature of the process as much as needed without overshoot. It can also compensate for differences in hysteretic heat values in different solvents. Just as much as you need, when you need it.
WHAT ARE THE HIDDEN COSTS OF RECOVERY? The hidden costs of solvent recovery manifest themselves in three phases; in setup, in day-to-day operation and in maintenance.
In setup, make sure that you know how a unit is being shipped, whether or not it has been tested as a unit, and how much labor it will take to get the unit operational in your plant. The best stills are of unitized construction and come pre-plumbed and prewired...and most importantly, all functions are tested in operation prior to shipment. Most such stills come shipped in as few as three pieces, and can be easily located, installed and started in one day. Many still manufacturers will dump a truckload of parts at your dock, and then move on to the next customer. Which do you prefer?
To ease day-to-day operation, it is necessary to have a well-integrated control package with all processes being controlled by a single computer. This allows many convenience features like ramped heating, vacuum distillation and auto—cool-down to be integrated on one machine, yielding outstanding efficiency and performance.
Maintenance can often be the single largest expense category for a processing machine. Here, look for features that make it easier to work on the still. Features like steps and a work platform make it easier when maintenance is required. Features like sufficient light to see the process and a sight port washing system allow for better understanding (and therefore control) of the process. An easily maintained scraper system is absolutely necessary, as these are the only wear parts on a still.