One question before proceeding any further with this thread: why have you selected Calcium Chloride as a dessicant rather than a classic, readily available adsorbent such as the likes of Activated Alumina, Molecular Sieves, or even Silica Gel? I cannot believe that you could economically justify the selection. I know this beforehand because I've been in that position more than once and the economic, practical answer was always an adsorbent.
And that was years ago when the value of the Propane was much cheaper than what it is going for today - and not to even mention the environmental costs and the labor costs today.
The client specified that we provide them with molecular sieve and CaCl2 dehydration designs. They seemed to imply a preference for CaCl2.
Personally, I know nothing about calcium chloride dehydration systems and nobody at my office has designed one. So I'm a bit lost.
About a year ago, I interned at a specialty chemical company doing scale control. It seems like that might be a problem depending on the composition of the water being removed.
A couple of years ago I was involved in a project where we also had CaCl2 based dryers for liquid propane. The CaCl2 brine is heavier than liquid propane so you get two liquid phases. The brine phase needs to be drained away every once in a while. This is also described here:
this would seem no different than draining water off any other light HC vessel. You could use a dP cell (there's a good difference between brine and propane for SG), a displacer or some other technology.
I'd be concerned what is the effect on my drain system if I should lose brine level and start to drain propane. If the propane flashes, does that create problem, can the drain system handle propane instead of brine. If draining propane creates significant operational or safety issues you might need another low level trip, independent shut-off valve etc.
A couple of years ago I was involved in a project where we also had CaCl2 based dryers for liquid propane. The CaCl2 brine is heavier than liquid propane so you get two liquid phases. The brine phase needs to be drained away every once in a while. This is also described here:
I did not design the towers, but they were 9.2 m high with a diameter of 3.4 m. The design propane flowrate was 480 m3/hr. So residence time based on propane was about 10 minutes.
Thanks for your quick reply and candor. It’s important to know just what the nature of the query is because of many trade-off factors, one of the most important being the safety issue. I have employed CaCl2 drying on several gases – one being Acetylene – and did a study on LPG. The LPG application was turned down on economics and practicality. There are many factors that you may not be aware of in such a seemingly benign and simple process and I’ll offer them to you for consideration:
1) The Process fluid is fed at the bottom of a packed bed of consumable desiccant within a tower. The operating pressure is not a practical factor on the products dewpoint, so if you have an atmospheric process, you can employ non-pressure vessels. However, if you are dealing with a liquefied gas – such as LPG – you are forced to design the vessel to the liquefied conditions which dictate a pressure above the fluid’s critical point. In the case of LPG, we all know immediately that we are talking about “bullet” storage tank design pressures: 200 to 300 psig. There are no savings in the pressure vessels.
2) The Calcium brine formed by the water content precipitates (against the LPG upward flow) down to the tower sump where it accumulates and is drained periodically to avoid entrainment with the product. Your superficial velocity through the tower must allow for efficient phase separation between the two liquids. This, in my case, turned out to be an empirical and conservative number in order to avoid a process upset. This resulted in a large diameter pressure vessel – something we all want to avoid from an economical point of view. This is a troublesome point in this type of process because it is obvious that you must provide an empirical contingency – and it turns out to affect the most cost-sensitive part of the process, the tower wall thickness.
3) The process is an inherent, semi-continuous batch process and requires two towers: one in operation and the other being recharged. This is no different than a standard adsorption system design. However, in this case manual shutdown and replenishment of desiccant is a requirement. Not only is expensive, highly trained, and senior labor required but you also inherit the obvious explosive and dangerous hazards of opening up an LPG vessel. And you have to do this repeatedly every time you have to replenish the desiccant consumed. There is no alternative to this ominous situation. You must be prepared with fully trained and supervised personnel who understand the potential hazards in full detail and can react to any danger during the re-charging operation. This is expensive, time consuming, dirty, and very risky work. You don’t locate this operation near other operating equipment or areas. This system is normally isolated on its own, away from other units. This runs up the original capital investment and the operating costs as well as the instrumentation, which must be remote.
4) Now comes the real dirty part of the story. As TD2K astutely noted, there is something inherently wrong in an engineer assuming that a 2-liquid phase drainer apparatus will work efficiently 100% of the time – especially one draining corrosive and troublesome brine like CaCl2. And that’s exactly what happens in practice: the brine cakes up and corrodes everything it comes in contact with. The drain valves constantly leak. But if that wasn’t enough, the dissolved LPG gases flash during the draining and constantly pose a dangerous explosive atmosphere around the unit 100% of the time – especially when the unit designer and its superintendent are home asleep with their families. No one needs this type of experience in their careers. Bear in mind that we are talking about a hydrocarbon process fluid that is heavier than air and will accumulate on the ground and we know how many LPG disasters have been caused by this very characteristic.
5) If you succeed in designing and operating a safe unit 100% of the time, you’ll be rewarded with all the waste, contaminated brine that was drained off and now must be disposed of. For this you need manual labor and money to pay for its safe disposal.
6) Don’t forget that since you’ve designed a unit that incorporates a consumable chemical, you must provide additional capital for the CaCl2 safe and dry storage. That also involves a lot of manual labor (jocks, not wimps) to incorporate forklifts and back labor in receiving, storing, and distributing the desiccant drums. All in a controlled atmosphere building.
7) When you do all the above safely and successfully, you wind up with an LPG product water content that doesn’t even come close to what your competitor down the road is producing with an Activated Alumina adsorption unit.
No, I don’t recommend you go down this route. Working with a consumable, semi-solid product or slurry is a hassle and, in this case, also a dangerous hazard. In my experience there are just too many opportunities for something to go tragically wrong. I would do it if there were no options left to me. Fortunately, you have options – relatively safe ones.
I wish you all the luck in world in beating this proposal to death and burying it where someone won’t find it and dig it up.
I happen to know a bit about propane dehydration with CaCl2 and an working on a proposal right now. It's not as difficult as Montemayor is making it out to be and is done quite often.
Please note or read my post. I've not said that CaCl2 drying is "difficult". Hell, it's easier than any other option. However, the tradeoff is a hassle: it requires a lot of maintenance and operating problems - most or all of them bordering on risk-taking in hazardous operations. Its a simple and easy unit operation to design for a process engineer; it's a hell of a hazardous challenge to a plant operations engineer and to the maintenance department. A process design engineer does his job and walks away from the finished design; the plant engineer and maintenance department have to live with the unit day-after-day-after-day. It's no picnic.
If you are going to operate the unit in a 3rd world country where the labor is dirt cheap and human lives often are not considered as critical, then I can see the justification. Doing this in beautiful downtown Deerpark, Pasadena, or Texas City, Texas is not going to come across as very economical or safe. Just venting and sniffing the unit prior to opening it is a major step, let alone the dessicant loading and all the supervision and testing required for normal, safety reasons.
I've used and had to live with CaCl2 dryers - in worse and more dangerous applications than ordinary LPG: in Acetylene service at 350 psig. I would have loved to inherit the LPG application instead of the Acetylene one. I would have certainly slept better.
Until you've seen the results of a Hydrocarbon explosion or fire you can't conceptualize the horror and the destruction it creates. My experience forces me to accentuate the safety aspects in this particular application.
Thats all well and good, and by no means meant to offend anyone, but I've been doing this for sometime now and just thought I'd share my opinion. I guess I won't try that again.
On these Forums everyone, including myself, puts a high value on someone else's opinion or experience - regardless of whether it conforms with our thoughts or findings. We can all learn more by listening or noting other's opinions. I seriously doubt if anyone would be offended by what I consider to be your valid and important opinion.
In such a large and diversified forum membership we have engineers of all types and degrees of experience or expertise. Many times we have opinions that don't dovetail because they come in from a different perspective. If you are in the process design field, I can easily respect and accept what you say; I spent many years in the operations field with hands-on responsibilities before dedicating the remaining years to process design. I hope you can understand my adament stand on safety or related issues that touch on operating risks or hazards. I hope you don't interpret my opinions as contrary to yours or opposed to your ideas. Without a complete and detailed scope of work, design and operations engineers have little to base their recommendations on and only opinions to air. And this is what I think we have done here.
To reenforce Montemayor's concern over hydrocarbon operational safety. Checkout the pictures. And this guy really didn't know how to show the real damage.
I have to admit you know me better than most. I have a brother and nephew working at the BP Texas City Refinery, located 70 miles from where I live. My nephew was not on shift the day of the tragedy, and my brother barely escaped serious injury or death. You're absolutely right; I have grave concerns when it comes to hydrocarbon operational safety. The same brother was burned with 2nd and 3rd degree burns 7 years ago, working with hot crude lines. That's what I mean about the horror and destruction these type of accidents create. Thanks for understanding my meaning.
