Countercurrent Exchange

Countercurrent exchange is a mechanism used to transfer some property of a fluid from one flowing current of fluid to another across a Semipermeable membrane or thermally-conductive material between them. The property transferred could be heat, concentration of a chemical substance, or others. Countercurrent exchange is used extensively in biological systems for a wide variety of purposes. For example, fish use it in their gills to transfer oxygen from the surrounding water into their blood, and birds use a countercurrent heat exchanger between blood vessels in their legs to keep heat concentrated within their bodies. In biology this is referred to as a Rete mirabile. Mammalian kidneys use countercurrent exchange to remove water from urine so the body can retain water used to move the nitrogenous waste products. Countercurrent exchange is also a key concept in chemical engineering thermodynamics and manufacturing processes, for example in extracting sucrose from sugar beet roots.

Concurrent exchange and countercurrent exchange

The diagram presents a generic representation of a countercurrent exchange system, with two parallel tubes containing fluid separated by a semipermeable or thermoconductive membrane. The property to be exchanged, whose magnitude is represented by the shading, transfers across the barrier in the direction from greater to lesser according to the second law of thermodynamics. With the two flows moving in opposite directions, the countercurrent exchange system maintain a constant gradient between the two flows over their entire length. With a sufficiently long length and a sufficiently low flow rate this can result in almost all of the property being transferred.

By contrast, in the concurrent (or co-current, parallel) exchange system the two fluid flows are in the same direction. As the diagram shows, a concurrent exchange system has a variable gradient over the length of the exchanger and is only capable of moving half of the property from one flow to the other, no matter how long the exchanger is. It can't achieve more than 50%, because at that point, equilibrium is reached, and the gradient declines to zero.

In a concurrent heat exchanger, the result is thermal equilibrium, with the hot fluid heating the cold, and the cold cooling the warm. Both fluids end up at around the same temperature, between the two original temperatures.

At the input end, we have a large temperature difference and lots of heat transfer; at the output end, we have a small temperature difference, and little heat transfer.

In a countercurrent heat exchanger, the hot fluid becomes cold, and the cold fluid becomes hot.

At the hot end, we have hot fluid coming in, warming further hot fluid which has been warmed through the length of the exchanger. Because the hot input is at its maximum temperature, it can warm the exiting fluid to near its own temperature.

At the cold end, because the cold fluid entering is still cold, it can extract the last of the heat from the now-cooled hot fluid, bringing its temperature down nearly to the level of the cold input.

Counter-current exchange of heat in organisms

Counter-current exchange is a highly efficient means of minimizing heat loss through the skin's surface because heat is recycled instead of being dissipated. This way, the heart does not have to pump blood as rapidly in order to maintain a constant body core temperature and thus, metabolic rate.

When animals like the leatherback turtle and dolphins are in colder water to which they are not acclimatized, they use this CCHE mechanism. Counter current heat exchangers are made up of a complex network of peri-arterial venous plexuses that run from the heart and through the blubber to peripheral sites (i.e. the tail flukes, dorsal fin and pectoral fins). Each plexus consists of a singular artery containing warm blood from the heart surrounded by a bundle of veins containing cool blood from the body surface. As these fluids run past each other they create a heat gradient in which heat is transferred. The warm arterial blood transfers most of its heat to the cool venous blood in order to conserve heat by recirculating it back to the body core. Since the arteries are losing a good deal of their heat, by the time they reach the periphery surface, there will not be as much heat lost through convection [1].

Counter-current exchange.

Transport processes often involve streams of material transport. For example, O2 transport across gill epithelium involves movement of material from a water current over the gills into a circulatory system that is also moving. Material exchange in these situations tends to be favoured when the streams are flowing in opposing directions (counter-current exchange). To see why this is so, consider first the co-current process.

Here, the maximum gradient occurs where the two streams first come into contact. Exchange between them lessens the gradient and the maximum exchange possible is an averaging of the concentrations in the two streams.

With a counter-current system, the gradient is lower than the initial co-current gradient, but the gradient is maintained throughout the length of association of the streams. In this way all but some fraction ( ) of the transported material is exchanged. The unexchanged fraction can be made smaller by increasing the length of contact between the two streams. This can be done by increasing the physical length of the zone of association of the two streams, or by decreasing flow rate in the streams. Counter-current mechanisms are almost always more efficient than co-current systems but the efficiency depends upon the time of association of the opposing streams. The long legs of wading birds contain very little metabolically active tissue, and represent a potentially extreme source of heat loss. Efficient counter-current blood flow in the legs allow much of this heat to be recuperated. In other systems, however, counter-current mechanisms may be only marginally more efficient than co-current mechanisms.

Heat exchange: legs of birds, body heating of fish such as Tuna and Mako shark; Salt and waste excretion in kidneys; O2 and CO2 exchange in gills of many invertebrates and fish, in lungs of birds (but not those of mammals).

Counter Current Multiplier Mechanism

Question - Can you please explain to me the counter-current multiplier

mechanism. I understand that cholride and sodium ions are filtered out

of the ascending loop of Henle into the interstial fluid, however, I'm

not sure exactly what happens from there and how this effects osmotic

pressure gradients in the nephron. Any help would be greatly appriciated.

This mechanism is very complex when it comes to writing a response. You

have to have a strong background in osmotic pressure understanding and the

anatomy of the kidney. It involves the cortex, outer and inner medula in

relationship to the vasa recta, interstitial fluids at two points, the

loop of Henle and the collecting duct. The size of the tubes and the

position in relations to the cortex and medulla is an essential part.

>> This function is dependent on the anatomical arrangement of the nephrons and

the vasa recta (blood vessels surrounding the nephron). The descending loop

of Henle carries urine filtrate down ward from the cortex into the medulla.

The ascending loop carries urine filtrate upward from the medulla into the

cortex. This sets up a situation where urine flowing in one tube is running

parallel and counter to fluid flowing in another tube. The descending tube

is permeable to water and impermeable to solutes. The fluid outside of the

tube is more concentrated and water leaves it by osmosis. This causes the

filtrate to become more concentrated. The ascending limb of Henle is

impermeable to water but permeable to solutes such as Na, Cl and urea. As

the filtrate moves up the limb the ions move out making the filtrate less

concentrated. The actual concentration is dependent on the hormone ADH or

anti-diuretic hormone. When ADH is present, water moves out of the

collecting ducts which are past the loops of Henle. This causes the urine to

be more concentrated. When ADH is NOT present, the water stays in the urine

and leaves the kidneys.

Continuous counter current process

Our SepTor continuous counter current process ensures optimal adsorbent utilization and leads to a far more efficient, compact and economical separation process compared to a conventional fixed column system.

The most common system for adsorption and ion exchange is the fixed bed process in which the adsorbent is being held in a stationary column.

The principle of SepTor Continuous Counter Current adsorption is explained below by evaluating the adsorption process in a fixed bed.

When a feed solution (φL) is being fed to a fixed bed vessel containing a resin, an adsorption or ion exchange reaction will take place. (Animation courtesy of Xendo Manufacturing)

During a continuous application of feed solution to the vessel, the adsorption or separation process moves as a front from the top to the bottom of the resin bed in the stationary vessel. This front that gradually moves through the bed is called the Mass Transfer Zone or active zone (MTZ).

For any set of parameters, such as the velocity of the feed flow, the particle size of the adsorbent, viscosity of the feed, temperature etc., this front or zone has a certain length, i.e. the “Mass Transfer Zone Length “ (MTZL). This MTZL usually comprises a very small part of the total length of the fixed bed vessel (the total resin bed length).

While the MTZ is gradually moving through the resin bed, it is only a small portion of the total resin bed that is active in the working zone. Thus one remaining big portion of the resin bed is exhausted resin (the adsorbent in the top part of the vessel is in equilibrium with the process fluid). The other part that is not yet exposed to the active ingredient in the feed solution is fresh resin. Both of these remaining resin portions are sitting idle, and here no activities are taking place.

Hence it the actual separation process takes place in only a small part of the columns resin volume.

When the MTZ has reached the exit of the adsorbent bed, the bed becomes saturated and needs to be washed and regenerated before it can be put in an adsorption operation again.

Since the washing and regeneration process consumes a certain amount of time, the continuous processing in a fixed bed will require a total of at least 2 to 3 beds.

Compared to the fixed bed process, SepTor continuous counter current adsorption aims to freeze the MTZ in the adsorption and elution sections. This is executed by physically rotating the adsorbent in small columns counter current in the opposite direction of the process fluids. (Animation courtesy of Xendo Manufacturing)

This approach ensures optimal adsorbent utilization and leads to a far more efficient, compact and economical separation process as compared to a fixed column system.

Counter-Current Liquid-Liquid Extraction

Computer Methods in Chemical Engineering

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Problem Statement: The following is a schematic diagram of an N-stage counter-current extraction operation, where E is the extract stream, R is the raffinate stream.

y1 +-----+ y2 yi +-----+ yi+1 yN +-----+ yN+1

E <----| |<----- ... <-----| |<----- ... <-----| |<----- E |stage| |stage| |stage| | 1 | | i | | N | R ---->| |-----> ... ----->| |-----> ... ----->| |-----> R

x0 +-----+ x1 xi-1 +-----+ xi xN-1 +-----+ xN

The following equations describe this counter-current extraction process.

xi-1 - (1+S)*xi + S*xi+1 = 0 for i=1..N

yi=K*xi

where x = the composition of the raffinate phase, R

y = the composition of the extract phase, E

K = the partition coefficient

S = K*E/R

N = the number of stages

Another equivalent form of the first equation is:

x0-xN SN+1-S

------ = -------

x0-xN+1 SN+1-1

Given E=200 lb/hr, R=300 lb/hr, K=3, and inlet raffinate and extract compositions of x0=0.03 and yN+1=0, respectively, write a program to determine the number of stages, N, required for the process to achieve an exit raffinate composition of xN=0.001. (Note that N is an integer -- you either have a stage or you don't.) Furthermore, output the composition of the raffinate phase at each stage.

Computer Methods in Chemical Engineering -- Counter-Current Liquid-Liquid Extraction

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