Distillation Column Pressure - Low Vs High

Recently Willis has initiated a question on cheresources forum that - Is there any energy saving if we reduce or increase the column pressure to the maximum possible extent. It was really an interesting post & I thought to write on this issue here also.


First we have to understand the question. The question is What is the impact on energy consumption during the separation of a mixture using distillation at different pressures? Let us consider a binary case of two components with fixed feed composition. So the question is what should be the pressure of distillation column for a desired recovery & purity of top & bottom products, so that energy consumption is low.

When you talk about energy consumption let us consider the gross energy required site wide so that overall impact can be assessed, I mean that every component of energy consumer should be converted finally to the fossil fuels.

We would like to simplify this problem so that justifications are easier. Let us consider that all the top product is @ 100% pure lighter key (to deal with pure compnent properties) and all the bottom is 100% pure heavy key. In this way we have fixed the top & bottom temperatures corresponding to operating pressure. Now assume that P1 is low pressure operation & P2 is high pressure operation.

Now starting from top, for maintaining the purity, you need to have higher amount of reflux or more no of stages in case of P2. Since discussion is not concerned about the capital investment it is better to consider more no of stages by keeping the same reflux amount. When reflux amount is same your vapor & liquid loads will remain same in the rectifying section of the column.

In such case, by internal heat balance, you will find that bottom vapors generation will also be more or less same and therefore, stripping section loads will also remain same.

After establishing the loads, let us focus on different energy consumption points.

1. Downstream system may generally be at higher pressure & accordingly you need more energy for pumping in case of P1.


2. When you decrease the pressure to P1, the latent heat of bottom product will go up & then you need more energy for the same amount of vaporization in the reboiler.


3. At lower pressure vapor densities goes down & you need bigger dia column at P1. Though it is not a concern in this case, but is applicable resulting in more cost.


4. At lower pressure saturation temperature goes low & you can use low level heat e.g. LP steam in place of HP steam. So if you compare with same HP steam at P2, you can generate power (Theoretically) & then use LP steam for reboiler.


5. But this is often misunderstood by energy saving, which is not true. In this case actual Kcal energy requirement goes up due to higher latent heat.


6. In case of low pressure you may need chilled water etc for condenser as condensing utility which is a costlier proportion than saving from HP to LP steam because of cascading effect of efficiencies in the process.


7. In such case, one should not limit himself to chilled water requirement but he should also consider its - generation cost including LP for absorption chilling / Power for compression chilling / pumping cost / system losses etc. In totality chilled water usage cost is much higher than LP steam generally.


8. If system is under vacuum, than the energy required for the operation of vacuum system is to be added in total energy requirement.


9. Total energy should be considered based on front end fossil fuel consumption e.g. for power we should consider fossil fuels consumed in the power plant at generation site than eqv transmission losses etc. Then only you will be able to truely compare the energy consumption

Next step is what happens to the VLE data in both the cases of P1 & P2. VLE is mostly favorable at low pressure for non-interactive / non-azeotrope type systems in general. The impact of VLE is visible for systems which have higher relative volatility (Alpha) of more than 2.0. Below this the differences on each stage are not very significant.

My view however is that if you have Alpha of more than 2.0 then you practically should avoid distillation as the preferred choice for separation. Other methods may be more cost & energy effective.

In above post, please make sure that you do not get confused by energy requirement, energy consumption & cost as all of them are different. You may consume more LP but still your cost may be lower if your steam generation cost is cheaper. At some plant sites power is cheaper than steam on the other hand steam is cheper than power in case of some specific sites. So overall site wide analysis is necessary before selecting the distillation column pressure.

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Flow Measurement - Errors & Corrections

Using flowmeters, but totally frustrated when they give inaccurate readings???

Considering flow meters, completing all mass balance & then finding the meter reading is wrong????

Calibrating, Checking Re-installing & still finding incorrect readings?????



Oh! GOD......What to do? Why....??
Read on for better understanding of flow measurement.

Let us start from basics of flow metering from orifice type devices which are the simplest one & most commonly used. Let me start this with an example so that its more clear........without covering too much theory.

So let us say that upstream conditions in case of an orifice are P1, T1 which are also the design conditions. Let us consider fixed mass flow rate m. So the density of this gas will be

rho1 = P1 x M / ( R x T1 ), where M is the mol wt of gas.

Let us assume that design condition for downstream pressure is P2.

Now the orifice equation is

m = Cv Y A (2 rho1 x (P1-P2))^0.5 Find it Here.

Now during the design based on above condition Cv & A are fixed, P1 & T1 varies so that rho1 also varies. so now let us simplify this equation for our use as below.

m = K x (DP x P1 x M / ( R x T1))^0.5

Ultimately, it will reduce to

m = k x ( DP x P1 / T1)^0.5.......................Eqn(1)

Now to understand the actual phenomenon, let us understand that how DP varies with the variation in P & T Or say collectively it is density (rho).

Remember there are two different things one is actual DP experienced by orifice due to changes in density. Orifice never measures volume or mass flow it measures only DP. Second thing is the indication of this DP in terms of volumetric flow or mass flow which is obatined by its full range DP (Volume flow) & then multiplying it by an assumed constant density figure (Mass flow) by your instrument people. This is the actual point of error.

So let us first understand what will be the new DP when mass flow m is fixed as per our assumption. There are two cases only....

1. Density increases (due to variation in P&T)


2. Density decreases (due to variation in P&T)

In case-1, when density increases, the volume flow will go down (mass flow remaining same) & hence velocity across orifice will go down. Therefore it will read lesser DP (more velocity means more DP). So indication will read less flow, but when you apply correct density your mass flow will remain same. Similarly, vice versa happens with the case-2. So there is a correction formula for reading actual figures as below.

Corrected actual flow = Indicated flow x ((Pa/Pd) x (Td/Ta) x (Ma/Md))^0.5

This is a general formula for any gas flow including variation in mol wt. Subscript 'a' indicates actual values during measurement & 'd' indicates design values for which orifice is designed.

This part is often missed by production, process engineers & instrument people. Once you apply this correction, you can have more accurate readings.

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Energy: Why important for Process Engineers

Fuel for energy is generated by the nature. For example, C1 (carbon in the form of CO2) is naturally synthesizes (photosynthesis in presence of natural UV radiation) to molecule Cn (like starch, sugar or cellulose), which are used directly /or indirectly as fuel/ or feed stocks for fuel generation. Consumption of available fuels higher than generation is creating not only fuel shortage but also leaving behind bad consequences to the environment due to exhausted excess carbon dioxide.

Being desired transformations can’t be avoided in the process industries, higher energy wastages in them demands higher input energy. How energy is wasted in the process industry? To understand it, consider following cases:

1. Doing more, thus wasting more



2. Doing more, but using less



3. Wasting more, thus doing more

Doing more, thus wasting more
For example, pump filling overhead constant level tank at higher flow rate than required, thus subsequently overflowing excess amount of liquid to ground tank. Being pump is operating at much higher capacity than required, excess energy is wasted.

Doing more, but using less
For example, use of higher capacity conveyor for small amount of material transportation. Underutilization of material conveyor wastes more energy for unit production rate.

Wasting more, thus doing more
For example, high number of pipe fittings used in the pipe routing leads to high fluid energy wastage in terms of frictional pressure drop, thus more work is to be done by the pump.

Right amount of energy for desired transformations is the ultimate aim of energy conservation. Reduction in energy wastage to minimum reduces the energy demands in the process plants. Following steps for energy conservations are discussed here:

1. Good Housekeeping



2. Optimization of Plant Operation



3. Hardware Modification or Replacement



4. Process Modification

Good Housekeeping
This is foremost simple step, which any process industry can easily adopt. Arresting steam leakages, stopping idle operating pumps or mixing agitators, etc. are the typical examples falls under this category. Being the simple steps, it absolutely doesn’t require any financial investment and can be implemented at any time. It requires only the energy conscious culture in the process industry. This step itself can save ~5% in energy bills of the industry.

Simply organizations need culture where everyone from shop floor to top management feels himself involved in the process.

Optimization of Plant Operation
Excess capacity, provided by the plant designer as safety margin, of the plant hardware provides the scope of using it during actual plant operation. Thus, process plant hardware need to be tuned for energy efficient operation. Liquid transportation with maximum flow capacity of the pump, minimizing reflux ratio for distillation column, etc. are the well known examples for process plant optimization. The optimization exercise require either no or minor financial investment. Though some operational time may be consumed for setting rightful optimum parameters, but often results attained are very positive as far as energy saving is concerned.

Hardware Modification or Replacement
Reduced efficiency of existing hardware e.g. scaled or blocked heat exchanger tubes, entry of innovative energy efficient devices in the market like high efficiency distillation column packing, poor selection of hardware at the time of first purchase e.g. lower pipeline size, etc. demands much higher input energy than actually required for desired transformation. Necessary hardware modification or replacement conserves energy in the process plant. Unlike previous two categories, it requires low to medium level financial investment along with process plant stoppage for changeover of the troubling hardware.

Process Modification
Many times overall or part of the process is modified for the sake of overall energy savings. Process routes as well as transformation steps are altered to minimize input energy supplies. It requires extensive studies at various levels starting from paperwork to laboratory, pilot plant & semi-commercial levels. Usually such process modifications require high investments as well as higher implementation time for gaining energy benefits. But they are essential at a stage when business need improvement due to tough market competition.

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