Make-up stream

Water and ethanol form a low boiling point azeotrope. So, water cannot be completely separated from ethanol by straight distillation. To produce absolute (100 per cent) ethanol it is necessary to add an entraining agent to break the azeotrope. Benzene is an effective entrainer and is used where the product is not required for food products. Three columns are used in the benzene process. Column 1. This column separates the ethanol from the water. The bottom product is essentially pure ethanol. The water in the feed is carried overhead as the ternary azeotrope of ethanol, benzene and water (24 per cent ethanol, 54 per cent benzene, 22 per cent water). The overhead vapour is condensed and the condensate separated in a decanter into, a benzene-rich phase (22 per cent ethanol, 74 per cent benzene, 4 per cent water) and a water-rich phase (35 per cent ethanol, 4 per cent benzene, 61 per cent water). The benzene-rich phase is recycled to the column as reflux. A benzene make-up stream is added to the reflux to make good any loss of benzene from the process. The water-rich phase is fed to the second column. 

The isobutane recycle from fractionation can also contain appreciable entrained free water. If the recycle contains no olefins (from outside make-up streams, for example) this stream can be chemically dried by contacting it with the discard acid from the unit. This is an efficient and a relatively inexpensive procedure. 

Manual calculations are also very useful when solving flowsheets that use recycle and purge. Purge streams are often withdrawn from recycles to prevent the accumulation of species that are difficult to separate, as described in Section 2.15. A typical recycle and purge flow scheme is illustrated in Figure 4.42. A liquid feed and a gas are mixed, heated, reacted, cooled and separated to give a liquid product. Unreacted gas from the separator is recycled to the feed. A make-up stream is added to the gas recycle to make up for consumption of gas in the process. If the make-up gas contains any inert gases, then over time these would accumulate in the recycle and eventually the reaction would be slowed down when the partial pressure of reactant... 

Elution efficiency. In flow systems with SPE involving derivatisation immediately after elution, it is possible to visually evaluate the efficiency of the elution process. In the ideal situation, a very abrupt colour formation is observed immediately after the inlet of the make-up stream. When elution is a limiting factor for optimum system performance, a slow fading of the colour is observed. 

Hydrogen make-up stream shown is the stoichiometric chemical consumption in the octafiner. Typical source of this stream is the off-gas produced from a catalytic reformer with a hydrogen content in the range of 75-95 mole %. 

Manipulate make-up streams, as solvent, acid/base solutions or catalyst. Example control the pFI in a precipitation tank. 

Let s take a look at the control loops. Now the make-up stream of reactant B is fed on level controller LCf of the buffer tank. The flow rate of the exit stream (Recycle) is set at a constant value by a simple specification on the stream s script. There are also two level controllers LCt and LCb for the top and bottom inventories of the distillation column, which manipulate the distillate and bottom products, respectively. Besides, the top column pressure is kept constant by means of the condenser duty. Quality control is implemented only for the bottom product, the reflux being fixed. We considered a composition measure with a first-order lag transmitted to a controller that manipulates the reboiler duty. 

Once fixed a flow in each recycle loop, one can determine what valve should be used to control each inventory variable. Inventory may be controlled with fresh reactant make-up streams but also with streams leaving the units. Liquid streams may be added to a location where the level varies with the amount of that component in the process. Similarly, gas fresh feed streams may be added to a location where the pressure gives a measure of the amount of that material in the process. 

The total raw material cost for each process is about the same. The cost of oxygen in the one-stage process is nearly offset by the higher cost for hydrochloric acid make up in the two stage system. The hydrochloric acid make-up stream is about seven times greater in the two-stage process. 

The molar stream flow rates in the stripping section can be made dependent on the rectifying section flow rates. This dependency is made by assuming that the relationship between the product streams D and B can be based on the feedstream composition, in that, ideally, all of component i makes up stream D and all of component / makes up stream B. Admittedly, this is a simplification, but the alternative is an increasing complexity that is probably not justified. 

Although the water make-up stream is much smaller than the total solvent flow, the 5 min holdup time in the base of the methanol column is able to handle dynamic changes in the water solvent eirculation rate. When the feed is increased, the ratio immediately increases the solvent fed to the extractive eolumn. The level in the base of the methanol column immediately begins to drop. However, the increased flows to the extractive column eventually produce an inerease in the feed to the methanol column, which brings the base level back up. 

The water in the absorber is thus recycled. What have we neglected to include with the recycle A purge. If the natural gas contains impurities that are soluble in water, but have boiling points comparable to water, the impurities will accumulate in the absorber-distiller loop. Which stream should we purge Obviously, it is easier to dispose of the stream after the distiller. And if we purge, we must replenish the loss of water in the loop. We add a combiner for the make-up stream. 

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