Let us see what happens in case of batch system.
Typical step Pharma industry: Product concentration, after reaction or washings, to evaporate solvent used in the process.
Conventional system used: Reaction kettle under vacuum to evaporate the solvent.
Batch time: 5 hrs to 20 hrs. [The product gets heated unnecessarily for so many hours]
Heating media used: Steam where possible or Hot water if product is much heat sensitive. [Hot water is poor heat transfer media, hence requires more heating hours to compensate the products sensitivity]
Condensing media used in condenser: Chilled water or brine [To compensate drawback of long heating hours in kettle, high vacuum is used to reduce operating temperatures. Hence solvent vapours are required to be condensed using low temperature chilled water or brine having higher operating cost ]
Typical problems: Residue generation as product polymerizes-reduces product yield, side reaction leading impurities in the product, affects product colour, solvent loss, inconsistent results due to human operation.
Let us see the mass balance if continuous process is used instead of batch process –
Take example of 5000 kg batch size containing 10% w/w product (500 kg) and 90% w/w solvent (4500 kg). All solvent is evaporated in say 10 hrs. A evaporator for same duty will be of just 500 kg/hr capacity [5000 kg/10hrs] evaporating solvent at constant rate of 450 kg/hr [450 x 10 hr = 4500 kg] giving product at the rate of 50 kg/hr [50 x 10 hrs =500 kg/hr]. So you see the end result is same.
Now let us see the advantages using continuous system –
In batch, the evaporation rate is quite high at initial period and then drops to very low level as it proceeds. It can be as high as 700-800 kg/hr in the early hours and then drop to 200-100 to even 50 kg/hr rate. In continuous system the same stands at constant rate of 450 kg/hr. Now, to take care of the peak load requirement – (1) you have to make big size vapour ducts and requires higher areas for condensers (2) You have to maintain high flow of cooling media in the condensers which is not required when evaporation rate drops. Ultimately you have to invest in larger cooling towers OR chillers which are highly underutilized when evaporation rate drops (3) You have to invest for high capacity steam boilers unnecessary when that quantum of steam is not required when evaporation rate drops. In continuous process, all the investment in utilities is for average & constant evaporation rate, thus requires lesser investment than batch system.
In most of the cases the drawback of high contact time in kettle is compensated by using low temperature heating i.e. by hot water and by using high vacuum in the system. The use of high vacuum brings down the solvent boiling points. Hence one has to use chilled water or brine in the condenser which has higher operating and maintenance cost than the normal cooling tower water. The hot water is poor heat transfer media too.
The continuous process offers short contact time. Hence we can use moderate vacuum and steam as heating media. Thus the process will be faster and economical.
There is always some air leakage in the system whether it is a continuous system or a batch system. Air gets saturated with solvent and thus solvent is lost. Now higher the system volume, higher the airleage rate and thus higher is the solvent loss. Thus the solvent is always higher in batch system due to its overall system volume.
The hold up in the continuous system is just few litres than the hole batch volume in batch process. Thus the impact on product, if any, due to disturbance in utility is minimal compared to complete batch at stake.
The residence time of the product (i.e. the contact time of product with heating media) is few seconds in continuous system due to small hold up. Whereas the product remains hot during the complete batch time.
Which process can be easily converted to continuous processes
Evaporation: As explained above, we can use evaporators like Falling Film Evaporator (FFE), Rising Film Evaporator (RFE) and Agitated Thin film Evaporators (ATFE) and their combinations for solvent evaporation.
Drying: It is seen that kettle is used for concentration. The concentrate is then filtered or centrifuged. The cake is then dried in batch dryers. Here we can use the evaporator for pre-concentration followed by ATFD. Thus the dry solids can be obtained from solution stage in a close loop without intermediate handling.
Extraction: Organic phase containing product is washed with solvent either to remove impurity from it or transferring the product from organic phase to solvent phase. Multiple solvent washes are given to achieve best possible results. Each wash involves – addition of solvent, mixing, settling and decantation. Here we offer our rotating disc type continuous extractor. The organic phase and solvent phase enters the system only at one point, opposite to each other. With counter current operation, the final extract and raffinate are obtained. The extractor offers excellent mixing feature. Less solvent consumption and better yield are the main advantages than batch system. Less solvent means less recovery cost at the downstream.