MOISTURE IN THE CHEMICAL PROCESS INDUSTRIES

Dr. D. M  Mohunta and P. V. Mohan

Commercial, Chemical And Development Company

No. 1, Umayal Street, Kilpauk, Chennai 600 010, Email: ccdc@arc-max.com

The Chemical Process Industries (CPI) employ solvents frequently for various reactions to produce bulk drugs, pesticides and other chemicals of industrial importance. These solvents, depending upon their properties, have a certain amount of moisture. This moisture often interferes with reactants or products or both and thus leads to loss of yields or additional processing and recovery costs. The presence of moisture may lead to any of these problems:

·        Reduce available reactants.

·        Reduce yields.

·        Produce unnecessary side products.

·        Corrosion

The choice of solvent as a reaction medium could be based on many factors the first of these being the ability to enhance reactions, inertness, ease of recovery and recycle, etc. If there is possibility of having a choice of solvents for a particular reaction there could be other considerations as discussed below. Further even if a single solvent is the choice its quality can affect yield, etc.

Solvents

Solvents normally used in the CPI have an equilibrium solubility of water in the range of 0.01 to 0.05 % at 25^o C (Table-1); there are cases of higher solubility and in some cases the water is soluble totally (Table-2 & 3). It should also be noted that even though freshly distilled or dehydrated solvent has a low moisture content, during prolonged storage in atmospheric storage tanks with open vents the moisture content could also increase. Moisture content could also increase during various processing operations.

Reactions in CPI

The overall efficiency of a multiple-step process depends on individual stage efficiencies. The efficiency as measured in yields, is affected by several factors, viz., process parameters like pressure, temperature, agitation, time, purity of reactants, etc. One factor, which contributes directly to the negative yield, is the moisture content, especially when reactants are halides, nitrogen and sulphur based compounds among several others.

Moisture can be deleterious for many reasons, in case of chlorination with thionyl chloride this itself breaks down, if phosgene is used it can form ureas, it can hydrolyze reactants as well as products. There are other consequences such as increased gaseous effluents, increased load of  liquid effluents and /or larger quantities of residues for incineration.

The economic consequences can be gauged by the following hypothetical example.

In a 8 step synthesis, let us suppose that 3 of the steps are moisture sensitive. It is further assumed that the 5 steps are giving an average yield of 90 % each and the yield of 3 moisture sensitive steps is 87 % for each step. Thus the total overall yield is 38.84 %. If by removal of moisture the yield in 3 steps rises to 95 %, the overall yield will now be 50.63 %.

There is on the average 30 % increase in productivity or decrease in cost. In addition, there is also decrease in fixed and overhead costs. Let us say the cost of raw material is 65 % and fixed & overhead costs are 35 %. The overall cost reduces by 24.3 %, a percentage that represents a substantial sum for costlier products in the range of Rs 500 + per Kg. In absolute terms the numbers can be very large if the production is in tonnages.

Moisture Loss Factor

It is often erroneously assumed that if a solvent has a low moisture content of say 0.02 %, it is least likely to affect the process yield. The percentage of loss as reactant (taken as loss of yield for convenience) is calculated by

Percentage loss of reactant = M_s x F_m

where M_s is the moisture content of solvent, % w/w

           F_m is a factor for moisture contribution; F_m  = F_SR  x  F_RM

where F_SR = mass of solvent / mass of reactant.

          F_RM = formula weight of reactant / formula weight of water.

Just as one tends to choose a suitable solvent based on its equilibrium moisture content, one should find out the F_m factor and base decisions on F_m also.

The overall loss of reactant is directly proportional to the moisture content of the solvent and F_m (or the combined effect of F_SR & F_RM).

As F_SR increases, i.e. as one increases the mass of solvent used for the process, even at the same moisture content, the yields would reduce. It is observed that in normal industrial practice, F_SR lies between 0.1 and 2.0; it can vary in certain cases. Similarly, as the F_RM increases, in other words, as the reactant molecule becomes larger (in formula weight) the loss of reactant, for the same moisture content, is higher. For most of the processes, F_RM lies between 5 and 20; exceptionally in rare cases it does go to 22 ~ 25 or lower than 5. Hence the combined effect of F_SR & F_RM on F_m can be taken to be in the range of 0.5 to 50, spread over a wide variety of combinations of reactants and solvents.

Based on the above, the yield loss (as measured in terms of unavailable reactant) has been computed for various initial moisture contents of solvents for a F _m factor up to 50. the results have been plotted (Fig 1 - F_m verses F_SR & Fig 2 - % Yield loss vs F_m).

Discussion

For convenience, assume a reactant of formula weight 270 (F_RM = 15) having a solvent to reactant ratio of 1.5; F_m as calculated would be 405. If a solvent with an initial moisture content of 0.02 % (w/w) is used, the loss of yield (as reactant) would be 8.1 % (w/w), as a direct contribution from moisture alone. Instead, if the solvent had been treated to remove moisture prior to process, to a moisture content of say 0.01 % (w/w), the loss, due to moisture alone reduces to 4.05 % (w/w) a 50% reduction in losses.

Alternately, the ratio of solvent to reactant can be reduced, to get a smaller F_SR; this of course, requires a check on other factors affecting the reaction. There is the option of changing to a solvent with lower equilibrium F_SM if this is permissible taking into consideration other factors.

The above presupposes that decrease in moisture content is not an option, there is additional processing cost if moisture content is lowered. Reduction of moisture content may be a better option for existing plants. It requires an additional processing loop rather than changes in a producing plant. The economic point to be considered is the cost of moisture removal versus the benefits.

Moisture Removal

1) It is suggested to reduce moisture to the minimum possible levels required for operation. Extremely low moisture content although desirable may not economic. The methods currently being practiced are,

·        Membrane based systems.

·        Adsorption systems (where applicable).

·        Molecular sieves.

·        Azeotropic distillation

2) As mentioned earlier, there is considerable moisture pick-up, when solvents are kept for extended periods, in storage vessels, day tanks & metering tanks, that have atmospheric vents. In order to reduce moisture pick-up during storage, it is suggested that the vessels be provided with conservation vents to reduce frequency of breathing or silica gel breathers. Wherever relevant, it may be gainful to use an inert purge in the vessel.

Benefits

There are both direct and indirect benefits:

·        Costly raw materials & their intermediates are saved.

·        Load to the Effluent Treatment Plant or Incinerator (Thermal oxidizer) is reduced, thereby reducing the impact on the environment.

·        Where applicable, costly methods of trying to recover the reactants can be reduced if not eliminated

·        In some cases, unwanted corrosive emissions are minimized.

·        Improved overall yields and hence productivity.

Table-1: Solvents having low water solubility

Table-2: Solvents that exhibit high solubility of water

Table-3: Solvents that exhibit total solubility in water