Process Synthesis

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Suspensions Processing Guide: GBHE SPG PEG 310

Process Synthesis

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Process Engineering Guide: Process Synthesis CONTENTS INTRODUCTION

1 A SUGGESTED GENERAL APPROACH 2 EXAMPLES OF PROCESS SELECTION 2.1 Harvesting and Thickening of Single Cell Protein

2.2 Dewatering of a Specialty Latex 3 REFERENCES TABLES 1 THE ADVANTAGES AND DISADVANTAGES OF DIFFERENT RANGE

OF PH FOR “PROTEIN” ORGANISM FLOCCULATION 2 THE ADVANTAGES AND DISADVANTAGES OF VARYING EXTENTS

OF CELL BREAKAGES 3 PREDICTED AND OBSERVED FILTER CAKE SOLIDS CONTENTS FOR

THE VARIOUS LATICES AFTER COAGULATION



FIGURES 1 THE “PROTEIN” BACTERIAL HARVESTING SYSTEM 2 PROCESS FOR MANUFACTURE OF CALCIUM CARBONATE FILTERS

3 H-ACID ISOLATION 4 A SUGGESTED APPROACH TO DETERMINING FEASIBLE PROCESS

OPTIONS, AND OPERATING CONDITIONS FOR SEPARATION OF FINE SOLIDS FROM SUSPENSION

5 MODULI VERSUS SOLIDS CONTENT FORTYPICAL FORWARD

FLOCCULATED “PROTEIN” SUSPENSIONS 6 DECISION TREE FOR SELECTION OF AS1 HARVESTING

CONDITIONS WHEN PRINCIPAL CONSTRAINT CONCERNS THE DEGREE OF THICKENING REQUIRED IN THE CONCENTRATE

7 DECISION TREE FOR SELECTION OF AS1 HARVESTING

CONDITIONS WHEN PRINCIPAL CONSTRAINT CONCERNS THE USE OF FLOTATION AS A UNIT OPERATION FOR THICKENING

8 DECISION TREE FOR SELECTION OF AS1 HARVESTING

CONDITIONS WHEN PRINCIPAL CONSTRAINT CONCERNS THE QUALITY OF THE RECYCLED LIQUOR

9 MODULUSSOLIDS CONTENT CURVES FOR THEVARIOUS

COAGULATED LATICES



INTRODUCTION One generally cannot consider the effects of different solid-liquid separation unit operations In Isolation from one another as: (a) In most potential solid-liquid separation processes, several unit operations

follow each other sequentially. Thus material properties have to be optimized for the whole range of steps, not just for a particular operation.

(b) At an early stage of design of a solids separation process, one will

generally want to consider various possible types of step (e.g. filtration, centrifugation) from which a final choice of method is made. To do this the outcome of different procedures must be directly comparable. Ideally, too, such a comparison should be capable of being made on the basis of simple, easily-executed laboratory tests, rather than time consuming semi-technical or pilot-plant trials.

The kinds of sequences of operations encountered in typical suspension separation operations are illustrated by the flow diagrams for three established commercial European processes shown in Figures 1-3. We will return to more detailed discussion of one of these examples in Section 2, after outlining procedures for process synthesis. 1 A SUGGESTED GENERAL APPROACH A basic procedure, which we believe is the most helpful route to determining appropriate choices of process, and process conditions, for separation of fine particles from suspension, is displayed in Figure 4. For the most part the flow diagram is self-explanatory but a few points are worthy of further comment: (a) The approach is an iterative one - infeasible options are steadily

eliminated as more data becomes available (cf the procedure for selection of flocculants in Section 2). As a rule it is found that, as might be expected, the greater the complexity of the problem, the more iterations are needed.



(b) With regard to the latter, it is important not to try to arrive at the final technological solution too quickly (i.e. in too few iterations) otherwise one may In fact end up performing an unnecessary amount of physical testing. Rather it is essential to perform some analysis, whittle down the options, and then proceed to the next, more detailed stage on a limited front. An example would be the thickening of slurry to prescribed solids content. Conceivably flotation, sedimentation, filtration or centrifugation could be used. Usually it is wise to first measure the network modulus/solids content relationship for samples at various degrees of thickening. Inspection of the curve will immediately tell one whether the first TWC options are "runners" or whether a significant pressure head is needed to give the desired solids content. Whatever the outcome of the experiment the number of possibilities will be approximately halved right away.

(c) The scheme given In Figure 4 should, if properly employed, yield both the

appropriate choices of unit operation (e.g. flotation, centrifugation) and of operating conditions (pH, pressures and so on). It should be noted that for any particular problem there will probably not be one “correct” answer; rather there will often be a series of possibilities (unit operations and (linked) operating conditions) which will have particular advantages in specific circumstances. For example, for thickening problems the route eventually chosen will depend very much on local factors such as equipment availability and cost of energy. Accordingly, the object of the scheme is to derive a series of decision trees from which a selection of processes can be made on a logical basis.

(d) Although Chapter 3 has dealt mainly with the physico-chemical factors

governing solid-liquid separation it Is, of course, essential to perform a parallel study of the process options based upon cost considerations. Implicitly in Figure 4 it is assumed that this aspect of solving the problem will be “built-in” right from the beginning to eliminate work on completely impractical routes. Process synthesis methods for solids separation and handling are, as yet, rather rudimentary compared with the techniques available for gas-liquid systems. However, some preliminary studies of possible methodology have recently been presented by Rossiter [1].



(e) One deficiency of Figure 4 is that at present the relative efficiency of membrane processes compared with the other mechanical separation techniques cannot be particularly easily assessed, except by inspection of the results of quite lengthy experimentation. There is no obvious way round this difficulty at the current time: the physical principles underlying membrane separation differ considerably from the (common) fundamentals which govern operations such as sedimentation, filtration, and flotation and so on. Further discussion of these physical principles pertaining to membrane processes may be found earlier in GBHE SPG PEG 302 and GBHE SPG PEG 307.

2 EXAMPLES OF PROCESS SELECTION 2.1 Harvesting and Thickening of Single Cell Protein This example, due to the complexity of the system, and the multipurpose nature of the objectives of work on the material, provides a particularly good illustration of how a separation problem may be dissected. As some aspects of single cell protein harvesting have already been discussed in GBHE SPG PEG 302 and GBHE SPG PEG 307, detail will be kept to a minimum here. Instead we will describe the general approach taken and why certain routes were followed. At the outset the aims of fundamental studies on this system may be described as:

(a) To provide a basis for operating the (existing) process outlined in Figure 1, in the most efficient manner.

(b) To provide a basis for design and operation of novel processing for

harvesting (ie descendents of 1). In particular to give a means of selecting the best options when economic and other conditions were very different from those prevailing in the early 1980s. Regimes which one would like to consider included those of high and low energy costs, variability of effluent disposal restrictions, and ease and simplicity of operation (for possible building of plants in areas where skilled technical staff would be scarce).

The first step in the procedure was "to go back to the beginning" and consider what unit processes could conceivably be used.



Attempts at membrane separation had previously proved extremely unpromising and, due to the strong fouling propensity of the biological suspension, were not considered further. Instead some kind of conventional mechanical dewatering seemed inevitable with sedimentation, flotation, centrifugation and filtration all being runners, either singly or in combination. The small size (~ 1 micron) and low density (only marginally greater than water) of the bacteria meant, however, that enlargement of species size, by some kind of flocculation operation, was required for any of the unit operations to be practical. However, even after flocculation, due to the near neutral buoyancy of the bacterial aggregates, sedimentation tended to be very slow probably (but not certainly) ruling it out as the thickening method. Filtration was also eliminated at an early stage: scouting filtration studies had been somewhat disappointing owing to floe breakup and material loss [2]. In addition, filtration was not an operation particularly lending itself to rapid continuous thickening of a perishable material. Thus at the end of the first assessment the options were clear: flocculation followed by flotation or centrifugation, singly or in combination. Sedimentation and filtration were relegated to a "reserve list", put aside but not entirely forgotten in case the preferred options proved flawed. Flocculation could either involve components of the broth (perhaps after pH adjustment to stimulate aggregation) or added floe agents. However, in the latter case, toxicology imposed severe constraints on the kinds of chemical additives which might be used. Costs also limited dose rates. Another factor to be considered In any solution was that the rheological characteristics of the thickened suspension had to allow for convenient handling and control of the drying process. When more detailed work was embarked upon, quite limited studies were sufficient to indicate that it was unlikely that an “all purpose” floc agent could be found which would satisfy the various restrictions. The best that could probably be hoped for was that a natural flocculant (e.g. a gum) could be found which would be added to retrieve the situation when the ‘normal” flocculation mechanism broke down. Costs, however, seemed likely to be prohibitive if continuous dosing were applied. Accordingly, study was concentrated on optimization of flocculation by the method of cell lysis (generally by heat shock) followed by pH adjustment to cause aggregation of the cells, cell fragments and released biopolymers. In the first stage of “coarse grained” testing studies were made of the flocculation mechanism, and of the floc and thickened suspension properties using simple, and now familiar, tools such as optical microscopy and shear modulus measurements.



In these investigations, the various properties were characterized as such parameters as flocculation pH, and degree of cell breakage were changed. Some results from the early parts of the study are displayed in Figure 5. The data from the investigations provided an indication of the likely key physico-chemical variables, e.g. pH, heat-shock temperature (through, amongst other things, its effect on cell breakage), and the balance of biochemical’s in the supernatant. It also provided some immediate conclusions concerning process options - for example above ~ 9% w/w solids the mechanism of thickening was consolidation of a cohesive structure (see e.g. Figure 5). As in flotation, there is only a marginal compressive effect upon the float blanket; flotation could not be used for thickening to, say, and 20% solids. For this, centrifugation (with or without primary thickening by flotation) was needed. In a second, more detailed, part of the investigation flotation testing (see Section 3.6) and sophisticated biochemical and colloid analysis were added to the comparatively simple techniques used in the first experiments. This enabled a picture of the effects of changing the key variables to be steadily built up. Tables of the positive and negative effects of selecting a particular value of a key process or material variable were now written down (see Tables 1 and 2). This was a particularly important step in this “case history” as there were clearly conflicting requirements for different stages of the process. For example, open-textured flocs suitable for flotation were facilitated by high pH whereas low pHs help give clear centrifugates. “Decision trees” for selection of design options and best operating conditions could now be constructed by inspection of the various PRO/COB tables. Three examples of the latter are shown in Figures 6-8. It should be noted that the decision trees cover both general and specific aspects of solving the problem, e.g. Figure 6 concerns the unit operations and conditions needed to give a particular final degree of thickening; Figure ‘7 deals with optimization of flotation alone. In the final stage of the work one of the more promising novel approaches, viz “reverse flocculation” (in which acidification is followed by heat shock, contrary to the standard process) with subsequent one-stage thickening by centrifuge, was examined on a semi-technical scale and data was obtained for a large-scale plant design package for potential licensing (Section 3.4). Further details on this example are given in GBHE SPG PEG 302 and GBHE SPG PEG 307.



2.2 Dewatering of a Specialty Latex Our second example has been briefly alluded to in an earlier chapter GBHE SPG PEG 302 in relation to predictive testing for filtration behavior. However it also illustrates well how different process options can be compared and sifted to give a limited number of practicable alternatives. The basic problem concerned dewatering of a specialty, methacrylate based, latex. An existing product (XC 32) was coagulated with electrolyte, subject to limited heat treatment to "condition" the flocs so that they could withstand mechanical abrasion better, and then filtered to - 50% solids. The resulting cake was then dried. However a new product (XC 37), when put through the same process train, gave only ~ 25% solids in the cake with a subsequent enormous increase in drying costs. Efforts to improve matters, for example by use of alternative coagulants, proved unsuccessful. Obviously the coagulated material was only likely to be concentrated to a satisfactory degree in equipment which could exert a significant concentrating pressure, e.g. a vacuum filter, a pressure filter or a centrifuge. Accordingly, as ultimate "dewaterabillty" for all these instances is governed by network strength, it was appropriate to measure the modulus solids content relationships for XC 32 and XC 37. Results are displayed in Figure 9, investigations being made of both heat-treated (as in the plant) and non-heat-treated systems. From these data estimates were also made of the solids content expected to be found in a filter cake at the pressures prevailing on the plant (Table 3). Agreement between prediction and observation is excellent. From even these limited measurements a number of important conclusions could be drawn. By far the most important was that the high moisture contents in the filter cake resulted from the fundamental material characteristics of the new product. Thus the only way forward was either to: (a) Use a higher concentrating pressure in the dewatering equipment; OR (b) Weaken the propensity of XC to form strong, open, relatively

incompressible networks. (NB: the above are the key process and material variables referred to in Figure 4)



With reference to (b), comparison of the network strength data with known results for other (coagulated) polymer latices of similar particle size (~ 0.3 µm) suggested that XC 37's behavior was in no way unexpected for the kind of dispersion involved. Rather than XC 32 being "bad", it was the case that the older product (due to a fortuitous combination of a particle size effect with the other processing conditions) was unusually “good". As a consequence it was unlikely that a simple change of coagulating agent would solve the problem, a conclusion supported by the negative results from the original screen for alternative floc agents, The comparative data for heat-treated and non-heat-treated suspensions did show that abandoning the conditioning would probably result in network weakening and higher filter cake solids, though perhaps there might be a price to be paid in terms of effects such as greater floc breakup and associated phenomena such as loss of fines. However, examination of the predicted moisture contents suggested that the degree of increase in solids would not nearly go far enough (ie within striking distance of the performance achieved for XC 32) to be satisfactory. Thus only two options remained open: (i) Use of a higher concentrating pressure, perhaps associated with

abandonment/modification of the heat treatment step. (ii) A search for a flocculant (most probably a high molecular weight polymeric

agent) which would give easily compressible flocs of the latex. Owing to the uncertainty of outcome of (ii), the first pathway was selected. A centrifuge was used to obtain a better degree of concentration than before, heat treatment being retained to minimize floc breakup prior to dewatering.



























